ANATOMY AND PHYSIOLOGY
CHAPTER XIII. THE URINARY SYSTEM
As body cells carry out metabolic activities they
consume oxygen and nutrients and produce waste
products such as carbon dioxide, urea, and uric acid.
Wastes must be eliminated from the body because
they can be toxic to cells if they accumulate. While the
respiratory system rids the body of carbon dioxide, the
urinary system disposes of most other wastes. The
urinary system performs this function by removing
wastes from the blood and excreting them into urine.
Disposal of wastes through the release of the urine is
not the only purpose of the urinary system. The urinary
system also helps regulate blood composition, pH,
volume, and pressure; maintains blood osmolarity; and
produces hormones.
Adopted from: Tortora, G.J. & Derrickson,
B., 2009. Principles of Anatomy and
Physiology, 12th edition. New Jersey: John
Wiley & Sons, Inc
This chapter of the module has been designed to
help you understand further how the urinary system
works. It is organized into 4 lessons.
Lesson 1: Overview of the Urinary System
Lesson 2: Components of the Urinary System
Lesson 3: Physiology of Urine Formation
Lesson 4: Fluids, Electrolytes, and Acid-Base
Homeostasis
LESSON 1. OVERVIEW OF THE URINARY SYSTEM
INTRODUCTION OF THE LESSON AND PRESENTATION OF OUTCOMES
You may already have an idea on how important the urinary system is. For you to be
able to understand the vitality of the urinary system, it is best that you should learn its
functions.
After studying the lesson, you must have:
1. defined the urinary system
2. identified the vital functions of the urinary system
CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
WARM-UP ACTIVITY
Word Hunt
Find the 7 terms that are related to the functions of the urinary system. You might find my
secret message too.
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PRESENTATION OF LEARNING INPUTS
The Urinary system contributes to homeostasis by excreting wastes; altering blood
composition, pH, volume, and pressure; maintaining blood osmolarity; and producing
hormones.
Functions of the Urinary System
The kidneys do the major work of the urinary system. The other parts of the system
are mainly passageways and storage areas. Functions of the kidneys include the following:
1. Excretion of wastes. By forming urine, the kidneys help excrete wastes from the
body. Some wastes excreted in urine result from metabolic reactions. These include urea and
ammonia from the deamination of amino acids; creatinine from the breakdown of creatinine
phosphate; uric acid from the catabolism of nucleic acids; and urobilin from the breakdown
of haemoglobin. Urea, ammonia, creatinine, uric acid, and urobilin are collectively known as
nitrogenous wastes because they are waste products that contain nitrogen. Other wastes
excreted in the urine are foreign substances that have entered the body, such as drugs and
environmental toxins.
2. Regulation of blood ionic composition. The kidneys help regulate the blood levels
of several ions, most importantly sodium ions (Na +), potassium ions (K+), calcium ions, (Ca2+),
chloride ions (Cl-), and phosphate ions (HPO42-). The kidney accomplish this task by adjusting
the amounts of these ions that are excreted into the urine.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
3. Regulation of blood pH. The kidneys excrete a variable amount of hydrogen ions
into the urine and conserve bicarbonate ions (HCO3-), which are an important buffer of
H+ in the blood. Both of these activities help regulate blood pH.
(H+)
4. Regulation of blood volume. The kidneys adjust blood volume by conserving or
eliminating water in the urine. An increase in blood volume increases blood pressure; a
decrease in blood volume decreases blood pressure.
5. Regulation of blood pressure. The kidneys also help regulate blood pressure by
secreting the enzyme renin, which activates the renin-angiotensin-aldosterone pathway.
Increased renin causes an increase in blood pressure.
6. Maintenance of blood osmolarity. By separately regulating loss of water and loss
of solutes in the urine, the kidneys maintain a relatively constant blood osmolarity close to
300 milliosmoles per liter (mOsm/liter).
7. Production of hormones. The kidneys produce two hormones. Calcitrol, the active
form of vitamin D, helps regulate calcium homeostasis, and erythropoietin stimulates the
production of red blood cells.
8. Regulation of blood glucose level. Like the liver, the kidneys can use the amino acid
glutamine in gluconeogenesis, the synthesis of new glucose molecules. They can then release
glucose into the blood to help maintain a normal blood glucose level.
ASSESSMENT (POST-ASSESSMENT)
Identify the terms described.
________________1. An enzyme that helps regulate blood pressure through activation of
renin-angiotensin-aldosterone pathway.
________________2. A process which describes the synthesis of new glucose molecules from
non-glucose materials.
_______________3. A hormone that stimulates the production of red blood cells.
_______________4. An active form of Vitamin D which helps regulate calcium homeostasis.
_______________5. A waste products that contain nitrogen.
_______________6-10. What are the important ions to which the kidneys help regulate blood
levels of the above mentioned ions.
REFERENCES
Tortora, G., & Derrickson B. 2017. Tortora’s Principles of Anatomy and Physiology 15th ed.
Singapore: John Wiley & Sons, Inc.
Tortora, G., & Derrickson. B. 2009. Principles of Anatomy and Physiology 12th ed. New
Jersey: John Wiley & Sons,Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
LESSON 2. COMPONENTS OF THE URINARY SYSTEM
INTRODUCTION OF THE LESSON AND PRESENTATION OF OUTCOMES
After learning the basic function of the urinary system, we proceed with studying the
structures of the urinary system. It is time for you to learn the anatomy of the urinary system.
After studying the lesson, you must have:
1. described the anatomy of the urinary system
2. identified the functions of the structures of the urinary system
WARM-UP ACTIVITY
Directions: Identify which of the following statements are True (T) and which are False (F) by
circling the appropriate letter.
T
F
1. The short length of the female urethra predisposes the females to urinary tract
infections.
T
F
2. The male urethra serves both excretory and reproductive functions.
T
F
3. The urinary bladder is located in the epigastric region.
T
F
4. The parasympathetic nerves are responsible for urinary retention.
T
F
5. Micturition is initiated by the stretch of reflex which occurs in the bladder wall.
T
F
6. The ureters begins at the neck of the urinary bladder and opens to the outside
of the body.
T
F
7. Micturition in adults is under the control of the central nervous system.
T
F
8. The desire to micturate occurs when the bladder accumulates about 700 ml of
urine.
T
F
9. The urinary bladder serves as temporary storage area of urine.
T
F
10. The ureters are tubes extending from the pelvis to the urinary bladder.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
PRESENTATION OF LEARNING INPUTS
Nitrogenous waste products are continuously produced by the body. The kidneys are
responsible for clearing the body of these products through the production of urine. The urine
produced is removed from the body by the process of micturition which is carried by the joint
functions of the urinary ureters, bladder, and urethra.
Figure 1. The Urinary System
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
A. KIDNEYS
1. Gross Structure. The kidneys are paired, bean-shaped organs located
on the posterior wall of the abdominal cavity on either side of the vertebral column
from the level of T12 to L3. They are 10 to 12 cm in length, 5 to 6 cm in width and
3 to 4 cm in thickness. The lateral border is convex while the medial is concave. In
the center of the concave border, there is deep longitudinal fissure through which
the vessels and nerves enter and leave. This fissure is known as the hilus. A ureter
merges from each hilus and courses downward to the urinary bladder which is
located behind the pubis.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
If the kidney is cut open into longitudinal halves, one will see that the hilus
expands into a central cavity called the renal sinus. In here lies the pelvis, a funnelshaped sac and cuplike tubes know as calices (sing, calyx). There are three major
calices which branch into about eight minor calices.
Each kidney is composed of a dark or reddish-brown cortex and a light
medulla. The medulla is composed of 8 to 18 cone-shaped subdivisions called renal
pyramids. Their bases are toward the cortex and their apices or papilla project into
the lumen of the minor calices. At the lateral boundaries of the pyramids are found
inward extensions of the cortex known as the renal columns of Bertin. From the
bases of the medullary pyramids, there are the medullary rays of Ferrein which
represent continuations of bundles of tubules from the pyramids into the cortex.
The medullary rays and the associated tissues of the cortex constitute a renal
lobule.
The renal papillae empty urine, which is formed in the nephrons, into the
calices, which drain into the renal pelvis, from there the urine flows through the
ureters.
Figure 2. Longitudinal Surface of the Kidney
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
2. Microscopic Structure. The unit of structure of the kidney is a nephron.
There are about two million nephrons in each kidney. A nephron consist of a renal
corpuscle and renal tubules.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
a. Renal corpuscle. If consists of a glomerulus, a tuft of capillaries
surrounded by a Bowman’s capsule. The corpus or body has a vascular
pole where the afferent arterioles enter and leave the glomerulus and
a urinary pole where the slit-like cavity in the Bowman’s capsule is
continuous with the proximal tubule.
Figure 3. Renal Corpuscle
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
b. Renal tubules. These consist of the following:
1) Proximal convoluted tubule. This makes up the bulk of
the renal cortex. The wall of the convoluted portion consists of a
single layer of epithelial cells with brush borders of microvilli. The
straight portion is linked with cuboidal epithelium.
2) Descending limb of the loop of Henle. The thin segment
of the loop is lined with squamous cells without brush border of
microvilli.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Figure 4.a. Nephron Unit
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Figure 4.b. Nephron Unit
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
3) Ascending limb of loop of Henle. The thick segment of
the loop has thicker walls than the descending limb. This limb is
continuous with the distal convoluted tubules.
4) Distal tubule. The straight portion of the distal tubules
enters the cortical tissues, returns to the renal corpuscle and
attaches to the afferent arteriole. The epithelial cells of the distal
tubule that come in contact with the afferent arteriole are more
dense than the other tubular epithelial cells and are collectively
called macula densa. These cells appear to be secreting some
substance toward the afferent arterioles. The smooth muscle
cells of the afferent arteriole that come in close contact with the
macula densa are swollen and contain heavy granules composed
mainly of inactive renin. These cells are called juxtaglomerular
cells; the whole complex of macula densa juxtaglomerular cells is
called the juxtaglomerular complex.
The substance renin is produced whenever the blood supply
is decreased. When renin is released into the blood, it acts on
plasma angiotensinogen and converts it to angiotensin I.
Angiotensin I is acted upon by a converting enzyme to angiotensin
II, a potent vasoconstrictor. It also stimulates the adrenal cortex
to secrete aldosterone which acts on the collecting tubules. This
causes reabsorption of more sodium and water which correct
reduction of plasma and interstitial tissue fluid volume (see
discussion on aldosterone).
5) Collecting tubules. The distal convoluted tubules empty
their contents here. The tubules unite with each other in the
medulla and the final convergence leads to the formation of the
papillary duct. This opens through the hole in the apex of the
papilla into a minor calyx.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 1. Histological Features of the Renal Tubule and Collecting Duct
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
3. Blood Supply. The blood supply of the kidneys comes from the renal
artery, a branch of the abdominal aorta. It branches at the hilus and enters the
kidney substance. The branches give rise to the afferent arterioles of the glomeruli.
Figure 5. Blood Supply of the Kidneys
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
4. Lymphatic Vessels. There are two sets of lymphatic vessels, the
capsular and those in the kidney substances. There are abundant anastomoses
between the two sets. The capsular lymphatics join the lymphatic vessels of the
nearby organs. The lymphatic vessels in the kidney substances from network
around the tubules; the large vessels accompany the blood vessels and make their
exit at the hilus. Lymphatic vessels are not present in the glomeruli or the medullary
rays.
5. Nerve Supply. The kidneys are supplied with visceral afferent fibers
that transmit impulse, from the kidneys to the central nervous system. Efferent
fibers are derived from the celiac plexus.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
B. URETERS
The ureters are tubes extending from the pelvis to the urinary bladder. They
are 26 to 28 cm long and are lined with the transitional type of epithelium. They consist
of a muscular layer, an inner longitudinal layer, and an outer circular layer. The
contraction of the muscles serve to propel the urine from the kidneys to the urinary
bladder.
Figure 6. Ureters
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
1. Blood Supply. The blood supply is derived from the renal artery, branch of
the testicular (male) or ovarian (female) arteries and the common iliac artery.
2. Nerve Supply. The afferent nerve fibers carry impulses from the ureters to
the central nervous system. Visceral afferent nerves from the autonomic system
transmit impulses to the smooth muscles in the walls of the ureter.
C. URINARY BLADDER
The urinary bladder is a hollow viscus organ which serves as temporary storage
area of urine. It is located in the hypogastric region, the ureters enter obliquely
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
through the bladder wall. The entrance of the two ureters and the exit of the urethra
at the inferior angle from the vesical trigone. The prostate gland in males attaches at
the inferior angle of the neck of the bladder.
The epithelium of the urinary bladder is also of the transitional type. The strong
smooth muscle, known as the detrusor muscle, forms the internal sphincter of the
bladder at the exit of the urethra. The urinary bladder has two sphincters: the internal
or involuntary sphincter and the external or voluntary sphincter. The internal
sphincter is located at the internal urethral opening while the external sphincter lies
about 2 cm beyond the internal sphincter.
Figure 7. Urinary Bladder
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
1. Blood Supply. The blood supply is derived from the vesical arteries, middle
rectal artery and in females, also from the uterine and vaginal arteries.
2. Nerve Supply. The parasympathetic nerves are responsible for bladder
emptying while the sympathetic nerves are concerned with urine retention.
3. Micturition (Urination). This is the process by which the urinary bladder
empties it contents. The peristaltic contraction of the ureters forces the urine
intermittently from the renal pelvis into the bladder. These peristaltic waves occur at
intervals of ten to twenty second. The muscles of the bladder exhibit a certain amount
of tone. As the bladder fills, the pressure rises but slightly due to the gradual relaxation
of muscles. When about 250 to 300 ml of urine have been collected and the pressure
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
reaches about 180 mm H2O, the sensory endings of the pelvic nerve in the wall of the
bladder are stimulated, and there arises a sense of fullness and a desire to urinate.
The impulses over the pelvic nerve are carried to the vesical center in the sacral cord.
In a very young child the center discharges efferent impulses over the parasympathetic
fibers in the pelvic nerves which stimulate the detrusor muscle and inhibit the
sphincters. In an infant this is purely a reflex action. Voluntary control of micturition
normally develop by age two or three years of age.
In the adult the same mechanism exist, but the impulses from the bladder also
pass upward to the brain. The cerebrum normally has a controlling influence over the
spinal center. The individual may either release the spinal center from this cortical
inhibition and void the urine, or he is able, for a certain length of time to reinforce the
inhibition. By voluntary contraction of the abdominal muscles (with glottis closed and
diaphragm fixed), external pressure may be applied to the bladder and the expulsion
of the urine facilitated. (We should call attention to this unusual situation of voluntary
control being exercised over a process governed by the autonomic nervous system).
The desire to micturate occurs when the bladder has accumulated about 300
ml of urine. Micturition, however, can be controlled until 700 ml of urine has
accumulated. By this time, micturition becomes urgent.
D. URETHRA
The urethra begins at the neck of the urinary bladder and opens to the outside
of the body. The male urethra is 18 to 20 cm long and is common to both urinary and
reproductive systems. It has three parts, namely:
1. Prostatic urethra, which is 2 to 5 cm long extending from the urinary
bladder to the pelvic floor and is completely surrounded by the prostate gland. This
portion of the urethra receives drainage from the prostate gland and the ejaculatory
ducts.
2. Membranous urethra, which is about 1.3 cm long, extending from apex of
the prostate gland to the bulb of the urethra.
3. Spongy or cavernous urethra, which is between 12.7 to 15.2 cm long
located in the penis and extends from the termination of the membranous portion to
the external orifice.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Figure 8. a. Male Urethra
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Figure 8. b. Female Urethra
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
The female urethra is 2.54 cm to 2.79 cm in length and belongs only to the
urinary system. The urethral orifice in the vestibule appears as a small slit. It is about
one inch from the clitoris. The urethral sphincter is composed of striated muscles
surrounding the orifice. The short length of the female urethra predisposes the
females to urinary tract infection.
The urethra conveys urine from the urinary bladder to the outside. In males, it
also transports semen during ejaculation.
ASSESSMENT (POST-ASSESSMENT)
Directions: Match each of the structures with the corresponding characteristics. Write the
letter of the correct answer to the blank provided before each number.
_______1. glomerulus
a.
funnel-shaped sac
_______2. calyx
b.
consist of brush border
_______3. descending limb of Henle
c.
bean-shaped organ
_______4. renal pelvis
d.
cup-like tube
_______5. proximal convoluted tubules
e.
thin segment without tubules brush border
_______6. ascending limb of Henle
f.
tuft of capillaries
_______7. ureters
g.
_______8. nephron
h. hollow viscus organ
_______9. hilus
i.
_______10. Bowman’s capsule
j. unit of structure of the kidneys
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thick segment, continuous with distal
tubules
extend from pelvis to the urinary bladder
k.
surrounds glomerulus
l.
fissure through which vessels and nerves
enter and leave.
17
CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Directions: Match each of the structures with the corresponding functions. Write the letter
of the correct answer on the blank provided before each number.
_______11. urethra
a. urine storage
_______12. urinary bladder
b. convey/s urine from kidney to urinary bladder
_______13. ureters
c. urine formation
_______14. kidneys
d. convey’s urine from the urinary bladder to the
outside
REFERENCES
Tortora, G., & Derrickson. B. 2017. Tortora’s Principles of Anatomy and Physiology. 15th ed.
Singapore: John Wiley & Sons, Inc.
Tortora, G., & Derrickson. B. 2009. Principles of Anatomy and Physiology. 12th ed. New
Jersey: John Wiley & Sons, Inc.
Gailan, T.K.P.1997. Self-Instructional Monograph. Urinary System. UP Manila: NTTC-HP.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
LESSON 3. PHYSIOLOGY OF URINE FORMATION
INTRODUCTION OF THE LESSON AND PRESENTATION OF OUTCOMES
As a future health care professional, it is very important to know the physiology of
urine formation. As mentioned earlier that the main functions of the kidneys are to clear the
plasma of waste products and to reabsorb those products which are useful to the body. The
waste products are released in the form of urine which is a watery solution of nitrogenous
waste and inorganic salts.
