Download Respiratory Care Anatomy and Physiology, 3rd

Beachey: Respiratory Care Anatomy and Physiology, 3rd Edition
Chapter 21: Filtration, Urine Formation, and Fluid Regulation
Answers to Workbook Questions
Key Terms and Definitions
1. Afferent arteriole—A group of blood vessels that branch from the renal artery and
supply blood to the glomerulus of the kidney.
2. Aldosterone—A hormone produced by the adrenal cortex that acts mainly on the
distal tubules and collecting ducts of the nephron. Aldosterone increases the
reabsorption of ions and water in the kidney, which increases blood volume and
therefore blood pressure.
3. Angiotension—A hormone in the renin-angiotensin system that causes
vasoconstriction, increased blood pressure, and release of aldosterone from the
adrenal cortex.
4. Antidiuretic hormone, ADH—A hormone secreted from the pituitary system that
promotes reabsorption of water into the blood by increasing the permeability of
the distal tubule and collecting ducts.
5. Atrial natriuretic hormone (ANH)—A hormone released by cells located in the
atria of the heart in response to overstretching of their fibers. ANH inhibits the
effects of aldosterone and prevents water reabsorption by promoting the loss of
Na+ in the urine, resulting in increased urine volume.
6. Bowman’s capsule—A hollow sphere at the beginning of the proximal tubule of
the nephron that performs the first step in the filtration of blood to form urine.
7. Collecting duct—A long straight tubule that extends into the medulla and is
involved in the regulation of fluid balance through the reabsorption of water and
sodium.
8. Cortex (adrenal cortex)—The outer portion of the kidney.
9. Efferent arteriole—Blood vessels formed by the convergence of the glomerular
capillaries that direct blood away from the glomerulus.
10. Filtration fraction—The ratio of the glomerular filtration rate (GFR) to the renal
plasma flow. It represents the portion of the fluid reaching the kidneys passing
into the renal tubules.
11. Glomerulus—A network of capillaries that is surrounded by Bowman’s capsule.
It receives its blood supply from the afferent arteriole and drains into the efferent
arteriole.
12. Isotonic—Refers to a solution that has the same sodium chloride concentration as
the normal cells of the body and the blood.
13. Juxtaglomerular apparatus—A microscopic structure in the kidney that regulates
the function of the nephrons.
14. Loop of Henle—The segment of the nephron located between the proximal and
distal tubules that is involved in ion and water transport and the concentration of
urine.
15. Macula densa—An area of densely packed distal tubular cells that are sensitive to
the concentration of sodium chloride.
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
Answers to Workbook Questions
21-2
16. Medulla (Renal medulla)—The innermost part of the kidney.
17. Nephron—The basic functional unit of the kidney. Its function is to eliminate
wastes from the body, regulate blood volume and blood pressure, control the
levels of electrolytes and metabolites, and regulate blood pH.
18. Osmotic pressure—Osmosis is the movement of solvent molecules through a
selectively permeable membrane into a region of higher solute concentration, with
the aim of equalizing the solute concentrations on the two sides. Osmotic pressure
is the pressure that needs to be applied to a solution to prevent the inward flow of
water across a semi-permeable membrane.
19. Peritubular capillaries—Tiny blood vessels that are branches of the efferent
arterial and that act as absorption vessels.
20. Renal corpuscle—The initial blood-filtering component of the nephron, consisting
of a glomerulus and a Bowman’s capsule.
21. Renin—A protein released by specialized kidney cells in response to low sodium
levels in the blood. It plays a role in the release of aldosterone to help control the
body’s sodium chloride and water balance.
22. Tubular transport maximum, Tm—The maximum rate at which actively
reabsorbed substances can be reabsorbed.
23. Vasa recta—A branch of the peritubular circulation that surrounds the loop of
Henle.
Labeling
1.
A.
B.
C.
D.
E.
F.
G.
Interlobular Artery
Interlobular vein
Renal pyramid
Medullary ray
Renal vein
Renal artery
Segmented artery
2.
