Capillary

From ArticleWorld

A capillary is a type of blood vessel and represents the smallest member of its class. Capillaries are microscopic, are approximately 5-10 micrometers in diameter and, in some cases, are often so tiny that the red blood cells passing within them must do so single-file. As small as capillaries are, the total length of all capillaries in the human body is 25,000 miles or 40,000 kilometers.

Anatomy

The capillaries are what separate the blood in the arteries from the blood in the veins. Ideally, all blood must pass from the arterial system and into the venous system by means of passing through the capillary system. Capillaries have extremely thin walls of only one cell in thickness. This is necessary for the effective exchange of molecules between the blood and the neighboring cells.

There are three anatomic types of capillaries. Continuous capillaries are completely sealed tubes that allow small molecules, minerals and water to pass through its walls. Fenestrated capillaries have small openings in the walls that allow for the passage of larger molecules to and from a neighboring cell. Sinusoidal capillaries have openings in the wall as well, but the openings are much bigger so that very large molecules and red blood cells can pass into the surrounding tissue.

Physiology

Capillaries provide a vital function to every body tissue and organ. Their small size and thin walls put a capillary within easy range to diffuse oxygen into nearby cells and to pick up, also by passive diffusion, waste products like urea and carbon dioxide for later disposal by other body organs.

Because some molecules, nutrients or enzymes cannot diffuse across the cell wall of capillaries, an active transport system is in place. One type of active transport system involves two interesting processes. In the process of “endocytosis”, a piece of cell membrane “grabs” a molecule, pinches itself off from the rest of the membrane and then, as a bubble-like vessel, it transports the molecule(s) to the opposite wall of the cell. Finally, in a process known as “exocytosis”, the contents of the membranous vessel or “vesicle” are spilled out into the external tissues.

Depending on need, the network of capillaries can increase their permeability or “leakiness” so that there is a greater degree of exchange occurring. The capillaries can expand and open to a greater degree if the tissue it is supplying sends signals indicating a greater need for oxygen, nutrients or for enhanced waste disposal.