The correct answer is angiotensin II. Renin-angiotensin-aldosterone system will be activated when there is a loss of blood volume (or decrease of blood pressure, such as in haemorrhage or dehydration. Renin is secreted by kidneys directly into the circulation when the blood flow is reduced. Renin then stimulates conversion of angiotensinogen, released by the liver, to angiotensin I which will then be converted to angiotensin II by the angiotensin-converting enzyme (ACE) found in the lungs. Angiotensin II is a vasoconstrictor (increases the blood pressure) and also stimulates the secretion of the aldosterone from the adrenal cortex. Aldosterone increases the reabsorption of sodium and water into the blood and the excretion of potassium (to maintain electrolyte balance). This also increases blood pressure and retention of water.
