The Kidneys and Regulation of Water and Inorganic Ions
517
SECTION B REVIEW QUESTIONS
1. What are the sources of water gain and loss in the body? What
are the sources of sodium gain and loss?
2. Describe the distribution of water and sodium between the
intracellular and extracellular fl
uids.
3. What is the relationship between body sodium and
extracellular fl
uid volume?
4. What is the mechanism of sodium reabsorption, and how is the
reabsorption of other solutes coupled to it?
5. What is the mechanism of water reabsorption, and how is it
coupled to sodium reabsorption?
6. What is the effect of vasopressin on the renal tubules, and what
are the sites affected?
7. Describe the characteristics of the two limbs of the loop of
Henle with regard to their transport of sodium, chloride, and
water.
8. Diagram the osmolarities in the two limbs of the loop of
Henle, distal convoluted tubule, cortical collecting duct,
cortical interstitium, medullary collecting duct, and medullary
interstitium in the presence of vasopressin. What happens
to the cortical and medullary collecting-duct values in the
absence of vasopressin?
9. What two processes determine how much sodium is excreted
per unit time?
10. Diagram the sequence of events in which a decrease in blood
pressure leads to a decreased GFR.
11. List the sequence of events leading from increased renin
secretion to increased aldosterone secretion.
12. What are the three inputs controlling renin secretion?
13. Diagram the sequence of events leading from decreased
cardiovascular pressures or from an increased plasma
osmolarity to an increased secretion of vasopressin.
14. What are the stimuli for thirst?
15. Which of the basic renal processes apply to potassium? Which
of them is the controlled process, and which tubular segment
performs it?
16. Diagram the steps leading from increased plasma potassium to
increased potassium excretion.
17. What are the two major controls of aldosterone secretion, and
what are this hormone’s major actions?
18. Contrast the control of calcium and phosphate excretion by PTH.
SECTION C
Hydrogen Ion Regulation
Metabolic reactions are highly sensitive to the hydrogen ion
concentration of the fl uid in which they occur. This sensitivity
is due to the infl uence that hydrogen ions have on enzyme func-
tion, which changes the shapes of proteins. Not surprisingly,
then, the hydrogen ion concentration of the extracellular fl
uid is
tightly regulated. At this point the reader might want to review
the section on hydrogen ions, acidity, and pH in Chapter 2.
This regulation can be viewed in the same way as the
balance of any other ion—that is, as matching gains and losses.
When loss exceeds gain, the arterial plasma hydrogen ion con-
centration decreases and pH exceeds 7.4. This is termed
alka-
losis
.
When gain exceeds loss, the arterial plasma hydrogen
ion concentration increases and the pH is less than 7.4. This is
termed an
acidosis
.
Sources of Hydrogen
Ion Gain or Loss
Table 14–6
summarizes the major routes for gains and losses
of hydrogen ions. As described in Chapter 13, a huge quantity
of CO
2
—about 20,000 mmol—is generated daily as the result
of oxidative metabolism. These CO
2
molecules participate in
the generation of hydrogen ions during the passage of blood
through peripheral tissues via the following reactions:
carbonic
anhydrase
CO
2
+ H
2
O
12
H
2
CO
3
12
HCO
3
+ H
+
(14–1)
This source does not normally constitute a net gain of hydro-
gen ions. This is because the hydrogen ions generated via these
reactions are reincorporated into water when the reactions
are reversed during the passage of blood through the lungs
(Chapter 13). Net retention of CO
2
does occur in hypoventila-
tion or respiratory disease and causes a net gain of hydrogen
ions. Conversely, net loss of CO
2
occurs in hyperventilation,
and this causes net elimination of hydrogen ions.
The body also produces both organic and inorganic acids
from sources other than CO
2
. These are collectively termed
nonvolatile acids.
They include phosphoric acid and sulfuric
acid, generated mainly by the catabolism of proteins, as well
Table 14–6
Sources of Hydrogen Ion Gain or Loss
Gain
1. Generation of hydrogen ions from CO
2
2. Production of nonvolatile acids from the metabolism of
proteins and other organic molecules
3. Gain of hydrogen ions due to loss of bicarbonate in diarrhea
or other nongastric GI fl
uids
4. Gain of hydrogen ions due to loss of bicarbonate in the
urine
Loss
1. Utilization of hydrogen ions in the metabolism of various
organic anions
2. Loss of hydrogen ions in vomitus
3. Loss of hydrogen ions in the urine
4. Hyperventilation
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