After studying the lesson, you must have:
1. explained the physiology of urine formation
2. identified the three processes of urine
formation
WARM-UP ACTIVITY
Directions: Identify which of the following statements are True (T) and which are False (F)
circling the appropriate letters.
T
F
1. The random specific gravity of urine is about 1.010.
T
F
2. The wide efferent arteriole offers resistance to inflow of urine.
T
F
3. The higher the hydrostatic pressure of the fluid in the Bowman’s capsule,
the higher the glomerular filtration rate.
T
F
4. Colloid osmotic pressure opposes glomerular filtration rate.
T
F
5. The normal quantity of urine for 24 hours is about one liter.
T
F
6. An increase in arterial pressure increases glomerular filtration rate.
T
F
7. The distal convoluted tubules reabsorb sodium and secrete hydrogen ions
and potassium.
T
F
8. Glucose is reabsorbed through the process of osmosis.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
T
F
9. The anti-diuretic hormone promotes obligatory water reabsorption by the
tubules.
T
F
10. Substances which are secreted from the blood are potassium, ammonia,
and certain drugs.
PRESENTATION OF LEARNING INPUTS
Physiology of Urine Formation
There are three main process by which urine is formed. These are glomerular
filtration, tubular reabsorption, and tubular secretion. The production of urine begins with
filtration through the glomerular filtrate passes down the tubules and its volume is reduced
and its composition. Altered by tubular reabsorption and tubular secretion the former
accounts for the removal of water and solutes; the latter for the addition or exchange of
solutes.
Figure 9. Relation of a nephron’s structure to its three basic functions: glomerular filtration,
tubular reabsorption, and tubular secretion.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
A. GLOMERULAR FILTRATION
Glomerular filtration is defined as the passage of protein-free plasma through the
glomerular capillaries into the Bowman’s capsule.
The fluid that filter through the glomerulus into the Bowman’s capsule is called
glomerular filtrate (plasma minus significant amount of proteins). The quantity of glomerular
filtrate formed each minute in the nephrons of both kidneys is called the glomerular filtration
rate (100-150 ml/minute in males and 105-132 ml/minute in females). This is equivalent to
7.5 L per hour or about 180 L per day.
Figure 10. Filtration Membrane
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Glomerular filtration occurs in the same manner that fluid filters out of any high
pressure capillary in the body. The blood passes through the afferent arteriole to the
glomerulus and leaves by way of the efferent arteriole. The anatomical structure of the
arterioles favors high blood pressure in the glomerulus and in effect favors the filtration of
blood. The high blood pressure in the glomerulus results from a wider afferent arteriole than
the efferent arteriole which gives resistance to the outflow in the glomerulus, a build-up of
pressure in the glomerular tuft and filtration into the Bowman’s capsule.
Another factor which effects glomerular filtration is the hydrostatic pressure of the
fluid in the Bowman’s capsule. Normally this is low (45-60mmHg) and does not impede
filtration into the capsule. The greater the capsular pressure, however, the lesser will be the
glomerular filtration rate.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Figure 11. The Pressures That Drive Glomerular Filtration.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Still another factor is the plasma colloid osmotic pressure. Proteins are substances
which do not penetrate the pores of the capillary membrane, therefore, they exert pressure
in the membrane and this pressure is the colloid osmotic pressure. The greater the plasma
colloid osmotic pressure the lesser the glomerular filtration rate.
1. Other Factors Affecting Glomerular Filtration Rate
a. Renal blood flow. The greater the rate of the flow of the blood into the
glomerulus, the greater the filtration rate.
b. Afferent arteriolar constriction. Afferent arteriolar constriction decreases
the rate of blood flow into the glomerulus and also decreases glomerular
pressure. This factor decreases glomerular filtration rate.
c. Efferent arteriolar constriction. Constriction of efferent arteriole offers
resistance to outflow thus increasing glomerular pressure and glomerular
filtration rate as well.
d. Arterial pressure. An increase in arterial pressure increases glomerular
filtration rate and vice-versa. This is the main reason why patients with high blood
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
pressure experienced frequency of urination even during night time. When
arterial pressure remains between 80 to 200 mmHg there is a little change in flow
rate. When arterial pressure is below 60mmHg, there is no urine output when a
patient is in shock.
e. Sympathetic stimulation. During mild to moderate sympathetic
stimulation, the efferent arterioles constrict thus reducing renal blood flow and
consequently, glomerular filtration rate.
This is the reason why during emergencies, a person experiences low urine output.
Table 2. Regulation of Glomerular Filtration Rate (GFR)
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12th edition. New
Jersey: John Wiley & Sons, Inc.
B. TUBULAR REABSORPTION
As the glomerular filtrate passes along the tubules, it is concentrated and essential
substance are conserved. About 97 per cent of the substances like water and selected
materials useful to the body are reabsorbed at the peritubular capillaries.
In the proximal convoluted tubules, about 70-80 per cent of the filtrate is reabsorbed.
This is enhanced by the presence of extensive brush border and the basal invagination of the
tubular cells.
The substance reabsorbed are sodium, potassium, bicarbonate chloride, glucose,
amino acids, Vitamin C, and water. The processes affecting reabsorption are diffusion,
osmosis, and active transport (explained in detail in the module cell). Next is a table showing
which process is responsible for reabsorbing specific substances.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
1. Active Transport
Glucose, amino acids, calcium ions,
phosphates, potassium ions, bicarbonate
ions
2. Diffusion
Chlorides, phosphates and bicarbonate
ions
3. Osmosis
Water
TABLE 3. Processes Involve in the Reabsorption of Substances by the Renal Tubules.
1. Absorptive Capabilities of Tubules.
a. Proximal tubules. About 65 per cent of all reabsorption occurs in these
tubules. This is so because of the presence of extensive brush border and basal
invaginations of tubular cells.
b. Thin segment of loop of Henle. This segment has no brush border but has an
extensive pore system. This segment is adapted for simple diffusion of substances.
The proximal convoluted tubules and loop of Henle reabsorb sodium, potassium,
glucose, and chloride.
c. Thick segment of loop of Henle and distal tubules. These have epithelial cells
and only rudimentary brush order. These are adapted for active transport of
sodium. The distal tubules absorb sodium and secrete hydrogen ions and
potassium. This segment is impermeable to water.
d. Collecting tubules. This is totally impermeable to urea. The urine becomes
acidic or basic in these tubules. These tubules reabsorb water by osmosis.
Factors Affecting Water Reabsorption
a) Anti-diuretic hormone (ADH). The hypothalamic cells are stimulated by baroreceptors
located in the vascular tree, probably by arterial and venous receptors in the thorax and
receptors in the left atrium. Their impulses are apparently carried by the vagus nerve. These
baroreceptors are activated by an increase blood pressure due to an expansion of the extra
cellular fluid volume. Under these circumstances, there is inhibition of secretion of ADH by
the posterior pituitary gland. This diminishes water reabsorption and contributes to the
desired reduction in extra-cellular volume. Conversely, a diminished extracellular volume
causes a release from inhibition by baroreceptors and results in an increase ADH secretion
and conservation of water for volume restoration. This type of reabsorption is known as
facultative reabsorption.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
b) Sodium Chloride (NaCl). The reabsorption of sodium chloride causes the blood to
become hypertonic, therefore, by osmosis water leaves the tubules to re-establish isotonicity.
The process is known as obligatory reabsorption
c) Blood Volume. A reduction in blood volume stimulates the volume receptors in the
vascular system. As a result, ADH is released.
d) Aldosterone. The volume of the extracellular fluids depends primarily on its sodium
content. The presence of aldosterone, a mineralocorticoid secreted by the adrenal cortex, in
the plasma enhances sodium retention by the kidneys, specifically the distal convoluted
tubules. As a solute, sodium attracts water consequently, there is expansion of the volume of
the extracellular fluid.
The plasma level of aldosterone is controlled by the kidneys. Specialized kidney cells
forming the juxtaglomerular complex are presumably sensitive to sodium ion concentration.
When this is low, the cells secrete into the blood an enzyme known as renin. This enzyme
catalyzes the production of angiotensin a small polypeptide from angiotensinogen, a protein
synthesized by the liver and always present in the blood. Angiotensin, in turn, stimulates the
release of aldosterone from the adrenal cortex. When aldosterone causes sodium retention,
the secretion of renin and the production of angiotensin are reduced.
Table 4. Hormonal Regulation of Tubular Reabsorption and Tubular Secretion
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
C. TUBULAR SECRETION
The tubules are secretory cells for certain substances. Among the substances which are
secreted to the filtrate from the blood are potassium, hydrogen ions, ammonia and certain
drugs.
Secretion occurs in the distal and collecting tubules. In the distal convoluted tubules,
excess hydrogen and potassium ions circulating in the peritubular blood are exchanged for
sodium ions in the urine. Since ions must not become too acidic, the direct exchange of
hydrogen for sodium ions would be restricted without compensatory mechanism to control
the acidity. The tubules accomplish this mechanism by synthesizing ammonia salts to keep
the blood and body fluids alkaline. The ammonia formed neutralizes the acidity thus, allowing
the hydrogen ion exchange to continue. This process is essential to the kidney’s function of
maintaining a normal blood pH by eliminating acid (in the form of hydrogen ions which are
considered to be acid-former) from the plasma and maintaining the base reserves (in the form
of bicarbonate ions which are considered as base-formers).
