The walls of capillaries are lined with tiny pores, which allow for the passage of water, electrolytes, and small molecules such as oxygen and carbon dioxide. Larger molecules, such as proteins, are unable to pass through the pores and must be transported through the endothelial cells by a process called transcytosis.
Capillaries are also involved in the regulation of blood pressure and fluid balance. When the blood pressure is too high, the capillaries can dilate to allow for more blood flow. When the blood pressure is too low, the capillaries can constrict to reduce blood flow.
The exchange of substances between the blood and the surrounding tissues occurs in the interstitial spaces, which are the spaces between the cells. The interstitial spaces are filled with a fluid called interstitial fluid, which contains nutrients, oxygen, and waste products.
The exchange of substances between the blood and the interstitial fluid occurs by diffusion, which is the movement of molecules from an area of high concentration to an area of low concentration. Oxygen diffuses from the blood into the interstitial fluid, and carbon dioxide diffuses from the interstitial fluid into the blood.
Capillaries are essential for the proper functioning of the body. They allow for the exchange of oxygen and carbon dioxide, the regulation of blood pressure and fluid balance, and the transport of nutrients and waste products.