CHAPTER 25-THE URINARY SYSTEM
I. THE URINARY SYSTEM
A. Includes the kidneys, ureters, urethra and urinary bladder.
B. The kidneys are the primary excretory in the human body. They function by removing
toxins from the body while returning necessary compounds back to the body.
1. The kidneys filter approximately 200 liters of fluid from the body everyday.
C. The Primary Functions of the Urinary System include:
1. Filtering wastes from the blood and removing the wastes from the body via
the urine.
2. Glucogenesis-during times of fasting.
3. Producing the enzyme Renin which regulates blood pressure and proper kidney
functioning.
4. Producing the hormone Erythropoietin which regulates and stimulates red
blood cell production.
5. Metabolizing vitamin D to its active form.
II. EXTERNAL KIDNEY ANATOMY
A. The Kidney is bean-shaped and located in the lumbar region of the body. The kidney
is described as being Retroperitoneal-that is, it is located between the dorsal body wall
and the parietal peritoneum.
1. An average human kidney weighs about 5 ounces.
2. Sitting on top of each kidney is a single adrenal gland that essentially has no
influence on the kidney.
B. The Renal Hilum-vertical cleft on the medial surface of the kidney, that leads into an
internal space within the kidney known as the Renal Sinus.
1. The ureter, the renal blood vessels, lymphatics and nerves all join each other at
the hilum and occupy the renal sinus.
C. There are Three Layers of Support Tissue Surrounding each kidney. The layers are:
1. The Fibrous Capsule-a capsule-like layer that prevents infections in
surrounding regions from spreading to the kidney.
2. The Perirenal Fat Capsule-a thick layer of adipose tissue that attaches the
kidney to the posterior body wall and cushions it against blows.
a. What is renal ptosis?
3. The Renal Fascia-an outer layer of fibrous connective tissue that anchors the
kidney and adrenal glands to surrounding tissues.
III. INTERNAL ANATOMY OF THE KIDNEY
A. Three Distinct Internal Segments in the Human Kidney:
1. The Renal Cortex-light colored, superficial region of the kidney. This area has
a granular appearance.
2. The Renal Medulla-a dark red or brown colored region in the kidney. The
medulla contains cone-shaped areas known as the Medullary or Renal Pyramids.
a. The base of each pyramid faces towards the cortex and the apex (Papilla)
points internally.
b. The pyramids contain bundles of microscopic urine-collecting tubules
and capillaries. Structures known as the Renal Columns separate the
pyramids from each other.
c. Each pyramid and its surrounding tissue makes up one of eight lobes of
a kidney.
3. The Renal Pelvis-a funnel-shaped tube that is continuous with the ureter leaving
the hilum.
a. Branching extensions of the Pelvis form two or three Major Calyces, each
of which subdivides to form several Minor Calyces.
b. The Minor Calyces are cup-shaped areas that enclose the papillae of the
pyramids.
1) The calyces collect urine, which drains from the papillae, and
empty into the renal pelvis. The urine then flows through the
renal pelvis and into the ureter which moves it to the bladder
where it is stored.
2) Smooth muscle lines the walls of the calyces, the pelvis and the
ureter. Urine is pushed through these areas via peristalsis.
c. What is Pyelitis? Pyelonephritis?
IV. BLOOD AND NERVE SUPPLY TO THE KIDNEY
A. Obviously, the kidneys have a huge blood supply. The renal artery carries one fourth
of the total cardiac output to the kidneys (1200ml) each minute!
B. As the renal arteries reach the kidney, they branch into smaller arteries in the
following fashion:
1.
Renal artery
↓
Segmental arteries (to the renal sinus)
↓
Interlobar arteries
↓
Arcuate arteries (to the base of the renal pyramids)
↓
Cortical radiate arteries (to the renal cortex)
C. Renal Plexus-a network of autonomic nerve fibers that provide much of the nerve
supply to the kidney and its ureter.
V. NEPHRONS-structural and functional units of the kidney. There are over 1 million of these
structures in each kidney. They are responsible for cleansing the blood to produce urine.
A. Each Nephron Consists of:
1. A Glomerulus-which is a tuft of capillaries.
2. A Renal Tubule-parts:
a. Glomerulur Capsule (Bowman’s Capsule)-cup-shaped end of the
renal tubule.
b. Collectively, the glomerular capsule and the enclosed glomerulus are
called the renal corpuscle.
c. Proximal convoluted tubule (PCT)-coiled structure that leaves the
glomerular capsule. The PCT makes a hairpin loop known as The
Loop of Henle which then twists (via ascending and descending limbs)
again as the Distal Convoluted Tubule (DCT). The DCT empties into a
collecting duct.
