Biology 12

116 MHR • Unit 2 Homeostasis

HO 2

H+

HO 2

solutes

collecting duct

capillary

network

drugs

creatinine

glucose

amino

acids

Bowman’s capsule

artery

loop of

Henle

vein

distal

tubule

proximal tubule

Figure 4.12Active and passive transport are both used to

maintain a balance of solutes and water. At A, the pressure

of the blood flowing into the glomerulus pushes solutes and

water into the Bowman’s capsule. At B, active transport is

used to recover amino acids and glucose. This makes the

filtrate relatively dilute, so water also moves out of the

nephron. At the bottom of the loop of Henle, the filtrate is

almost isotonic. Diffusion of urea and the active transport of

sodium ions out of the ascending loop of Henle creates a

relatively hypertonic environment. At C, water can leave the

distal tubule, resulting in more concentrated urine.

Glomerular Filtration

Water, salts, nutrient molecules,

and waste molecules move

from the glomerulus to the

inside of the Bowman’s capsule.

These small molecules are

called the glomerular filtrate.

A Active Recovery

ATP is used to actively transport amino acids

and glucose out of the filtrate back into the

body. This makes the filtrate more dilute, so

water leaves passively by osmosis as the fluid

flows through the descending loop of Henle.

B Water Recovery

Removing more water

concentrates the urine. Active

transport pumps sodium ions

outside the descending loop

of Henle to create a

hypertonic environment.

C

is actively pulled from the filtrate into the

surrounding tissue. At the same time, the water

that left the descending loop cannot re-enter the

ascending loop because this loop is impermeable

to water.

Chloride ions tend to follow the sodium ions

because of the electrical attraction between the

negative chloride ions and the positive sodium

ions. In addition, as the water concentration in the

filtrate decreases, the chloride ion concentration in

the filtrate increases, resulting in still more

chloride diffusion out of the ascending loop.

As shown in Figure 4.12, the distal tubule is

responsible for a process called tubular secretion.

Tubular secretioninvolves active transport to pull

substances such as hydrogen ions, creatinine, and

drugs such as penicillin out of the blood and into

the filtrate. The fluid from a number of nephrons

moves from the distal tubules into a common

collecting duct, which carries what can now be

called urine into the renal pelvis. At that point,

99 percent of the water that entered the proximal

tubule as nephric filtrate has been returned to the

body. In addition, nutrients such as glucose and

amino acids have been reclaimed.