Which of the following occurs during the expiratory phase of respiration?

A. The stomach plays a key role in digestion by breaking down food mechanically and chemically but does not play a significant role in nutrient absorption.

B. The large intestine is primarily involved in water and electrolyte absorption, as well as the formation of feces. It does not significantly contribute to nutrient absorption.

C. While some digestion begins in the mouth, particularly with enzymes in saliva, nutrient absorption does not occur there.

D. The small intestine is the primary site of nutrient absorption in the digestive system. Its walls are lined with villi and microvilli that increase surface area for the absorption of nutrients such as carbohydrates, proteins, fats, vitamins, and minerals.

A. Respiratory bronchioles are part of the lower respiratory tract and lead to alveolar sacs, but they themselves are not primarily responsible for gas exchange. Their role is in the conduction of air.

B. Alveolar sacs are the primary site of gas exchange in the lungs. These sacs contain alveoli, where oxygen and carbon dioxide are exchanged between the air and the blood. This is the correct answer.

C. The pleura are double-layered membranes surrounding the lungs and do not play a direct role in gas exchange. They reduce friction and allow for smooth lung expansion.

D. Pulmonary veins carry oxygenated blood from the lungs back to the heart, but gas exchange occurs in the alveolar sacs, not the veins.