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The Human Heart- Everything You Need to Know

Published on Oct 06, 2022   -  6 min read


Read this article to know about the characteristics, properties, functions, and regulations of the human heart in detail.


The heart is situated in the thoracic cavity between the two lungs, in a position protected by the sternum. The lower border of the heart, known as the apex, points towards the left of the thorax. The heart is tightly contained in a membranous sac, the pericardium. The pericardial fluid secreted by the pericardium reduces friction between the beating heart and the surrounding stationary tissue. The outer part of the pericardium is made up of non-distensible white fibrous tissue, which gives the pericardium its strongly inelastic nature and thus prevents the heart from being overfilled with blood and consequently overstretched.

The wall of the heart is made up of three distinct layers.

The cardiac muscle consists of many branching myofibrils, each of which contains one or two centrally placed nuclei and many large mitochondria and is surrounded by a sarcolemma. Adjacent fibers are connected by structures known as intercalated discs. The discs have gap junctions where adjacent fibers are closer together. These junctions form areas of low electrical resistance and thus both transmit the force of contraction and also allow the rapid and uniform movement of electrical excitation through the myocardium and thus ensure a uniform contraction.

Cardiac muscle is richly supplied with blood through a very dense capillary network originating from the left and right coronary arteries. After passing through the capillary network, the blood returns mainly through a series of cardiac veins that drain into the right atrium through a channel known as the coronary sinus. Some smaller veins from the right ventricle drain directly into the right atrium. The myocardium does not possess any neurons.

What Is the Structure of the Heart?

The interior of the heart is divided into four chambers: two upper thin-walled atria and two lower thick-walled ventricles. The right side of the heart is completely separated from the left side by the interventricular and interauricular septa. The atria and ventricles are separated by the atrioventricular septum.

The right atrium receives deoxygenated blood from the head and shoulders through the superior or anterior vena cava and from the rest of the body through the inferior or posterior vena cava and pumps it to the right ventricle. The right atrioventricular opening is guarded by the tricuspid valve, which consists of three cup-shaped flaps. The right ventricle pumps the deoxygenated blood through the pulmonary arteries to the lungs. Blood oxygenated in the lungs returns to the left atrium through the two pulmonary veins and is then pumped into the left ventricle.

The left atrioventricular opening is guarded by the bicuspid or mitral valve, which consists of two cup-shaped flaps. It prevents the backflow of blood from the left ventricle to the left atrium during ventricular systole. Attached to the ventricular side of the flaps of the bicuspid and tricuspid valves are tough, fibrous, non-elastic cords called chordae tendineae, which are anchored to the papillary muscles on the ventricular wall.

The bicuspid and tricuspid valves are pushed open by the pressure of the blood when the atria contract and are shut tightly by the pressure of the blood when the ventricles contract, thus allowing blood to flow from the atria to the ventricles and preventing backflow from the ventricles to the atria during ventricular systole. The papillary muscles contract simultaneously with the ventricular myocardium, tightening the chordae tendineae and preventing the valve flaps from turning inside out.

The left ventricle pumps the oxygenated blood to all body parts through the aorta. The opening of the right ventricle into the pulmonary artery and that of the left ventricle into the aorta are guarded by pocket-like semilunar valves, which close and prevent the backflow of blood into the ventricles during ventricular diastole.

The contraction of the atria need not be strong as they have to send the blood through a very short distance into the ventricles, and therefore the atrial myocardium is thin. Since the left ventricle pumps blood through the systemic circuit, its force of contraction must be very strong, and therefore the left ventricular myocardium is very thick. The right atrium supplies the pulmonary circuit, which is shorter than the systemic circuit. Therefore, the myocardium of the right ventricle is thicker than that of the atria but not as thick as that of the left ventricle. Therefore, the pressure of blood in the aorta is much higher than that in the pulmonary artery.

What Is the Cardiac Cycle?

The rhythmic sequence of events that occur each time the heart beats is known as the cardiac cycle. At rest, the heart beats about 72 times per minute, so each cycle lasts about 0.83 seconds.