By passing through the convoluted tubules, the glomerular filtrate changes from
isotonic to hypertonic fluid (urine).
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 5. Summary of Filtration, Reabsorption, and Secretion in the Nephron and
Collecting Duct.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 6. Substances Filtered, Reabsorbed, and Excreted in Urine
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Table 7. Characteristics of Normal Urine
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
ASSESSMENT (POST-ASSESSMENT)
Directions: Indicate which of the following statements are True (T) and which are False (F) by
encircling the appropriate letter.
T
F
1. Increase in arterial pressure causes increase in glomerular filtration rate.
T
F
2. Water is reabsorbed mainly by active transport.
T
F
3. A large percentage of reabsorption occurs in the distal tubules.
T
F
4. The collecting tubules are totally impermeable to urea.
T
F
5. Water reabsorption resulting from the action of anti-diuretic secretion is
facultative.
T
F
6. Urine output is low during emergencies because of parasympathetic
stimulation.
T
F
7. Constriction of the afferent arteriole increases pressure in the glomerulus.
T
F
8. Hemorrhage will stimulate release of anti-diuretic hormone to compensate
for blood loss.
T
F
9. Renin is secreted as a response to a decrease in sodium ion concentration
on the blood.
T
F
10. As the hydrostatic pressure of the fluid in the Bowman’s capsule increases,
glomerular filtration rate decreases.
T
F
11. The reabsorption of sodium ions causes obligatory reabsorption of water.
T
F
12. Aldosterone indirectly influences water reabsorption by acting the tubules
to absorb sodium ions.
T
F
13. When a person takes very little amount of fluids, the specific gravity of his
urine decreases.
T
F
14. Renin is an enzyme that catalyses the production of angiotensin.
T
F
15. A reduction in blood volume stimulates the volume receptors in the
vascular system, as a result, ADH is released
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
REFERENCES
Tortora, G., &. Derrickson. B. 2017. Tortora’s Principles of Anatomy and Physiology. 15th ed.
Singapore: John Wiley & Sons, Inc.
Tortora, G., & Derrickson. B. 2009. Principles of Anatomy and Physiology. 12th ed. New
Jersey: John Wiley & Sons,Inc.
Gailan, T.K.P. 1997. Self-Instructional Monograph. Urinary System. UP Manila: NTTC-HP.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
LESSON 4. FLUIDS, ELECTROLYTES, AND ACID-BASE HOMEOSTASIS
INTRODUCTION OF THE LESSON AND PRESENTATION OF OUTCOMES
As you have learned in the previous lesson, how the kidneys form urine. One
important function of the kidneys is to help maintain fluid balance of the body. Regulatory
mechanisms involving the kidneys and other organs normally maintain homeostasis of body
fluid. Malfunction in any or all of them may seriously endanger the functioning of the organs
throughout the body. In this lesson you will explore the mechanisms that regulate the volume
and distribution of body fluids and examine the factors that determine the concentration of
solute and the pH of body fluids.
After studying the lesson, you must have:
1. described the various fluid compartments of the body
2. explained the regulation of both water and solutes
3. explained the movement of water between body fluid
compartments
4. compared the electrolyte composition of the three
major fluid compartments
5. discussed the functions and regulation of sodium,
chloride, potassium, bicarbonate, calcium, phosphate
and magnesium ion
6. compared the roles of buffers, exhalation of carbon
dioxide, and kidney excretion of H+ in maintaining pH of
body fluids
7. described the different types of acid - base imbalances
WARM-UP ACTIVITY: Word Hunt
Find the 20 terms that are related to fluids, electrolytes and acid-base homeostasis. You
might find my secret message too.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
PRESENTATION OF LEARNING INPUTS
Fluid Compartments and Fluid Homeostasis
A body fluid is a substance, usually a liquid that is produced by the body in consists of
water and dissolved solutes. In lean adults, body fluids constitute between 55 & 60% of total
body mas in females and males respectively. Body fluids are present in two main
“compartments” – inside cells and outside cells. About two-thirds of body fluid is intracellular
fluid (ICF) (intra=within) or cytosol, the fluid within cells. The other third called extracellular
fluid (ECF) (extra= outside), is outside cells and includes all other body fluids. About 80% of
the ICF is interstitial fluid (inter=between), which occupies the microscopic spaces between
tissue cells , and 20% of the ECF is blood plasma, the liquid portion of the blood. Other
extracellular fluids that are grouped with interstitial fluid include lymph in lymphatic vessels;
cerebrospinal fluid in the nervous system; synovial fluid in joints’ aqueous humor and vitreous
body in the eyes; endolymph and perilymph in the ears and pleural, pericardial, and
peritoneal fluids between serous membranes.
Two general “barriers” separate intracellular fluid, interstitial fluid, and blood plasma.
1. The plasma membrane of individual cells separates intracellular fluid from the
surrounding interstitial fluid. You learned in previous chapter that the plasma membrane is a
selectively permeable barrier: It allows some substances to cross but blocks the movement of
other substances. In addition, active transport pumps work continuously to maintain different
concentrations of certain ions in the cytosol and interstitial fluid.
2. Blood vessel walls divide the interstitial fluid from blood plasma. Only in capillaries,
the smallest blood vessels, are the walls thin enough and leaky enough to permit the
exchange of water and solutes between blood plasma and interstitial fluid.
The body is in fluid balance when the required amounts of water and solutes are
present and are correctly proportioned among the various compartments. Water is by far the
largest single component of the body, making up 45–75% of total body mass, depending on
age and gender, and the amount of adipose tissue (fat) present in the body. Obese people
have proportionally less water than leaner people because water comprises less than 20% of
the mass of adipose tissue. Skeletal muscle tissue, by contrast is about 65% water. Infants
have the highest percentage of water, up to 75% of body mass. The percentage of body mass
that is water decreases until about 2 years of age. Until puberty, water accounts for about
60% of body mass. However, lean adult females have more subcutaneous fat than do lean
adult males. Thus, their percentage of total body water is lower, accounting for about 55% of
body mass.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Figure 12. Body fluid compartments.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
The processes of filtration, reabsorption, diffusion, and osmosis allow continual
exchange of water and solutes among body fluid compartments. Yet the volume of fluid in
each compartment remains remarkably stable. The pressures that promote filtration of fluid
from blood capillaries and reabsorption of fluid back into capillaries can be reviewed in Figure
10 on page 21. Because osmosis is the primary means of water movement between
intracellular fluid and interstitial fluid, the concentration of solutes in these fluids determines
the direction of water movement. Because most solutes in body fluids are electrolytes,
inorganic compounds that dissociate into ions, fluid balance is closely related to electrolyte
balance. Because intake of water and electrolytes rarely occurs in exactly the same
proportions as their presence in body fluids, the ability of the kidneys to excrete excess water
by producing dilute urine, or to excrete excess electrolytes by producing concentrated urine,
is of utmost importance in the maintenance of homeostasis.
Sources of Body Water Gain and Loss
The body can gain water by ingestion and by metabolic synthesis (Figure 13). The main
sources of body water are ingested liquids (about 1600 mL) and moist foods (about 700 mL)
absorbed from the gastrointestinal (GI) tract, which total about 2300 mL/day. The other
source of water is metabolic water that is produced in the body mainly when electrons are
accepted by oxygen during aerobic cellular respiration and to a smaller extent during
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
dehydration synthesis reactions. Metabolic water gain accounts for only 200 mL/day. Daily
water gain from these two sources totals about 2500 mL. Normally, body fluid volume
remains constant because water loss equals water gain. Water loss occurs in four ways. Each
day the kidneys excrete about 1500 mL in urine, the skin evaporates about 600 mL (400 mL
through insensible perspiration, sweat that evaporates before it is perceived as moisture, and
200 mL as sweat), the lungs exhale about 300 mL as water vapor, and the gastrointestinal
tract eliminates about 100 mL in feces. In women of reproductive age, additional water is lost
in menstrual flow. On average, daily water loss totals about 2500 mL. The amount of water
lost by a given route can vary considerably over time. For example, water may literally pour
from the skin in the form of sweat during strenuous exertion. In other cases, water may be
lost in diarrhea during a GI tract infection.
Figure 13. Sources of daily water gain and loss under normal conditions..
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12th edition. New
Jersey: John Wiley & Sons, Inc.
The volume of metabolic water formed in the body depends entirely on the level of
aerobic cellular respiration, which reflects the demand for ATP in body cells. When more ATP
is produced, more water is formed. Body water gain is regulated mainly by the volume of
water intake, or how much fluid you drink. An area in the hypothalamus known as the thirst
center governs the urge to drink. When water loss is greater than water gain, dehydration—
a decrease in volume and an increase in osmolarity of body fluids—stimulates thirst (Figure
14). When body mass decreases by 2% due to fluid loss, mild dehydration exists. A decrease
in blood volume causes blood pressure to fall. This change stimulates the kidneys to release
renin, which promotes the formation of angiotensin II. Increased nerve impulses from
osmoreceptors in the hypothalamus, triggered by increased blood osmolarity, and increased
angiotensin II in the blood both stimulate the thirst center in the hypothalamus. Other signals
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
that stimulate thirst come from (1) neurons in the mouth that detect dryness due to a
decreased flow of saliva and (2) baroreceptors that detect lowered blood pressure in the
heart and blood vessels. As a result, the sensation of thirst increases, which usually leads to
increased fluid intake (if fluids are available) and restoration of normal fluid volume. Overall,
fluid gain balances fluid loss. Sometimes, however, the sensation of thirst does not occur
quickly enough or access to fluids is restricted, and significant dehydration ensues. This
happens most often in elderly people, in infants, and in those who are in a confused mental
state. When heavy sweating or fluid loss from diarrhea or vomiting occurs, it is wise to start
replacing body fluids by drinking fluids even before the sensation of thirst occurs.
