Anatomy of the human heart

Introduction to heart 

Heart is a wonderful organ works tirelessly from birth till the death. The human heart is a hollow muscular organ measures about one`s own fist. It has 3 three layers;  the inner layer is called endocardium which is made up of endothelium ( a type of epithelium), the middle layer is the muscular myocardium (responsible for the pumping action)  and the outermost layer of the heart is the pericardium. Pericardium is further divided into an outer parietal layer and the inner visceral layer. The space between the parietal and visceral layer forms pericardial space, filled with pericardial fluid. A thick and strong outermost layer of tissues surrounding the pericardium is called epicardium. Epicardium protects the chest cavity from external forces.

 

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

 

Anatomical location of the heart

The human heart lies within the mediastinum of the thoracic cavity slightly tilted towards left side. The space between the left and right  lung is called the mediastinal space within which our heart is leaned. Precisely, it is positioned a bit posterior and slightly towards the left side of the sternum (or breastbone). From the basal portion of the  heart (generally called as apex), the diaphragm helps to hold the heart in place and it also helps to separate the thoracic cavity from the abdominal cavity.

 

Chambers of heart

Human heart is 4 chambered structure constituting the 2 right and 2left chambers. 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 constitutes the left atrium and left ventricle help in distributing 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 depending upon the direction of blood flow and the type of blood it carries. In general, arteries demonstrates high pressure as they have to push the blood to the whole body against gravity whereas the veins feel lesser pressure because they only receive the blood from body parts in a passive manner. The varying degree of thicknesses of the atrial and ventricular walls  is correlated with the workload required by each chamber. In general, atria are thin-walled because they only receive the deoxygenated blood as well as blood returning from the lungs after oxygenation. In contrast, the ventricular walls are thicker because they have to generate greater pressures in order to push the blood to each cell of our body. However, right ventricle is relatively thinner than the left ventricle.

 

Valves of the heart

A valve acts as a gate valve that allows the blood to move only in one direction instead of backflow. There are 4 valves in the heart, and they  are classified into two types ,i,e atrioventricular valves and semilunar valves. Atrioventricular valves separate the right atria from its corresponding ventricle. AV valves 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. Moreover, there are 2 semilunar valves made up of three half-moon-like leaflets, and they are aortic and pulmonic valves.  The valve between the right ventricle and the pulmonary artery is called the pulmonic valve;similarly,  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 supplies blood to the different cells of the heart. Our heart needs a huge amount of oxygen as it has to beat tirelessly till the death. Coronary arteries are divided into left and right coronary arteries which gets deeply branched to supply arterial blood to each cell of the heart. The coronary arteries are branched out from the aorta just above the aortic valve leaflets. The inability of the coronary arteries to constantly supply blood to the heart is called heart attack which is fatal and a medical emergency.

 

Cardiac Muscles       

Cardiac muscles are generally known as the myocardium. Myocardium is made up of specialized muscle tissue known as cardiac muscles. Myocardial muscles resembles striated (skeletal) muscle however, unlike striated muscles, cardiac muscles are involuntary in nature. The sequential pattern of contraction and relaxation of individual muscle fibres of heart ensure the rhythmic behaviour of the myocardium. This can make certain that the whole heart functions as an effective pump.

 

Functions of Heart

The function of the heart in any organism is to maintain a constant flow of blood throughout the body. The heart helps to exchange gases and nutrients to each cell of our body. The 4 main functions of the heart are:

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.

 

Physiology of heart

Our heart beats are regulated by the electrical activity called impulse mechanism. Electrical impulses are generated by the nodes located at different locations 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 called bundle of His. Bundle of His  travel in between the septum separating the left and right ventricles and it divides itself 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, pure blood 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 pure blood enters the heart through pulmonary veins. In the flow diagram below, the blue colour indicated impure blood(deoxygenated blood) and the red colour indicates ( oxygenated blood).

 

Blood flow  of the human heart-Image from Vecteezy

 

 

Read More

1 .    

Cardiac cycle          

2.

Cardiac muscles

3.

Blood vessels in human beings

4.

Blood and circulatory system

 

 

Check your understanding

  1. Name the thickest chamber of heart.

  2. What is myocardium, explain?

  3. What are the functions of SA node?

  4. Where is semilunar valve present ?

  5. What is pericardial space?

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