The Circulatory System

Source: Wikipedia.

The circulatory system (scientifically known as the cardiovascular system) is an organ system that moves substances to and from cells; it can also help stabilize body temperature and pH (part of homeostasis). There are three types of circulatory systems (from simplest to most complex): no circulatory system, open circulatory system, and closed circulatory system.

Mammalian circulation

Having circulated through the body, all the relatively de-oxygenated blood collects in the venous system which coalesces into two major veins: the superior vena cava (roughly speaking from areas above the heart) and the inferior vena cava (roughly speaking from areas below the heart). These two great vessels empty into the right atrium of the heart. The coronary sinus empties the heart's veins themselves into the right atrium. The right atrium is the larger of the two atria, although both receive the same amount of blood. The blood is then pumped through the tricuspid valve, or right atrioventricular valve, into the right ventricle. From the right ventricle, blood is pumped through the pulmonary semi-lunar valve into the pulmonary artery. This blood enters the two pulmonary arteries (one for each lung) and travels through the lungs, where it is oxygenated and then flows into the pulmonary veins. This oxygenated blood then enters the left atrium, which pumps it through the bicuspid valve, also called the mitral or left atrioventricular valve, into the left ventricle. The left ventricle is thicker and more muscular than the right ventricle because it pumps blood at a higher pressure.

From the left ventricle, blood is pumped through the aortic semi-lunar valve into the aorta, a massive and thick-walled artery. The aorta arches and gives off major arteries to the upper body before piercing the diaphragm in order to supply the lower parts of the body with its various branches. Once the blood enters the peripheral tissues oxygen and nutrients are extracted from it and carbon dioxide and wastes added, and it will again be collected in the veins and the process will be repeated. Peripheral tissues do not fully deoxygenate the blood, so venous blood does have oxygen, but in a lower concentration than in arterial blood.

The release of oxygen from red blood cells or erythrocytes is regulated. It increases with an increase of carbon dioxide in tissues, an increase in temperature, or a decrease in pH. Such characteristics are exhibited by tissues undergoing high metabolism, as they require increased levels of oxygen. This is for mammals only.

Flow of blood in the human heart

First the blood enters through the right atrium of the heart. It flows into the right ventricle and into the pulmonary arteries. The oxygen-poor blood flows through to the lungs. The pulmonary veins take the oxygen-rich blood to the left atrium and into the left ventricle. From the left ventricle the oxygen-rich blood flows into the aorta. The aorta pumps the blood to the rest of the body.