Final answer:
Capillaries are the smallest blood vessels that connect arterioles and venules, facilitating the exchange of oxygen, nutrients, and waste in the capillary beds. The capillaries lead into venules, which progress into veins, returning blood back to the heart.
Step-by-step explanation:
Capillaries are the smallest blood vessels in the body, and they serve the crucial function of connecting arterioles and venules. Arterioles branch out into capillary beds, which are extensive networks consisting of tens to hundreds of capillaries interwoven among the body's cells.
These narrow-diameter tubes allow red blood cells to pass through in a single file and are the primary sites for vital exchanges, including oxygen, nutrients, and waste, with the body's tissues at the cellular level. Meanwhile, fluid can also pass from the blood into the interstitial space through the capillaries.
The capillary network converges to form venules, which are extremely small veins that lead to larger veins. The walls of these venules consist of endothelium, a likely thin middle layer with some muscle cells and elastic fibers, and a thin outer layer of connective tissue.
They, along with capillaries, are key locations for the process of diapedesis, where white blood cells migrate into surrounding tissue fluid. Veins, larger than venules and with valves to prevent backflow, ultimately return deoxygenated blood to the heart.