What is the primary function of capillaries in the human body?

The primary function of capillaries is to connect arteries to veins and facilitate the exchange of gases, nutrients, and waste products between blood and tissues. Capillaries are the smallest and most numerous blood vessels in the body, featuring thin walls that allow substances to pass easily in and out. This is essential for maintaining cellular functions, as cells require a continuous supply of oxygen and nutrients while expelling carbon dioxide and other metabolic wastes.

Transportation of blood away from the heart is a role performed by arteries, while pumping blood throughout the body is done by the heart itself. Storing excess oxygen is not a function of capillaries; instead, oxygen is transported in the blood bound to hemoglobin in red blood cells. Thus, the primary role of capillaries as connectors and exchange sites is what distinguishes them from other types of blood vessels.