Final answer:
Capillaries are the smallest blood vessels with thin walls that facilitate A. oxygen exchange, holding about A. 5% of the body's total blood volume.
Step-by-step explanation:
The capillary structure consists of the smallest blood vessels in the body with extremely thin walls, composed of endothelial cells surrounded by a basement membrane.
The diameter of a capillary lumen ranges from 5-10 micrometers, allowing just enough space for red blood cells to pass through in single file.
This design is perfect for their primary function, which is to facilitate the exchange of substances, such as oxygen, nutrients, and wastes, between the blood and tissue cells.
The capillaries are the major site of oxygen exchange (Answer A), where oxygen is delivered to the tissues from the blood, and carbon dioxide is taken up by the blood from the tissues.
This process is essential for cellular respiration, which generates the energy required for cell function. Concerning the percentage of the total blood volume they contain, capillaries hold approximately 5% (Answer A) of the blood in the body at any given time.
Despite their small individual size, the vast network of capillaries across the body makes their collective volume quite significant.