Question

Explain glomerular filtration and the mechanisms that control its pressure and rate. Why is GFR critical to normal urine formation?

Answer 1

GLOMERULAR FILTRATION

Glomerular filtration is the first step in the formation of urine. Here, about 20% of the blood flowing through the glomeruli is fitered out of the capillaries into the renal tubules. In the second step of urine formation, tubular reabsorption of nutrients and other substances takes place in the renal tubules by active or passive transport; and excess fluid and waste products are eliminated from the  body through urine.

In glomerular filtration, the hydrostatic pressure of the blood in the glomerulus, pushes the filtrate from the glomerular capillaries into the filtration slits in the nephron and to the renal tubules.

Glomerular filtration rate (GFR) represents the blood flowing from the glomerulus into Bowman's space in a given time. The rate of glomerular filtration gives a direct idea aout the renal function. The normal adult GFR is 125 ml / minute.

Factors affecting glomerulat filtration

1. Glomerular capillary hydrostatic pressure

Glomerular capillary hydrostatic pressure is the  pressure exerted by fluid on the capillary walls of the glomerulus. This force is opposed by the hydrostatic pressure in the Bowmans capsule. So, when the Glomerular capillary hydrostatic increases, the GFR increases.

2. Oncotic pressure of plasma in glomerular capillaries

Oncotic pressure of plasma in glomerular capillaries is the pressure exerted by proteins (such as albumin) within the glomerulus. The oncotic tries to hold water back in the glomerulus, reducing glomerular filtration.

'' So, an increase in the Glomerular hydrostatic pressure and decrease in glomerular oncotic pressure favours filtration ang hence increases Glomerular filtration rate ''

3. Changes in effective filtratration surface area.

The effective filtratration surface area is altered by the measnagial cells in the kidneys. Contraction of the mesangial cells causes a decrease in effective filtration area hence reducing GFR and vice versa.

4. Changes in glomerular capillary permeability.

The glomeruli allows water and small solutes to pass readily into the bowman's space, wheras albumin and other large proteins are not allowed to pass through. The permeability of the glomerular capillaries can be altered in various diseases, hence affecting the GFR.

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Importance of Glomerular filtration in urine formation

Glomerular filtration is the first step in the formation of urine. About 20% of the blood flowing through the glomeruli at any time is fitered out of the capillaries into the renal tubules. In the second step of urine formation, tubular reabsorption of nutrients and other substances takes place in the renal tubules by active or passive transport; and excess fluid and waste products are eliminated from the  body through urine.