Biol 12
Excretion Review
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Chapter Overview.
The lungs excrete carbon dioxide and the liver excretes bile pigments. The skin excretes
perspiration, which contains urea, and the kidneys excrete ammonia, urea, uric acid, and
creatinine, all nitrogenous wastes.
The path of urine is through the kidneys, ureters, urinary bladder, and finally, the
urethra.
Macroscopically, the kidneys are divided into the cortex, medulla, and pelvis.
Microscopically, they contain the nephrons .
Each nephron has it’s own blood supply; the afferent arteriole approaches the glomerular
capsule and divides to become the glomerulus. The spaces between the podocytes of the
glomerular capsule allow small molecules to enter the capsule from the glomerulus, a
capillary tuft. The efferent arteriole, leaves the capsule and immediately branches into the
peritubular capillaries.
Each region of the nephron is anatomically suited to its task in urine formation. The
spaces between the podocytes of the glomerular capsule allow small molecules to enter
the capsule from the glomerulus, a papillary knot. The cuboidal epithelial cells of the
proximal convoluted tubule have many mitochondria and microvilli to carry out active
transport (following passive transport) from the tubule to blood. IN contrast, the cuboidal
epithelial cells of the distal convoluted tubule have numerous mitochondria but lack
microvilli. They carry out active transport from the blood to the tubule.
Urine is composed primarily of nitrogenous waste products and salts in water.
The steps in urine formation are glomerular filtration; tubular reabsorption and tubular
secretion, as explained in Figure 16.6. Water is reabsorbed from all parts of the tubule,
and the loop of the nephron establishes an osmotic gradient that draws water from the
descending loop of the nephron and also the collecting duct.
The kidneys contribute to homeostasis not only by ridding the body of nitrogenous
wastes but also by helping control the pH and saltwater balance of the blood. The latter
also determines blood volume, which is controlled by several hormones, including ADH,
aldosterone, and ANH.
Various types of problem, including repeated urinary infections, can lead to kidney
failure, which necessitates receiving a kidney from a donor or undergoing dialysis by
utilizing a kidney machine or CAPD.
Study Questions
16.1 Four Excretory Organs
Concepts:
- Excretion rids the body of unwanted substances, particularly the waste product so
metabolism.
- Several organs assist in the process of excretion, but only the kidneys produce
urine.
1. List the four organs of excretion and their products of excretion.
a. liver – bile pigments
b. lungs – CO2
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Excretion Review
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c. skin – water, urea, salt
d. kidneys – water, urea, excess salts, excess ions
Where in the body is urea produced? liver What molecules are used to form urea?
Ammonium + Carbon dioxide
2. Creatine phosphate in muscles results in the end product creatinine. The breakdown
of nucleotides produces uric acid, which in the joints produces the ailment called
gout.
16. 2 – Urinary System
Concept: The urinary system consists of organs that produce, store, and rid the body
of urine.
3. What is the function of the kidney? Produces urine (part of homeostasis). The
ureters are muscular tubes that convey urine from the kidneys toward the bladder.
The urinary bladder is a hollow, muscular organ that can hold urine. The urethra
extends from the urinary bladder to an external opening. In males, the urethra
transports both urine and semen .
4. The three regions of the kidney include the renal cortex, an outer granulated layer,
the renal medulla , which consists of cone-shaped renal pyramids, and the renal
pelvis.
16.3: Anatomy of a Nephron
Concepts:
- The work of an organ is dependent on its microscopic anatomy; nephrons within a
kidney produce urine.
- Structure suits the function, and this can be illustrated by studying the microscopic
anatomy of a nephron.
5. Blood that supplies the nephron comes from the afferent arteriole and goes to the
glomerulus, to the efferent arteriole, to the peritubular capillaries, to the venule, and
to the renal vein.
6. The cuplike structure called the glomerular capsule, or Bowman’s capsule, has an
inner layer made up of cells termed podocytes that form pores for passage of small
molecules from the glomerulus into the glomerular capsule. This process is called
pressure (glomerular) filtration .