B. Glomerular capillaries are described as being fenestrated (containing many pores).
This allows large amounts of solute-rich materials to pass from the blood into the
glomerular capsule. This material, known as filtrate, is the material that the renal
tubules process to form urine.
C. The external parietal layer of the glomerular capsule is simple squamous epithelial
tissue. The visceral layer, which attaches to the glomerular capillaries, consists of
podocytes that cling to the glomerulus. The podocytes form filtration slits through
which filtrate enters the capsular space within the glomerular capsule.
D. Collecting Ducts-receive filtrate from many nephrons, run through the medullary
pyramids and give them their striped appearance. As the collecting ducts approach
the renal pelvis, they fuse to form the large papillary ducts which deliver urine into the
minor calyces via papillae of the pyramids.
E. The walls of the PCT are covered by cuboidal epithelial cells that contain microvilli;
thus, these cells offer a great surface area for reabsorbing water and solutes from the
filtrate. The walls of the DCT are also covered by cuboidal epithelial cells, however,
microvilli are less abundant in this region.
F. Cortical Nephrons-are located entirely within the cortex of the kidney.
G. Juxtamedullary Nephrons-are located close to the cortex-medulla junction and they
play a role in producing concentrated urine.
H. Every nephron is closely associated with two capillary beds: the glomerulus and the
peritubular capillaries.
1. In the glomerulus, the capillaries run parallel and are specialized for filtration.
a. The glomerulus is fed by and drained by arterioles-the afferent arteriole
and the efferent arteriole. This is different from all other capillary beds
in the body.
b. The afferent arterioles arise from the interlobular arteries and run
through the renal cortex.
c. Blood pressure in the glomerulus is extremely high for a capillary bed.
This pressure forces fluid and solutes out of the blood into the
glomerular capusule. Much of this filtrate is reabsorbed by the renal
tubule cells and returned to the blood in the peritubular capillaries.
d. What creates this great pressure in the arterioles of the glomerulus?
1) Arterioles are highly resistant vessels
2) The afferent arteriole has a larger diameter than the efferent
arteriole.
2. Peritubular capillaries-arise from the efferent arterioles draining the glomeruli.
a. These attach to renal tubules and empty into venules.
b. These capillaries are low pressure vessels that specialize in absorbing
solutes and water that is reclaimed from filtrate produced in the tubules.
c. Juxtamedullary nephrons do not contain peritubular capillaries. Instead,
they contain straight vessels known as the vasa recta, that specialize in
concentrating urine.
3. In general, nephrons contain two capillary beds that are separated by efferent
arterioles. The glomeruli produces filtrate and the peritubluar capillaries
reclaim much of the filtrate.
I. Every nephron contains a region known as the Juxtagomerular Apparatus (JGA). This
is a region where the DCT lies against the afferent arteriole.
1. Juxtaglomerular cells (JG)-line the walls of arterioles in the nephron. These
cells can store and secrete renin. These cells also act to monitor blood pressure
in the afferent arteriole.
2. The Macula Densa-cells that lie adjacent to the JG cells. These respond to the
solute concentration of filtrate. These two sets of cells play a role in regulating
the rate of filtrate formation and systemic blood pressure.
3. The Filtration Membrane-lies between the blood and the interior of the
glomerular capsule. It is extremely porous and allows water and small solutes
to pass through.
a. The capillary pores of this membrane allow passage of all plasma
components but not blood cells.
VI. KIDNEY PHYSIOLOGY
A. Our kidneys filter the volume of our plasma more than 60 times each day. Our kidneys
require 20-25% of all oxygen used by the body at rest to accomplish this.
B. Filtrate vs. Urine
1. Filtrate contains everything found in blood plasma except for proteins. As
filtrate moves into the collecting ducts, it has lost most of its water, ions and
nutrients. The material that remains at this point is known as urine
a. Urine contains metabolic wastes and unneeded compounds.
C. Urine formation proceeds through three major processes in the kidney:
1. Glomerular filtration-by the glomeruli.
2. Tubular reabsorption and secretion in the renal tubules.
3. Tubular secretion
D. Glomerular Filtration-passive process in which hydrostatic pressure forces fluids and
solute through a membrane.
