Question

Explain how blood pressure influences the glomerular filtration rate? (In detail).

Answer 1

Glomerular filtration rate (GFR) is a measure of how much blood flows into Bowman’s capsule of kidneys. Plasma enters the glomerulus via the afferent arterioles. Bowman’s capsule surrounds the glomerulus and has podocytes in its visceral epithelial layers. These podocytes allow fluid to pass through via the slits present. However, albumin, RBCs and platelets cannot pass through podocyte membrane. These compounds leave via efferent arterioles.

Normally, renal plasma flow is not majorly affected by a wide variety of mean arterial pressure. When there is increase in blood pressure, hydrostatic pressure increases in the afferent arteriole as well. As a result, there is stretching of vascular smooth muscles, which causes activation of inward directed ion channels. The renal blood flow is not increased because there is contraction of these smooth muscles Decreased blood pressure will cause dilation of smooth muscle and prevents renal blood flow from damaging the kidneys. However, if there is higher increased in blood pressure, the renal blood flow will be increased too as it is beyond the capacity of the arteriole smooth muscles to contract. This will cause increased GFR. Conversely, if the blood pressure decreases significantly, there will be decreased GFR as less blood flows through the afferent arterioles.

In case of large decrease in blood pressure, the renin-angiotensin-aldosterone system will be activated. The decrease GFR will cause stretch receptors in macula causes conversion of prorenin to renin in juxtamedullary cells in the afferent arteriole. Renin then acts on angiotensinogen in blood to form angiotensin I. Angiotensin I is converted to angiotensin II in lung. This conversion is by the angiotensin converting enzyme or ACE. Angiotensin II can act on kidney tubules to cause vasoconstriction of blood vessels. Angiotensin II will cause secretion of aldosterone from adrenal gland. Aldosterone will cause increased reabsorption of sodium and loss of potassium in the kidney tubules by action of sodium potassium channels. There is also increased secretion of vasopressin that caused increased reabsorption of water by adding aquaporin channels. All these measure will cause increased blood pressure. Reverse occurs when the blood pressure is high. There is decreased aldosterone secretion that causes decreased sodium reabsorption by the kidneys.