In a capillary, what substances can pass through the walls into and out of the interstitial fluid?

In a capillary, the walls are structured to allow for the selective movement of various substances between the blood and the interstitial fluid. This movement is facilitated by the thin endothelial layer that forms the capillary walls, which is permeable to numerous small molecules and gases.

Oxygen and carbon dioxide, as well as nutrients like glucose and amino acids, can freely diffuse through the capillary walls due to their small size and solubility. Additionally, waste products such as urea and creatinine also move out of the capillary into the interstitial fluid for elimination.

Consequently, the correct answer indicates that a variety of substances, including nutrients, gases, and waste products, can pass through the capillary walls, reflecting the dynamic and multifaceted exchange process that occurs at the capillary level. This exchange is crucial for maintaining homeostasis in the body, as it ensures that tissues receive essential substances and can eliminate waste effectively.