Question

Describe the importance of the renin-Ang II-aldosterone system. Identify the effects of Ang II. Identify the effects of aldosterone and its mechanism of action.

Answer 1

Answer.

The renin angiotensin aldosterone system is critical regulator for decreased arterial pressure and systemic vascular resistance.

In contrast to short term regulation by baroreceptors this component regulates the blood pressure on long term basis. The renin angiotensin aldosterone system consists of 3 components- renin, angiotensin-II and aldosterone. This complex acts to elevate arterial blood pressure in response to reduced renal blood pressure and sodium (Na+) to distal convulated tubule.

This complex acts on renal, pulmonary, systemic vascular system and brain.

Mechanism of action:

The JG (juxta glomerular cells) of afferent arteriole of renal system secretes prorenin which remains in inactive state in resting stage. On activation prorenin cleaves into renin in response to decreased blood pressure and activation of macula densa cells in response to reduced Na+ concentration in distal convulated tubule.

Once renin released in to the systemic circulation it acts on the angiotensinogen released by liver to form angiotensin-I. The angiotensin is converted into the angiotensin-II by ACE (Angiotensiogen converting enzyme) to act on the AT-I and AT-II receptors.

Effect of angiotensin-II:

On Proximal convulated tubule, it acts by increased Na+ reabsorption, which increases the osmolarity of the blood, by elevating the blood volume and ECF (Extracellular fluid). Thereby increasing the arterial blood pressure.

On adrenal cortex, angiotensin-II acts on the zona glomerulosa cells to release aldosterone, this in turn increases the Na+ reabsorption and potassium excretion. This increases the blood and ECF volume by elevating the osmolarity of the blood and arterial blood pressure.

On Brain, the angiotensin-II acts on the hypothalamus to increase water intake by increasing the thirst.

Overall effect of this complex renin angiotensin and aldosterone is to increase the Na+ reabsorption and body water and vascular tone.