7. The proximal convoluted tubule has cuboidal epithelial cells that have microvilli
that increase the surface area for absorption. In the loop of Henle, the tube narrows
and makes an U-turn. The cells of the distal convoluted tubule have many
mitochondria but lack microvilli. Molecules move from the blood into the tubule, a
process called secretion. Distal convoluted tubules enter one collecting duct.
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16.4: Urine Formation
Concept:
- Like many physiological processes, urine formation is a multi-step process.
8. IN glomerular (pressure) filtration, blood pressure in the glomerulus forces water
and small molecules from the glomerulus into the glomerular (Bowman’s) capsule.
This is a filtration process.
9. The glomerular filtrate contains small dissolved molecules in approximately the
same concentration as plasma. Tubular reabsorption occurs as molecules are
reabsorbed from the tubule to the blood in the peritubular capillary. When sodium
ions are actively reabsorbed, Chlorine ions follow passively from the tubule into the
blood.
10. Nutrients such as glucose and amino acids are selectively returned to the blood at the
proximal convoluted tubule because those molecules are recognized by carrier
molecules. After all of a substance’s carriers are in use, any excess of the substance
in the filtrate will appear in the urine. Tubular secretion occurs when substances are
removed from blood and added to tubular fluids in the distal convoluted tubule. Such
substances include hydrogen and ammonium ions.
11. Excretion of hypertonic urine is dependent upon what two sites in the nephron?
Loop of Henle collecting Duct
In the ascending limb of the loop of the nephron (loop of Henle), the thick portion
actively transports out sodium chloride into the tissue of the outer renal medulla. The
concentration of salt is greater in the direction of the medulla. The ascending limb is
impermeable to water. Urea leaks from the collecting duct and contributes to the
high solute concentration. Water leaves the descending limb of the loop of the
nephron. Due to the osmotic gradient, water diffuses out of the collecting duct into
the renal medulla so that urine becomes hypotonic.
16.5: Regulatory Functions of the Kidneys
Concept:
- the kidneys rid the body of nitrogenous wastes and help regulate the pH and the
salt/water balance of blood.
12. Write True or False for each of the statements below.
a. the juxtaglomerular apparatus is between the afferent arteriole
and distal convoluted tubule. T
b. Increased blood pressure causes the afferent arteriole cells to secrete renin. F
c. Renin converts angiotensinogen into angiotensin II. F
d. Angiotensin-converting enzyme changes angiotensin I into angiotensin II T
e. Angiotensin I stimulates the adrenal cortex to release aldosterone. F
f. Aldosterone decreases Na+ reabsorption into the blood, which leads to greater
blood volume. F
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13. The antidiuretic hormone is released by the posterior pituitary gland . When ADH is
present, more water is reabsorbed, and a decreased amount of urine results. The
atrial natriuretic hormone is released by the atria of the heart when stretched due to
increased blood volume. ANH inhibits secretion of renin and aldosterone and causes
the excretion of sodium, which causes the blood volume and blood pressure to
decrease.
14. Diuretics are agents that increase the flow of urine. Alcohol causes diuresis because
it inhibits the secretion of ADH. If blood is acidic, the nephrons of the kidney will
excrete H+ in combination with NH3, while Na+ and bicarbonate ions are reabsorbed
since NaHCO3 is a base. If blood is basic, fewer H+ are excreted.
16.6: Problems with Kidney Function
Concept: The kidneys are vital body organs, and malfunction causes illness and even
death.
15. Infection of the urethra is called urethritis. Infection of the bladder is called cystitis.
Infection of the kidneys is called pyelonephritis. Accumulation of urea in the blood
is called uremia. Which is worse: accumulation of urea in the blood or retention of
water and salts? Water/salts. Presence of albumin in the urine is detected by a
urinalysis .
16. Dialysis is defined as the diffusion of dissolved molecules through a semipermeable
membrane. During hemodialysis , the patient’s blood is passed through a
semipermeable membranous tube, which is in contact with a balanced salt solution. I
CAPD, a fresh amount of dialysate is introduced directly into the abdominal cavity
from a bag attached to a permanently implanted plastic tube.
Objective Questions: Completion and Short Answer Questions:
1. The removal of metabolic wastes from the body is called excretion. The primary
excretory organ of the body is the kidney, but other excretory organs include the skin,
lungs, and the liver.
