Which type of white blood cell is primarily responsible for fighting bacterial infections?

A. To prevent backflow of blood: Vein valves are one-way structures that ensure blood flows toward the heart and prevent it from pooling or flowing backward. This is especially important in the extremities, where blood must move against gravity to return to the heart.

B. To increase blood pressure: Vein valves do not generate or increase blood pressure. Blood pressure is primarily maintained by cardiac output and arterial tone, not by the presence of venous valves.

C. To regulate body temperature: While veins contribute indirectly to temperature regulation through blood flow, valves themselves do not regulate body temperature. Their primary role is mechanical, controlling blood direction.

D. To transport nutrients: Nutrient transport occurs via blood plasma and red blood cells. Vein valves do not facilitate nutrient transport; they simply maintain unidirectional blood flow toward the heart.

A. Arteries: Arteries carry oxygenated blood away from the heart under high pressure but are not the primary sites for nutrient and waste exchange.

B. Veins: Veins return deoxygenated blood to the heart and serve as reservoirs, but they do not facilitate significant exchange with tissues.

C. Capillaries: Capillaries are thin-walled vessels that connect arterioles and venules, allowing efficient diffusion of oxygen, nutrients, and waste products between blood and surrounding tissues.

D. Lymph vessels: Lymph vessels transport lymph and assist in immune function but do not directly exchange nutrients or wastes with tissues.