HUMAN BIOLOGY

the UrInary SyStem 229

filtration In nephrons,

blood pressure forces water

and small solutes in blood

plasma out of glomerular

capillaries and into the

Bowman’s capsule.

tubular reabsorption

Substances move from

the filtrate inside a kidney

tubule into the peritubular

capillaries.

tubular secretion

Substances move out of

peritubular capillaries and

into the filtrate in kidney

tubules.

pumped out of the nephron tubule and then enter peritu-
bular capillaries and so return to the bloodstream.
Most reabsorption takes place across the walls of proxi-
mal tubules. As in all parts of the tubule, the walls in this
area are only one cell thick. Step 2 in Figure 12.5 shows
what happens with water, glucose, and salt (ions of sodium,
Na^1 , and chloride, Cl^2 ). All these substances can diffuse
from the filtrate in a tubule into and through the cells of the
tubule wall. On the outer side of the cells, active transport
(through proteins in the cells’ plasma membranes) moves
glucose and Na^1 into the tissue fluid. Sodium ions (Na^1 ) are
positively charged, and negatively charged ions, including
chloride (Cl^2 ), follow the sodium.
As the concentration of solutes rises in the fluid, water
moves out of the tubule cells by osmosis. In a final step,
solutes are actively transported into peritubular capillaries
and water again follows by osmosis. These substances now
have been reabsorbed. The solutes and water that remain in
the tubule become part of urine.
Reabsorption usually returns almost 99 percent of the
filtrate’s water, all of the glucose and most amino acids, all
but about 0.5 percent of the salt (sodium and chloride ions),
and 50 percent of the urea to the blood (Table 12.2).

secretion rids the body of excess hydrogen
ions and some other substances

Tubular secretion takes up unwanted substances that
have been transported out of peritubular capillaries and
adds them to the urine that is forming in nephron tubules.
It is summarized in step 3 of Figure 12.5. Among other
functions, this highly controlled process rids the body
of urea and of excess hydrogen ions (H^1 ) and potassium
ions (K^1 ).
Secretion is crucial to maintaining the body’s acid–base
balance, which you will read about in Section 12.5. It also
helps ensure that some wastes (such as uric acid and some
breakdown products of hemo globin) and foreign sub-
stances (such as antibiotics and some pesticides) do not
build up in the blood. The drug testing noted in the chapter
introduction relies on the use of urinalysis to detect drug
residues that have been secreted into urine.

What is urine and hoW does it form?

• Urine consists of water and solutes that are not needed to
  maintain the chemical balance of extracellular fluid, as well
  as water-soluble wastes.


• Urine forms through the steps called filtration, reabsorption,
  and secretion.


• In filtration, water and other small molecules are filtered from
  the blood and into the nephron.


• Reabsorption recaptures needed water and solutes.


• Secretion adds unwanted substances into urine, including
  hydrogen ions and foreign substances such as antibiotics.

taKe-Home message

amount Percentage Percentage

Filtered excreted Reabsorbed

Water 180 liters 1 99

Glucose 180 grams 0 100

Amino acids 2 grams 5 95

Sodium ions 630 grams 0.5 99.5

Urea 54 grams 50 50

Table 12.2 Average Daily Reabsorption Values

for a Few Substances

Figure 12.6 Sphincter muscles control the passage of urine

into the urethra. The external sphincter is skeletal muscle and

allows voluntary control of urination. (© Cengage Learning)

Urethra

Ureter

Bladder wall

Internal

sphincter

Opening to exterior

Urinary bladder

External

sphincter

Homeostasis requires that the

total volume of fluid in the blood

and tissues stays fairly stable.

Blood and tissue fluid are mostly

water, and while your kidneys are

removing impurities from your

blood they are also adjusting the

amount of water that is excreted

in urine or returned to the blood-

stream. These adjustments, which

we will look at in more detail in

Section 12.4, are represented by step

4 in Figure 12.5.

urination is a

controllable reflex

You probably don’t need to be told that urination is urine

flow from the body. Urination is a reflex response. As the

bladder fills, tension increases in the smooth muscle of its

strong walls. Where the bladder joins the urethra, an inter-

nal urethral sphincter built of smooth muscle helps prevent

urine from flowing into the urethra (Figure 12.6). As ten-

sion in the bladder wall increases, though, the sphincter

relaxes; at the same time, the bladder walls contract and

force urine through the urethra.

Skeletal muscle forms an external urethral sphincter closer

to the urethral opening. Learning to control it is the basis

of urinary “toilet training” in young children.

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