Heart
In the human body, the heart is normally situated slightly to the left of the middle of the thorax, underneath the sternum (breastbone). The heart is usually felt to be on the left side because the left heart (the left part of the heart) is stronger (it pumps the blood out). The heart is enclosed by a sac known as the pericardium and is surrounded by the lungs.Betwen the pericardium and the heart there is a serous fluid to reduce friction during heart contractions. The apex is the blunt point at the base of the heart. A stethoscope can be placed directly over the apex and count the beats. In normal adults, its mass is 250-350 g (8-11 oz), but extremely diseased hearts can be up to 1000 g (2 lb) in mass. It consists of four chambers, the two upper atria (singular: atrium ) and the two lower ventricles. On the right is a picture of a fresh human heart which was just removed from a 64 year old British gentleman. The mass for a thin white male is usually 270-300 grams (9-11 ounces) with little fat.
A septum divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. Valves between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles. The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.
The function of the right side of the heart (see right heart) is to collect deoxygenated blood from the body and pump it into the lungs (pulmonary circulation)so that carbon dioxide can be dropped off and oxygen picked up. This happens through a process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs and pumps it out to the body (systemic circulation). On both sides, the lower ventricles are thicker than the upper atria.
Oxygen-depleted or deoxygenated blood from the body enters the right atrium through two great veins, the superior vena cava which drains the upper part of the body and the inferior vena cava that drains the lower part which drains the portal cyrculation too full with nutrients to nourish the body. The blood then passes through the tricuspid valve to the right ventricle. The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. In the lungs gaseous exchange takes places and the blood releases carbon dioxide into the lung cavity and picks up oxygen which is called external respiration. The oxygenated blood then flows through pulmonary veins to the left atrium. From the left atrium this newly oxygenated blood passes through the mitral valve (also called the bicuspid valve) to enter the left ventricle. The left ventricle then pumps the blood through the aorta to the entire body. Even the lungs take some of the blood supply from the aorta via bronchial arteries.
The left ventricle is much more muscular, which is 1.3 - 1.5 cm (.5-.6 in) thick, than the right, 0.3 - 0.5 cm ( .1-.2 in) thick, as it has to pump blood around the entire body, which involves exerting a considerable force to overcome the vascular pressure. As the right ventricle needs to pump blood only to the lungs, it requires less muscle.
Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top side.
The contractile nature of the heart is due to the presence of cardiac muscle in its wall which can work continuously without fatigue. The heart wall is made of three distinct layers. The first is the outer epicardium which is composed of a layer of flattened epithelial cells and connective tissue. Beneath this is a much thicker myocardium made up of cardiac muscle. The endocardium is a further layer of flattened epithelial cells and connective tissue which lines the chambers of the heart.
The blood supply to the heart itself is supplied by the left and right coronary arteries, which branch off from the aorta.