The correct answer is D. By a decrease in the blood pressure.
Renin is an enzyme produced by the kidneys in response to a decrease in blood pressure. It helps to regulate blood pressure by stimulating the production of angiotensin II, which causes the blood vessels to constrict. Renin also helps to regulate the production of aldosterone, which causes the kidneys to retain sodium and water.
A decrease in blood pressure can occur for a number of reasons, including dehydration, blood loss, and heart failure. When blood pressure decreases, the kidneys sense a decrease in the amount of blood flowing through them. This triggers the release of renin, which helps to restore blood pressure to normal levels.
The other options are incorrect because they do not directly affect blood pressure. Option A is incorrect because the pH of urine is not directly related to blood pressure. Option B is incorrect because the specific gravity of urine is a measure of the concentration of dissolved substances in urine, and it is not directly related to blood pressure. Option C is incorrect because the peritubular capillaries are located in the kidneys, and their dilation or constriction does not directly affect blood pressure.