It is hollow, muscular organ that contracts at regular intervals, forcing blood through the circulatory system.
It is cone-shaped, about the size of a fist, and is located in the centre of the thorax, between the lungs, directly behind the sternum (breastbone). The base is tilted to the left.
Pericardium (heart enclosure): Its function is protection of heart. It has two layers a) Fibrous pericardium: It is tough, inelastic and fibrous connective tissue. b) Serous pericardium: It is made up of two layers parietal layer (outer layer) and visceral layer attached to the heart. Between these two layers pericardial fluid is present, which acts as a shock absorber.
Heart wall: It has three layers. a) Epicardium: It is made up of single layer flat epithelial cells called mesothelium b) Myocardium: It is made up of cardiac muscles responsible for beating of heart c) Endocardium: It is made up of single layer flat epithelial cells called endothelium.
The cardiac muscles are not under the conscious control of the nervous system, and can generate its own electrical rhythm (myogenic). For the same reasons, cardiac muscle cannot respire anaerobically and so the muscle cannot get tired (or develop cramp!)
Cardiac muscle has a rich supply of blood, which ensures that it gets plenty of oxygen. This is brought to the heart through the coronary artery.
Since the heart relies on aerobic respiration to supply its energy needs, cardiac muscle cells are richly supplied with mitochondria.
Our hearts beat about once every second of every day of our lives, or over 2.5 million times in an average life span. The only time the heart gets a rest is between beats.
Internal Structure of Human Heart:
Sketch of Internal Structure of Heart:
Structure of Human Heart
The heart can be thought of as two pumps sitting side by side each of which has an upper atrium and a lower ventricle. Thus there are two atria and two ventricles. Thus heart has total of 4 chambers.
The right side of the heart pumps ‘deoxygenated blood’ (blood low in oxygen) from the body into the lungs, where gas exchange takes place. In that process, carbon dioxide is lost to the air and oxygen is absorbed. This oxygen is almost all carried by the Red Blood Cells (RBC’s). The left side of the heart pumps oxygenated blood from the lungs to the rest of the body.
The opening of inferior vena cava in right atrium is guarded by Eustachian valve and that of coronary sinus by Thebesian valve. The atria (upper chambers) of the heart receive blood coming into the heart. They have thin walls, so allowing them to be filled easily. They pump the blood into the ventricles (lower chambers), thus filling them.
The ventricles pump blood out of the heart and the left ventricle has the thickest walls of the heart because it has to do most of the work to pump blood to all parts of the body. This is where the blood has the highest pressure.
Vertically dividing the two sides of the heart is a wall, known as the septum (interatrial and interventricular). The septum prevents the mixing of oxygenated (left side) and deoxygenated (right side) blood. It also carries electrical signals instructing the ventricles when to contract. These impulses pass down specially-modified muscle cells (Purkinje fibres), collectively known as the Bundle of His.
Each atrium opens into the ventricle of its side through atrioventricular aperture guarded by valves made up of connective tissue. The right atrioventricular valve has three flaps and called tricuspid valve. The left atrioventricular valve has two flaps and called bicuspid valve or mitral valve. Tricuspid valve and bicuspid valve are attached to papillary muscles of ventricles by chordae tendinae. Chordae tendinae prevents the valves from turning back into atria during contraction of ventricles.
The inner surface of ventricle is thrown into series of irregular muscular ridges called columnae carnae or trabeculae carnae.
Pulmonary trunk or aorta arises from right ventricle carrying deoxygenated blood to the lungs for oxygenation. Systemic aorta arises from left ventricle carrying oxygenated blood to different parts of body. Pulmonary trunk and systemic aorta are guarded by three semilunar valves.
Physiologically heart can be myogenic or neurogenic
In this heart the cardiac movement is initiated by cardiac muscles themselves. This type of heart is found in molluscus and vertebrates.
The cardiac movement is initiated by the nerves arising from the brain. This type of heart is found in invertebrates except moluscus.