3
4
1
5
2
6
Loop of Henle
Distal tubule
Bowman’s capsule
Juxtaglomerular apparatus
Proximal tubule
Collecting duct
Matching
1. F
2. C
3. A
4. D
5. B
6. E
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
Answers to Workbook Questions
21-3
Short Answer/Critical Thinking Questions
1. The kidneys play an important role in the compensation of deficiencies in other
organ system since they control fluid volume and composition. For example, the
kidneys will compensate for a decrease in blood pressure by retaining fluid. They
also adjust acid and base excretion to compensate for acid-base abnormalities
caused by respiratory compromise.
2. Glomerular capillary blood pressure is higher than pressures in other capillary
beds due to the high resistance of the efferent arterioles. This pressure gradient
allows glomerular filtration to occur more rapidly than filtration in other tissue
capillaries.
3. Efferent arteriole constriction increases glomerular filtration pressure, causing
more water to be filtered out of the blood. This increases the osmotic pressure of
the blood entering the efferent arteriole and ultimately the peritubular capillaries.
High osmotic pressure enhances the blood’s reabsorption capacity; thus, more
water is returned to the general circulation, restoring blood volume and pressure.
4. The daily volume of substances filtered by the kidneys is greater than the volume
of substances excreted as urine (180 L of water is filtered, and 0.5–3.0 L is
excreted).
The filtered volume is reabsorbed into the blood unless it is present in amounts
greater than normal.
5. Osmotic pressure in the efferent arterioles’ blood is much higher than in afferent
arteriole blood because the blood loses much water in the glomerular filtration
process. Thus, efferent blood is highly concentrated and has great reabsorption
capacity as it passes through the peritubular capillaries.
6. Two specialized feedback mechanisms automatically maintain a constant GFR
between arterial blood pressures of 75 and 160 mm Hg: (1) the afferent arteriolar
vasodilator feedback mechanism increases glomerular blood flow when sodium
chloride concentration in the filtrate is low, and (2) the efferent arteriolar
vasoconstrictor feedback mechanism decreases glomerular blood flow in response
to low sodium chloride concentration of the filtrate. The juxtaglomerular
apparatus controls both mechanisms. A constant GFR is important in
appropriately eliminating waste products and reabsorbing solutes.
7. Threshold substances in the filtrate are totally reabsorbed unless they are present
in amounts greater than normal. Threshold substances include glucose, amino
acids, phosphates, and sulfates.
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
Answers to Workbook Questions
21-4
Non-threshold substances pass through the glomerular membrane into the filtrate
and are not reabsorbed, regardless of their plasma concentration. Creatinine is an
example of a non-threshold substance.
8. A. The descending loop of Henle is freely permeable to water and moderately
permeable to most ions. As it passes down the descending limb of the loop of
Henle, filtrate fluid loses a great deal of water because of the tubule’s
permeability and the surrounding interstitial fluid’s high osmotic pressure. Thus,
at the tip of the loop of Henle, the filtrate’s volume is reduced and highly
concentrated, contributing to an increase in osmotic pressure.
B. Under the influence of ADH, the kidney excretes a low-volume, highly
concentrated urine. This conserves water and restores the body’s extracellular
fluid volume. In the absence of ADH, water remains trapped in the collecting duct
and the kidney excretes a large volume of dilute urine. Therefore, maintanence of
the high osmotic pressure gradient is essential for excreting concentrated urine.
9. If the body’s vascular volume is too low, decreased renal blood flow causes the
macula densa cells of the juxtaglomerular apparatus to secrete renin. This
ultimately results in angiotensin II formation, which causes the cortex of the
adrenal glands to secrete the hormone aldosterone. Aldosterone causes sodium to
be reabsorbed from the distal tubular filtrate. When positively charged sodium
ions leave the filtrate, negatively charged chloride ions passively follow. These
events combine to create an osmotic pressure gradient that reabsorbs water from
the filtrate back into the blood. In this way the kidney restores intravascular
volume toward normal.
