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The cardiovascular system is part of the circulatory system that circulates blood. The circulatory system also includes the lymphatic system, which circulates lymph, but the terms circulatory system and cardiovascular system are commonly used interchangeably to describe blood circulation.
The cardiovascular system consists of the heart, blood, and blood vessels. The heart is essentially a pump that moves blood through the vessels. It has 2 sides, each of which has 2 chambers.
The best-known function of the circulatory system is perhaps the transport of inhaled oxygen from the lungs to body’s tissues, and removal of carbon dioxide in the opposite direction to be exhaled. Basically, oxygen-poor blood from the body returns to the right side of the heart, where it is pumped to the lungs. In the lungs, blood picks up oxygen and releases carbon dioxide. Oxygen-rich blood then returns to the left side of the heart. This part of the system is called the pulmonary circuit.
The left side of the heart pumps oxygen-rich blood to body’s tissues, where it unloads oxygen and picks up carbon dioxide. The resulting deoxygenated blood again returns to the heart’s right side to complete the cycle. This part is the systemic circuit. Because the heart’s left side has to pump blood to the entire body, it has much thicker muscle than the right side.
There are 4 valves which serve to ensure one-way blood flow through the heart: oxygen-poor blood flows from right atrium to right ventricle to pulmonary arteries; while oxygen-rich blood moves from left atrium to left ventricle to the aorta.
The heart is enclosed in a double-walled protective sac called the pericardium. The pericardial cavity contains a fluid which serves as lubricant and allows the heart to contract and relax with minimum friction. The heart wall has 3 layers:
– the outer layer, epicardium, lines the pericardial cavity,
– the inner layer, endocardium, lines heart chambers and valves and is continuous with the endothelium of blood vessels,
– and the thick middle layer, myocardium, is the muscle tissue responsible for the beating of the heart.
The contraction of the heart muscle is initiated by electrical impulses, known as action potentials. Unlike skeletal muscles, which have to be stimulated by the nervous system, the heart generates its own electrical stimulation. The impulses start from a small group of cells called the pacemaker cells, which constitute the cardiac conduction system. The primary pacemaker is the SA node, it initiates all heartbeats and controls heart rate.
Apart from transporting gases, the blood also supplies body’s tissues with nutrients and removes metabolic wastes. It receives nutrients from the digestive system, where digested substances are absorbed through the walls of the small intestine into the bloodstream. These substances are then passed through the liver to be screened for toxins before joining the general circulation. In tissues, nutrients are exchanged for wastes. Wastes are then filtered from the blood in the kidneys and removed in urine.
The blood also carries hormones from endocrine glands to target organs, and plays an important role in the body’s immune defense.
The blood has two main components: a clear extracellular fluid called plasma, and the so-called formed elements which include red blood cells, white blood cells and platelets. Red blood cells transport oxygen and carbon dioxide; white blood cells participate in various defense mechanisms against invading organisms; while the platelets are responsible for blood clotting, minimizing blood loss during an injury.
The blood circulatory system is a closed loop, meaning the blood itself never leaves the vessels. Instead, substances diffuse through the walls of blood vessels to move to and from the surrounding tissues.
Vessels that move blood away from the heart are called arteries, while those that bring blood back to the heart are veins. Arteries usually carry oxygenated blood while veins carry deoxygenated blood. For pulmonary arteries and veins, however, the reverse is true.
The usual route of blood flow is: heart to large arteries, smaller arteries, then even smaller arteries, called arterioles, then smallest blood vessels called capillaries, where the exchange of substances takes place. Blood then collects into small veins, called venules, then to larger veins and back to the heart.
Arteries and veins essentially serve to conduct blood, their walls consist of 3 layers:
– an outer layer of loose connective tissue serves to anchor the vessels to the surroundings.
– a middle layer of mostly smooth muscles allows the vessels to constrict or dilate, regulating blood flow.
– and an inner layer consisted of thin squamous endothelium, separated from outer layers by a basal membrane.
In general, larger vessels have more connective tissue and smooth muscle. In addition, arteries have more muscles than veins because they carry blood away from the heart and must withstand higher pressures generated by the beating of the heart.
The walls of capillaries, whose function is to exchange substances between the blood and surrounding tissue, consist solely of a thin endothelium with its basement membrane, thus permitting easy diffusion of blood solutes.