The Circulatory System
Click here to hear a heartbeat.
The Circulatory System has two major subdivisions- the cardiovascular system and the lymphatic system. The cardiovascular system can be compared to a muscular pump equipped with one-way valves and a system of large and small plumbing tubes within which blood travels. In this section I will discuss the circulatory system: the heart and the blood vessels.
The Heart: Location and Size
Approximately the size of a person's fist, the hollow, cone-shaped heart weighs less than a pound. The heart is located within the bony thorax and is flanked on each side by the lungs. Its apex is directed toward the left hip and rests on the diaphram. Its base points toward the right shoulder and lies beneath the second rib.
Covering and Wall
The heart is enclosed in a double sac of serous membrane called the pericardium. The pericardium is composed of the visceral epicardium, which is actually a part of the heart wall, and the parietal pericardium, which protects the heart and anchors it to its surrounding structures. This pericardial membrane produces a serous fluid that helps the heart beat and move in a nearly frictionless area. The heart walls are composed of three layers: the outer epicardium, the myocardium, and the endocardium.
Chambers and Associated Great Vessels
The heart has four hollow chambers- two atria and two ventricles. Each of these chambers is lined with endocardium, which helps blood flow smoothly through the heart. The atria are superior to the ventricles and are primarily receiving chambers- they are not important in the pumping activity of the heart. Blood flows into them under low pressure from the veins of th body and then continues on the fill the ventricles below. The thick wall ventricles are dicharging chambers, or ctual pumps of the heart. When they contract, blood is propelled out of the heart and through the body. Although the heart is a single organ, it acts as a double pump. The right side works as the pulmonary circuit pump. It receives oxygen-poor blood from the veins of the body through the large superior and inferior vena cavae, and it pumps it out through the pulmonary trunk. The pulmonary trunk splits into the right and left pulmonary arteries, which carry blood to the lungs to pick up oxygen and to release carbon dioxide. Oxygen-rich blood drains from the lungs and returns to the left side of the heart, through the pulmonary veins. This it the pulmonary circulation. The second circuit from the left side of the heart through the body tissues and back to the right side of the heart is called the systematic circulation. It supplies oxygen and nutrient-rich blood to all the body parts. Its walls are substantially thicker than the walls of the right ventricle and is a much more powerful pump.
Blood Vessels
Like a system of roads, the circulatory system has its freeways, secondary roads, and alleys. As the heart beats, blood is propelled into the large arteries leaving the heart. It then moves into successively smaller and smaller arteries and then into the arterioles, which feed the cpillary beds in the tissues. Capillary beds are drained by venules, which in turn empty into veins that finally empty into the great veins entering the heart.
Blood Vessel Anatomy
Except for the capillaries, the walls of blood vessels have three coats- the tunica intima, the tunica media, and the tunica externa. The tunica intima, which lines the lumen or interior of the vessels, is a thin layer of endothelium. The tunic media is the thick middle part of the vessel that is conntected to the nervous system. The nervous system controlls the diameter of the vessel by making the vessel contract. The tunica externa is the outermost tunic. Its function is to support and protect the vessel. Arteries are usually a lot thicker than veins. This is because that arteries are close to the heart (as in they are the first thing that blood flows through), so they have to be very flexible to deal with the fluctuating pressures going on in the heart. Blood also has to get to all of the body tissues all overr the body, so arteries have muscle so that they can contract and help the blood flow through the body. Veins on the other hand are far from the heart and don't really feel that much of a pressure difference in them. Since there isn't a lot of pressure in the veins there is the possibility of blood pooling in them. To keep this from happening, there are valves in veins that aren't in arteries. This prevents blood from going backwards through the system. The transparent walls of the tiny capillaries are only one cell layer thick- just the tunica intima. Because of this exceptional thinness, exchanges are easily made between the blood and tissue cells. It is here at the capillaries where cells get rid of their carbon dioxide and get oxygenated blood.
written by Michael Simpson
