Human Physiology, 14th edition (2016)

Physiology of the Kidneys 585

A number of interlobular arteries radiate from the arcuate arteries

into the cortex and subdivide into numerous afferent arterioles

( fig. 17.5 ), which are microscopic. The afferent arterioles deliver

blood into glomeruli —capillary networks that produce a blood

control the switch from the guarding to the voiding reflex,
thereby allowing the person to have voluntary control of mic-
turition. When the decision to urinate is made, the micturition
center in the pons becomes activated by sensory information
monitoring bladder stretch. As a result, pudendal nerve activ-
ity is inhibited so that the external urethral sphincter can relax.
Then the parasympathetic nerve to the detrusor muscle is acti-
vated, causing contraction of the bladder and voiding of urine.
The ability to voluntarily inhibit micturition generally devel-
ops between the ages of two and three.

Microscopic Structure of the Kidney

The nephron (see fig.  17.2 ) is the functional unit of the kid-
ney responsible for the formation of urine. Each kidney con-
tains more than a million nephrons. A nephron consists of small
tubes, or tubules, and associated small blood vessels. Fluid
formed by capillary filtration enters the tubules and is subse-
quently modified by transport processes; the resulting fluid that
leaves the tubules is urine.

Renal Blood Vessels

Arterial blood enters the kidney through the renal artery, which
divides into interlobar arteries ( fig.  17.4 ) that pass between the
pyramids through the renal columns. Arcuate arteries branch from
the interlobar arteries at the boundary of the cortex and medulla.

Interlobular artery

and vein

Renal cortex

Arcuate artery

and vein

Renal medulla

Interlobar artery

and vein

Renal artery

Renal pelvis

Renal vein

Ureter

Figure 17.4 Major blood vessels of the kidney. The

vessels carrying blood into the renal medulla and cortex, and

those carrying blood out of the kidney, are illustrated.

Glomerulus

Efferent arteriole

Afferent arteriole

Interlobular artery

Arcuate artery

and vein

Peritubular

capillaries

(vasa recta)

Collecting

duct

Interlobular vein

Peritubular capillaries Distal convoluted tubule

Interlobar

artery and vein

Proximal

convoluted

tubule

Nephron loop

(of Henle)

Descending

limb

Ascending

limb

Glomerular capsule

Figure 17.5 The nephron tubules and associated blood vessels. In this simplified illustration, the blood flow from a
glomerulus to an efferent arteriole, to the peritubular capillaries, and to the venous drainage of the kidneys is indicated with arrows. The
names for the different regions of the nephron tubules are indicated with boldface type.