The circulation of blood in closed blood vessels was discovered by William Harvey, an English physiologist.
The heart is a hollow, fibromuscular conical pyramidal organ made up of cardiac muscle fibers measuring about 12 cm in length, 9 cm in breadth, and thickness of about 6 cm.
It is the pumping organ of the circulatory system.
It is dark red in color and lies towards the left in the thoracic cavity between the two lungs.
The weight is about 300 gms in adult males and about 250 gms in adult females.
The heart apex rests obliquely upon the superior surface of the diaphragm. The heart is enveloped by a double-layered dense fibrous sac called the pericardium. In between the two layers, there is a very narrow space called the pericardial cavity which is filled with pericardial fluid.
Pericardial Fluid keeps the surface of the heart moist and prevents it from friction between heart walls and surrounding tissues.
It has four chambers, the right and left auricles (or atria), and the right and left ventricles.
The upper chamber, the auricles, which are relatively thin-walled, receive blood from the veins. Oxygenated blood from the lungs enters the left atrium via the pulmonary veins and deoxygenated blood from the body enters the right atrium from the vena cavae (i.e. precava and postcava).
Both the auricles open into their respective ventricles through auriculo-ventricular apertures.
These apertures are guarded by a tricuspid valve on the right side and bicuspid valve on the left side.
The pointed ends of the flaps of these valves are attached to the inner walls of the ventricles by tendon-like cords called chordae tendinae and papillary muscle.
Relaxation of the ventricular muscle allows the lower chambers, the ventricles, to expand and fill with blood which flows in from atria and veins.
Simultaneous contraction of both atria forces the blood they contain into the corresponding ventricles and about 0.1 seconds later, both ventricles contract simultaneously, expelling their blood into the arteries.
Both ventricles have thick muscular walls but those of left are thicker having to pump blood all around the entire body via the aorta.
The right ventricle pumps blood to the lungs through the pulmonary arteries.
The heart beats rhythmically throughout one’s life. The periodic contraction and relaxation of the heart is called the heartbeat.
The normal human heartbeat is 72 times a minute.
The total volume of blood in the system is about 5 to 6 liters.
The human heart pumps about 5 liters of blood per minute. This can increase to about 20 liters per minute during strenuous exercise.
Doctors use a stethoscope to record the heartbeat.
The force that blood exerts against the wall of a vessel is called blood pressure. This pressure is much greater in arteries than in veins.
The pressure of blood inside the artery during ventricular systole (contraction) is called systolic pressure and pressure in an artery during ventricular diastole (relaxation) is called diastolic pressure.
The normal systolic pressure is about 120 mm of Hg and diastolic pressure is 80 mm of Hg.
Blood pressure is measured with an instrument called a sphygmomanometer.
Electrocardiogram (ECG) is a graphic recording of an electric potential produced by cardiac impulse during its origin at Sinoatrial Node (SA node), transmission to Atrioventricular Node (AV node), and from there to Purkinje fibers.
The instrument employed for ECG is called an electrocardiograph. Einthoven is called the father of electrocardiography.
In fishes, the heart is 2-chambered. It has a sinus venosus, a single atrium, and a ventricle. It pumps only deoxygenated blood which is oxygenated by the gills and supplied to the body parts from where deoxygenated blood is returned to the heart.
Amphibians and the reptiles (except crocodiles) have a 3-chambered heart with two atria and a single ventricle. The left atrium receives oxygenated blood from the gills/ lungs/ skin and the right atrium gets the deoxygenated blood from the other body parts. They get mixed up in the Single ventricle which pumps out mixed blood (incomplete double circulation).