The fluid that enters the glomerular capsule is called ultrafiltrate (fig. 17.10) because it is formed under pressure—the hydrostatic pressure of the blood. This process is similar to the formation of tissue fluid by other capillary beds in the body in response to Starling forces (chapter 14; see fig. 14.9). The force favoring filtration is opposed by a counterforce developed by the hydrostatic pressure of fluid in the glomerular capsule. Also, since the protein concentration of the tubular fluid is low (less than 2 to 5 mg per 100 ml) compared to that of plasma (6 to 8 g per 100 ml), the greater colloid osmotic pressure of plasma promotes the osmotic return of filtered water. When these opposing forces are subtracted from the hydrostatic pressure of the glomerular capillaries, a net filtration pressure of only about 10 mmHg is obtained.
Because glomerular capillaries are extremely permeable and have an extensive surface area, this modest net filtration pressure produces an extraordinarily large volume of filtrate. The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys per minute. The GFR averages 115 ml per minute in women and 125 ml per minute in men. This is equivalent to 7.5 L per hour or 180 L per day (about 45 gallons)! Since the total blood volume averages about 5.5 L, this means that the total blood volume is filtered into the urinary tubules every 40 minutes. Most of the filtered water must obviously be returned immediately to the vascular system, or a person would literally urinate to death within minutes.
■ Figure 17.8 The structure of the glomerulus and capsule. An illustration of the relationship between the glomerular capillaries and the inner layer of the glomerular (Bowman's) capsule. Notice that filtered molecules pass out of the fenestrae of the capillaries and through the filtration slits to enter the cavity of the capsule.
Proximal convoluted tubule
Glomerular (Bowman's) capsule
Podocyte of visceral layer of glomerular capsule
Parietal layer of glomerular capsule
■ Figure 17.9 An electron micrograph of the filtration barrier. This electron micrograph shows the barrier separating the capillary lumen from the cavity of the glomerular (Bowman's) capsule.
■ Figure 17.10 The formation of glomerular ultrafiltrate. Only a very small proportion of plasma proteins (green spheres) are filtered, but smaller plasma solutes (purple spheres) easily enter the glomerular ultrafiltrate. Arrows indicate the direction of filtration.
Physiology of the Kidneys jBlood pressure
Baroreceptor reflex f Sympathetic nerve activity f Sympathetic nerve activity
t Blood volume
■ Figure 17.11 Sympathetic nerve effects. The effect of increased sympathetic nerve activity on kidney function and other physiological processes is illustrated.
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