Anatomy of the human heart

Introduction to heart 

Heart is a wonderful organ that works tirelessly from birth to death. It is a hollow muscular organ having the size of one`s own fist. The heart is composed of three layers. The inner layer is called endocardium made up of epithelium, the middle layer is called myocardium which is made up of muscle fibres responsible for the pumping action. The outer layer of the heart is called the epicardium.  The heart is surrounded by a thin, fibrous sac called the pericardium. 


Table Of Contents

1. Introduction to heart 

2. Anatomical location of the heart

3. Heart Chambers

4. Heart Valves

5. Blood vessels of the heart

6. Cardiac Muscles       

7. The Functions of Heart

8. Heart physiology

9. Human blood circulation



Human heart- image from US national library

Anatomy of the human heart( Image from US  National Library)​​


Anatomical location of the heart

The human heart is situated in the chest space called mediastinum (within the thoracic cavity or chest cavity). This space lies between the lungs, behind and slightly towards the left side of the sternum (or breastbone). The heart is tilted towards left and very rarely there may be a slight change in its position but in most cases, the majority of the parts of the heart are situated on the left side of the chest, and the remaining part is leaned towards the right side.


Heart Chambers

The four chambers of the heart constitute the right- and left-sided pumping systems. The right side of the heart is made up of the right atrium and right ventricle which distributes deoxygenated blood to the lungs via the pulmonary artery. The left side of the heart, composed of the left atrium and left ventricle distributes oxygenated blood to the remainder of the body via the aorta (systemic circulation). The left atrium receives oxygenated blood from the pulmonary circulation via the pulmonary veins whereas the right atrium receives deoxygenated (impure) blood from body parts. There are different levels of pressure in different parts of the heart. This depends upon the blood flow direction and purpose of the flow. In general, arteries will have high pressure because they have to push the blood to the whole body against gravity and the veins will have lesser pressure because they only receive the blood from body parts. The varying thicknesses of the atrial and ventricular walls relate to the workload required by each chamber and this is the reason there is a variation in the blood pressures at different points of the heart (see diagram). The atria are thin-walled because blood returning to these chambers generates low pressures. In contrast, the ventricular walls are thicker because they generate greater pressures during contraction. The right ventricle is relatively less thick than the left ventricle.


Heart Valves

There are 4 valves in the heart. A valve acts as a gate valve that allows the blood to move only in one direction and doesn’t allow the backflow. There are two types of valves namely atrioventricular valves and semilunar valvesatrioventricular separate the atria from the ventricles are termed. These have 3  cusps or leaflets, hence they are called tricuspid valves. On the other hand, the mitral, or bicuspid (two cusps) valve, lies between the left atrium and the left ventricle (see Fig). The two semilunar valves are composed of three half-moon-like leaflets. The valve between the right ventricle and the pulmonary artery is called the pulmonic valve; the valve between the left ventricle and the aorta is called the aortic valve.


Blood vessels of the heart

Coronary Arteries and coronary veins

Coronary arteries are those that supply blood to the heart itself because the heart needs a huge amount of oxygen as it always keeps beating tirelessly. The left and right coronary arteries and their branches supply arterial blood to the heart. These arteries come out from the aorta just above the aortic valve leaflets. Any issues with these blood vessels are known as coronary artery disease (CAD) which can develop a heart attack.


Cardiac Muscles       

The myocardium is composed of specialized muscle tissue known as cardiac muscles. Microscopically, myocardial muscle resembles striated (skeletal) muscle, that work on a voluntary control. Functionally, however, myocardial muscle resembles smooth muscle because its contraction is involuntary. The sequential pattern of contraction and relaxation of individual muscle fibres ensure the rhythmic behaviour of the myocardium as a whole and enables it to function as an effective pump.


The Functions of Heart

The function of the heart in any organism is to maintain a constant flow of blood throughout the body. The heart replenishes oxygen and circulates nutrients among all the cells and tissues. Also, because the human heart is a homologous organ, it functions no differently from any other vertebrates that possess a heart. Following are the main functions of the heart:

1. One of the primary functions of the human heart is to pump blood throughout the body.

2. Blood delivers oxygen, hormones, glucose and other components to various parts of the body, including the human heart.

3. The heart also ensure that adequate blood pressure is maintained in the body

4. There are two types of circulation within the body, namely pulmonary circulation and systemic circulation.


Heart physiology

Herat function is regulated by electrical activity called impulse mechanism. Electrical impulses are generated by nodes located at different locations within the chambers of the heart. The most important one is the sinoatrial (SA) node, located at the junction of the superior vena cava and the right atrium. The SA node has a firing rate of 60 to 100 impulses per minute, but the rate can change in response to the metabolic demands of the body. The AV node (located in the right atrial wall near the tricuspid valve) consists of another group of specialized muscle cells similar to those of the SA node. The AV node coordinates the incoming electrical impulses from the atria and, after a slight delay (allowing the atria time to contract and complete ventricular filling), relays the impulse to the ventricles. This impulse is then conducted through a bundle of specialized conduction cells (bundle of His) that travel in the septum separating the left and right ventricles. The bundle of His divides into the right bundle branch (conducting impulses to the right ventricle) and the left bundle branch (conducting impulses to the left ventricle). To transmit impulses to the largest chamber of the heart, the left bundle branch gets bifurcated into the left anterior and left posterior bundle branches. Impulses travel through the bundle branches to reach the terminal point in the conduction system, called the Purkinje fibres. This is the point at which the myocardial cells are stimulated, causing ventricular contraction.


Human blood circulation

The flow diagram below illustrates the blood flow for a healthy and normal person. There are 2 channels of blood flow. Oxygenated blood flow( pure blood) and the deoxygenated blood flow( impure blood). When the ventricles of the heart contracting, pureblood leaves from the left ventricle and blood gets into body cells. After the utilization of oxygen, body cells,  release carbon dioxide into the blood and this blood will become deoxygenated at this stage. The deoxygenated blood from the body pushed into the right atrium and from the right atrium, blood gets into the right ventricle. From the right ventricle, blood moves into the lungs for purification. After the purification, the pureblood enters the heart through pulmonary veins. Blue colour here indicated impure blood( deoxygenated blood) and the red colour indicates ( oxygenated blood).

Blood flow  of the human heart-Image from Vecteezy




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