1. This is an extremely efficient process at filtering the blood. This efficiency is in
part due to the high permeability of the filtration membrane and the great
surface area that this membrane offers.
2. In glomerular filtration, small molecules (water, glucose, nitrogen wastes) can
pass from the blood into the renal tubule. Larger molecules tend to remain in
blood capillaries, thus maintaining appropriate vessel pressures to prevent the
complete loss of water form the capillaries.
a. Protein in the urine is often an indicator of problems with the filtration
membrane.
3. Net Filtration Pressure (NFP)-refers to the forces acting at the glomerular beds.
a. Glomerular Hydrostatic Pressure (HPg)-is essentially glomerular blood
pressure. This is the primary force involved in pushing water and
solutes out of blood and across the filtration membrane.
b. Colloid osmotic pressure of glomerular blood and capsular hydrostatic
pressure both act to counter the effects of glomerular hydrostatic
pressure. In the end, the NFP responsible for producing renal filtrate is
10mmHg.
4. Glomerular Filtration Rate-the volume of filtrate formed each minute by the
activity of all 2 million glomeruli of the kidneys. In adults, the GFR is
120-125ml/min.
a. Any changes in pressure on the filtration membrane may lead to a
change in NFP.
5. Factors That Regulate Glomerular Filtration
a. Intrinsic Controls:
1) Renal Autoregulation-refers to the ability of the kidney to adjust
its own resistance to blood flow. This allows for a constant
GFR. This is created by:
a) The Myogenic Mechanism-the ability of vascular
smooth muscle to contract when stretched. This
contraction reduces blood flow into the glomerulus and
prevents an increase in glomerular blood pressure, thus
avoiding damage to the kidney. The smooth muscle can
relax when systemic pressure is low, thus increasing
blood flow and raising pressure in the glomerulus.
b) Tubuloglomerular Feedback Mechanism-involves
macula densa cells which can promote either dilation or
constriction of afferent arterioles to increase or decrease
blood flow as needed.
2) Intrinsic controls do not work when systemic blood pressure
drops below 90 mmHg.
b. Extrinsic Controls-involves neural and hormonal devices that seek to
maintain systemic blood pressure.
1) Sympathetic Nervous System Controls-this portion of the ANS
can act to slow the flow of blood to the kidneys during times of
of extreme stress or emergency. This reduces filtrate formation.
2) Renin-Angiotensin Mechanism-can be triggered by several
stimuli, including the Sympathetic Division of the ANS.
a) In this system, JG cells are stimulated to release renin
which is an enzyme that acts on the protein
angiotensinogen to produce a compound known as
angiotensin II. This compound is a powerful
vasoconstrictor; therefore it acts to raise blood pressure.
1) Angiotensin II also stimulates the kidney to
release aldosterone which forces the renal tubules
to reclaim more sodium ions. Water follows the
sodium via osmosis, thus, blood pressure increases
due to the increase in blood volume.
3) What is anuria?
E. Tubular Reabsorption-this process involves the reclamation of much of the material
that collects in the renal tubule during glomerular filtration.
1. Reabsorption begins when filtrate reaches the proximal tubules.
2. For reabsorption to occur, transported substances pass through three barriers:
a. The Luminal Membrane of the Tubule Cells
b. The Basolateral Membrane of the Tubule Cells
c. Endothelium of the Peritubular Capillaries
3. Under normal conditions, most organic nutrients (including glucose, amino
acids) are reabsorbed into the plasma. However, water and ion absorption is
often regulated by hormones.
4. Reabsorption can be passive (no ATP required) or active (requiring ATP to
occur).
5. Sodium Reabsorption-is almost always an active process.
a. Sodium ions are the most abundant cation in the filtrate.
b. Typically, sodium ions enter tubule cells from the filtrate and are
pumped via a Sodium-Potassium Pump out of the tubule cells.
1) Sodium ions are then swept into the peritubular capillaries
by the bulk flow of water.
6. Water, Nutrient and Ion Reabsorption-these materials are typically reabsorbed
via passive process such as diffusion, osmosis, and facilitated diffusion.
a. As noted earlier, water moves into peritubular capillaries via osmosis.
1) This occurs due to the presence of abundant sodium ions in the
peritubular capillaries.
b. Aquaporins-water filled channels located at certain areas along the PCT.
