Where is the SA node is located?

A. Reticular cells:Reticular cells provide structural support within lymphoid tissues by producing the stroma, but they do not engage in direct destruction of abnormal cells. Their role centers on organizing the tissue microenvironment rather than conducting immune surveillance.

B. Macrophages:Macrophages phagocytose pathogens and debris and help present antigens, but their actions depend on encountering targets rather than performing continuous nonspecific monitoring. Their surveillance is limited compared to NK cells’ rapid response capabilities.

C. Natural killer (NK) cells:NK cells constantly patrol the body and nonspecifically kill virus-infected and cancerous cells through perforin and granzyme release. They do not require prior antigen exposure, making them central to immune surveillance and early defense against abnormal cells.

D. T lymphocytes (T cells):T cells require antigen presentation and activation before responding, so they do not provide the immediate, nonspecific surveillance carried out by NK cells. Their function is part of adaptive immunity rather than constant innate monitoring.

E. Dendritic cells:Dendritic cells specialize in capturing antigens and migrating to lymph nodes to activate T cells. They contribute to immune activation but are not responsible for nonspecific detection and destruction of diseased cells.

A. A decrease in membrane thickness:Thinner respiratory membranes allow gases to diffuse more rapidly between alveolar air and blood. A decrease in thickness enhances gas exchange rather than slowing it.

B. An increase in alveolar surface area:A larger surface area provides more sites for diffusion, improving the efficiency of gas exchange. This change accelerates, rather than impedes, oxygen and carbon dioxide transfer.

C. An increase in membrane thickness:Thickening of the alveolar-capillary membrane, as occurs in conditions like pulmonary fibrosis or edema, increases the diffusion distance. This slows down the movement of oxygen and carbon dioxide between alveoli and blood, reducing gas exchange efficiency.

D. A decrease in nitrogen solubility:Nitrogen is largely inert and not actively exchanged in the lungs. Changes in its solubility have minimal effect on oxygen or carbon dioxide diffusion, so it does not significantly slow gas exchange.

E. An increase in respiratory rate:A higher respiratory rate enhances the ventilation of alveoli and maintains the concentration gradient for diffusion. This actually promotes faster gas exchange rather than slowing it.