The Circulatory System

The circulatory system is also known as the cardiovascular system. It transports important substances such as oxygen and nutrients to the organs and tissues around the body and removes waste products from the body at the same time, making use of the Pulmonary circulation and Systemic circulation. It is made up of : the heart and blood vessels, which include arteries and arterioles, veins, capillaries and sinusoids.

Heart

About the size of a clenched fist, it is the engine of the circulatory system and is divided into 4 chambers, the right atrium, right ventricle, left atrium, and left ventricle. The upper chambers are the atria while the lower ones are the ventricles. The walls of these chambers are made up of a special muscle called myocardium. They undergo alternate periods of relaxation (diastole) and contraction (systole) to pump blood and receive blood from the veins. Blood is prevented from flowing back into the atria by valves. Heart Valves are one-way sentry points that allow blood to pass through between chambers and into the various blood vessels. There are 4 main heart valves, namely, the Tricuspid valve, Pulmonary valve, Bicuspid valve (Mitral valve) and Aortic valve.



Blood vessels

Three types of blood vessels form a complex network of tubes around the body to transport the blood pumped by the heart to the various organs and tissues.

Arteries and arterioles- carries blood away from the heart. They have thicker walls than veins to withstand the pressure of blood being pumped from the heart. Arterioles are branches of arteries that are smaller.




Veins— carries blood towards the heart . Blood in the veins is at a lower pressure, so the veins make use of the one-way valves to prevent blood from flowing backwards, away from the heart.


Capillaries— the tiny links between arteries and veins where oxygen and nutrients diffuse into body tissues. They have extremely thin walls that permit these dissolved oxygen and nutrients from the blood to diffuse across the body fluid, known as interstitial fluid, that fills the gaps between the cells of tissues or organs.


Sinusoids- have functions that are greatly similar to the capillaries in which they are directly involved in the transfer of substances between the blood and the body cells. Sinunoids are usually found in the liver, spleen and the bone marrow.

The inner layer of blood vessels is lined with endothemic cells that create a smooth passage for the transit of blood. It is surrounded by connective tissue and smooth muscle that allows the blood vessels so expand or contract as the heart pumps the blood. The blood vessels expand during exercise to let more blood flow through when the body requires more food and nutrients and to cool the body. They contract after an injury to reduce bleeding and to conserve body heat.

These blood vessels are divided into two systems of circulation—the pulmonary and systemic.


Pulmonary circulation

The pulmonary system carries deoxygenated blood carrying waste products, such as carbon dioxide, from the heart to the lungs. It travels from the right atrium to the right ventricle, then is pushed into the lungs through the pulmonary artery. In the lungs, blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart before transferring to the systemic circulation.


Systemic circulation

The systemic circulation carries oxygenated blood from the heart to all the tissues in the body except for the lungs and returns deoxygenated blood carrying waste to the heart. It also transports nutrients derived from digested food to the body. These nutrients enter the bloodstream by passing through the walls of the intestine. They are absorbed through a network of capillaries and veins that drain the intestines.

The heart ejects oxygen-rich blood under high pressure out of the heart's main pumping chamber, the left ventricle, through the largest artery, the aorta. Smaller arteries branch off from the aorta, leading to various parts of the body. These smaller arteries in turn branch out into even smaller arteries, called arterioles. Branches of arterioles become progressively smaller in diameter, eventually forming the capillaries. Once blood reaches the capillary level, blood pressure is greatly reduced.

Oxygen and nutrients diffuse through the capillaries into the various body tissues and cells. Meanwhile, carbon dioxide and other wastes leave the cell, and enter the blood.

After delivering oxygen to tissues and absorbing wastes, the deoxygenated blood in the capillaries then starts the return trip to the heart. The capillaries merge to form tiny veins, called venules. These veins in turn join together to form progressively larger veins. Ultimately, the veins converge into two large veins: the inferior vena cava, bringing blood from the lower half of the body; and the superior vena cava, bringing blood from the upper half. Both of these two large veins join at the right atrium of the heart.

Because the pressure is dissipated in the arterioles and capillaries, blood in veins flows back to the heart at very low pressure, often running uphill when a person is standing. Flow against gravity is made possible by the one-way valves, located several centimeters apart, in the veins.


Circulatory system disorders

Disorders of the circulatory system include any injury or disease that damages the heart, the blood, or the blood vessels. The three most common circulatory diseases are hypertension, arteriosclerosis, and atherosclerosis.

Hypertension, or elevated blood pressure, develops when the body's blood vessels narrow, making the heart to pump harder than normal to push blood through the narrowed openings. If untreated, this may cause heart enlargement and thickening of the heart muscle. The heart is made to work harder and if the heart is unable to pump enough food needed by the body, hypertension can lead to heart failure, brain stroke, or kidney impairment. Hypertension can be prevented if one has a low-salt diet, maintenance of ideal weight, aerobic exercise, and a diet rich in fruits, vegetables, plant fiber, and the mineral potassium. Medication may help as well.

In arteriosclerosis, commonly known as hardening of the arteries, the walls of the arteries thicken, harden, and lose their elasticity. The heart must work harder than normal to deliver blood, and in advanced cases, it becomes impossible for the heart to supply sufficient blood to all parts of the body. There is no known causes of arteriosclerosis, but heredity, obesity, smoking, and a high-fat diet all appear to play roles.

Atherosclerosis, a form of arteriosclerosis, is the reduction in blood flow through the arteries caused by greasy deposits called plaque that form on the insides of arteries. This plaque deposits partially restrict the flow of blood. Plaque deposits are associated with high concentrations of cholesterol in the blood. Blood flow is often further reduced by the formation of blood clots, which are most likely to form where the artery walls have been roughened by plaque. These blood clots can also break free and travel through the circulatory system until they become lodged somewhere else and reduce blood flow there. Reduction in blood flow can cause organ damage. When brain arteries become blocked and brain function is impaired, the result is a stroke. A heart attack occurs when a coronary artery becomes blocked and heart muscle is destroyed.

Factors that contribute to atherosclerosis include physical inactivity, smoking, a diet high in fat, high blood pressure, and diabetes. Some cases of atherosclerosis can be corrected with healthy lifestyle changes, aspirin to reduce blood clotting, or drugs to lower the blood cholesterol concentration. For more serious cases, surgery would be needed to remove these blood clots.