Position, Shape and Size of the Heart:
The human heart is located between the lungs in the thoracic cavity (i.e. it is not situated on the left side of the chest). The heart is a hollow, fibro-muscular organ and is somewhat conical in shape. It is about the size of one’s fist, measures about 12 cm in length and 9 cm in breadth. Its weight is about 300 g in an adult.
The heart is enclosed in pericardium (a double- layered membranous sac). It protects the heart from mechanical injury. The space between two pericardial membranes is the pericardial space, which is filled with the pericardial fluid. The pericardial fluid keeps the heart moist and reduces friction between the heart wall and the surrounding tissues when the heart beats. Human heart is made of cardiac muscles or myocardia which contract rhythmically by self generated impulse.
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External Structure of the Heart:
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The human heart is a four-chambered organ divided by septa into two halves—the right half and the left half. Each half consists of two chambers—the upper, small-sized auricle or atrium and the lower, large- sized ventricle. The ventricles form the larger, lower part of heart.
Internal Structure of the Heart:
Internally, the heart has the following main components—two auricles, two ventricles, great blood vessels that carry blood to the heart and away from it, and various apertures and valves.
i. Auricles the Receiving Chambers:
The auricles or atria are thin-walled chambers and are separated from each other by an inter-auricular septum. The septum has an oval, thin area called the fossa ovalis. It marks the position of an opening, the foramen ovale between the two atria in a foetus.
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ii. Ventricles the Discharging Chambers:
The ventricles are thick-walled chambers and are separated from each other by an obliquely placed inter-ventricular septum. The wall of the left ventricle is thicker than that of the right ventricle, because the left ventricle has to pump blood into vessels, which in turn carry the blood to the entire body. The right ventricle pumps blood to the pulmonary arteries, which carry it to the lungs.
The walls of the atria are thinner than that of ventricles because they only have to pump blood into the ventricles.
Great Blood Vessels of the Heart:
The blood vessels that enter or leave the heart are called great blood vessels.
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Blood Vessels Entering the Heart:
The right auricle receives three blood vessels.
i. Superior (Anterior) Vena Cava or Precaval:
Brings deoxygenated blood from the head and upper region of the body.
ii. Inferior (Posterior) Vena Cava or Postcaval:
Brings deoxygenated blood from lower region of the body.
iii. Pulmonary Veins:
The left auricle receives two pairs of pulmonary veins, one pair from each lung. These bring oxygenated blood from the lungs.
iv. Coronary sinus:
Brings deoxygenated blood from the heart’s wall itself. It consists of two coronary arteries arising from the base of the aorta. These supply blood to the heart muscles. If coronary arteries get blocked, then it can cause heart attack.
Blood Vessels Leaving the Heart:
i. Pulmonary Artery:
Arises from the right ventricle and carries deoxygenated blood to the lungs for purification.
ii. Systemic Aorta:
Arises from the left ventricle and supplies oxygenated blood to all body parts, except the lungs.
Shows the location and functions of major blood vessels of human cardiovascular system.
Apertures and Valves in the Heart:
There are four valves in the heart which control the flow of blood within the heart and its passage to various parts of the body through the great blood vessels.
i. The bicuspid valve also called the mitral valve or left auriculoventricular valve guards the opening of the left auricle into the left ventricle.
ii. The tricuspid valve also called the right auriculo- ventricular valve guards the right auriclo-ventricular aperture.
iii. Semilunar or pulmonary valves are present at the base of aortic and pulmonary arches. These valves check the back flow of blood into the ventricles.
iv Aortic semilunar valve is present at the point of origin of aorta from the left ventricle. In all, there are three semilunar valves in the vessels.
Circulation of Blood through the Heart:
The circulation of blood in the human heart is called double circulation because the blood centers and leaves the heart twice in each heart beat. Circulation of blood between the heart and body organs (except lungs) is called systemic circulation. Circulation of blood between the heart and the lungs is called pulmonary circulation.
Systemic Circulation:
The left ventricle pumps the oxygenated blood into the systemic aorta. Aorta gives off branches to all the organs of the body except lungs. From these arteries, oxygen is diffused into the tissues. Deoxygenated blood from visceral organs is brought to the right auricle by superior and inferior venae cavae. This part of circulation from the left ventricle to the right auricle of the heart via body tissues (except lungs) is called systemic circulation.
Pulmonary Circulation:
The circulation of blood from the right ventricle to the left auricle of the heart via lungs is called pulmonary circulation. The right ventricle pumps deoxygenated blood to the lungs for oxygenation (purification). Oxygenated blood from the lungs is returned to the left auricle by four pulmonary veins.