Structure of the mammalian heart

The heart is a large, muscular and pear-shaped organ that pumps blood through the body of mammals‘. it is reddish in colour and enveloped in a thin membrane called pericardium, which keeps the heart in position in the thoracic cavity.

 The cavity between the pericardium and the heart is filled with a fluid called pericardial fluid, which reduces friction between the heart and the two  lungs that ilank it in the thoracic cavity. It is made up of cardiac muscles, which enable the heart to continuously contract rhythmically without being fatigued.

The heart consists of four chambers: two upper chambers called atria (singular: atrium) or auricles (singular: auricle) and two lower chambers known as ventricles. The walls of the ventricles are thicker than those of the atria or auricles. The heart is divided into two halves by a thick central wall called septum, which completely separates deoxygenated blood from oxygenated blood. The right side of the heart is separated into the right upper chamber, right auricle or right atrium and the right lower chamber, right ventricle, by the tricuspid valves, consisting of three flaps or cusps. Similarly, the left side is separated into the left upper chamber, left auricle or left atrium and left loWer chamber, left ventricle by the bicuspid valves or mitral valves which consist, of two flaps or cusps. The cusps or flaps of the tricuspid and bicuspid valves are attached to the inner walls of the ventricles by the chordae tendineae (singular: chordae tendinea). These are tough. non elastic fibrous cords, which prevent the valves from turning inside out. The chordae tendineae also known as tendinous cords, heart strings or valve tendons, are themselves attached to conical muscular projections on the inner walls of the ventricles called papillary muscles.

Originating in the ventricles and traversing the outside walls of the auricles are the trunks of the two main arteries of the body: aorta, on the left side of the heart, and pulmonary artery on the right. Semi-lunar valves or pocket valves, at the base or opening of the aorta and pulmonary artery, prevent the backilow of blood into the ventricles when the walls of the ventricles relax.

The right auricle receives deoxygenated blood from two anterior venae cavae and a posterior vena cava. The left auricle also receives oxygenated blood from the lungs through the pulmonary veins.