Figure 14. Pathways through which dehydration stimulates thirst.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12th edition. New
Jersey: John Wiley & Sons, Inc.
Regulation of Water and Solute Loss
Even though the loss of water and solutes through sweating and exhalation increases
during exercise, elimination of excess body water or solutes occurs mainly by control of their
loss in urine. The extent of urinary salt (NaCl) loss is the main factor that determines body
fluid volume. The reason for this is that “water follows solutes” in osmosis, and the two main
solutes in extracellular fluid (and in urine) are sodium ions (Na+) and chloride ions (Cl-). In a
similar way, the main factor that determines body fluid osmolarity is the extent of urinary
water loss. Because our daily diet contains a highly variable amount of NaCl, urinary excretion
of Na+ and Cl- must also vary to maintain homeostasis. Hormonal changes regulate the urinary
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
loss of these ions, which in turn affects blood volume. Figure 15 depicts the sequence of
changes that occur after a salty meal. The increased intake of NaCl produces an increase in
plasma levels of Na+ and Cl- (the major contributors to osmolarity of extracellular fluid). As a
result, the osmolarity of interstitial fluid increases, which causes movement of water from
intracellular fluid into interstitial fluid and then into plasma. Such water movement increases
blood volume.
The three most important hormones that regulate the extent of renal Na+ and Clreabsorption (and thus how much is lost in the urine) are angiotensin II, aldosterone, and
atrial natriuretic peptide (ANP). When your body is dehydrated, angiotensin II and
aldosterone promote urinary reabsorption of Na+ and Cl- (and water by osmosis with the
electrolytes), conserving the volume of body fluids by reducing urinary loss. An increase in
blood volume, as might occur after you finish one or more supersized drinks, stretches the
atria of the heart and promotes release of atrial natriuretic peptide. ANP promotes
natriuresis, elevated urinary excretion of Na + (and Cl-) followed by water excretion, which
decreases blood volume. An increase in blood volume also slows release of renin from
juxtaglomerular cells of the kidneys. When renin level declines, less angiotensin II is formed.
Decline in angiotensin II from a moderate level to a low level increases glomerular filtration
rate and reduces Na+, Cl-, and water reabsorption in the kidney tubules. In addition, less
angiotensin II leads to lower levels of aldosterone, which causes reabsorption of filtered Na +
and Cl- to slow in the renal collecting ducts. More filtered Na + and Cl- thus remain in the
tubular fluid to be excreted in the urine. The osmotic consequence of excreting more Na+ and
Cl- is loss of more water in urine, which decreases blood volume and blood pressure.
The major hormone that regulates water loss is antidiuretic hormone (ADH). This
hormone, also known as vasopressin, is produced by neurosecretory cells that extend from
the hypothalamus to the posterior pituitary. In addition to stimulating the thirst mechanism,
an increase in the osmolarity of body fluids stimulates release of ADH. ADH promotes the
insertion of water-channel proteins (aquaporin-2) into the apical membranes of principal cells
in the collecting ducts of the kidneys. As a result, the permeability of these cells to water
increases. Water molecules move by osmosis from the renal tubular fluid into the cells and
then from the cells into the bloodstream. The result is production of a small volume of very
concentrated urine. Intake of water in response to the thirst mechanism decreases the
osmolarity of blood and interstitial fluid. Within minutes, ADH secretion shuts down, and soon
its blood level is close to zero. When the principal cells are not stimulated by ADH, aquaporin2 molecules are removed from the apical membrane by endocytosis. As the number of water
channels decreases, the water permeability of the principal cells’ apical membrane falls, and
more water is lost in the urine. Under some conditions, factors other than blood osmolarity
influence ADH secretion. A large decrease in blood volume, which is detected by
baroreceptors (sensory neurons that respond to stretching) in the left atrium and in blood
vessel walls, also stimulates ADH release. In severe dehydration, glomerular filtration rate
decreases because blood pressure falls, so that less water is lost in the urine. Conversely, the
intake of too much water increases blood pressure, causing the rate of glomerular filtration
to rise, and more water to be lost in the urine. Hyperventilation (abnormally fast and deep
breathing) can increase fluid loss through the exhalation of more water vapor. Vomiting and
diarrhea result in fluid loss from the GI tract. Finally, fever, heavy sweating, and destruction
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
of extensive areas of the skin from burns can cause excessive water loss through the skin. In
all of these conditions, an increase in ADH secretion will help conserve body fluids.
Figure 15. Hormonal regulation of renal Na+ and Cl- reabsorption.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 8. Summary of Factors That Maintain Body Water Balance
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Movement of Water between Body Fluid Compartments
Normally, cells neither shrink nor swell because intracellular and interstitial fluids have
the same osmolarity. Changes in the osmolarity of interstitial fluid, however, cause fluid
imbalances. An increase in the osmolarity of interstitial fluid draws water out of cells, and
they shrink slightly. A decrease in the osmolarity of interstitial fluid, by contrast, causes cells
to swell. Changes in osmolarity most often result from changes in the concentration of Na. A
decrease in the osmolarity of interstitial fluid, as may occur after drinking a large volume of
water, inhibits secretion of ADH. Normally, the kidneys then excrete a large volume of dilute
urine, which restores the osmotic pressure of body fluids to normal. As a result, body cells
swell only slightly, and only for a brief period. But when a person steadily consumes water
faster than the kidneys can excrete it (the maximum urine flow rate is about 15 mL/min) or
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
when renal function is poor, the result may be water intoxication, a state in which excessive
body water causes cells to swell dangerously (Figure 16). If the body water and Na lost during
blood loss or excessive sweating, vomiting, or diarrhea is replaced by drinking plain water,
then body fluids become more dilute. This dilution can cause the Na concentration of plasma
and then of interstitial fluid to fall below the normal range. When the Na concentration of
interstitial fluid decreases, its osmolarity also falls. The net result is osmosis of water from
interstitial fluid into the cytosol. Water entering the cells causes them to swell, producing
convulsions, coma, and possibly death. To prevent this dire sequence of events in cases of
severe electrolyte and water loss, solutions given for intravenous or oral rehydration therapy
(ORT) include a small amount of table salt (NaCl).
Figure 16. Series of events in water intoxication
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
ELECTROLYTES IN BODY FLUIDS
The ions formed when electrolytes dissolve and dissociate serve four general
functions in the body. (1) Because they are largely confined to particular fluid compartments
and are more numerous than nonelectrolytes, certain ions control the osmosis of water
between fluid compartments. (2) Ions help maintain the acid–base balance required for
normal cellular activities. (3) Ions carry electrical current, which allows production of action
potentials and graded potentials. (4) Several ions serve as cofactors needed for optimal
activity of enzymes.
Concentration of Electrolytes in Body Fluids
To compare the charge carried by ions in different solutions, the concentration of ions
is typically expressed in units of milliequivalents per liter (mEq/liter). These units give the
concentration of cations or anions in a given volume of solution. One equivalent is the positive
or negative charge equal to the amount of charge in one mole of H; a milliequivalent is
onethousandth of an equivalent. Recall that a mole of a substance is its molecular weight
expressed in grams. For ions such as sodium (Na +), potassium (K+), and bicarbonate (HCO3 ), which have a single positive or negative charge, the number of mEq/liter is equal to the
number of mmol/liter. For ions such as calcium (Ca2 +) or phosphate (HPO4 2-), which have
two positive or negative charges, the number of mEq/liter is twice the number of mmol/liter.
Figure 17 compares the concentrations of the main electrolytes and protein anions in
blood plasma, interstitial fluid, and intracellular fluid. The chief difference between the two
extracellular fluids—blood plasma and interstitial fluid—is that blood plasma contains many
protein anions, in contrast to interstitial fluid, which has very few. Because normal capillary
membranes are virtually impermeable to proteins, only a few plasma proteins leak out of
blood vessels into the interstitial fluid. This difference in protein concentration is largely
responsible for the blood colloid osmotic pressure exerted by blood plasma. In other respects,
the two fluids are similar.
The electrolyte content of intracellular fluid differs considerably from that of
extracellular fluid. In extracellular fluid, the most abundant cation is Na +, and the most
abundant anion is Cl-. In intracellular fluid, the most abundant cation is K +, and the most
abundant anions are proteins and phosphates (HPO4 2-). By actively transporting Na+ out of
cells and K+ into cells, sodium–potassium pumps (Na+- K+ ATPase) play a major role in
maintaining the high intracellular concentration of K + and high extracellular concentration of
Na+.
