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Support of Renal Function 481

URINE PRODUCTION, REGULATION OF
GFR AND FILTRATE REABSORPTION
Bowman’s capsule IN THE NEPHRON
Proximal Urine production consists of a three-stage process, which
Glomerulus convoluted occurs in the nephron: glomerular filtration, tubular
tubule reabsorption and tubular secretion (see Figure 18.3). As
Efferent
arteriole previously noted, the production of urine is highly
dependent on delivery of blood under pressure to the
Afferent glomerulus. This results in the first step of the urine pro-
arteriole
duction process, glomerular filtration. The glomerular
filtration rate (GFR) is about 125 mL/min under normal
conditions. Changes in the diameter of the afferent and
Collection efferent arteriole help regulate glomerular blood flow, but
tube this is unable to compensate for large variations of mean
blood pressure; hence, filtration rates may rise or fall
Descending markedly over the course of a day. 10
limb of loop
Interlobular As the filtrate transgresses the glomerulus it is collected
artery Distal into the Bowman’s capsule and delivered into the proxi-
convoluted
tubule mal convoluted tubule, loop of Henle and then the distal
convoluted tubule, where a number of processes result in
Interlobular Ascending the reabsorption of about 99% of the glomerular filtrate.
vein limb of loop The remaining fluid within the tubule drains into the
collecting tubule to form urine. This fluid has substan-
Loop of Henle
tially different properties from the original glomerular
filtrate, as fluid and many electrolytes and glucose are
FIGURE 18.2 Nephron and glomerulus. reabsorbed by the peritubular capillaries. 10
107

Glomerular Tubular Tubular
filtration reabsorption secretion peritubular
capillary

glomerulus distal
convoluted
proximal
glomerular convoluted tubule
capsule tubule

H 2 O collecting
duct

H O
2
Reabsorption
of water

loop of
the nephron

renal pelvis

Excretion

108
FIGURE 18.3 Urine formation: filtration, reabsorption and excretion.