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luman Body
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.lary is composed of a single thin layer of endothelial cells which permits the passage of water and other small molecular substances. Their diameter is approximately that of a red blood cell.
Capillaries are so small (.006 mm. dia.) that the red blood cells have to pass through them in single file. They permeate the tissues and service the body-cells directly. Oxygen, food materials and hormones transported in the blood are transferred to the cells, while cell-products, produced for export, diffuse into the blood. This exchange is so rapid that each given unit of blood spends only one to three seconds in a particular capillary. Total length of capillaries is more than 99% of the entire length of the circulatory system.
The capillaries coalesce into venules, which are the branches of the larger veins, which eventually drain into the right auricle through two venae cavae. Contraction of skeletal muscles puts pressure on the veins pusbing the blood towards the heart. Many veins of the limbs and abdomen have one-way valves in their interior, so arranged that they allow the blood to flow only towards the heart and prevent it to flow in the opposite direction, Usually the total diameter of veins returning blood from an organ is at least twice the total diameter of the arteries. This is because of the sluggish flow of blood in the veins. which requires the greater capacity to maintain the balance between the incoming and the outgoing blood volume. All the veins of the systemic circulatory system (except for the cardiac veins) empty into either the superior or inferior vena cava which drains into the right auricle. Cardiac Cycle
A full cardiac cycle takes less than a second. Yet during that brief time a complicated sequence of events takes place. The working part of the heart is its muscular walls. They contract and relax rhythmically. This rhythm is self-initiated requiring no nervous or chemical stimulation. During relaxation, blood from the tissues with a high carbon dioxide content carried by veins, enters the upper right chamber (right auricie). At the same time, oxygenated blood from the lungs carried by the pulmonary veir
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