Permitting the exchange of nutrients and gases between the blood and tissue cells is the primary function of

A. Veins:Veins primarily function as capacitance vessels, returning blood to the heart and serving as blood reservoirs. They are not specialized for nutrient or gas exchange because their walls are thicker and less permeable.

B. Arteries:Arteries carry blood away from the heart and help maintain blood pressure, but their thick muscular walls prevent direct exchange of nutrients and gases with surrounding tissues. Their primary role is transport and pressure regulation.

C. Capillaries:Capillaries are the smallest blood vessels with thin walls composed of a single layer of endothelial cells. This structure allows efficient exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and tissue cells, making them the primary site of exchange.

D. Arterioles:Arterioles regulate blood flow into capillary beds through vasoconstriction and vasodilation. While they influence perfusion, they are not the primary site for actual exchange of gases and nutrients.

A. Cytokines:Cytokines are signaling molecules secreted by various immune cells to coordinate immune responses. While NK cells do secrete cytokines to activate other immune cells, cytokines themselves are not the enzymes that directly kill target cells.

B. Interferons:Interferons are proteins released by virus-infected cells that help inhibit viral replication and activate immune responses. They do not directly destroy target cells and are not classified as enzymes secreted by NK cells.

C. Selectins:Selectins are adhesion molecules that mediate the rolling of leukocytes along blood vessel walls during inflammation. They are unrelated to NK cell cytotoxic activity and do not function as killing enzymes.

D. Granzymes and perforins:NK cells secrete granzymes and perforins to induce apoptosis in virus-infected or abnormal cells. Perforins form pores in the target cell membrane, allowing granzymes to enter and trigger programmed cell death, providing a rapid innate immune defense.