2. The gross structure of the kidney, when cut lengthwise, shows three regions. The
outermost portion is granular and is called the cortex. It dips down between a radially
striated inner layer called the medulla, which consists of cone-shaped tissue masses
termed renal pyramids. The renal pelvis is a central space continuous with the ureter.
Microscopically, the kidney is composed of over one million nephrons.
3. Label the following parts of the nephron in the diagram shown below.
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a.
b.
c.
d.
e.
f.
g.
Excretion Review
glomerular (Bowman’s) capsule
glomerulus
afferent arteriole
efferent arteriole
proximal convoluted tubule
loop of Henle (ascending loop)
loop of Henle (descending loop)
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h. Loop of Henle (loop of nephron)
i. Peritubular capillary
j. distal convoluted tubule
k. collecting duct
l. renal medulla
4. The structure of each part of the nephron suits its function. The inner layer of the
glomerular capsule is composed of squamous epithelial cells called podocytes that have
long processes forming pores for filtration. The cells of the proximal convoluted tubule
have numerous microvilli that increase the surface area for tubular reabsorption. The
descending limb of the loop of the nephron allows water to leave, but the ascending limb
is imperious to water. The cells of the distal convoluted tubule have numerous
mitochondria that provide energy for active transport in tubular secretion.
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5. The process that is used to divide the blood into the filterable and non-filterable
components is called glomerular filtration . The process that transports molecules from
the nephron tubules into the surrounding capillaries is called tubular reabsorption. In
tubular secretion, molecules are removed from the blood and added to the tubular fluids.
6. Complete the following table to illustrate what takes place in different parts of the
nephron. For the heading molecules, use the following terms: protein, blood cells, water,
glucose, salt, urea, amino acids, H+, and ammonia.
Nephron Structure
Glomerulus
Major Event (process)
Filtration
Glomerular Capsule
Pressurization
Proximal convoluted
tubule
Tubular reabsorption
Loop of the Nephron
Water reabsorption
Distal convoluted tubule
Collecting Duct
Tubular secretion
Selective Concentration of
urine (reabsorption of
water if desired)
Molecules (present, to be
removed, to be added)
Most Small molecules
forced into tubules.
Afferent arteriole enters,
efferent arteriole leaves
Nutrients, salts, amino
acids actively transported
back to blood, water/other
small molecules passively
follow
Water, salt removed; urea
is concentrated – remains
in tubule
H+ /ammonia/drugs are
actively removed from
blood and into the tubule
Water leaves tubule (if
desired); urea leaks from
tubule into medulla
7. The juxtaglomerular apparatus helps to maintain blood volume by releasing renin
from the affferent arteriole cells whenever there is decreased blood volume or blood
pressure. Renin, an enzyme, converts angiotensinogen into angiotensin I, which in turn
is converted to angiotensin II by the converting enzyme in the lungs. Angiotensin II
constricts blood vessels, thereby increasing blood pressure, and stimulates the adrenal
cortex to release aldosterone . This hormone promotes the reabsorption of sodium ions
into the blood at the distal convoluted tubule, followed by increased water reabsorption,
which leads to increased blood volume and blood pressure.
8. Another hormone that regulates blood volume is antidiuretic hormone (ADH), which
comes from the posterior lobe of the pituitary gland. If an individual does not drink
much water on a particular day, ADH is released and more water is reabsorbed from the
renal tubules into the surrounding peritubular capillaries, which means a(n) decreased
(increased/decreased) urine volume. ADH secretion is inhibited by alcohol
True (T) or False (F) Questions
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If you believe the statement to be false, rewrite the statement as a true one.
9. There is a greater osmotic concentration in the outer renal medulla region than in the
inner renal medulla region.
AnswerF
region.
. Restatement: greater osmotic concentration on the inner renal medulla
10. The ascending limb of the loop of the nephron is impermeable to water.
Answer T
. Restatement:
11. Hangovers after drinking alcohol may be due to dehydration as a result of alcohol
stimulating ADH secretion.
AnswerF
. Restatement: alcohol inhibits ADH secretion
12. The hormone ADH is released from the adrenal cortex, whereas aldosterone is
released from the posterior pituitary.
