Structure of Human Heart
Heart is a muscular pumping organ that pumps out the blood into the blood vessels. Human heart is situated between the lungs, slightly tilted toward the left and above the diaphragm in the thoracic cavity. The heart is made of specialized type of muscle called the cardiac muscle.
The heart is enclosed in a double walled sac called pericardium. It contains lubricating pericardial fluid which reduces friction during heart beat and protects it from mechanical injuries.
The human heart is four chambered. The two upper thin walled chambers of the heart are called auricle or atria (sing: atrium) and two lower thick walled chambers are called ventricles. The chambers are separated by partition called septum. The septum between auricles and ventricles prevents the mixing of oxygenated and deoxygenated blood.
The two auricles are separated from each other by interatrial septum.The left atrium is smaller than the right atrium. The right atrium receives deoxygenated blood from different parts of the body through the main veins superior vena cava, inferior vena cava and coronary sinus. Pulmonary veins bring oxygenated blood to the left atrium from the lungs. The right and left auricles pump blood into the right and left ventricles respectively.
The ventricles form the lower part of the heart. The two ventricles are separated from each other by an interventricular septum. The left and right ventricles have thick walls because the ventricles have to pump out blood with force away from the heart. From the right ventricle arises the pulmonary trunk which bifurcates to form right and left pulmonary arteries. The right and left pulmonary arteries supply deoxygenated blood to the lungs of the respective side. The left ventricle is longer and narrower than the right ventricle. The walls are about three times thicker than the right ventricle. The left ventricle gives rise to aorta. The oxygenated blood is supplied by the aorta to various organs of the body. The coronary arteries supply blood to the heart.
Valves: The valves are the muscular flaps that regulate the flow of blood in a single direction and prevent back flow of blood. The heart contains three types of valves.
Right atrioventricular valve: It is located between the right auricle and right ventricle. It has three thin triangular leaf like flaps and therefore called tricuspid valve. The apices of the flaps are held in position by chordae tendinae arising from the muscular projection of the ventricle wall known as papillary muscles.
Left atrioventricular valve: It is located between the left auricle and left ventricle. It has two cusps and therefore called bicuspid or mitral valve.
Semilunar valves: The major arteries (pulmonary artery and aorta) which leave the heart have semilunar valves which prevent backward flow of blood into the ventricles. They are the pulmonary and aortic semilunar valves.
The blood circulates in our body as oxygenated and deoxygenated blood. The types of circulation are:
Circulation of oxygenated blood from the left ventricle of the heart to various organs of the body and return of deoxygenated blood to the right atrium. Aorta carries oxygenated blood to all the organs of the body.
ii. Pulmonary circulation: The path of pulmonary circulation starts in the right ventricle. Pulmonary artery arises from the right ventricle and reaches the lungs with deoxygenated blood. Pulmonary veins collect the oxygenated blood from the lungs and supplies it to the left atrium of the heart.
iii. Coronary circulation: The supply of blood to the heart muscles (cardiac muscles) is called as coronary circulation. Cardiac muscles receive oxygenated blood from coronary arteries that originate from the aortic arch. Deoxygenated blood from the cardiac muscles drains into the right atrium by the coronary sinuses.
When the blood circulates twice through the heart in one complete cycle it is called double circulation. In double circulation the oxygenated blood do not mix with the deoxygenated blood.
However, in some animals the oxygenated and deoxygenated blood are mixed and pass through the heart only once. This type of circulation is called single circulation. e.g., fishes, amphibians and certain reptiles.
One complete contraction (systole) and relaxation (diastole) of the atrium and ventricles of the heart constitute heartbeat. The heart normally beats 72 – 75 times per minute.
The human heart is myogenic in nature. Contraction is initiated by a specialized portion of the heart muscle, the sino-atrial (SA) node which is situated in the wall of the right atrium near the opening of the superior vena cava. The SA node is broader at the top and tapering below. It is made up of thin fibres.
Sino-atrial node acts as the ‘pacemaker’ of the heart because it is capable of initiating impulse which can stimulate the heart muscles to contract. The impulse from the sinoatrial node spreads as a wave of contraction over the right and left atrial wall pushing the blood through the atrioventricular valves into the ventricles. The wave of contraction from SA node reaches the atrioventricular (AV) node which is stimulated to emit an impulse of contraction spreading to the ventricular muscle via the atrioventricular bundle and the Purkinje fibres.
Pulse: When the heart beats the blood is forced into the arteries. The expansion of the artery every time the blood is forced into it is called pulse. It can be felt by placing the fingertip on the artery near the wrist. Normal pulse rate ranges from 70 – 90 / min.
The sequence of events occurring from the beginning to the completion of one heart beat is called cardiac cycle. During cardiac cycle blood flows through the chambers of the heart in a specific direction. Each cardiac cycle lasts about 0.8 second. The events during a single cardiac cycle involves
(a) Atrial systole: Contraction of auricles (0.1 sec)
(b) Ventricular systole: Contraction of ventricles (0.3 sec)
(c) Ventricular diastole: Relaxation of ventricles (0.4 sec)
The rhythmic closure and opening of the valves cause the sound of the heart.
The first sound LUBB is of longer duration and is produced by the closure of the tricuspid and bicuspid valves after the beginning of ventricular systole. The second sound DUPP is of a shorter duration and produced by the closure of semilunar valves at the end of ventricular systole.