The circulatory system is responsible for the distribution of blood throughout the body. The system mainly consists of the vessels responsible for this flow and the heart, but when looking at the function of the system, other parts can be identified.
The essence of the circulatory system is what it carries – the blood. Blood consists of cells and plasma. Plasma is mostly water, but also carries nutrients and waste to and from the tissues of the body and hormones among tissues. The cells include red blood cells, which carry oxygen, and white blood cells, which aid in immunity and protective functions. Platelets are also found in the blood and are responsible for healing responses when blood leaks from a vessel, effectively clotting the break to prevent blood loss.
The circulatory system would not function without the heart. This organ is made of a specialized muscle tissue capable of converting electric potential into mechanical movement, specifically acting as a pump. The human heart is divided into four chambers – the upper atria and lower ventricles, one each on the right and left sides. Blood is squeezed from chamber to chamber and into blood vessels, forcing the flow through the body. Valves between the chambers and external vessels prevent backflow, maintaining the pressure needed to keep the blood moving.
The major component of the circulatory system is the vessels themselves. Various types of blood vessels are found in the human body. The arteries carry blood away from the heart; veins carry blood to the heart. The large vessels are connected directly to the heart: the aorta is the largest artery through which the left ventricle pumps blood (ascending aorta, aortic arch, and descending aorta which consists of the thoracic aorta and abdominal aorta), and the vena cava are the large veins (superior and inferior, draining from the upper body and lower body, respectively).
The large vessels branch into medium-sized vessels, those commonly referred to as arteries and veins. These medium vessels further branch into smaller vessels: arterioles and venules. The smallest blood vessels are the width of a single red blood cell – the capillaries. Blood from the arteries traveling to the tissues is oxygenated. The red blood cells carrying oxygen enter the capillaries for exchange. The deoxygenated blood then goes from the capillary to the venules for transport back to the heart, where the pulmonary circulation replenishes the oxygen.
The pulmonary circulation consists of the heart, lungs, and two large blood vessel systems. Deoxygenated blood from the body enters the heart from the vena cava, which is then pumped through the pulmonary artery to the lungs. In the multitude of tiny blood vessels branching into the lung tissue, carbon dioxide is released and oxygen taken up. The oxygenated blood is transported through the pulmonary vein back to the heart to be pumped through the aorta to the body.
The heartbeat, breathing, and circulation are involuntary actions controlled by a respiration center in the brain. Blood pressure also influences circulation and heart rate, and is controlled by both the brain and hormones that act on the blood vessels. The kidneys play an additional role in the hormonal control of circulation by influencing and detecting the concentration of ions in the blood. All together, circulation is a complex system with an intricate web of vessels to achieve its function.