The circulatory system is the process in the human body where blood is cycled through blood vessels, the heart and the lungs. The circulatory keeps tissue in the body filled with oxygen, which is used for energy in cellular respiration. Without oxygen, the body is unable to produce adenosine triphosphate (ATP), which is the energy currency of the body.
All the oxygen and nutrients that travel using the circulatory system is contained in the blood. The blood is mainly comprised of white blood cells, red blood cells and platelets. Red blood cells are responsible for carrying oxygen. White blood cells are the main structures of the immune system. Platelets are responsible for clotting factors when damage to tissue occurs.
The heart is a muscle that creates pressure in the circulatory system to cycle the blood to and from the tissue. The heart is made up of two ventricles, two atria and the valves that open and close as blood is circulated. The heart is also responsible for cycling the blood to and from the lungs, which is where the blood is able to bond to oxygen and remove carbon dioxide.
The vessels are the medium in which the blood travels. The vessels are made up of arteries, arterioles, capillaries and veins. These vessels branch as large diameter structures from the heart into small vessels like capillaries. Capillaries are the smallest of the blood vessels and they are the site of oxygen and carbon dioxide exchange in cells.
The lungs fill with oxygen when a person inhales. The oxygen travels to the bronchioles and into alveoli where gas exchange occurs with red blood cells. Red blood cells bond to the oxygen molecules in the alveoli when a person inhales. When a person exhales, the carbon dioxide that was in the blood is sent into the alveoli where it's sent out of the body through the mouth.
The path of the circulatory system is starts at the heart in this example. The circulatory system is a cycle, so no one point is a start or an end. Rather, the blood travels in a cycle throughout the body. The heart pumps the deoxygenated blood from the right ventricle to the lungs. Red blood cells pick up oxygen where they are sent back to the heart's left ventricle. The left ventricle contracts and blood is pushed into arteries. Arteries branch into smaller units called capillaries, where oxygen is dropped off to cells. Deoxygenated blood is transferred to veins and dropped back into the right ventricle. This is the entire path blood uses in the circulatory system.