Question

What is the importance of capillaries in the circulatory system?

Answer 1

The fine, single-cell layered tubular structure forming a network around the tissues is called capillaries. Each capillary is lined by a single layer of endothelium cells placed on the basement membrane. The positioning of the capillaries is in between the arterioles and venules. From the arteriole, the capillaries are formed known as arteriolar capillaries which converge to venular capillaries and finally to venules.

Complete answer:
The walls of capillaries are very thin layered and are a single-layered structure hence the exchange of material between the blood and the tissues occurs in this area. This includes the transport of oxygen from the blood to the tissues and the transport of carbon dioxide from the tissues to the blood.
Capillaries are one of the sources of formation of the tissue fluids. The factors that influence the formation of tissue fluid from the blood plasma include – capillary permeability, the difference of pressure between the capillaries and the tissue fluids( at the arterial end Blood Pressure is $32$mm of Hg and the plasma protein osmotic pressure is $25$mm of Hg, thus the filtration pressure is found to be $7$ mm of Hg. Due to this pressure tissue fluid is formed at the arteriolar end. At the venous end, the osmotic pressure is $25$ mm of Hg, and Blood Pressure is $16$mm of Hg which helps in the returning of the tissue fluid in the venous end.), the difference in the colloidal osmotic pressure and the tissue fluid.

Note :
In the case of capillaries the presence of tunica interna is observed. At the intervals along the capillary wall, some large cells of unknown function called pericytes or Rouget cells are present. In the case of arteries and veins along with the tunica intima or interna $2$ more layers are observed, they are tunica adventitia or tunica externa and tunica media. The coats of the arteries are thicker than the vein so the lumen is smaller than that of the veins.