The Circulatory System
The circulatory system is extremely important in sustaining life. It’s proper functioning is responsible for the delivery of oxygen and nutrients to all cells, as well as the removal of carbon dioxide, waste products, maintenance of optimum pH, and the mobility of the elements, proteins and cells, of the immune system. In developed countries, the two leading causes of death, myocardial infarction and stroke are each direct results of an arterial system that has been slowly and progressively compromised by years of deterioration.
Arteries are muscular blood vessels that carry oxygenated blood away from the heart to the body. The only exception being the pulmonary artery that carries deoxygenated blood to the lungs. Arteries have a thick wall that consists of three layers. The inside layer is called the endothelium, the middle layer is mostly smooth muscle and the outside layer is connective tissue. The artery walls are thick so that when blood enters under pressure the walls can expand.
Capillaries are the smallest of a body’s vessels; they connect arteries and veins, and most closely interact with tissues. They are very prevalent in the body; total surface area is about 6,300 square meters. Because of this, no cell is ever very far from a capillary. The walls of capillaries are composed of a single layer of cells, the endothelium. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the tissues. Waste products such as carbon dioxide and urea can diffuse back into the blood to be carried away for removal from the body. The "capillary bed" is the network of capillaries present throughout the body. These beds are able to be “opened” and “closed” at any given time, according to need. This process is called autoregulation and capillary beds usually carry no more than 25% of the amount of blood it could hold at any time. The more metabolically active the cells, the more capillaries it will require to supply nutrients.
Veins carry deoxygenated blood to the heart. The only exception to this is in the pulmonary vein that carries oxygenated blood to the heart. Most of the blood volume is found in the venous system; about 70% at any given time.The veins outer walls have the same three layers as the artery, differing only because there is a lack of smooth muscle in the inner layer and less connective tissue on the outer layer. Veins have low blood pressure compared to arteries and need the help of skeletal muscles to bring blood back to the heart. Most veins have one- way valves called venous valves to prevent backflow caused by gravity. They also have a thick collagen outer layer, which helps maintain blood pressure and stop blood pooling. If a person is standing still for long periods or is bedridden, blood can accumulates in veins and can cause varicose veins. The hollow internal cavity in which the blood flows is called the lumen. A muscular layer allows veins to contract, which puts more blood into circulation
A venule is a small vein that allows deoxygenated blood to return from the capillary beds to the larger blood veins. Venules have three layers; they have the same makeup as arteries with less smooth muscle, making them thinner.