In the case of the left ventricle, the afterload is a consequence of the blood pressure, since the pressure in the ventricle must be greater than the blood pressure in order to open the aortic valve. The fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries. An adult male of average size normally has about 6 quarts (5.6 liters) of blood. The blood carries oxygen and nutrients to the body tissues and removes carbon dioxide and other wastes. The colorless fluid of the blood, or plasma, carries the red and white blood cells, platelets, waste products, and various other cells and substances.
The blood vessels are elastic tubular canals through which blood circulates in the body. Different types of blood vessels include Arteries, veins and Capillaries.
The blood vessels are part of the circulatory system and function to transport blood throughout the body. The most important types, arteries and veins, are so termed because they carry blood away from or towards the heart, respectively. The arteries carry blood away from the heart; the main arterial vessel, the aorta, branches into smaller arteries, which in turn branch repeatedly into still smaller vessels and reach all parts of the body. Within the body tissues, the vessels are microscopic capillaries through which gas and nutrient exchange occurs. Blood leaving the tissue capillaries enters converging vessels, the veins, to return to the heart and lungs.
All blood vessels follow the same histological makeup. The arterial layer that is in direct contact with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells. Just deep to this layer is the tunica media, known as the media. This "middle layer" is made up of smooth muscle cells and elastic tissue. The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.
The tunica intima is the inner layer of arteries and veins. In arteries this layer is composed of an elastic membrane lining and smooth endothelium that is covered by elastic tissues. A layer of endothelial cells covers the lumen of the vessel, as well as a sub-endothelial layer made up of mostly loose connective tissue. Often, the internal elastic lamina separates the tunica intima from the tunica media.
This layer is comprised of a smooth lining of endothelial cells, which also forms the valves, and a basement membrane. Due to this layer the arteries are able to cope with the high-pressure surges of blood from the heart because they have very thick muscular walls. The single layer of cells that make up a capillary enables the passage of oxygen and nutrients from capillaries into the surrounding tissue. Veins have loose, slack walls as the blood inside them is under very little pressure.
The blood vessels are capable of either constricting (Vasoconstriction) and decreasing the lumen size or dilating (vasodilatation) and increasing the lumen size. Vasoconstriction is the major contributing factor leading to high blood pressure.
Capillaries consist of little more than a layer of endothelium and occasional connective tissue.
Blood vessels exist in varying calibers: