Final answer:
Angiotensin II is a vasoconstrictor that decreases medullary blood flow and GFR. It aids in preserving blood volume and pressure, primarily in response to blood loss or dehydration.
Step-by-step explanation:
Angiotensin II has significant effects on kidney function, specifically on the medullary blood flow. It is a potent vasoconstrictor, which leads to a decrease in renal blood flow and glomerular filtration rate (GFR). This response is particularly noticeable during states of blood loss or dehydration. The mechanism operates through the renin-angiotensin-aldosterone system. Angiotensin II acts to preserve blood volume by reducing kidney excretion of fluids, ultimately aiming to maintain systemic blood pressure. Moreover, it stimulates the release of aldosterone from the adrenal cortex, which results in increased sodium and water reabsorption, further aiding the conservation of blood volume and pressure. Medicinally, angiotensin-converting enzyme (ACE) inhibitors can be used to manage blood pressure by interrupting this pathway.