Where does the circulating lymph return to cardiovascular circulation?

a. Forming the contractile component of the digestive, urinary, and reproductive systems: This describes smooth muscle, not skeletal muscle.

b. Contracting to cause involuntary muscle movements: This is a characteristic of smooth and cardiac muscles. Skeletal muscle is primarily involved in voluntary movements.

c. Pumping blood throughout the body: This is the function of cardiac muscle, not skeletal muscle.

d. Shivering for heat production: Correct. Skeletal muscles generate heat through shivering, which is an involuntary contraction response to cold.

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.