Blood Vessels Of The Heart
The human heart pumps blood through the arteries, which connect to smaller arterioles and then even smaller capillaries. It is here that nutrients, electrolytes, dissolved gases, and waste products are exchanged between the blood and surrounding tissues.
The capillaries are thin-walled vessels interconnected with the smallest arteries and smallest vein about 7000 liters of blood is pumped by the heart every day.
In an average person’s life, the heart will contract about 2.5 billion times. Blood flow throughout the body begins it’s return to the heart when the capillaries return blood to the venules and then to the larger veins.
The cardiovascular system consists of a closed circuit including:
The venules and veins are part of the pulmonary circuit because they send deoxygenated blood to the lungs to receive oxygen and unload carbon dioxide.
The arteries and arterioles are part of the systemic circuit because they send oxygenated blood and nutrients to the body cells while removing wastes. All body tissues require circulation to survive.
The blood vessels of the human body carry blood to every type of tissue and organ. Vessels decrease in size as they move away from the heart, ending in the capillaries, and then increase in size as they move toward the heart. The largest artery in the body is the aorta, with the largest veins being the venae cavae, each being about 1 in wide.
The Blood Vessels
There are five general classes of blood vessels in the cardiovascular system: arteries, arterioles, capillaries, venules, and veins.
Arteries are elastic vessels that are very strong, able to carry blood away from the heart under high pressure. They subdivide into thinner tubes that give rise to branched, finer arterioles. An artery’s wall consists of three distinct layers, the inner-most tunica interna is made up of a layer of simple squamous epithelium known as endothelium.
It rests on a connective tissue membrane with many elastic collagen fibers. The endothelium helps prevent blood clotting and may also help in regulating blood flow. It releases nitric oxide to relax the smooth muscle of the vessel. Vein walls are similar but not identical to artery walls.
The middle tunica media makes up most of an arterial wall, including smooth muscle fibers and a thick elastic connective tissue layer. The outer tunica externa is thinner, mostly made up of connective tissue with irregular fibers, it is attached to the surrounding tissues. Smooth artery and arteriole muscles are innervated by the sympathetic nervous system.
Vasomotor fibers receive impulses to contract and reduce blood vessel diameter. When inhibited, the muscle fibers relax and the vessel’s diameter increases. Changes in artery and arteriole diameters greatly affect blood flow and pressure.
Larger arterioles also have three layers in their walls, which gets thinner as arterioles leads to capillaries. Very small arteriole walls only have an endothelial lining and some smooth muscle fibers, with a small amount of surrounding connective tissue.
The smallest diameter blood vessels are capillaries, which connect the smallest arterioles to the smallest venules. The walls of capillaries are also composed of endothelium and form the semipermeable layer through which substances in blood are exchanged with substances in tissue fluids surrounding cells of the body. Capillary walls have thin slits where endothelial cells overlap.
These slits have various sizes, affecting permeability. Capillaries of muscles have smaller openings than those of the glands, kidneys, and small intestine. Tissues with higher metabolic rates have many more capillaries than those with slower metabolic rates. Some capillaries pass directly from arterioles to venules while others have highly branched networks.
Precapillary sphincters control blood distribution through capillaries. Based on the demands of cells, these sphincters constrict or relax so that blood can follow specific pathways to meet tissue cellular requirements. Gases, metabolic products, and nutrients are exchanged between capillaries and the tissue fluid surrounding body cells.
Capillary walls allow the diffusion of blood with high levels of oxygen and nutrients. They also allow high levels of carbon dioxide and other wastes to move from the tissues into the capillaries. Plasma proteins usually cannot move through the capillary walls due to their large size, so they remain in the blood. Blood pressure generated when capillary walls contract provides force for filtration through hydrostatic pressure.
Blood pressure is strongest when blood leaves the heart and weaker as the distance from the heart increases because of friction between the blood and the vessel walls.
Therefore, blood pressure is highest in the arteries, less in the arterioles, and lowest in the capillaries. Filtration occurs mostly at the arteriolar ends of capillaries because the pressure is higher than at the venular ends. Plasma proteins trapped in capillaries create an osmotic pressure that pulls water into the capillaries. Capillary blood pressure favors filtration while plasma colloid osmotic pressure favors reabsorption.
At the venular ends of capillaries, blood pressure has decreased due to resistance so that reabsorption can occur. More fluid usually leaves capillaries than returns to them. Lymphatic capillaries have closed ends and collect excess fluid to return it through lymphatic vessels to the venous circulation. Unusual events may cause excess fluid to enter spaces between tissue cells, often in response to chemicals such as histamine. If enough fluid leaks out, lymphatic vessels can be overwhelmed, and affected tissues swell and become painful.
Venules are microscopic vessels that link capillaries to veins, which carry blood back to the atria. Vein walls are similar to arteries but have poorly developed middle layers. Because they have thinner walls that are less elastic than arteries, their lumens have a greater diameter. Many veins have flaplike valves projecting inward from their linings. These valves often have two structures that close if blood begins to back up in the vein.
They aid in returning blood to the heart, opening if blood flow is toward the heart, but closing if it reverses. Unlike the arteries, veins do not have sufficient pressure from the contractions of the heart to keep blood moving through them. To keep blood flowing, the veins rely on the movement of nearby skeletal muscles, as well as the opening and closing of the valves within them. Therefore, a major structural difference between veins and arteries is that arteries do not have valves.
Veins also act as reservoirs for blood in certain conditions, such as during arterial hemorrhage. Resulting venous constrictions help to maintain blood pressure by returning more blood to the heart, ensuring an almost normal blood flow even when up to one-quarter of the blood volume is lost.
Functions of the blood vessels
Blood vessels of the heart and their function:
- Arteries – Carries relatively high
pressure blood from the heart to the arterioles.
- Aorta – Carries blood from left ventricle.
- Arterioles – Helps control blood flow from arteries to capillaries by
- Capillaries – Has a membrane allowing nutrients, gases, and wastes to be exchanged between blood and tissue fluid.
- Venules – Collects blood from capillaries.
- Veins – Carries relatively low-pressure blood from venules to the heart and it also serve as blood reservoirs.
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