Smallest pathway to and from the heart

The human body depends on capillaries as a route for blood to transport oxygen and carbon dioxide along with other nutrients and wastes to and from the lungs and heart. The capillaries are the smallest blood vessel in the body at only 5-10 microns in diameter. The lumen, or the center opening which allows the flow of blood, is only the width of a single red blood cell. The human body contains billions of capillaries in three different forms.

The most common capillary is the continuous capillary which supplies the skin and muscles. Its endothelium cells are in a continuous wall around the lumen of the capillary and are met by tight junctions between cells. Where a tight junction is not present between endothelium cells there is an intercellular cleft. Water and small ions are able to pass through the intercellular clefts into the skin and muscles. Pinocytotic vesicles run the length of the endothelium cells and supply the capillary with nutrients. Pericytes on the exterior of the basement membrane are large and cover an outstanding amount of the capillary.

The endocrine system, kidneys and pancreas contain fenestrated capillaries. These capillaries contain the same basement membrane and endothelium cells as the continuous capillary, however they contain small pores, fenestrations, within the endothelium which allow passage of small molecules and a trace amount of protein in and out of the capillary.

The last of the capillary types is called a sinusoid or discontinuous capillary. Although there are similarities between a sinusoid and the other types of capillaries, it is very distinctive. There are very large intercellular clefts within the capillary walls that split the basement membrane and endothelium cells far apart. The liver, spleen and bone marrow contain discontinuous capillaries. These large openings allow larger molecules and even blood cells to pass through the capillary walls. In the liver, macrophages are present on the exterior of the capillary which allow the destruction of foreign materials. The spleen has phagocytes present outside the capillary walls with extensions that reach through the intercellular clefts into the capillary to destroy foreign materials and prevent them from flowing through the blood stream and throughout the entire body. Each type of capillary is adapted for the type of tissue in which is accommodating.