Which of the following physiological responses occurs in response to a decrease in blood pressure?

a. Exchanging of gas: Alveoli are the primary sites for gas exchange in the lungs, where oxygen is absorbed into the blood and carbon dioxide is expelled.

b. Removing pathogens that are breathed in: This is a function of the respiratory tract's immune components, like macrophages in the alveoli, but not the primary function.

c. Containing mucus-producing goblet cells: Goblet cells are found in the respiratory tract but not in the alveoli.

d. Secreting surfactant to reduce surface tension: Alveolar cells do secrete surfactant, which is crucial for reducing surface tension and preventing alveolar collapse, but the primary function is gas exchange.

a. Decreased blood pressure due to increased blood viscosity: Increased viscosity makes blood flow more difficult, raising blood pressure, not lowering it.

b. Increased blood pressure due to decreased blood viscosity: Decreased viscosity would make blood flow easier, potentially lowering blood pressure, not raising it.

c. Decreased blood pressure due to decreased blood viscosity: Same error in reasoning as b.

d. Increased blood pressure due to increased blood viscosity: Red blood cells are the main components of blood that affect its viscosity (thickness). An increased concentration of red blood cells (erythrocytosis) makes the blood more viscous. Thicker blood increases resistance to flow within blood vessels, making the heart work harder to pump blood, which can lead to increased blood pressure.