The Human Heart

Overview 

The human heart is a four-chambered muscular organ. It is a mesodermally derived organ of the human circulatory system, also called the blood vascular system. The heart pumps blood throughout the body through the network of closed branched blood vessels.  

The heart weighs between 7 and 15 ounces (200g to 425g) and is a little larger than the size of your fist.

Functions of the human heart: 

• It acts as the major organ through which double circulation occurs in the human body 
• It supplies oxygen and nutrients to the tissues and removes carbon dioxide 

Location of the human heart in the human body

The human heart is located in the thoracic cavity, slightly towards the left of the sternum and in between the two lungs. The size of the human heart is that of a clenched fist. 

Anatomy of the human heart 

• The outer protective layer of the heart is a double-layer membranous bag called the pericardium, filled with pericardial fluid
• Out of the heart’s four chambers, two upper, relatively smaller sections are called atria, and the two lower and larger chambers are called ventricles
• The two atria are separated by a thin muscular septum called the interatrial septum. A thick-walled interventricular septum separates the two ventricles
• The atrioventricular septum, a thick fibrous tissue, separates the atrium and ventricle on the same sides. The atrioventricular septa have openings through which the chambers on the same side are connected
• A tricuspid valve with three muscular cusps or flaps regulates the opening and closing of the right atrioventricular septum
• A bicuspid or mitral valve regulates the opening and closing of the left atrioventricular septum
• The aorta and the pulmonary artery of the human heart are provided with the semilunar valves
• The valves assure the unidirectional flow of blood in the heart, i.e., from the atria to the ventricles and from ventricles to the aorta and the pulmonary artery thereby, preventing backward flow of blood 
• There is a specialised nodal tissue, a cardiac musculature distributed in the heart. A patch of this tissue called a sinoatrial node (SAN) is present in the upper right corner of the right atrium. Another patch of the nodal tissue called the atrioventricular node (AVN) is located in the lower-left corner of the right atrium

• Atrioventricular bundles (AV bundles) are nodal fibres originating from the AVN. AV bundles emerge on top of the interventricular septum via the atrioventricular septa and further divide into left and right bundles
• These left and right bundles give rise to minute Purkinje fibres, distributed throughout the ventricular musculature
• The nodal musculature is auto-excitable, i.e., it generates action potential without external stimuli. The number of action potentials generated differs in different parts of the nodal system. The maximum action potential is generated by SAN (Sinu atrial node). It initiates and maintains the rhythmic contraction and relaxation of the human heart; that is why SAN is also called a pacemaker

Physiology of the human heart 

• Our heart normally beats 70-75 times in a minute (average 72 beats per minute) 
• When the heart chambers are in a relaxed state, it’s called diastole, and when they are in contraction, it’s called systole

 How does the cardiac cycle work?

Joint diastole:

When all four chambers of the human heart are in a relaxed state, it’s called joint diastole. During joint diastole, the tricuspid and the bicuspid valves are open. As a result, blood flows from the pulmonary veins to the left ventricle through the left atria and from the vena cava to the right ventricle through the right atria. The semilunar valves are closed at this stage.

Atrial systole:

During atrial systole, the SAN generates action potential to stimulate the atria for simultaneous contraction. The atrial systole increases the blood flow into the ventricles by 30 percent. 

Ventricular systole:

The AVN and AV bundles generate an action potential. Bundle of His transmits the action potential to the entire ventricular musculature. This causes muscles of the ventricle to contract (ventricular systole). Ventricular systole also stops the backward flow of blood into the atria as the ventricular pressure increases, causing the closure of bicuspid and tricuspid valves. 

Atrial diastole:

The atrial diastole (relaxation of atria) coincides with the ventricular systole. 

Ventricular diastole:

As the ventricular pressure keeps on increasing, the semilunar valves open up, allowing blood from the ventricles to reach the circulatory pathways via the pulmonary artery (right) and the aorta (left). This leads to relaxation of the ventricles (ventricular diastole). The ventricular pressure falls, causing the semilunar valves to close and preventing the backflow of blood in the ventricles. The ventricles and atria are now in a joint diastole state again. These sequential steps continue, and the cycle known as the cardiac process is repeated. 

• The human heart beats 72 times per minute, i.e., 72 cardiac cycles happen per minute
• Therefore the duration of a single cardiac cycle is 0.8 seconds
• Stroke volume: The volume of blood (70mL) pumped by each ventricle during a cardiac cycle is called stroke volume 
• Cardiac output: Stroke volume x heart rate = 70 x 72⋍5000 ml or 5 litres in a healthy individual. Cardiac output is the volume of the blood pumped by each ventricle per minute
• Two major sounds are produced during a cardiac cycle, which can be heard using a stethoscope. These two sounds are important from a clinical diagnostic perspective

Lub sound: This is the first sound heard when the tricuspid and the bicuspid valves close.

Dub sound: It is the second sound heard on the closure of the semilunar valves.

Double circulation in humans

The dual circulation in the human body consists of pulmonary circulation and systemic circulation. Both these circulations occur simultaneously. 

Pulmonary circulation:

The blood flows in the human body through arteries and veins. The right ventricle pumps the deoxygenated blood in the pulmonary artery, and the left ventricle pumps oxygenated blood into the aorta. The deoxygenated blood is passed on to the lungs by the pulmonary artery; the pulmonary veins carry the oxygenated blood into the left atrium. This pathway from the right ventricle to the left atrium is called the pulmonary circulation. 

Systemic circulation:

The oxygenated blood from the aorta is passed on to the body tissues via a system of arteries, arterioles, and capillaries. The deoxygenated blood is collected from tissues by the method of veins, venula, and vena cava and emptied into the right atrium. 

The systemic circulation provides oxygen, nutrients, and other essential substances to the body tissues and eliminates carbon dioxide and other harmful substances from the body tissues. 

High blood pressure (Hypertension):

Hypertension occurs when the pressure rises than the normal blood pressure—120/80 mm Hg. 120mm Hg is the normal systolic pressure, while 80 mm Hg is the normal diastolic pressure. If a person’s blood pressure differs from the range 140/90 mm Hg, they have hypertension. Prolonged hypertension can lead to heart attack and affect kidneys and the brain.

Coronary artery disease (CAD):

Also called atherosclerosis, it is caused when calcium, fats, cholesterol, and fibrous tissues get deposited in the coronary artery (blood vessels that supply blood to the heart muscles) and make the streets narrower. 

Angina: Also called ‘Angina Pectoris’, angina is a symptom that indicates acute chest pain. It occurs due to a lack of oxygen supply to the heart muscles.

Heart failure: Heart failure occurs when the heart stops pumping enough blood in the body. It is also termed congestive heart failure in case lungs get congested. Heart failure is not the same as cardiac arrest.

Cardiac arrest: When the heart stops beating, the situation can be called a cardiac arrest.

Heart attack: when the heart muscle suddenly gets damaged due to inadequate blood supply.

Conclusion

The heart is a very important organ of the human body. It comprises four chambers and a very intricate network of blood vessels that carry blood to and from the human body. It is responsible for the flow of blood and eventually of nutrients and respiratory gases through the body.