Not only is the relationship between the kidney and cardiovascular system import

Question

Not only is the relationship between the kidney and cardiovascular system important in the exchange of materials, but this relationship is also critical in maintaining homeostatic blood volumes that allow for proper perfusion of gases in the lung, exchange of nutrients, oxygen, and glucose to the brain, and filtration of wastes in the kidney—stuff that keeps us alive. The kidney is the main activator of a complex feedback mechanism to regulate blood volume and perfusion pressure by detecting deficits in blood volume and sodium levels that could deleteriously affect perfusion and lead to dehydration and death. Cells near the glomerulus, juxtaglomerular cells, detect deficits in pressure and sodium concentration and activate the renin-angiotensin system (RAS) to correct these deficits. From the juxtaglomerular cell detection of low BP and Na, explain step by step how the RAS returns the body to its homeostatic levels.

Explanation / Answer

The Renin-Angiotensin System (RAS) or sometime considered as Renin-Angiotensin-Aldosterone system (RAAS) is a hormonal system that controls blood pressure, nutrient level and other important function. A simple cascade of hormones and enzymes regulates the fluid balance and thus homeostasis of our system. The key components of this system are in the order of Renin, Angiotensin I, Angiotensin II and Aldosterone. The system works in a orderly manner which is,

1. The system starts when blood pressure drops, which are sensed by baroreceptors present in renal arteries. Also when the Na filtration in the distal tubules decreases in kidney, the RAS system takes off. The decrease of Na or Cl concentration is detected by macula densa of distal tubule in nephrone. Macula densa then sends signal to Juxtaglomerular cells (JG) which is the storage compartment of Renin, to release renin. The low blood pressure decreases the afferent arteriole pressure in kidney resulting decrease in glomerular filtration rate, hence reduction in NaCl concentration.

2. Once renin is released in blood by JG cells it converts Angiotensinogen to Angiotensin I (a deca peptide, meaning having 10 aminoacids in the peptide chain) by proteolytic cleavage. Angiotensinogen is released by liver into blood. So it is already present in blood to be activated by renin.

3. The activated Angiotensin I then converts into Angiotensin II (an octapeptide) by an enzyme released by lungs, called Angiotensin Converting Enzyme (ACE). Now this Angiotensin II has various important functions to be done to return body in homeostatic levels.

4. Angiotensin II (A-II) is a potent vasoconstrictor, which constricts blood vessels to increase the blood pressure by interacting with AT1 receptors. Thus A-II keeps up the glomerular filtration although the blood pressure is low in other part of the body.

5. A-II also signals adrenal cortex to release Aldosterone. Aldosterone then acts on tubules of nephrone to reabsorb Sodium and water from blood, and thus increasing Na concentration back to normal. This effectively increases blood volume and also the blood pressure.

6. As A-II is in circulation, it can reach to brain. There it stimulates release of Anti-Diuretic hormone or Vasopressin from posterior pituitary gland. Vasopressin also constricts blood vessels, increasing blood pressure; but the main function is increment of water retention in blood, also leading to blood pressure rise.

7. Additionally A-II stimulates release of norepinephrine from sympathetic nerve endings. Norepinephrine enhances sympathetic nerve stimulation, creating an adrenergic effect which helps vasoconstriction of blood vessels along the skin and digestive system.

8. The ultimate effect is systematic blood pressure level increase which then acts as inhibitory effect on RAS, preventing release of further renin into blood. Thus RAS acts as feedback mechanism.

Thus an effective RAS maintains homeostatic level in body, as proper blood volume and pressure helps in nutrient exchange in digestive system, gas exchange in lungs, nutrient transport and exchange in brain and other organs and also proper blood filtration through kidney.

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