Excessive extracellular fluid volume has the opposite effect: aldosterone is not
secreted, and sodium in the distal tubular filtrate is not reabsorbed. Consequently,
a larger quantity of water remains in the tubule with sodium and is excreted in the
urine.
Other hormones, atrial natriuretic hormone (ANH) and B-type natriuretic peptide
(BNP), also influence water reabsorption. Specialized muscle fibers in the heart’s
atria and ventricles secrete ANH and BNP in response to overstretching of their
+
fibers. As their names imply, these hormones promote natriuresis, or loss of Na
in the urine. In other words, ANH and BNP inhibit the effects of aldosterone and
thus tend to prevent water reabsorption and increase urine volume.
10. Loop diuretics block active sodium and chloride transport out of the ascending
+
–
limb of Henle’s loop (the main mechanism for Na and Cl reabsorption). This
+
–
causes diuresis for two reasons: (1) Na and Cl remain in the tubules and act as
osmotic diuretics, and (2) because sodium transport out of the ascending limb of
Henle’s loop is blocked, the osmotic pressure of medullary interstitial fluid
diminishes. Consequently, the medulla’s interstitium reabsorbs less water from
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
Answers to Workbook Questions
21-5
the collecting duct, and more fluid passes into the urine. These drugs are effective,
quick-acting agents useful in treating edema of congestive heart failure.
Case Study
1. a. A weakly contracting heart may not generate enough arterial pressure for
adequate renal perfusion and urine output. The body responds to low renal
perfusion pressure by secreting aldosterone and ADH. Consequently, the kidney
reabsorbs large quantities of water, increasing blood volume. The kidneys
continue retaining the fluid volume until the heart either fails from fluid overload
or responds by increasing cardiac output sufficiently to achieve a normal urine
output.
An increased blood volume is evidenced by this patient’s breath sounds, pedal
edema, and recent weight gain. Bilateral crackles are characteristic of pulmonary
edema; pedal edema and weight gain are characteristics of systemic edema.
b. The rationale for starting Lasix is to decrease the patient’s work of breathing
(due to the pulmonary edema) by decreasing blood volume, which will decrease
hydrostatic pressure and in turn decrease the amount of pulmonary interstitial
fluid. Lasix works quickly and is effective in treating the dyspnea associated with
pulmonary edema.
Lasix works by blocking active sodium and chloride transport out of the
+
–
ascending limb of Henle’s loop, the main mechanism for Na and Cl
reabsorption (see Critical Thinking Question 10).
c. Hypokalemia and dehydration are common complications of loop diuretics
such as Lasix. Because Na/Cl reabsorption is reduced, more sodium and chloride
reach the distal tubule and collecting duct. At these later points, sodium is
reabsorbed in exchange for potassium; thus, the more sodium there is in the
tubule, the more sodium is reabsorbed and the more potassium is lost, causing
hypokalemia. Excessive administration of loop diuretics can also lead to
dehydration. Severe dehydration is most likely to occur in the elderly and in
patients under prolonged sodium restriction.
Key Concept Questions.
1. D. When a person is at rest, the renal arteries usually carry approximately 15–
30% of the total cardiac output to the kidneys.
2. B. Water, electrolytes, and glucose are all threshold substances that can be
reabsorbed by the nephron. Creatinine is a non-threshold substance and is not
reabsorbed.
3. C. A GFR of 125 mL/min produces about 180 L of filtrate. Of this filtrate,
approximately 99% is reabsorbed into the blood for a total urine output of
approximately 1500 mL per day.
4. B. Renin secretion results in secretion of aldosterone. Both aldosterone and
antidiuretic hormone decrease the volume of urine in order to conserve blood
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
Answers to Workbook Questions
volume. Atrial natriuretic hormone promotes the loss of sodium in the urine,
thereby increasing the volume of urine.
Copyright © 2013, 2007, 1998 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.
21-6