Heart :
Human heart is situated almost in the middle of thoracic cavity in a space called mediastinum, between the two lungs. Heart is hollow, muscular, conical organ about the of one's fist with broad base and narrow apex tilted towards left. Heart is mesodermal in origin. In measures about 12 cm in length, 9 cm in breadth and weighs about 250 to 300 grams.
Pericardium - Heart is enclosed is double layered peritoneum called pericardium. It consists of two laters -
1. Fibrous pericardium - It is the outer layer made up of tough, inelastic fibrous connective tissue.
2. Serous pericardium - It is the inner layer made up of outer parietal layer and inner visceral layer. The parietal layer forms inner lining of fibrous pericardium whereas the visceral layer or epicardium adheres to heart forming its outer covering.
In between the parietal and visceral layers of serous pericardium is present a pericardial space filled with pericardial fluid.
Heart wall - The wall of heart is composed of three layers viz. outer epicardium , middle myocardium and inner endocardium.
1. Epicardium - is composed of single layer of flat epithelial cells called mesothelium.
2. Myocardium - is composed of cardiac muscle fibres responsible for contraction (systole) and relaxation (diastole) of heart.
3. Endocardium - is composed of single layer of flat epithelial cells called endothelium.
External structure of Heart :
Human heart consists of four chambers - two superior, small, thin walled receiving chambers called atria; and two inferior, large, thick, walked, distributing chambers called ventricles.
Externally a transverse groove is present between the atria and the ventricles called atrioventricular groove or coronary sulcus. The interventricular sulcus is present between the right and left ventricles. In these sulci, there are situated coronary arteries and coronary veins. The coronary arteries supply oxygenated blood to the heart while coronary veins collect deoxygenated blood from the heart. The coronary veins join to form coronary sinus which open into the right atrium.
Right atrium is larger in size than the left atrium.
From the right ventricle, arises pulmonary trunk, while from the left ventricle arises systemic aorta.
The pulmonary trunk and systemic aorta are connected by ligamentum arteriosum that represents remnant of ductus arteriosus of foetus.
 |
| Human heart- Ventral View |
 |
| Human heart- Dorsal View |
Internal structure if Heart :
Internally the heart is four chambered with two atria and two ventricles.
Atria :
The atria are two thin walled receiving chambers placed superiorly and separated from each other by interatrial septum. The right atrium receives deoxygenated blood from all over the body through superior vena cava, inferior vena cava and and from heart through coronary sinus. The opening of inferior vena cava is guarded by Eustachian valve while the opening of coronary sinus is guarded by Thebesian valve.
An oval depression, the fossa ovalis is present on the right side of interatrial septum. It represents the remnant of foramen ovale, am oval opening in the interatrial septum of the foetus.
The left atrium receives oxygenated blood from the lungs through four openings of pulmonary veins.
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 hence called tricuspid valve while left atrioventricular valve gas two flaps hence called bicuspid valve or mitral valve. These valves are attached to papillary muscles of ventricles by chordate tendinae. The chordate tendinae prevent the valves from turning back into the atria during the contraction of ventricles.
 |
| Human heart Internal Structure |
Ventricles :
The ventricles are two thick walled distributing chambers placed inferiorly and separated from each other by interventricular septum. Left ventricle has thickest wall as it has to pump blood to all parts of the body. The inner surface of the ventricle is thrown into a series of irregular muscular ridges called columnae carnae or trabeculae carnae.
Pulmonary trunk or aorta arises from the right, ventricle carrying deoxygenated blood to lungs for oxygenation. Systemic aorta arises from left ventricle carrying oxygenated blood to all parts of the body.
Pulmonary aorta and systemic aorta has three semilunar valves at the base which prevent backward flow of blood during ventricular diastole.
Pumping action of Heart :
The heart acts as a pumping organ. The rhythmic contraction (systole) and relaxation (diastole) of heart is called heart beat. The heart beats about 72 times per minute and pumps about 5 litres of blood per minute.
Conducting system of heart : Human heart is myogenic (myo-muscle, genic-originating from). The heart beat originates in modified cardiac muscles called Sinoatrial node (SA node) which lies in the wall of right atrium near the opening of superior vena cava. The SA node is called pace maker because it has power of generation of wave of contraction. The wave of contraction or cardiac impulse generated by SA node is contracted by cardiac muscle fibres to both the atria causing their contraction (atrial systole).
The atrioventricular node (AV node) is located in the wall of right atrium near the opening of coronary sinus receives the wave of contraction generated by SA node through internodal pathways.
 |
| Conducting System of heart |
Bundle of His - arises from AV node and divides into right and left bundle branches located in the interventricular septum. The bundle branches give rise to Purkinje fibres which penetrate into myocardium of ventricles. The bundle of His and Purkinje fibres conduct the wave of contraction from AV node to myocardium of ventricles causing their contraction (ventricular systole).
Working of heart (Cardiac cycle) : The events associated with one heart beat is called cardiac cycle. It lasts for 0.8 seconds. Each heart beat includes atrial systole, ventricular systole and joint diastole.
Atrial systole : During atrial systole, the deoxygenated blood from the right atrium enters the right ventricle through atrioventricular aperture, the back flow is prevented by tricuspid valve. The oxygenated blood from left atrium enters the left ventricle through atrioventricular aperture, the back flow is prevented by bicuspid valve. The atrial systole lasts for 0.1 second.
Ventricular systole : during ventricular systole, the deoxygenated blood from the right ventricle enters the pulmonary trunk, which carries blood to lungs for oxygenation. The oxygenated blood from the left ventricle enters the aorta, which carries blood to all parts of the body. The back flow of blood is prevented by semilunar valves. The ventricular systole lasts for 0.3 seconds.
Joint diastole or complete cardiac diastole : During joint diastole both atria and ventricles undergo relaxation. The right atrium receives deoxygenated blood from all parts of the body through superior vena cava, inferior vena cava and coronary sinus. The left atrium receives oxygenated blood from the lungs through two pairs of pulmonary veins. The joint diastole lasts for 0.4 seconds.
Regulation of Cardiac Activity :
Normal activities of the heart are regulated by specialized muscles (auto regulated), hence the heart is called myogenic.
The cardiovascular centre lies in the medulla oblongata of the brain. Sino auricular node receives sympathetic and para sympathetic nerves which secretes adrenaline and acetylecholine respectively. Adrenaline stimulates and increases heart beat whereas acetylecholine decreases the heart rate.
During inspiration heart rate increases and during expiration it falls. This phenomenon is known as sinus arrhythmias.