1) In areas where these are located, water is automatically
reabsorbed, no matter the body’s water requirement (under or
overhydrated). This is known as Obligatory water reabsorption.
2) As water exits the tubules, the concentration of solutes in the
filtrate increases. Many of these compounds follow their
concentration gradients into the peritubular capillaries.
c. Some substances, including amino acids, glucose and vitamins are
reabsorbed via Secondary Active Transport. This often involves the
cotransport of the above materials out of the filtrate. The substances
are often carried by cotransport into the peritubular capillaries.
d. Some materials are not reabsorbed because they are too large or because
they do not attach efficiently to a carrier molecule.
1) Nitrogenous compounds produced during the metabolism of
proteins and nucleic acids are compounds that are generally not
reabsorbed. Specifically, this includes substances such as urea,
uric acid and creatinine.
7. Absorptive Capabilities of the Renal Tubules and Collecting Ducts
a. The entire renal tubule is involved in reabsorption, however, the PCT
cells are the most active reabsorbers in the renal tubules. What is
reabsorbed in this region of the renal tubules?
b. The Loop of Henle
1) The general rule here is that water exits the descending limb but
not the ascending limb. The opposite is true for solutes.
c. The Distal Convoluted Tubule and Collecting Ducts
1) Typically, reabsorption in these regions is regulated by hormonesincluding aldosterone for sodium reabsorption and PTH for
calcium reabsorption.
F. Tubular Secretion
1. As stated before, some compounds are not reabsorbed into the peritubular
capillaries. In tubular secretion, some substances (creatinine, certain organic
compounds) either move into the filtrate from the peritubular capillaries or
they are synthesized by tubule cells and secreted into the filtrate. Urine, therefore,
contains filtered and secreted substances.
2. The PCT is the primary site of secretion. The collecting ducts are also involved in
secretion.
3. Tubular Secretion is important for the following reasons:
a. Disposing of substances that are not easily filtered from the blood. This
includes certain drugs and substances that are tightly attached to plasma
proteins.
b. Eliminating toxic compounds that have been reabsorbed by passive
processes. This would include urea and uric acid.
c. Removing excess potassium ion from the body.
d. Regulating blood pH.
1) When blood pH drops towards the acidic end, the renal
tubules secrete more hydrogen ions into the filtrate and retain
bicarbonate. As a result, the blood pH increases and the urine
removes excess hydrogen ions. The opposite also occurs to
reduce pH.
G. Regulation of Urine Concentration and Volume
1. One of the primary functions of the kidneys is to maintain a constant solute level
in body fluids (at about 300milliOsmol). This is accomplished by the kidney as
it regulates the concentration and volume of urine.
a. Recall that Osmolality is the number of solutes dissolved in 1 kg of water
and it reflects the solution’s ability to cause osmosis.
2. The kidneys regulate the concentration and volume of urine along with the solute
level in body fluids via countercurrent mechanisms.
3. The term countercurrent refers to the flow of fluid through adjacent tubes in
opposite directions.
4. Countercurrent mechanisms are found in two primary areas in the kidneys:
a. Between the ascending and descending limbs of the loops of Henle of
Juxtamedullary nephrons (this system is known as the countercurrent
multiplier).
b. Between the ascending and descending portions of the vasa recta
blood vessels (this system is known as the countercurrent exchanger).
5. Countercurrent mechanisms function by creating an osmotic gradient from the
cortex of the kidney through the medulla. This gradient allows the kidney to
vary urine concentrations as needed.
6. The Countercurrent Exchanger-is associated with the vasa recta.
a. This system involves the cycling of salt.
b. Blood flow through the vasa recta is sluggish. These blood vessels are
also freely permeable to water and NaCl. As blood flows deep into the
medulla, it loses water and gains salt (hypertonic). As blood emerges
from the medulla, this process is reversed.
1) Blood entering and leaving the vasa recta; therefore, has the same
solute concentration. This system does not create an osmotic
gradient, but it protects it by preventing the rapid removal of salt
from the tissue spaces of the medulla and by removing reabsorbed
water.
c. Formation of Dilute Urine
1) Filtrate is diluted as it travels through the ascending limb of the
loop of Henle. This filtrate is allowed to travel to the renal pelvis
to produce dilute urine. This process occurs when ADH is not
released.
a) This essentially stops water reabsorption.
d. Formation of Concentrated Urine
1) Antidiuretic hormone (ADH) inhibits urine output (diuresis).