Sodium
Sodium ions (Na+) are the most abundant ions in extracellular fluid, accounting for
90% of the extracellular cations. The normal blood plasma Na + concentration is 136–148
mEq/liter. As we have already seen, Na + plays a pivotal role in fluid and electrolyte balance
because it accounts for almost half of the osmolarity of extracellular fluid (142 of about 300
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
mOsm/liter). The flow of Na through voltage-gated channels in the plasma membrane also is
necessary for the generation and conduction of action potentials in neurons and muscle
fibers. The typical daily intake of Na + in North America often far exceeds the body’s normal
daily requirements, due largely to excess dietary salt. The kidneys excrete excess Na+, but they
also can conserve it during periods of shortage.
The Na level in the blood is controlled by aldosterone, antidiuretic hormone (ADH),
and atrial natriuretic peptide (ANP). Aldosterone increases renal reabsorption of Na +. When
the blood plasma concentration of Na+ drops below 135 mEq/liter, a condition called
hyponatremia, ADH release ceases. The lack of ADH in turn permits greater excretion of water
in urine and restoration of the normal Na level in ECF. Atrial natriuretic peptide (ANP)
increases Na excretion by the kidneys when Na level is above normal, a condition called
hypernatremia.
Figure 17. Electrolyte and protein anion concentrations in plasma, interstitial fluid, and
intracellular fluid.
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Chloride
Chloride ions (Cl-) are the most prevalent anions in extracellular fluid. The normal
blood plasma Cl- concentration is 95–105 mEq/liter. Cl- moves relatively easily between the
extracellular and intracellular compartments because most plasma membranes contain many
Cl- leakage channels and antiporters. For this reason, Cl - can help balance the level of anions
in different fluid compartments. One example is the chloride shift that occurs between red
blood cells and blood plasma as the blood level of carbon dioxide either increases or
decreases. In this case, the antiporter exchange of Cl - for HCO3- maintains the correct balance
of anions between ECF and ICF. Chloride ions also are part of the hydrochloric acid secreted
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
into gastric juice. ADH helps regulate Cl- balance in body fluids because it governs the extent
of water loss in urine. Processes that increase or decrease renal reabsorption of sodium ions
also affect reabsorption of chloride ions.
Potassium
Potassium ions (K+) are the most abundant cations in intracellular fluid (140 mEq/liter).
K+ plays a key role in establishing the resting membrane potential and in the repolarization
phase of action potentials in neurons and muscle fibers; K + also helps maintain normal
intracellular fluid volume. When K+ moves into or out of cells, it often is exchanged for H+ and
thereby helps regulate the pH of body fluids.
The normal blood plasma K+ concentration is 3.5– 5.0 mEq/liter and is controlled
mainly by aldosterone. When blood plasma K+ concentration is high, more aldosterone is
secreted into the blood. Aldosterone then stimulates principal cells of the renal collecting
ducts to secrete more K+ so excess K+ is lost in the urine. Conversely, when blood plasma K +
concentration is low, aldosterone secretion decreases and less K+ is excreted in urine. Because
K+ is needed during the repolarization phase of action potentials, abnormal K + levels can be
lethal. For instance, hyperkalemia (above-normal concentration of K+ in blood) can cause
death due to ventricular fibrillation
Bicarbonate
Bicarbonate ions (HCO3 -) are the second most prevalent extracellular anions. Normal
blood plasma HCO3- concentration is 22–26 mEq/liter in systemic arterial blood and 23–27
mEq/ liter in systemic venous blood. HCO3- concentration increases as blood flows through
systemic capillaries because the carbon dioxide released by metabolically active cells
combines with water to form carbonic acid; the carbonic acid then dissociates into H and
HCO3-. As blood flows through pulmonary capillaries, however, the concentration of HCO3decreases again as carbon dioxide is exhaled. Intracellular fluid also contains a small amount
of HCO3-. As previously noted, the exchange of Cl- for HCO3- helps maintain the correct
balance of anions in extracellular fluid and intracellular fluid.
The kidneys are the main regulators of blood HCO3- concentration. The intercalated
cells of the renal tubule can either form HCO3- and release it into the blood when the blood
level is low or excrete excess HCO3- in the urine when the level in blood is too high. Changes
in the blood level of HCO3- are considered later in this chapter in this lesson on acid–base
balance.
Calcium
Because such a large amount of calcium is stored in bone, it is the most abundant
mineral in the body. About 98% of the calcium in adults is located in the skeleton and teeth,
where it is combined with phosphates to form a crystal lattice of mineral salts. In body fluids,
calcium is mainly an extracellular cation (Ca2+). The normal concentration of free or
unattached Ca2+ in blood plasma is 4.5–5.5 mEq/liter. About the same amount of Ca2+ is
attached to various plasma proteins. Besides contributing to the hardness of bones and teeth,
Ca2+ plays important roles in blood clotting, neurotransmitter release, maintenance of muscle
tone, and excitability of nervous and muscle tissue.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
The most important regulator of Ca2+concentration in blood plasma is parathyroid
hormone (PTH). A low level of Ca2+ in blood plasma promotes release of more PTH, which
stimulates osteoclasts in bone tissue to release calcium (and phosphate) from bone
extracellular matrix. Thus, PTH increases bone resorption. Parathyroid hormone also
enhances reabsorption of Ca2+ from glomerular filtrate through renal tubule cells and back
into blood, and increases production of calcitriol (the form of vitamin D that acts as a
hormone), which in turn increases Ca2+ absorption from food in the gastrointestinal tract.
Recall that calcitonin (CT) produced by the thyroid gland inhibits the activity of osteoclasts,
accelerates Ca2+ deposition into bones, and thus lowers blood Ca2+ levels.
Phosphate
About 85% of the phosphate in adults is present as calcium phosphate salts, which are
structural components of bone and teeth. The remaining 15% is ionized. Three phosphate
ions (H2PO4 -, HPO4 2-, and PO4 3-) are important intracellular anions. At the normal pH of
body fluids, HPO4 2- is the most prevalent form. Phosphates contribute about 100 mEq/liter
of anions to intracellular fluid. HPO4 2- is an important buffer of H, both in body fluids and in
the urine. Although some are “free,” most phosphate ions are covalently bound to organic
molecules such as lipids (phospholipids), proteins, carbohydrates, nucleic acids (DNA and
RNA), and adenosine triphosphate (ATP).
The normal blood plasma concentration of ionized phosphate is only 1.7–2.6
mEq/liter. The same two hormones that govern calcium homeostasis—parathyroid hormone
(PTH) and calcitriol—also regulate the level of HPO4 2-in blood plasma. PTH stimulates
resorption of bone extracellular matrix by osteoclasts, which releases both phosphate and
calcium ions into the bloodstream. In the kidneys, however, PTH inhibits reabsorption of
phosphate ions while stimulating reabsorption of calcium ions by renal tubular cells. Thus,
PTH increases urinary excretion of phosphate and lowers blood phosphate level. Calcitriol
promotes absorption of both phosphates and calcium from the gastrointestinal tract.
Fibroblast growth factor 23 (FGF 23) is a polypeptide paracrine (local hormone) that also helps
regulate blood plasma levels of HPO4 2-. This hormone decreases HPO4 2- blood levels by
increasing HPO4 2- excretion by the kidneys and decreasing absorption of HPO4 2- by the
gastrointestinal tract
Magnesium
In adults, about 54% of the total body magnesium is part of bone matrix as magnesium
salts. The remaining 46% occurs as magnesium ions (Mg2 +) in intracellular fluid (45%) and
extracellular fluid (1%). Mg2+ is the second most common intracellular cation (35 mEq/liter).
Functionally, Mg2+ is a cofactor for certain enzymes needed for the metabolism of
carbohydrates and proteins and for the sodium–potassium pump. Mg2+ is essential for
normal neuromuscular activity, synaptic transmission, and myocardial functioning. In
addition, secretion of parathyroid hormone (PTH) depends on Mg2 +.
Normal blood plasma Mg2+ concentration is low, only 1.3–2.1 mEq/liter. Several
factors regulate the blood plasma level of Mg2+ by varying the rate at which it is excreted in
the urine. The kidneys increase urinary excretion of Mg2 + in response to hypercalcemia,
hypermagnesemia, increases in extracellular fluid volume, decreases in parathyroid hormone,
and acidosis. The opposite conditions decrease renal excretion of Mg2+.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 7 describes the imbalances that result from the deficiency or excess of several
electrolytes.
People at risk for fluid and electrolyte imbalances include those who depend on others
for fluid and food, such as infants, the elderly, and the hospitalized; individuals undergoing
medical treatment that involves intravenous infusions, drainages or suctions, and urinary
catheters; and people who receive diuretics, experience excessive fluid losses and require
increased fluid intake, or experience fluid retention and have fluid restrictions. Finally,
athletes and military personnel in extremely hot environments, postoperative individuals,
severe burn or trauma cases, individuals with chronic diseases (congestive heart failure,
diabetes, chronic obstructive lung disease, and cancer), people in confinement, and
individuals with altered levels of consciousness who may be unable to communicate needs or
respond to thirst are also subject to fluid and electrolyte imbalances.
