Human Circulatory System
The circulatory system (or 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). While the most primitive animal phyla lack circulatory systems, some invertebrate groups have an open circulatory system. Vertebrates have a closed circulatory system.
De-oxygenated blood enters the right atrium of the heart and flows into the right ventricle where it is pumped through the pulmonary arteries to the lungs. Pulmonary veins return the now oxygen-rich blood to the heart, where it enters the left atrium before flowing into the left ventricle. From the left ventricle the oxygen-rich blood is pumped out via the aorta, and on to the rest of the body.
As blood circulates through the body, oxygen diffuses from the blood into cells surrounding the capillaries, and carbon dioxide diffuses into the blood from the capillary cells. 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.
Oxygenation from the Lungs
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.
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 left ventricle is thicker and more muscular than the right ventricle because it pumps blood at a higher pressure. The left ventricle pumps blood to the entire body whereas the right ventricle pumps all of its blood directly to the lungs.
Oxygen and Red Blood Cells
The release of oxygen from red blood cells or erythrocytes is regulated in mammals. 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.