2) ADH functions by allowing aquaporins to insert into the luminal
membrane. Due to this, water passes through the cells into the
interstitial space. Overall, water and urea leave the filtrate and
pass into the medulla.
3) The amount of water exiting the filtrate is related to the amount of
ADH that is released. With maximal ADH secretion, as much as
99% of the water in the filtrate may be reabsorbed; thus producing
highly concentrated urine.
4) Facultative water reabsorption refers to water uptake influenced
by the presence of ADH.
e. Diuretics-chemicals that enhance urinary output.
1) Osmotic diuretics are compounds that are not reabsorbed and
that carry water out with it (for example, the high glucose level
in the urine of a diabetes mellitus patient).
2) Alcohol acts as a diuretic by inhibiting the release of ADH.
3) Caffeine and certain drugs act as diuretics by interfering with
sodium reabsorption.
H. Renal Clearance-refers to the volume of plasma that is cleared of a particular
substance in a given time (usually 1 minute).
1. Renal clearance tests are often done to examine the GFR.
2. What is the equation for Renal Clearance (RC)?
VII. CHARACTERISTICS OF URINE
A. Color and Transparency-freshly voided urine ranges in color from clear to deep yellow.
1. The yellow color is the result of urochrome, a pigment that results from the
breakdown of hemoglobin (bilirubin, bile pigments). The more concentrated the
urine, the deeper the yellow color.
2. Abnormal colors such as pink or brown urine or smoky urine may result from
eating certain foods (beets, rhubarb). Additionally, some drugs and vitamins may
alter the color of urine.
B. Odor-freshly voided urine may have a slight smell. If allowed to stand, it will begin to
develop an ammonia smell as bacteria metabolize urea in the urine. Some drugs, foods
and illnesses may alter the odor of urine. For example, the urine of an individual with
diabetes mellitus may smell fruity because of its acetone content.
C. pH-urine is slightly acidic. Changes in urine pH may indicate a variety of issues
including, bacterial infection, an extreme protein diet, a vegetarian diet.
D. Specific Gravity-refers to the mass of a substance to the mass of an equal volume of
water. Urine has a fairly high specific gravity.
E. Chemical Composition of Urine-water accounts for about 95% of the volume of urine.
1. Other compounds in urine include urea and the nitrogenous wastes uric acid and
creatinine.
VIII. THE URETERS-tubes that carry urine from the kidneys to the bladder.
A. When the bladder fills, the distal ends of the ureters are compressed and closed.
B. The Walls of the Ureters are Trilayered:
1. Mucosa-inner layer.
2. Muscularis-middle layer, composed of smooth muscle. This layer contracts to
force urine into the urinary bladder.
3. Adventitia-covers the external surface of the ureters.
C. What are renal calculi?
IX.THE URINARY BLADDER-collapsible muscular sac that temporarily stores urine.
A. The interior of the bladder has openings for both ureters and the urethra. The region of
the bladder outlined by the three openings is known as the Trigone.
B. The Urinary Bladder has three Layers:
1. Mucosa-inner layer.
2. Thick muscular layer-known as the Detrusor Muscle, is composed of smooth
muscle.
3. Adventitia-outer covering.
C. When empty, the bladder has a triangular shape. The bladder expands as urine
accumulates. The maximum capacity of the bladder is approximately 1000ml
(2 pints).
X. THE URETHRA-thin muscular tube that drains urine from the urinary bladder out of
the body.
A. Internal Urethral Sphincter-thickening of the detrusor muscle at the bladder-urethra
junction. This involuntary sphincter keeps the urethra closed when urine is not
being passed and prevents leaking between voiding.
1. This sphincter is unusual in that contraction opens it and relaxation closes it.
B. What is urethritis? What is cystitis?
XI. MICTURITION-the act of emptying the bladder. Is also known as voiding or urination.
A. When about 200 ml of urine accumulates in the bladder, impulses are transmitted to
the brain, creating the urge to void. We can fight this urge up to a point.
1. Micturition occurs when urine volume exceeds 500-600ml, whether one
wants it to or not.
B. About 10 ml of urine remains in the bladder after micturition.
C. What is incontinence? What is urinary retention?
XII. RELATED CLINICAL TERMS-at end of chapter.