Acid- Base Balance
From our discussion thus far, it should be clear that various ions play different roles
that help maintain homeostasis. A major homeostatic challenge is keeping the H +
concentration (pH) of body fluids at an appropriate level. This task—the maintenance of acid–
base balance—is of critical importance to normal cellular function. For example, the threedimensional shape of all body proteins, which enables them to perform specific functions, is
very sensitive to pH changes. When the diet contains a large amount of protein, as is typical
in North America, cellular metabolism produces more acids than bases, which tends to acidify
the blood. In a healthy person, several mechanisms help maintain the pH of systemic arterial
blood between 7.35 and 7.45. (A pH of 7.4 corresponds to a H+ concentration of 0.00004
mEq/liter 40 nEq /liter.) Because metabolic reactions often produce a huge excess of H +, the
lack of any mechanism for the disposal of H+ would cause H level in body fluids to rise quickly
to a lethal level. Homeostasis of H+ concentration within a narrow range is thus essential to
survival. The removal of H from body fluids and its subsequent elimination from the body
depend on the following three major mechanisms:
1. Buffer systems. Buffers act quickly to temporarily bind H, removing the highly
reactive, excess H from solution. Buffers thus raise pH of body fluids but do not remove H
from the body.
2. Exhalation of carbon dioxide. By increasing the rate and depth of breathing, more
carbon dioxide can be exhaled. Within minutes this reduces the level of carbonic acid in blood,
which raises the blood pH (reduces blood H level).
3. Kidney excretion of H. The slowest mechanism, but the only way to eliminate acids
other than carbonic acid, is through their excretion in urine.
We will examine each of these mechanisms in more detail in the following sections.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
*Values are normal ranges of blood plasma levels in adults.
Table 9. Blood Electrolyte Imbalances
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
The Actions of Buffer System
Most buffer systems in the body consist of a weak acid and the salt of that acid, which
functions as a weak base. Buffers prevent rapid, drastic changes in the pH of body fluids by
converting strong acids and bases into weak acids and weak bases within fractions of a
second. Strong acids lower pH more than weak acids because strong acids release H + more
readily and thus contribute more free hydrogen ions. Similarly, strong bases raise pH + more
than weak ones. The principal buffer systems of the body fluids are the protein buffer system,
the carbonic acid–bicarbonate buffer system, and the phosphate buffer system.
Protein Buffer System
The protein buffer system is the most abundant buffer in intracellular fluid and blood
plasma. For example, the protein hemoglobin is an especially good buffer within red blood
cells, and albumin is the main protein buffer in blood plasma. Proteins are composed of amino
acids, organic molecules that contain at least one carboxyl group (-COOH) and at least one
amino group (- NH2); these groups are the functional components of the protein buffer
system. The free carboxyl group at one end of a protein acts like an acid by releasing H when
pH rises; it dissociates as follows:
The H is then able to react with any excess OH- in the solution to form water. The free
amino group at the other end of a protein can act as a base by combining with H + when pH
falls, as follows:
So proteins can buffer both acids and bases. In addition to the terminal carboxyl and
amino groups, side chains that can buffer H+ are present on seven of the 20 amino acids.
As we have already noted, the protein hemoglobin is an important buffer of H in red
blood cells. As blood flows through the systemic capillaries, carbon dioxide (CO2) passes from
tissue cells into red blood cells, where it combines with water (H2O) to form carbonic acid
(H2CO3). Once formed, H2CO3 dissociates into H and HCO3 -. At the same time that CO2 is
entering red blood cells, oxyhemoglobin (Hb--O2) is giving up its oxygen to tissue cells.
Reduced hemoglobin (deoxyhemoglobin) picks up most of the H +. For this reason, reduced
hemoglobin usually is written as Hb-H. The following reactions summarize these relations:
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Carbonic Acid–Bicarbonate Buffer System
The carbonic acid–bicarbonate buffer system is based on the bicarbonate ion (HCO3
which can act as a weak base, and carbonic acid (H2CO3), which can act as a weak acid. As
you have already learned, HCO3- is a significant anion in both intracellular and extracellular.
Because the kidneys also synthesize new HCO3- and reabsorb filtered HCO3 -, this important
buffer is not lost in the urine. If there is an excess of H +, the HCO3 - can function as a weak
base and remove the excess H+ as follows:
-),
Then, H2CO3 dissociates into water and carbon dioxide, and the CO 2 is exhaled from
the lungs. Conversely, if there is a shortage of H+, the H2CO3 can function as a weak acid and
provide H+ as follows:
At a pH of 7.4, HCO3- concentration is about 24 mEq/liter and H2CO3 concentration is
about 1.2 mmol/liter, so bicarbonate ions outnumber carbonic acid molecules by 20 to 1.
Because CO2 and H2O combine to form H2CO3, this buffer system cannot protect against pH
changes due to respiratory problems in which there is an excess or shortage of CO 2.
Phosphate Buffer System
The phosphate buffer system acts via a mechanism similar to the one for the carbonic
acid–bicarbonate buffer system. The components of the phosphate buffer system are the ions
dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO4 2-). Recall that
phosphates are major anions in intracellular fluid and minor ones in extracellular fluids. The
dihydrogen phosphate ion acts as a weak acid and is capable of buffering strong bases such
as OH-, as follows:
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
The monohydrogen phosphate ion is capable of buffering the H + released by a strong
acid such as hydrochloric acid (HCl) by acting as a weak base:
Because the concentration of phosphates is highest in intracellular fluid, the
phosphate buffer system is an important regulator of pH in the cytosol. It also acts to a smaller
degree in extracellular fluids and buffers acids in urine. H2PO4 - is formed when excess H+ in
the kidney tubule fluid combines with HPO4 2- .The H that becomes part of the H2PO4- passes
into the urine. This reaction is one way the kidneys help maintain blood pH by excreting H+ in
the urine.
Exhalation of Carbon Dioxide
The simple act of breathing also plays an important role in maintaining the pH of body
fluids. An increase in the carbon dioxide (CO 2) concentration in body fluids increases H+
concentration and thus lowers the pH (makes body fluids more acidic). Because H 2CO3 can be
eliminated by exhaling CO2, it is called a volatile acid. Conversely, a decrease in the CO2
concentration of body fluids raises the pH (makes body fluids more alkaline). This chemical
interaction is illustrated by the following reversible reactions:
Changes in the rate and depth of breathing can alter the pH of body fluids within a
couple of minutes. With increased ventilation, more CO 2 is exhaled. When CO2 levels
decrease, the reaction is driven to the left (blue arrows), H+ concentration falls, and blood pH
increases. Doubling the ventilation increases pH by about 0.23 units, from 7.4 to 7.63. If
ventilation is slower than normal, less carbon dioxide is exhaled. When CO2 levels increase,
the reaction is driven to the right (red arrows), the H + concentration increases, and blood pH
decreases. Reducing ventilation to one-quarter of normal lowers the pH by 0.4 units, from 7.4
to 7.0. These examples show the powerful effect of alterations in breathing on the pH of body
fluids.
The pH of body fluids and the rate and depth of breathing interact via a negative
feedback loop (Figure 18). When the blood acidity increases, the decrease in pH (increase in
concentration of H+) is detected by central chemoreceptors in the medulla oblongata and
peripheral chemoreceptors in the aortic and carotid bodies, both of which stimulate the
inspiratory area in the medulla oblongata. As a result, the diaphragm and other respiratory
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
muscles contract more forcefully and frequently, so more CO2 is exhaled. As less H2CO3 forms
and fewer H+ are present, blood pH increases. When the response brings blood pH (H +
concentration) back to normal, there is a return to acid–base homeostasis. The same negative
feedback loop operates if the blood level of CO 2 increases. Ventilation increases, which
removes more CO2, reducing the H+ concentration and increasing the blood’s pH.
By contrast, if the pH of the blood increases, the respiratory center is inhibited and
the rate and depth of breathing decreases. A decrease in the CO 2 concentration of the blood
has the same effect. When breathing decreases, CO2 accumulates in the blood so its H +
concentration increases.
Figure 18. Negative feedback regulation of blood pH by the respiratory system
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Kidney Excretion of H+
Metabolic reactions produce nonvolatile acids such as sulfuric acid at a rate of about
1 mEq of H+ per day for every kilogram of body mass. The only way to eliminate this huge acid
load is to excrete H+ in the urine. Given the magnitude of these contributions to acid–base
balance, it’s not surprising that renal failure can quickly cause death.
As you learned in the previous lessons, cells in both the proximal convoluted tubules
(PCT) and the collecting ducts of the kidneys secrete hydrogen ions into the tubular fluid. In
the PCT, Na+/H+ antiporters secrete H+ as they reabsorb Na+ .Even more important for
regulation of pH of body fluids, however, are the intercalated cells of the collecting duct. The
apical membranes of some intercalated cells include proton pumps (H+ ATPases) that secrete
H+ into the tubular fluid. Intercalated cells can secrete H + against a concentration gradient so
effectively that urine can be up to 1000 times (3 pH units) more acidic than blood. HCO 3produced by dissociation of H2CO3 inside intercalated cells crosses the basolateral membrane
by means of Cl-/HCO3- antiporters and then diffuses into peritubular. The HCO 3 - that enters
the blood in this way is new (not filtered). For this reason, blood leaving the kidney in the
renal vein may have a higher HCO3- concentration than blood entering the kidney in the renal
artery.
