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Clinical Note Chapter 26 The Urinary System 1023
Diuretics Diuresis (dī-ū-RĒ-sis; dia, through ouresis, system. As we have noted, these actions are opposed by the
urination) is the elimination of urine. Urination is an equiva- natriuretic peptides.
lent term in a general sense, but diuresis typically indicates
the production of a large volume of urine. Diuretics (dī-ū- By ADH, which controls the permeability of the DCT and
RET-iks) are drugs that promote water loss in urine. The
usual goal in diuretic therapy is to decrease blood volume, collecting system to water. The secretion of ADH is sup-
blood pressure, extracellular fluid volume, or all three. The pressed by the natriuretic peptides, and this—combined
ability to control renal water losses with relatively safe and with the effects of natriuretic peptide on aldosterone secre-
effective diuretics has saved the lives of many people, es- tion and action—can dramatically increase urinary water
pecially those with high blood pressure or congestive heart losses.
failure.
The collecting system also has other reabsorptive and secre-
Diuretics have many mechanisms of action. However, tory functions. Many of them are important to the control of
all such drugs affect transport activities or water reabsorp- body fluid pH.
tion along the nephron and collecting system. For exam-
ple, consider the class of diuretics called thiazides (THĪ-uh- Reabsorption in the Collecting System
zīdz). These drugs increase water loss by decreasing so-
dium and chloride ion transport in the proximal and distal The collecting system reabsorbs sodium ions, bicarbonate ions,
convoluted tubules. and urea as follows:
Diuretic use for nonclinical reasons is on the rise. For S odium Ion Reabsorption. The collecting system contains
example, some bodybuilders take large doses of diuretics
to improve muscle definition temporarily. Some fashion aldosterone-sensitive ion pumps that exchange Na+ in
models or horse jockeys do the same. Their goal is to tubular fluid for K+ in peritubular fluid (Figure 26–14b).
reduce body weight for brief periods. This practice of “cos-
metic dehydration” is extremely dangerous. It has caused B icarbonate Reabsorption. Bicarbonate ions are reabsorbed
several deaths due to electrolyte imbalance and conse-
quent cardiac arrest. in exchange for chloride ions in the peritubular fluid
(Figure 26–14c).
deaminate amino acids in reactions that strip off the amino
groups ( ¬ NH2). The reaction sequence ties up H+ and yields Urea Reabsorption. The concentration of urea in the tubular
both ammonium ions (NH4+) and HCO3−. As indicated in
Figure 26–14c, the ammonium ions are then pumped into fluid entering the collecting duct is relatively high. The fluid
the tubular fluid by sodium-linked countertransport, and the entering the papillary duct generally has the same osmotic
bicarbonate ions enter the bloodstream by way of the peritu- concentration as that of interstitial fluid of the medulla—
bular fluid. about 1200 mOsm/L—but contains a much higher concen-
tration of urea. As a result, urea tends to diffuse out of the
Tubular deamination thus has two major benefits. It pro- tubular fluid and into the peritubular fluid in the deepest
vides carbon chains suitable for catabolism. It also generates portion of the medulla.
bicarbonate ions that add to the buffering capacity of plasma.
Secretion in the Collecting System
Reabsorption and Secretion along the
Collecting System The collecting system is important in controlling the pH of body
fluids through the secretion of hydrogen or bicarbonate ions.
The collecting ducts receive tubular fluid from many nephrons If the pH of the peritubular fluid decreases, carrier proteins
and carry it toward the renal sinus, through the concentration
gradient in the medulla. The normal amount of water and sol- 26pump hydrogen ions into the tubular fluid and reabsorb bi-
ute loss in the collecting system is regulated in two ways:
carbonate ions that help restore normal pH. If the pH of the
B y aldosterone, which controls sodium ion pumps along peritubular fluid rises (a much less common event), the col-
lecting system secretes bicarbonate ions and pumps hydrogen
most of the DCT and the proximal portion of the collecting ions into the peritubular fluid. The net result is that the body
eliminates a buffer and gains hydrogen ions that lower the pH.
We examine these responses in more detail in Chapter 27,
when we consider acid–base balance.
The Control of Urine Volume and Osmotic
Concentration
Urine volume and osmotic concentration are regulated through
the control of water reabsorption. Water is reabsorbed by
osmosis along the proximal convoluted tubule and the de-
scending limb of the nephron loop. The water permeabilities
