HUMAN BIOLOGY

228 Chapter 12

How urine Forms: Filtration,

reabsorption, and secretion

plasma into Bowman’s capsule. Blood cells, platelets,

proteins, and other large solutes stay in the blood. Every-

thing else—water and small solutes such as glucose,

amino acids, sodium, urea, and vitamins—can filter out

of the glomerular capillaries and into Bowman’s capsule.

From there the filtrate flows into the proximal tubule

(Figure 12.5, step 2), where the next phase of urine forma-

tion can begin.

next, reabsorption returns useful

substances to the blood

The body cannot afford to lose the huge amounts of water

and valuable solutes such as glucose, amino acids, and

electrolytes that are filtered from the blood by the kidneys.

Fortunately, most of the filtrate is recovered by tubular

reabsorption. In this process, substances leak or are

n    The processes that form urine normally ensure that only

unneeded substances are excreted from the body.

n    Links to Diffusion and osmosis 3.10, Other ways substances

cross membranes 3.11, Liver functions 11.5

Urine forms in a sequence of three steps called filtration,

reabsorption, and secretion. Figure 12.5 gives you an over-

view of these steps.

Filtration removes a large amount of fluid

and solutes from the blood

Blood pressure is the driving force for filtration, the first

step in forming urine. Afferent arterioles are narrow, so

they deliver blood to the glomerulus under high pres-

sure. This pressure forces about 20 percent of the blood

12.3

Bowman’s

capsule

renal cortex

renal medulla

collecting

duct

increasing solute

concentration in

interstitial fluid

Glomerular

filtration

proximal tubule

ascending arm

of loop of Henle

H+

Tubular

reabsorption

Tubular

secretion

distal tubule

peritubular capillary

H 2 O

H 2 O

Na+, Cl–, K+,

nutrients, H 2 O

Na+, Cl–,

H 2 O

descending arm

of loop of Henle

H+, K+

urine to

renal pelvis

Na+ urea

H 2 O

Na+

filtered plasma

(^4) Hormones that alter
permeability of distal
tubules and collecting
ducts adjust urine
concentration.
(^1) Glomerular filtration
Filtered plasma forced out
of glomerular capillaries
by blood pressure enters
Bowman’s capsule.
(^2) Tubular reabsorption
Essential ions, nutrients,
water, and some urea in
the filtrate return to the
blood. Green arrows
indicate reabsorption.
(^3) Tubular secretion
Wastes and excess ions
are moved from the blood
into the filtrate for elimina-
tion in urine. Blue arrows
indicate secretion.
1 2 3
4
Figure 12.5 Animated! Urine forms in a sequence of three steps called filtration, tubular reabsorption,
and tubular secretion. For the sake of clarity, this diagram includes only a small portion of a peritubular
capillary (see Figure 12.4). (© Cengage Learning)
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