Interestingly, a second type of intercalated cell has proton pumps in its basolateral
membrane and Cl-/HCO3 - antiporters in its apical membrane. These intercalated cells secrete
HCO3 - and reabsorb H+. Thus, the two types of intercalated cells help maintain the pH of body
fluids in two ways—by excreting excess H+ when pH of body fluids is too low and by excreting
excess HCO3 - when pH is too high.
Some H+ secreted into the tubular fluid of the collecting duct are buffered, but not by
HCO3-, most of which has been filtered and reabsorbed. Two other buffers combine with H +
in the collecting duct. The most plentiful buffer in the tubular fluid of the collecting duct is
HPO4 2- (monohydrogen phosphate ion). In addition, a small amount of NH 3 (ammonia) also
is present. H+ combines with HPO4 2- to form H2PO4 - (dihydrogen phosphate ion) and with
NH3 to form NH4 (ammonium ion). Because these ions cannot diffuse back into tubule cells,
they are excreted in the urine.
Table 8 summarizes the mechanisms that maintain pH of body fluids.
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
Table 10. Mechanisms That Maintain pH of Body Fluids
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
Acid-Base Imbalances
The normal pH range of systemic arterial blood is between 7.35 ( 45 nEq of H +/liter)
and 7.45 ( 35 nEq of H+/liter). Acidosis (or acidemia) is a condition in which blood pH is below
7.35; alkalosis (or alkalemia) is a condition in which blood pH is higher than 7.45.
The major physiological effect of acidosis is depression of the central nervous system
through depression of synaptic transmission. If the systemic arterial blood pH falls below 7,
depression of the nervous system is so severe that the individual becomes disoriented, then
comatose, and may die. Patients with severe acidosis usually die while in a coma. A major
physiological effect of alkalosis, by contrast, is overexcitability in both the central nervous
system and peripheral nerves. Neurons conduct impulses repetitively, even when not
stimulated by normal stimuli; the results are nervousness, muscle spasms, and even
convulsions and death
A change in blood pH that leads to acidosis or alkalosis may be countered by
compensation, the physiological response to an acid–base imbalance that acts to normalize
arterial blood pH. Compensation may be either complete, if pH indeed is brought within the
normal range, or partial, if systemic arterial blood pH is still lower than 7.35 or higher than
7.45. If a person has altered blood pH due to metabolic causes, hyperventilation or
hypoventilation can help bring blood pH back toward the normal range; this form of
compensation, termed respiratory compensation, occurs within minutes and reaches its
maximum within hours. If, however, a person has altered blood pH due to respiratory causes,
then renal compensation—changes in secretion of H and reabsorption of HCO3- by the kidney
tubules—can help reverse the change. Renal compensation may begin in minutes, but it takes
days to reach maximum effectiveness. In the discussion that follows, note that both
respiratory acidosis and respiratory alkalosis are disorders resulting from changes in the
partial pressure of CO2 (PCO2) in systemic arterial blood (normal range is 35–45 mmHg). By
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
contrast, both metabolic acidosis and metabolic alkalosis are disorders resulting from changes
in HCO3- concentration (normal range is 22–26 mEq/liter in systemic arterial blood).
Respiratory Acidosis
The hallmark of respiratory acidosis is an abnormally high PCO 2 in systemic arterial
blood—above 45 mmHg. Inadequate exhalation of CO 2 causes the blood pH to drop. Any
condition that decreases the movement of CO 2 from the blood to the alveoli of the lungs to
the atmosphere causes a buildup of CO2, H2CO3, and H+. Such conditions include emphysema,
pulmonary edema, injury to the respiratory center of the medulla oblongata, airway
obstruction, or disorders of the muscles involved in breathing. If the respiratory problem is
not too severe, the kidneys can help raise the blood pH into the normal range by increasing
excretion of H and reabsorption of HCO3 (renal compensation). The goal in treatment of
respiratory acidosis is to increase the exhalation of CO 2, as, for instance, by providing
ventilation therapy. In addition, intravenous administration of HCO3- may be helpful.
Respiratory Alkalosis
In respiratory alkalosis, systemic arterial blood PCO 2 falls below 35 mmHg. The cause
of the drop in PCO2 and the resulting increase in pH is hyperventilation, which occurs in
conditions that stimulate the inspiratory area in the brain stem. Such conditions include
oxygen deficiency due to high altitude or pulmonary disease, cerebrovascular accident
(stroke), or severe anxiety. Again, renal compensation may bring blood pH into the normal
range if the kidneys are able to decrease excretion of H and reabsorption of HCO3 -. Treatment
of respiratory alkalosis is aimed at increasing the level of CO 2 in the body. One simple
treatment is to have the person inhale and exhale into a paper bag for a short period; as a
result, the person inhales air containing a higher-than-normal concentration of CO2.
Metabolic Acidosis
In metabolic acidosis, the systemic arterial blood HCO 3 - level drops below 22
mEq/liter. Such a decline in this important buffer causes the blood pH to decrease. Three
situations may lower the blood level of HCO 3 -: (1) actual loss of HCO3 -, such as may occur
with severe diarrhea or renal dysfunction; (2) accumulation of an acid other than carbonic
acid, as may occur in ketosis or (3) failure of the kidneys to excrete H+ from metabolism of
dietary proteins. If the problem is not too severe, hyperventilation can help bring blood pH
into the normal range (respiratory compensation). Treatment of metabolic acidosis consists
of administering intravenous solutions of sodium bicarbonate and correcting the cause of the
acidosis.
Metabolic Alkalosis
In metabolic alkalosis, the systemic arterial blood HCO 3- concentration is above 26
mEq/liter. A nonrespiratory loss of acid or excessive intake of alkaline drugs causes the blood
pH to increase above 7.45. Excessive vomiting of gastric contents, which results in a
substantial loss of hydrochloric acid, is probably the most frequent cause of metabolic
alkalosis. Other causes include gastric suctioning, use of certain diuretics, endocrine
disorders, excessive intake of alkaline drugs (antacids), and severe dehydration. Respiratory
compensation through hypoventilation may bring blood pH into the normal range. Treatment
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CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
of metabolic alkalosis consists of giving fluid solutions to correct Cl -, K+, and other electrolyte
deficiencies plus correcting the cause of alkalosis.
Table 9 summarizes respiratory and metabolic acidosis and alkalosis.
Table 11. Summary of Acidosis and Alkalosis
Adopted from: Tortora, G.J. & Derrickson, B., 2009. Principles of Anatomy and Physiology, 12 th edition. New
Jersey: John Wiley & Sons, Inc.
ASSESSMENT (POST-ASSESSMENT)
Directions: Indicate which of the following statements are True (T) and which are False (F) by
encircling the appropriate letter.
T
F
1. Two thirds of body fluids is in the extracellular fluid compartment.
T
F
2. Blood vessel walls divide the interstitial fluid from blood plasma.
T
F
3. Female has a lower body fluids than the male considering the amount of
adipose tissue present in the body.
T
F
4. Hypothalamus is considered as the thirst center of the body.
T
F
5. Dehydration occurs when water loss is lesser than water gain.
T
F
6. Aldosterone promotes urinary absorption of sodium and chloride.
T
F
7. Antidiuretic hormone is also known as vasopressin.
Mariano Marcos State University
College of Health Sciences
Department of Nursing
53
CN 100 ANATOMY & PHYSIOLOGY: CHAPTER XIII: URINARY SYSTEM
T
F
8. Water intoxication can lead to convulsions, coma and possible death.
T
F
9. Sodium is the most abundant ion in ICF.
T
F
10. Potassium is the most abundant anions in ECF.
T
F
11. The normal serum calcium level is 4.5 – 5.5mEq/liter.
T
F
12. Buffer systems takes the slowest mechanism to maintain acid-base
homeostasis
T
F
13. The protein buffer system is the most abundant buffer in intracellular fluid
and blood plasma.
T
F
14. Phosphate buffer system acts via a mechanism similar to the one for the
carbonic acid – bicarbonate buffer system.
T
F
15. In metabolic acidosis, the systemic arterial blood HCO 3- level is above 22
mEq/liter
T
F
16. Metabolic alkalosis, the systemic arterial blood bicarbonate level drops
below 22 mEq/liter.
T
F
17. The hallmark of respiratory acidosis is an abnormally high PCO2 in systemic
arterial blood – above 45 mmHg.
T
F
18. In respiratory alkalosis, systemic arterial blood PCO2 falls below 35mmHg
T
F
19. Changes in the rate and depth of breath can alter the pH of body fluids
within a couples of minutes
T
F
20. Chloride ions are the most prevalent anions in ICF.
REFERENCES
Tortora, G., &. Derrickson. B. 2017. Tortora’s Principles of Anatomy and Physiology. 15 th ed.
Singapore: John Wiley & Sons, Inc.
Tortora, G., & Derrickson. B. 2009. Principles of Anatomy and Physiology. 12th ed. New
Jersey: John Wiley & Sons,Inc.
Mariano Marcos State University
College of Health Sciences
Department of Nursing
54