In the kidneys, the hydrostatic pressure in the glomerular capillaries (GC) is g

Question

In the kidneys, the hydrostatic pressure in the glomerular capillaries (GC) is greater than most capillaries in order to facilitate filtering of the blood to the Bowman?s space (BS) through reverse osmosis. The hydrostatic pressure of blood entering in the GC is 6OmmHg = 8000Pa, while the hydrostatic pressure is slightly negative in the BS at -15mmHg = -2000Pa. Water and small molecules like glucose and urea can pass freely through the capillary walls, but these walls are impermeable to plasma proteins. Describe in words and/or equations how blood is concentrated as it passes through the glomerular capillaries.

Explanation / Answer

Blood enters the kidney via the renal artery, eventually forming many afferent arterioles, each of which delivers blood to an individual kidney nephron. The diameter of the afferent (incoming) arteriole is greater than the diameter of the efferent arteriole (by which blood leaves the glomerulus). The pressure of the blood inside the glomerulus is increased due to the difference in diameter of the incoming and out-going arterioles. This increased blood pressure helps to force water, salts, glucose and urea from blood out of the glomerular capillaries. These components are filtered on the basis of particle size that should be less than 68000 Da; forming the glomerular filtrate. Plasma proteins ad blood cells have size greater than this and are not filtered.

This glomerular filtrate is formed at a rate of above 125 cm3 per minute in humans. This volume is approx. 20% of the plasma delivered during that time.

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