CIRCULATORY SYSTEM

Circulatory system -----is an organ system that passes nutrients (such as amino acids, electrolytes and lymph), gases, hormones, bloods, cells, etc. to and from cells in the body to help fight diseases, stabilize body temperature and pH, and to maintain homeostasis.

This system may be seen strictly as a blood distribution network, but some consider the circulatory system as composed of the cardiovascular system, which distributes blood, and the lymphatic system, which returns excess filtered blood plasma from the interstitial fluid (between cells) as lymph.

The lymphatic system, on the other hand, is an open system providing an accessory route for excess interstitial fluid to get returned to the blood. Two types of fluids move through the circulatory system: blood and lymph. Lymph is essentially recycled blood plasma after it has been filtered from the blood cells and returned to the lymphatic system.

The blood, heart, and blood vessels form the cardiovascular (from Latin words meaning 'heart'-'vessel') system. The lymph, lymph nodes, and lymph vessels form the lymphatic system. The cardiovascular system and the lymphatic system collectively make up the circulatory system.

Human cardiovascular systemThe main components of the human cardiovascular system

are the heart, blood, and blood vessels. It includes: the pulmonary circulation, a "loop" through the lungs where blood is oxygenated; and the systemic circulation, a "loop" through the rest of the body to provide oxygenated blood.

An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, which consists of plasma, red blood cells, white blood cells, and platelets. Also, the digestive system works with the circulatory system to provide the nutrients the system needs to keep the heart pumping.

Pulmonary Circulation

The pulmonary circulatory system is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart, via the pulmonary artery, to the lungs and returned, oxygenated, to the heart via the pulmonary vein.

Oxygen deprived blood from the vena cava, enters the right atrium of the heart and flows through the tricuspid valve (right atrioventricular valve) into the right ventricle, from which it is then pumped through the pulmonary semilunar valve into the pulmonary artery to the lungs. Gas exchange occurs in the lungs, whereby CO2 is released from the blood, and oxygen is absorbed. The pulmonary vein returns the now oxygen-rich blood to the heart.

Systemic Circulation

Systemic circulation is the portion of the cardiovascular system which transports oxygenated blood away from the heart, to the rest of the body, and returns oxygen-depleted blood back to the heart. Systemic circulation is, distance-wise, much longer than pulmonary circulation, transporting blood to every part of the body.

View from the front, which means the right side of the heart is on the left of the diagram (and vice-

versa)

Coronary circulation

The coronary circulatory system provides a blood supply to the heart. As it provides oxygenated blood to the heart, it is by definition a part of the systemic circulatory system.

HeartThe heart pumps oxygenated blood to the body and

deoxygenated blood to the lungs. In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle. The right atrium is the upper chamber of the right side of the heart.

The blood that is returned to the right atrium is deoxygenated (poor in oxygen) and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.

Oxygen transportation

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with haemoglobin molecules.

About 1.5% is physically dissolved in the other blood liquids and not connected to haemoglobin. The haemoglobin molecule is the primary transporter of oxygen in mammals and many other species.

Measurement techniques

Electrocardiogram—for cardiac electrophysiology

Sphygmomanometer and stethoscope—for blood pressure

Pulse meter—for cardiac function (heart rate, rhythm, dropped beats)

Pulse—commonly used to determine the heart rate in absence of certain cardiac pathologies

Heart rate variability -- used to measure variations of time intervals between heart beats

Nail bed blanching test—test for perfusion

Vessel cannula or catheter pressure measurement—pulmonary wedge pressure or in older animal experiments.

Other vertebrates

Two-chambered heart of a fish

The circulatory systems of all vertebrates, as well as of annelids(for example, earthworms) and cephalopods(squids, octopuses and relatives) are closed, just as in humans. Still, the systems of fish, amphibians, reptiles, and birds show various stages of the evolution of the circulatory system.

In fish, the system has only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. The heart of fish is therefore only a single pump (consisting of two chambers).

In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.

HeartThe human heart is about the size of a clenched fist. It contains

four chambers: two atria and two ventricles. Oxygen-poor blood enters the right atrium through a major

vein called the vena cava. The blood passes through the tricuspid valve into the right ventricle.

