Each alveolus is surrounded by a web of blood capillaries supplied by the

A. Granzymes:Granzymes are proteolytic enzymes released by NK cells into target cells after perforin creates entry pores. They trigger apoptosis by activating caspases and degrading intracellular components, allowing NK cells to eliminate virus-infected or cancerous cells efficiently.

B. Selectins:Selectins are adhesion molecules that help leukocytes migrate during inflammation. They are not enzymes and are not secreted by NK cells, as their role involves cell-to-cell binding rather than inducing apoptosis.

C. Cytokines:Cytokines are signaling molecules used for communication within the immune system. Although NK cells secrete cytokines like IFN-γ, cytokines are not proteolytic enzymes and do not directly induce programmed cell death.

D. Interferons:Interferons are antiviral signaling proteins that enhance immune defense, but they are not proteolytic and do not mediate direct killing. NK cells may secrete interferons, yet these are regulatory rather than destructive molecules.

E. Perforins:Perforins create pores in the target cell membrane, enabling granzymes to enter. While essential to NK-cell function, perforins themselves are not proteolytic enzymes, but rather pore-forming proteins.

A. Right atrioventricular:The right atrioventricular valve, also called the tricuspid valve, regulates blood flow between the right atrium and right ventricle. It does not control flow from the ventricle to the pulmonary vessels.

B. Pulmonary:The pulmonary valve controls blood flow from the right ventricle into the pulmonary artery, which carries deoxygenated blood to the lungs for oxygenation. It prevents backflow into the ventricle during diastole, making it the correct valve for this function.

C. Aortic:The aortic valve regulates blood flow from the left ventricle into the aorta. It manages systemic circulation, not pulmonary circulation, so it does not serve the right ventricle.

D. Left atrioventricular:The left atrioventricular valve, also known as the mitral valve, controls flow from the left atrium to the left ventricle. It is unrelated to the right ventricular outflow to the lungs.

E. Mitral:The mitral valve is another name for the left atrioventricular valve. Like option D, it regulates flow into the left ventricle and is not involved in pulmonary circulation.