Which structural feature is unique to arteries compared to veins?

A. The neuroendocrine system releases ADH: During shock, the neuroendocrine system responds to decreased perfusion by releasing antidiuretic hormone (ADH) from the posterior pituitary. ADH promotes water reabsorption in the kidneys, helping to maintain blood volume and support blood pressure, which is a key compensatory mechanism.

B. The cardiovascular system directs blood to the skin muscles: In shock, the cardiovascular system prioritizes perfusion to vital organs such as the heart, brain, and kidneys, while reducing blood flow to nonessential areas like the skin and skeletal muscles. Directing blood to the skin and muscles would worsen tissue perfusion, making this statement false.

C. The hematologic system initiates the coagulation cascade: Activation of the coagulation system is a compensatory response during shock, particularly in hemorrhagic or hypovolemic states, to minimize blood loss. Platelet aggregation and clot formation help preserve circulating volume and prevent further compromise.

D. The renal system activates renin to convert angiotensinogen to angiotensin I: The renin-angiotensin-aldosterone system (RAAS) is activated in shock when renal perfusion drops. Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II, causing vasoconstriction and aldosterone release to retain sodium and water, maintaining blood pressure.

Correct answer: True

Larger veins, particularly in the extremities, contain valves that prevent the backflow of blood and ensure unidirectional movement toward the heart. These valves are necessary because venous blood is often moving against gravity, especially from the legs and lower body. The combination of venous valves and the skeletal muscle pump, where contracting muscles compress veins, facilitates venous return. Without these valves, blood could pool in the lower extremities, leading to venous insufficiency, edema, and varicose veins.