Which tissue layer is absent in capillaries?

The correct answer states that the adventitia is the tissue layer absent in capillaries. Capillaries are the smallest blood vessels in the body, primarily responsible for the exchange of oxygen, carbon dioxide, nutrients, and waste products between blood and surrounding tissues.

Capillaries consist of only an endothelial layer, also known as the intima, which allows for efficient diffusion of substances. This single layer of endothelial cells provides a minimal barrier for exchange. The media, which is the middle layer typically found in larger vessels comprised of smooth muscle and elastic fibers, is also absent in capillaries, as capillaries do not require these components for their function.

In contrast, the adventitia, which is the outer layer of larger blood vessels that provide structural support and protection, is not present in capillaries. The absence of both the media and adventitia in capillaries distinguishes them from larger vessels while still allowing them to effectively perform their primary role in the circulatory system.