Arteries, veins, and capillaries are all vessels that form part of the body's circulatory system. Each has its particular function to keep blood moving through the system, bringing oxygen and nutrients and carrying away waste products. While arteries and veins are typically visible to the eye, capillaries are microscopic in size and there are more than a billion of them in a single human being.
Arteries are the vessels that carry blood rich in oxygen to all the parts of the body to supply the processes in the body that require oxygen to function. The largest blood vessel in the body is the aorta, which carries oxygenated blood away from the heart. The arteries divide many times and get smaller and smaller as they spread out to cover every section of the body. There is one artery that carries blood without oxygen, the pulmonary artery, which takes blood from the heart to the lungs to be recharged with oxygen.
Arteries are the strongest blood vessels in the body because they carry blood that is being pumped away from the heart with the highest amount of force and pressure. They have three layers, an outer layer of elastic connective tissue, a middle layer of muscle, and an inner layer covered with endothelial cells, which are very smooth to allow the free flow of blood through the vessel.
Veins are the vessels that carry the blood, full of carbon dioxide, back to the heart, where it is pumped through the pulmonary artery to the lungs to receive fresh oxygen. There is one vein carrying oxygenated blood, the pulmonary vein, which takes blood from the lungs to the heart for circulation to the rest of the body. Veins also have one-way valves to prevent the blood from flowing in the wrong direction. By the time the blood reaches the veins, it has less pressure. In addition, blood flowing in the veins of the arms and legs is flowing upward against gravity, and the one-way valves help to overcome the force of gravity.
Veins have a similar physical structure to arteries, but the middle muscular layer is not nearly as strong, because the veins are only subject to about one fiftieth of the pressure found in the arteries. In the reverse of the arterial structure, blood leaving the capillaries flows into veins that combine into larger and larger vessels until it reaches either the superior vena cava, the vessel collecting blood from tissues above the heart, or the inferior vena cava, the vessel collecting blood from tissues below the heart.
When the diameter of the arteries goes below 10 micrometers, the vessels are known as capillaries. These capillaries are only as thick as one cell and their primary function is to allow the exchange of many materials between them and the surrounding tissues. Substances such as water, carbon dioxide, oxygen, nutrients, and waste elements pass back and forth between the capillaries and the tissues they pass through. The capillaries have no connective or muscle tissue to contain the blood. The huge area provided by the numerous capillaries and the reduced internal pressure aid the transfer of materials to and from the tissues.
All these vessels work in harmony with the heart and lungs to circulate about five liters of life-giving blood throughout the human body. The circulatory system is just one of the major systems that support human life.