Biology 12

114 MHR • Unit 2 Homeostasis

down the ureters. These sections are shown to the

left of Figure 4.10. Within the cortex and medulla

of each kidney are about one million tiny filters

called nephrons. As Figure 4.10 illustrates, each

nephron consists of five parts — the Bowman’s

capsule, the proximal tubule, the loop of Henle,

the distal tubule, and the collecting duct. The

upper portions of the nephron are found in the

renal cortex, while the loop of Henle is located in

the renal medulla. The tubes of the nephron are

surrounded by cells, and a network of blood vessels

spreads throughout the tissue. Any material that

leaves the nephron enters the surrounding cells

and eventually returns to the bloodstream through

the network of blood vessels. By controlling what

leaves and what remains in the nephrons, the

kidneys keep the levels of water, ions, and other

materials nearly constant and within the limits

necessary to maintain homeostasis.

Blood enters the cavity of the ball-shaped

Bowman’s capsule through a tiny artery that

branches to form a network of porous, thin-walled

capillaries called the glomerulus. Under the

influence of blood pressure, some blood plasma

and small particles are forced out of the capillaries

and into the surrounding capsule. Larger blood

components, such as blood cells and proteins,

remain in the capillaries. The fluid in the

Bowman’s capsule is called nephric filtrate, and

it is pushed out of the capsule into the proximal

tubule. About 20 percent of the blood plasma that

enters the kidney becomes nephric filtrate.

When the nephric filtrate enters the proximal

tubule, re-absorption begins. Re-absorptionis the

People who are trying to increase muscle mass sometimes

use a diet high in proteins or amino acids. The problem with

a diet like this is that it creates an excess of amino acids,

which are broken down in the liver to form the carbohydrates

necessary for metabolism. The excess amine groups in turn

produce high levels of urea, which is released into the

bloodstream to be removed by the kidneys. However, high

urea levels can damage the kidneys, so it is necessary to

find ways to bring these levels down. The simplest way to

accomplish this is to drink plenty of fluids.

BIO FACT

renal artery

renal vein

capillary

network

glomerulus

proximal tubule

distal tubule

Renal Cortex

Renal Medulla

collecting duct

loop of

Henle

Bowman’s capsule

descending

loop

ascending

loop

Figure 4.10A nephron is composed of the Bowman’s

capsule, the proximal tubule, the loop of the nephron

(called the loop of Henle), the distal tubule, and the

collecting duct.

outer layer

(cortex)

inner layer

(medulla)

inner

collecting

area (pelvis)

ureter