Which of the following vitamins is synthesized in the skin?

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.

A. While phagocytes can move to areas of damage, their primary function is not to clump together and adhere to tissues, but rather to engulf and digest foreign materials.

B. Phagocytes, such as macrophages and neutrophils, are specialized white blood cells that ingest and digest foreign pathogens, dead cells, and debris within damaged tissues. This is their main characteristic and function.

C. The release of histamine is typically associated with mast cells, not phagocytes. Histamine helps to initiate inflammatory responses, but phagocytes are involved in engulfing pathogens rather than releasing histamine.

D. Transporting oxygen from the lungs to tissues is the role of red blood cells, not phagocytes. Phagocytes are involved in immune defense, not oxygen transport.