Human Circulatory System
The heart has four chambers. The upper or superior chambers are called the left and right atria, and the lower or inferior chambers are called the left and right ventricles. On the anterior surface of atrium is wrinkled pouchlike structure called an auricle, so named because of its resemblance to a dog’s ear and its functions is to increase the capacity of an atrium. A wall of muscle called the septum separates the left and right atria and the left and right ventricles. The surface of the heart is also a series of grooves, called sulci which contain coronary blood vessels and variable amount of fat. The two atria are thin-walled chambers that receive blood from the veins. The right atrium is about 2-3mm in average thickness and receive deoxygenated blood from three veins are superior vena cave, inferior vena cava and coronary sinus. Whereas, left atrium is about same thickness as the right atrium and it receive oxygenated blood from the pulmonary veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. The right ventricle is about 4-5mm in average thickness. Inside the right ventricle contain a series of ridges formed by raised bundles of cardiac muscle fiber called trabeculae carneae. Some of the trabeculae carneae convey part of the conduction system of the heart. Blood passes from the right ventricle through the pulmonary valve into a large artery called pulmonary trunk. The left ventricle is the thickest chamber of the heart, averaging 10-15 mm and forms the apex of the heart. Blood passes from the left ventricle through the aortic valve into ascending aorta.
In postnatal circulation, the heart pumps blood into two closed circuits with each beat, systemic circulation and pulmonary circulation. The two circuits are arranged in series. The output of one becomes the input of the other, as would happen if you attached two garden hoses. The left side of the heart is the pump for systemic circulation, it receives bright red, oxygen-rich blood from the lungs. The left ventricle ejects blood into the aorta. From the aorta, the blood divides into separates streams, entering progressively smaller systemic arteries that carry it to all organs throughout the body except for the air sacs of the lungs, which are supplied by pulmonary circulation. In systemic tissues, arteries give rise to smaller diameter arterioles, which finally lead into extensive beds of systemic capillaries. Exchange of nutrients and gases occurs across the thin capillary walls. Blood unloads oxygen and picks up carbon dioxide. In most cases, blood flows through only one capillary and then enters a systemic venule. Venules carry deoxygenated blood away from tissues and merge to form larger systemic veins. Ultimately the blood flows back to the right atrium.
The right side of the heart is the pump for pulmonary circulation. It receives all the dark red, deoxygenated blood returning from systemic circulation. Blood ejected from the right ventricle flows into the pulmonary truck, which branches into pulmonary arteries that carry blood to the right and left lungs. In pulmonary capillaries, blood unloads carbon dioxide, which is exhaled, and picks up inhaled oxygen. The freshly oxygenated blood then flows into pulmonary monary veins and returns to the left atrium.
In conclusion, differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate.
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