What are capillaries?
Capillaries are the smallest of blood vessels in the circulatory system and are responsible for distributing oxygenated blood from arteries to the tissues of the body and feeding deoxygenated blood from the tissues back into the veins. Depending on the organ where they are located, capillaries vary in diameter. Smaller capillaries, being 5 do 10 micrometers (symbol "uM", equal to one thousandth of a millimeter) are typically found in the lungs, retina and striated muscle; while larger capillaries (rising up to about 25-30mM) are mainly found in glands and bone marrow.
Being cylindrical and microscopic in dimension, capillaries were first discovered by Marcello Malpighi, who was an Italian physician and biologist. He first spotted them in the seventeenth century and gave them their name in reference to their subtlety. He was also the first person to discover the link between arteries and veins, as well as the earliest person to observe red blood cells under a microscope.
Both the venous system (large veins, medium veins and small venules) and the arterial system (large arteries, medium arteries, and small arterioles) branch into smaller and smaller blood vessels until they terminate into capillaries, where nutrients and waste products are exchanged. The capillaries then join and widen to become venules and further develop into veins, which then return blood back to the heart through the large veins.
From the perspective of functional capillaries, they represent the most fundamental part of the vascular system. While all other blood vessels (arteries and arterioles, venules and veins) simply pass through blood, the capillaries instead, go through metabolic and respiratory exchanges between blood and intercellular fluid which ensures the supply of oxygen and nutrients to the tissues and organs (by means of the capillaries resulting from the arteries) as well as the removal of waste substances (by means of the capillaries resulting from the veins). The function of the capillaries is made possible by their special structural features which differ from arteries and veins in a sense that their walls are constituted by a single layer of flattened endothelial cells, making them permeable. In addition to the networks of capillaries arranged between arteries and veins, there are also networks of capillaries between two arteries or two veins. In the case of arteries, this is referred to as “admirable arterial networks”, and in the case of veins, it is referred to as "marvelous venous networks”.
Capillary formations can also result in certain disorders. The formation of additional capillaries and larger blood vessels (angiogenesis) is a major mechanism by which a cancer may increase in growth. Disorders of retinal capillaries contribute to the pathogenesis of age related macular degeneration. On the other hand, reduced capillary density (also called capillary rarefaction) is associated with cardiovascular risk factors such as high blood pressure or cholesterol as well as coronary heart disease, which can lead to complications such as heart failure or an irregular heartbeat.
What function do capillaries serve?
Capillaries are responsible for ensuring metabolic and respiratory changes between blood and intercellular fluid in order to guarantee that all tissues will be supplied with oxygen and nutrients, as well ensure the removal of carbon dioxide waste substances and fluids.