The Circulatory System
Circulatory System or cardiovascular system, in humans, is the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products (Encarta 2000). The heart, blood, and blood vessels are the three structural elements that make up the circulatory system.
The Heart: The heart is the engine or the pump of the circulatory system. It is divided into four chambers: the right atria, the right ventricle, the left atria, and the left ventricle. The walls of these chambers are made of a special muscle called myocardium, which contracts continuously and rhythmically to pump blood. The pumping action of the heart occurs in two stages for each heartbeat:
+ Diastole, when the heart is at rest or receiving blood from the blood vessels.
Blood: consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma.
+ Red Blood Cell or Erythrocyte: is the oxygen-carrying part of the blood. Also referred to as red corpuscles, they are by far the most numerous type of blood cell. Blood carries oxygen from the lungs to all the other tissues in the body and, in turn, carries waste products, predominantly carbon dioxide, back to the lungs.
+ Platelets: or Thrombocytes are the smallest cells in the blood, which are designed for a single purpose to begin the process of coagulation, or forming a clot, whenever a blood vessel is broken. Without Platelets animals would simply bleed to death from a small wound. (This disease is called Hemophilia)
+ Arteries carry blood away from the heart
+ Veins carry blood to the heart.
+ Aorta is the principal artery of the body that carries oxygenated blood to most other arteries in the body. The aorta rises from the left ventricle (lower chamber) of the heart, arches back and downward and goes to the legs and dived into smaller blood vessels into the lower body.
Superior and Inferior Vena Cava are the two principal veins that carry deoxygenated blood to the right atrium of the heart. Superior Vena Cava carries all the deoxygenated blood form the upper body and the Inferior Vena Cava carries all the deoxygenated blood from the lower body.
The inner layer of blood vessels is lined with endothelial cells, which create a smooth passage for the transit of blood. This inner layer is surrounded by smooth muscle that enables the blood vessel to expand or contract during the pumping of blood. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.
The Circulatory System consists of two types of Circulations.
+ Systemic Circulation: carries oxygenated blood from the heart to all the tissues in the body except the lungs and returns deoxygenated blood carrying waste products, such as carbon dioxide, back to the heart. (This is the first loop the blood makes).
+ Pulmonary Circulation: carries the deoxygenated blood from the heart to the lungs. In the lungs, the blood releases its carbon dioxide and absorbs oxygen. The oxygenated blood then returns to the heart and the Systemic circulation starts again. (This is the second loop the blood makes making us have a double loop circulatory system)
Blood Pressure is the pressure generated by the pumping action of the heart of the blood to the arteries (Encarta 2000). In order to maintain an adequate flow of blood to all parts of the body, a certain level of pressure is needed, called Blood pressure. Blood pressure enables a person to rise quickly from a horizontal position without blood collecting in the lower body, which would cause fainting from loss of blood to the brain. Normal blood pressure is regulated by a number of factors, such as the contraction of the heart, the elasticity of arterial walls, blood volume, and resistance of blood vessels to the passage of blood. Blood pressure is the ratio between the Systolic (When heart pumps the blood) and diastolic (when the heart is resting) pressures of the blood (Encarta 2000). It is measured in units of millimeters of mercury (abbreviated mm Hg) and the average normal blood pressure is about 120/80 mm Hg.