Capillary hydrostatic pressure during filtration is built in the glomerulus as: (a) the size of Bowman’s capsule is significantly large (b) the afferent arteriole is narrow compared to the efferent arteriole (c) Bowman’s capsule is cup-shaped (d) the efferent arteriole is narrow compared to the afferent arteriole
(d) the efferent arteriole is narrow compared to the afferent arteriole
The process of glomerular filtration, the first step in urine formation, relies on a high-pressure gradient to force water and small solutes from the blood into the Bowman’s capsule. This crucial pressure, known as the glomerular blood hydrostatic pressure (GBHP), is maintained at a remarkably high and constant level (approximately 55 mmHg) due to a unique anatomical arrangement of the arterioles supplying and draining the glomerulus.
The key to building this pressure lies in the relative diameters of the two arterioles. The afferent arteriole, which delivers blood to the capillary network of the glomerulus, has a relatively large diameter. This allows a substantial volume of blood to enter the glomerulus under high pressure from the renal artery. In contrast, the efferent arteriole, which carries blood away from the glomerulus, has a significantly narrower diameter.
This difference in caliber creates a “bottleneck” effect, providing considerable resistance to the outflow of blood from the glomerulus. As blood enters through the wide afferent arteriole more easily than it can exit through the narrow efferent arteriole, it effectively “dams up” or backs up within the glomerular capillaries. This damming effect is the direct cause of the elevated hydrostatic pressure within the glomerular tuft.
This high GBHP is the primary force promoting filtration and must be sufficient to overcome the two main opposing pressures: the capsular hydrostatic pressure (the pressure of the filtrate already in the Bowman’s capsule) and the blood colloid osmotic pressure (the tendency of water to move back into the capillaries due to the presence of plasma proteins). The unique vascular architecture, with a wider inlet and a narrower outlet, is a critical physiological adaptation ensuring that the net filtration pressure remains positive, allowing the kidneys to filter the blood efficiently and continuously.
