HUMAN BIOLOGY

226 Chapter 12

glomerulus Cluster of
blood capillaries in a neph-
ron, where substances
move from the blood into
the nephron.

kidneys Organs that adjust
fluid balance and filter
wastes from blood. The
kidneys also perform other
physiological functions.

nephrons Blood-filtering
units in the kidneys.

ureter Tube that carries
urine from kidneys to the
bladder.

urethra Tube that carries
urine to the outside of the
body.

urinary bladder Hollow
organ that stores urine.

the urinary system: Built for Filtering

and Waste Disposal

intestine to absorb calcium in food. In addition, kidneys

make the enzyme renin, which helps regulate blood pres-

sure, as you will read later in this chapter. The main func-

tion of kidneys, however, is to remove metabolic wastes

from the blood and adjust fluid balance in the body.

In addition to the two kidneys, the urinary system

includes “plumbing” that transports or stores urine. Once

urine has formed in a kidney, it flows into a tubelike

ureter, then on into the urinary bladder, where it is stored

until you urinate. Urine leaves the bladder through the

urethra, a tube that opens at the body surface.

nephrons are the kidney filters

Each kidney lobe contains blood vessels and more than a

million slender tubes called nephrons. Nephrons are the

structures that filter water and solutes from blood.

A nephron is shaped a bit like the piping under a sink

(Figure 12.4A). Its wall is a single layer of epithelial cells,

but the cells and junctions between them vary in different

parts of the tube. Water and solutes pass easily through

some parts, but other parts block solutes unless they are

moved across by active transport (Section 3.11).

As shown in Figure 12.4B, the nephron wall balloons

around a cluster of blood capillaries called the glomerulus

n The urinary system consists of filtering organs—the

kidneys—and structures that carry and store urine.

n Links to Metabolism 3.13, Blood exchanges with tissues 7.7,

Red blood cell production 8.3

Each kidney is a bean-shaped organ

about the size of a rolled-up pair

of socks (Figure 12.3). It has sev-

eral roughly triangular internal

lobes. In each lobe, an outer cortex

wraps around a central region, the

medulla, as you can see sketched in

Figure 12.3C. The whole kidney is

wrapped in a tough coat of connec-

tive tissue, the renal capsule (from

the Latin renes, meaning kidneys). A

kidney’s central cavity is called the

renal pelvis.

Our kidneys have several func-

tions. They produce the hormone

erythropoietin, which stimulates the

production of red blood cells (Sec-

tion 8.3). They also convert vitamin

D to a form that stimulates the small

Figure 12.3 Animated! The urinary system consists of the kidneys and several other parts. a Structure and function of the urinary

system. B The two kidneys, ureters, and urinary bladder are located between the abdominal cavity’s wall and its lining, the peritoneum.

c Internal structure of a kidney. (© Cengage Learning)

Kidney (one of a pair)

Blood-filtering organ;

filters water, all solutes

except proteins from

blood; reclaims only

amounts body requires,

excretes rest as urine

Ureter(one of a pair)

Channel for urine flow

from a kidney to

urinary bladder

Urinary Bladder

Stretchable urine

storage organ

Urethra

Urine flow channel

between urinary

bladder and

body surface

right

kidney

backbone

peritoneum abdominal

cavity

left

kidney

(back of body)

(front of body)

heart

diaphragm

adrenal

gland

abdominal

aorta

inferior^

vena cava

renal

medulla

renal

pelvis

renal

artery

ureter

renal

cortex

renal

capsule

renal

vein

a

B

c

12.2

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