Next, the blood is pumped through the pulmonary artery to the lungs for gas exchange. Oxygen-rich blood returns to the left atrium via the pulmonary vein.

The oxygen-rich blood flows through the bicuspid (mitral) valve into the left ventricle, from which it is pumped through a major artery, the aorta. Two valves called semilunar valves are found in the pulmonary artery and aorta.

The ventricles contract about 70 times per minute, which represents a person's pulse rate. Blood pressure, in contrast, is the pressure exerted against the walls of the arteries. Blood pressure is measured by noting the height to which a column of mercury can be pushed by the blood pressing against the arterial walls.

A normal blood pressure is a height of 120 millimeters of mercury during heart contraction ( systole), and a height of 80 millimeters of mercury during heart relaxation ( diastole). Normal blood pressure is usually expressed as “120 over 80.”

Coronary arteries------ supply the heart muscle with blood. The heart is controlled by nerves that originate on the right side in the upper region of the atrium at the sinoatrial node. This node is called the pacemaker.

It generates nerve impulses that spread to the atrioventricular node where the impulses are amplified and spread to other regions of the heart by nerves called Purkinje fibers.

Blood

Blood is the medium of transport in the body. The fluid portion of the blood, the plasma, is a straw-colored liquid composed primarily of water. All the important nutrients, the hormones, and the clotting proteins as well as the waste products are transported in the plasma. Red blood cells and white blood cells are also suspended in the plasma. Plasma from which the clotting proteins have been removed is serum.

Red blood cells

Red blood cells are erythrocytes. These are disk-shaped cells produced in the bone marrow. Red blood cells have no nucleus, and their cytoplasm is filled with hemoglobin.

Hemoglobin is a red-pigmented protein that binds loosely to oxygen atoms and carbon dioxide molecules. It is the mechanism of transport of these substances. (Much carbon dioxide is also transported as bicarbonate ions.) Hemoglobin also binds to carbon monoxide. Unfortunately, this binding is irreversible, so it often leads to carbon-monoxide poisoning.

A red blood cell circulates for about 120 days and is then destroyed in the spleen, an organ located near the stomach and composed primarily of lymph node tissue. When the red blood cell is destroyed, its iron component is preserved for reuse in the liver.

The remainder of the hemoglobin converts to bilirubin. This amber substance is the chief pigment in human bile, which is produced in the liver.

Red blood cells commonly have immune-stimulating polysaccharides called antigens on the surface of their cells. Individuals having the A antigen have blood type A (as well as anti-B antibodies); individuals having the B antigen have blood type B (as well as anti-A antibodies); individuals having the A and B antigens have blood type AB (but no anti-A or anti-B antibodies); and individuals having no antigens have blood type O (as well as anti-A and anti-B antibodies).

White blood cells

White blood cells are referred to as leukocytes. They are generally larger than red blood cells and have clearly defined nuclei. They are also produced in the bone marrow and have various functions in the body. Certain white blood cells called lymphocytes are essential components of the immune system. Other cells called neutrophils and monocytes function primarily as phagocytes; that is, they attack and engulf invading microorganisms.

About 30 percent of the white blood cells are lymphocytes, about 60 percent are neutrophils, and about 8 percent are monocytes. The remaining white blood cells are eosinophils and basophils. Their functions are uncertain; however, basophils are believed to function in allergic responses.

Platelets

Platelets are small disk-shaped blood fragments produced in the bone marrow. They lack nuclei and are much smaller than erythrocytes. Also known technically as thrombocytes, they serve as the starting material for blood clotting.

The platelets adhere to damaged blood vessel walls, and thromboplastin is liberated from the injured tissue. Thromboplastin, in turn, activates other clotting factors in the blood. Along with calcium ions and other factors, thromboplastin converts the blood protein prothrombin into thrombin.

Thrombin then catalyzes the conversion of its blood protein fibrinogen into a protein called fibrin, which forms a patchwork mesh at the injury site. As blood cells are trapped in the mesh, a blood clot forms.

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