The capillaries are the smallest of the blood vessels within the human body. They measure approximately 7 microns in diameter and form a capillary network that links arteries and veins. Arteries transport blood away from the heart and veins carry blood towards the heart.
Substances such as nutrients, hormones and oxygen are transported within the blood vessels and it is the capillaries that are responsible for the transference of the necessary materials into the various cells within the body. Substances are transported through the blood vessels in the blood plasma which is the extracellular fluid that surrounds blood cells. When substances reach the capillaries, they are transferred from the plasma through the capillary membrane into the extracellular fluid, and ultimately to the cell interior. The procedures involved in this depend upon the properties that are being transported, and it occurs by way of diffusion, osmosis or active transport.
Diffusion is a process that permits substances with small molecules such as the amino and fatty acids, vitamins, some of the mineral salts and the gases oxygen and carbon dioxide to travel through the capillary membrane into the extracellular fluid and ultimately to the cell interior. These substances diffuse along a concentration gradient, meaning that they always travel from an area where there is a higher concentration of the same substance to an area of lower concentration.
An example of diffusion is the exchange between carbon dioxide and oxygen across the capillary membrane within the lungs. The capillary network surrounds the walls of the alveoli which are tiny air sacs within the lungs. Both the alveoli and capillary walls consist of a very fine membrane of flattened epithelial cells, allowing for diffusion to take place. The level of carbon dioxide is greater within capillaries than in alveoli; therefore the carbon dioxide crosses both membranes and enters into the alveolus. Oxygen enters the capillary blood from the alveolus as the oxygen concentration is greater within alveoli.
Osmosis works in the same way as diffusion, however the term is only used to refer to water and its movement through the capillary and cell membranes along a water potential gradient. This means that water molecules will pass through the membrane from an area of higher water concentration to a place of lower concentration.
Osmosis and diffusion are passive forms of transport, meaning that no other energy is needed for the action to take place.
The term active transport implies that further energy is needed. This is necessary when materials must move against a concentration gradient. This applies to the movement of substances such as the mineral salts sodium and potassium. An example being that intracellular fluid has a higher concentration of potassium than extracellular fluid, therefore in order for potassium to cross the capillary membrane and ultimately to the cell interior it must move against a concentration gradient. The extra energy needed for the transfer involved in active transport is provided by adenosine triphosphate (ATP) which is a high energy molecule within cells and it enables substances to be pumped across the cell and capillary membranes in both directions. ATP is the energy behind a carrier molecule that takes a substance across the membrane.
Cell waste products do not cross into to the capillary blood; they are transported within the lymph capillaries, although some waste materials are eventually returned to the blood stream.
Capillaries situated in the skin's dermis help to maintain normal body temperature. Whenever there is a rise in body temperature the capillaries dilate, and this allows for the blood to be cooled as it is circulated.
In conclusion capillaries play an important role in the circulation as they link arteries and veins, they help to maintain body temperature, and they transfer the required substances into the appropriate cells.