At first glance, the cardiovascular system appears to consist of only a single organ--the heart. This organ is certain a key player in the system; it’s responsible for pumping blood rich in nutrients and oxygen to the body cells, and its pumping action helps return oxygen-deficient blood back to the heart for re-oxygenation at the lungs. However, the heart is not the only organ of the cardiovascular system; other organs synthesize blood, process and clean the blood, and assist in distribution.
Bone Marrow
The primary purpose of the circulatory system--to distribute blood--would be rendered impossible if there were no blood to distribute. As such, the bone marrow, or soft, active tissue found inside the long bones of the body, is a crucial component of this system. Red blood cells, called erythrocytes, are responsible for transporting oxygen to the tissues. Dr. Gary Thibodeau notes in his text, “Anatomy and Physiology,” that the erythrocytes are synthesized in bone marrow localized in the ends of long bones. White blood cells, or leukocytes, are also important components of blood, and help to fight infection. They’re made in the marrow localized in smaller ends of bones.
Blood Vessels
The arteries, capillaries, and veins are necessary components of the cardiovascular system as well. These are the vessels designed to carry blood to and from the heart, and their morphology reflects their function. Arteries, which carry blood from the heart, must withstand the force of high-pressure blood. They are muscular and tough. Arteries branch into smaller and smaller vessels the further they get from the heart, eventually branching into the tiny capillaries, which are so narrow that blood can only pass through a single cell at a time. Capillaries have very thin walls, to allow nutrient, waste, and gas exchange between cells outside the vessel and blood inside the vessel. Veins, then, return blood to the heart. Venous blood is at much lower pressure than arterial blood, so veins are not as tough as arteries. They do, however, have one-way valves inside to prevent backflow of blood, notes Dr. Thibodeau.
Heart
The heart is the circulatory system’s pump, and its job is to receive blood from body cells, send that blood to the lungs for oxygenation, receive oxygenated blood back from the lungs, and then send the blood to the body cells once more. To accomplish this without mixing of blood, the heart has two sides. The right side receives blood from the body and sends it out to the lungs, while the left side receives blood from the lungs and sends it out to the body. Since the left side has to push blood through a much longer network of vessels, this side of the heart is much more muscular and much larger than the right side.
Spleen
Located near the top of the abdomen, the spleen is blood storage and cleaning organ. It is filled with cells of the immune system, so as blood passes through, it is checked for pathogens and impurities. In addition, however, the spleen acts as a source of an emergency blood transfusion; if a person is bleeding badly, the spleen contracts and increases blood volume and blood pressure to try to maintain consciousness, notes Dr. Lauralee Sherwood in her book “Human Physiology.” The spleen’s tendency to store large quantities of blood makes it prone to rupture during traumatic injuries, however. Fortunately, it’s a nonessential organ, and ruptured spleens can be removed surgically with no negative long-term consequences.
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