  • Atrial Systole (0.0 – 0.1seconds): The cardiac cycle begins with the contraction of the atria, during which time the ventricles are in diastole. In the beginning, the valves in the venae cavae and in the pulmonary veins are open, and the atrioventricular valves are shut; atrial and ventricular blood pressures are low; the walls of the atria are distended due to being filled with blood. As the atrial myocardium contracts, it squeezes inward on the blood in the atria, decreasing the volume and increasing the pressure. These forces open the atrioventricular valves, allowing the blood to flow into the ventricles. During this process, the arterial blood pressure falls due to the atria emptying their contents. Contraction of the atria shuts the valves in the venae cavae and in the pulmonary veins, preventing the backflow of blood from the atria to these veins.

  • Ventricular Systole (0.1 – 0.4 seconds): The blood-filled ventricles now contract very forcefully, resulting in the ventricular blood pressure reaching very high. The pressure in the left ventricle is around four times that in the right ventricle because the left ventricular myocardium is much thicker than the right ventricle and thus creates more force. The high pressure in the ventricles slam shuts the atrioventricular valves, which are prevented from turning inside out by the contraction of the chordae tendineae and papillary muscles, and forces open the aortic and pulmonary semilunar valves and pump the blood into these arteries.

The ventricular blood pressure rises initially due to the decrease in volume brought about by the squeezing action of systole, but it falls as the ventricles empty their contents into the aorta and the pulmonary artery. Consequently, the blood pressure in the aorta and pulmonary artery rises. The right ventricle pumps blood into the pulmonary artery to supply the pulmonary circuit, and the left ventricle pumps blood into the aorta to supply the systemic circuit. The sound of the closing of the atrioventricular valves constitutes the lubb sound of the heartbeat. During ventricular systole, the atria are in diastole and are getting filled with blood.

  • Complete Cardiac Diastole (0.4 – 0.8 seconds): The ventricles now relax, along with the already relaxed atria, and ventricular blood pressure falls rapidly to zero. Thus, blood pressures in the aorta and in the pulmonary artery are now higher than that in the left and right ventricles, respectively. This causes the semilunar valves in the aorta and in the pulmonary artery to be pushed shut, preventing the backflow of blood from these arteries to the ventricles and causing the dup sound of a heartbeat. The closure of the valves, along with the elastic recoil of the arterial walls, helps to maintain relatively high blood pressure in the aorta and pulmonary arteries throughout the cardiac cycle. The left atrium receives blood through the pulmonary veins, and the right atrium receives blood from the venae cavae, causing the atria to distend and blood pressure in the atria to rise slightly gradually. The atrioventricular valves remain closed.

What Is Myogenic Control of the Human Heart?

Muscles contract when they are subjected to an action potential. Cardiac muscle is myogenic rather than neurogenic, which means that the action potential originates in the myocardium itself and not in any nerve. The conducting system of the heart consists of specialized plexuses of cardiac muscle cells that constitute the sinoatrial node (SAN), the atrioventricular node (AVN), and the bundle of His and its branches.

The stimulation for contraction of the heart originates in the SAN, which is situated near the opening of the superior vena cava. The cells of the SAN maintain a differential ionic concentration of -90 milliVolts across their membranes. These cells have a permanently high sodium conductance, enabling Na+ions (sodium ions) to diffuse into them continually. This produces a depolarization which leads to a propagated action potential being set up in the cells adjacent to the SAN. As this wave of excitation passes across the muscle fibers of the heart, it causes them to contract. The atrial muscle fibers are completely separated from the ventricular muscle fibers by the atrioventricular septum, which cannot conduct the electrical impulse. The impulse reaches the ventricles through the AVN, which is situated in the wall of the right atrium near the opening of the coronary sinus. The AVN picks up the impulse and transmits it to the ventricles through the branches of the bundle of His, causing the ventricular myocardium to contract. This provides a delay of about 0.15 seconds between atrial systole and ventricular systole, allowing the ventricles to be filled up with blood from the atria before starting to contract.

The bundle of His and its branches consist of modified cardiac muscle known as Purkinje tissues. The left and right branches of the bundle of His run down the atrioventricular septum to the apex of the heart and then radiate upwards throughout the ventricles. The ventricles contract simultaneously, and the wave of ventricular contraction begins at the apex and spreads upwards, allowing the blood to be squeezed more efficiently up into the arteries than would be possible if the whole ventricular wall were to contract at once.


The heart is the main organ of the cardiovascular system. It also works in coordination with other systems of the body to maintain the heart rate and blood pressure of an individual. It is responsible for the purification of the blood. Family history, personal medical history, and an individual’s lifestyle are some of the factors that affect how well the heart works in an individual.


Last reviewed at:
06 Oct 2022  -  6 min read




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