Answer F
. Restatement:
ADH  p.pituitary gland
aldosterone  adrenal cortex
13. The kidneys help to regulate salt balance, blood volume, and the pH of the blood.
Answer T
. Restatement:
14. During continuous ambulatory peritoneal dialysis, a dialysate is placed into the
abdominal cavity.
Answer T
. Restatement:
For questions 15-19, match the following excretory products to each of the sources
below.
a. urea
c. uric acid
e. urochrome
b. creatinine d. bile pigments
15.
16.
17.
D
E
A
hemoglobin
heme
CO2 + ammonia
18.
19.
C
B
nucleotides
Creatine phosphate
For questions 20-24, match the following excretory organs to each of the excretory
products below.
a. skin
c. lungs
b. liver
d. kidneys
20.
21.
22.
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B
D
A
bile pigments
urochrome
water, salts, some urea
23.
24.
C
D
CO2 + water
water, urea, salts
For questions 25-29, match the following enzymes or hormones to each of their
functions below.
a. renin
b. aldosterone
c. angiotensin converting enzyme
25.
26.
27.
28.
29.
E
D
C
A
D
d. antidiuretic hormone
e. atrial natriuretic hormone
inhibits secretion of renin and aldosterone
promotes water reabsorption, decreases urine
converts angiotensin I into angiotensin II
converts angiotensinogen to angiotensin I
promotes Na+ reabsorption in the distal tubule
For questions 30-35, match the following conditions to each of the statements below.
a. uremia
b. urinalysis
c. pyelonephritis
d. urethritis
e. cystitis
f. dialysis
30. infection of the urinary bladder
31. infection of the urethra
32. determination of urine content
33. infection of the kidneys
34. accumulation of urea in the blood
35. diffusion of dissolved molecules through a semipermeable membrane
E
D
B
C
A
F
Multiple Choice Questions.
36. Which of the following describes the flow of urine through the urinary system
correctly?
a. urethra, urinary bladder, ureter, kidney
b. kidney, ureter, urinary bladder, urethra
c. kidney, urethra, urinary bladder, ureter
d. urinary bladder, ureter, kidney, urethra
e. kidney, urinary bladder, ureter, urethra
37. Which of the following statements is NOT correct about water reabsorption in the
kidney?
a. Salt is actively transported out of the ascending limb
b. Water leaves the ascending limb
c. Water leaves the descending limb
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d. Water may leave the collecting duct
e. Urea contributes to the high solute concentration in inner medulla
38. Which process accounts for H+ and ammonium ions to be removed from the blood
and added to the distal convoluted tubule?
a. glomerular filtration
b. tubular reabsorption
c. tubular secretion
d. osmosis
e. diffusion
39. The composition of the glomerular filtrate is about the same as that found in the
a. proximal convoluted tubule
b. distal convoluted tubule
c. ascending limb of loop of nephron
d. collecting duct
e. plasma
40. Which sequence correctly describes the flow through various parts of the nephron?
a. glomerulus, glomerular capsule, proximal convoluted tubule, nephron
loop.
b. Glomerular capsule, nephron loop, proximal convoluted tubule, distal
convoluted tubule
c. Proximal convoluted tubule, distal convoluted tubule, nephron loop,
collecting duct
d. Nephron loop, collecting duct, distal convoluted tubule, proximal convoluted
tubule
e. Distal convoluted tubule, collecting duct, nephron loop, glomerulus,
glomerular capsule.
41. The juxtaglomerular apparatus is found between the
a. glomerular capsule and afferent arteriole
b. efferent arteriole and distal convoluted tubule
c. afferent arteriole and distal convoluted tubule
d. afferent arteriole and proximal convoluted tubule
e. proximal convoluted tubule and collecting duct
42. If blood becomes acidic,
are excreted and
a. H+ and Na+ ; NH3 and HCO3b. H+ and HCO3-; NH3 and Na+
c. H+ and NH3 ; Na+ and HCO3d. H+ and glucose; Na+ and HCO3-
are reabsorbed.
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a.
b.
c.
d.
kidney
ureter
urinary bladder
urethra
a.
b.
c.
d.
e.
f.
ureter
renal vein
renal artery
renal pelvis
renal cortex
renal medulla
(renal
pyramids)