Page 1040 - Fundamentals of anatomy physiology
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Chapter 26 The Urinary System 1027
The Function of the Vasa Recta Table 26–5 General Characteristics of Normal Urine
The solutes and water reabsorbed in the renal medulla must Characteristic Normal Range
be returned to the bloodstream without disrupting the con-
centration gradient. This return is the function of the vasa pH 4.5–8 (average: 6.0)
recta. Recall that the vasa recta are long, straight capillar- Specific gravity 1.003–1.030
ies that parallel the long nephron loop of juxtamedullary Osmotic concentration (osmolarity) 855–1335 mOsm/L
nephrons. Water content 93%–97%
Volume 700–2000 mL/day
Blood entering the vasa recta from the peritubular Color Clear yellow
capillaries has an osmotic concentration of approximately Odor Varies with composition
300 mOsm/L. As the blood descends into the medulla, it gradu- Bacterial content None (sterile)
ally increases in osmotic concentration as the solute concentra-
tion in the peritubular fluid increases. This increase in blood creatinine, are missed by filtration but are actively secreted into
osmotic concentration involves both solute absorption and the tubular fluid.
water loss. Solute absorption predominates, however, because
the plasma proteins limit the osmotic flow of water out of the Filtration, reabsorption, and secretion determine the kinds
blood. p. 765 and amounts of substances excreted in urine. The concentration
of these substances in a given urine sample depends on the
Blood ascending toward the cortex gradually decreases in osmotic movement of water across the walls of the tubules and
osmotic concentration as the solute concentration of the peri- collecting ducts. Because the composition and concentration
tubular fluid decreases. Again, this decrease involves both sol- of urine vary independently, you can produce a small volume
ute diffusion and osmosis. In this case osmosis predominates, of concentrated urine or a large volume of dilute urine and
because the presence of plasma proteins does not oppose the still excrete the same amount of dissolved substances. For this
osmotic flow of water into the blood. reason, physicians who are interested in a detailed assessment
of renal function commonly analyze the urine produced over a
The net results are that (1) some of the solutes absorbed 24-hour period rather than a single urine sample.
in the descending portion of the vasa recta do not diffuse out
in the ascending portion and (2) more water moves into the Urinalysis is the analysis of a urine sample. It is an im-
ascending portion of the vasa recta than moves out in the portant diagnostic tool, even in high-technology medicine.
descending portion. Thus, the vasa recta carries both water A standard urinalysis includes an assessment of the color and
and solutes out of the medulla. Under normal conditions, appearance of urine. These two characteristics can be deter-
the removal of solutes and water by the vasa recta precisely mined without specialized equipment. In the 17th century,
balances the rates of solute reabsorption and osmosis in the physicians classified the taste of the urine as sweet, salty, and
medulla. so on, but quantitative analytical tests have long since replaced
the taste-bud assay.
The Composition of Normal Urine
Normal urine is a clear, sterile solution. Its yellow color
As we have seen, more than 99 percent of the 180 liters of
filtrate produced each day by the glomeruli is reabsorbed. 26comes from the pigment urobilin. The kidneys generate this
It never reaches the renal pelvis for elimination. General
characteristics of the remaining filtrate—normal urine—are pigment from the urobilinogens produced by intestinal bac-
listed in Table 26–5. However, the composition of the urine teria and absorbed in the colon (see Figure 19–4, p. 688). The
produced each day varies with the metabolic and hormonal characteristic odor of urine is due to the evaporation of small
events under way. molecules, such as ammonia. Other substances not normally
present, such as acetone or other ketone bodies, can also impart
The composition and concentration of urine are two re- a distinctive smell.
lated but distinct properties. The composition of urine reflects
the filtration, reabsorption, and secretion activities of the neph- Table 26–6 gives some typical values obtained from uri-
rons. Some substances (such as urea) are neither actively ex- nalysis. Spotlight Figure 26–16 provides a summary of kidney
creted nor reabsorbed along the nephron. In contrast, organic function showing the major steps in the reabsorption of water
nutrients are completely reabsorbed. Other substances, such as and the production of concentrated urine.
