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Anatomy of Heart

A heart is a hollow muscular organ roughly shaped like a cone. It measures about 10 centimeters in length, about the size of someone's fist.
A heart is a hollow muscular organ roughly shaped like a cone. It measures about 10 centimeters in length, about the size of someone's fist. In women, it weighs about 225 g, while in men, it weighs more.

The heart has its base in the mediastinum that is present in the thoracic cavity (It is present in between two lungs). There is a base and an apex above it, and it lies obliquely, slightly to the left of the right. Just below the nipple and a bit below the midline, the apex of the 5th intercostal space is located 9 cm to the left of the middle line. Toward the level of the 2nd rib, the base continues.

Structure
The heart walls

There are three layers of the wall of the heart. Firstly, there is pericardium, secondly myocardium, and lastly, there is endocardium.

1. Pericardium
The pericardium is the outermost layer of the heart and is composed of two sacs. There are two layers of the serous membrane lining each sac: the outer one is fibrous and the inner double-layered. On the right is the fibrous outer sac that adheres to the diaphragm and is connected to the inner tunica adventitia of the great vessels. As a fibrous, inelastic tissue, it prevents excessive heart expansion. The serous membrane's outermost layer, the fibrous sac, is lined by the parietal pericardium. The pericardium is a two-layer structure, one on top of the other, with the inner layer, called the epicardium, surrounding the heart. As with pleura, the membrane that encloses the lungs, there is also a structure of a double membrane that forms a closed compartment. Flat epithelial cells make up the serous membrane. With every heartbeat, there is a serous fluid that is secreted to facilitating flow between the space between the visceral and parietal layers. Parietal pericardium and visceral pericardium do not differ much but rather by their potential. The two layers of skin, serous fluid, and epidermis are closely bound together in health.

2. Myocardium
The myocardium is exclusive to the heart and is composed of specialized cardiac muscles. A striated muscle, like skeletal muscle, is not controlled by voluntary movements. There is either a nucleus or one or more branches in every fiber (cell). A very close connection exists between the stems and branches of each cell and the stems and branches of adjacent cells. On a microscopic scale, these can be seen as a thicker, darker line than the striations. A sheet of muscle rather than many individual cells gives the appearance that cardiac muscle is a sheet. There is no need to have a separate nerve supply for each fiber because they are end-to-end connected. The impulse propagates from cell to cell over the entire 'sheet' of muscle by way of branches and intercalated discs, thereby causing contraction. As a result of the myocardium's 'sheet' arrangement, atria and ventricles can contract efficiently under synchronized conditions.

Electrical signals for the heart are also transmitted through the myocardium's network of specialized conducting fibers. Towards the base, the myocardium becomes thinner as it becomes more elongated. In other words, the output of each chamber is reflected in how much work it exerts in pumping blood. The left ventricle gets the most work and is thickest here.

The fibrous tissue of the heart
Fine fibers run through all of the heart muscle of the myocardium to support it. Fibrous tissue makes up the heart's skeleton. Moreover, a ring of fibrous tissue separates the atria from the ventricles, which does not conduct electricity. Since electrical activity can only spread between the atrium and the ventricles through a fibrous ring that spans the atrium, waves of an electric signal can only propagate over the atrium to the ventricles.

3. Endocardium
The blood vessels in the heart are lined by this substance. Located inside the heart, this thin, smooth membrane offers smooth blood flow. A continuous endothelium encloses the vessel walls, on either side of which are flattened epithelial cells. By the septum, which is an endocarditis-covered myocardium partition that separates the left from the right side of the heart, the heart is divided into two parts. A septum protects the body after birth and prevents blood from crossing sides. By the atrioventricular valve, the upper atrium is divided into the upper and lower ventricles during conduction. An atrioventricular valve is composed of a double fold of endocardium reinforced by small amounts of fibrous tissue. Three flaps form the right atrioventricular valve (tricuspid valve) while two cusps form the left atrioventricular valve (mitral valve).

In the heart, blood circulates in one direction; it enters via the atria and leaves via the ventricles underneath. The ventricles and atria are connected by passive valves that open and close in response to changes in pressure across each chamber. An atrioventricular pressure differential opens the heart valves during an atrioventricular beat. To prevent blood pooling within ventricles, the pressure of the ventricles exceeds the pressure of the atria in atrioventricular systole (contraction). There are tendinous cords that prevent each valve from opening upward into the atria, called chordee tendineae. Papillary muscles are little flaps of myocardium covered in the endothelium that project into the ventricles.

In the right atrium are located the inferior and superior venae cavae, both of which are the largest veins of the body. In addition to being transported by the right atrioventricular valve, blood from the right ventricle helps oxygenate the pulmonary artery (the only artery in the body with deoxygenated blood). In the pulmonary artery, the three semilunar cusps protect the opening to the pulmonary valve. This valve prevents blood from flowing into the right ventricle by relaxing the ventricular muscle. When the pulmonary artery leaves the heart, the venous blood travels down the left and right pulmonary arteries to the lungs, where combustion occurs with carbon dioxide being exhaled and oxygen being taken in.

As oxygenated blood flows from the pulmonary veins as each lung has two, so, the blood flow to the left atrium, each lung has two. Aorta is the first artery of the body's circulatory system. As blood travels from the left ventricle to the left atrium, it is absorbed into these vessels in the meantime. Three semilunar cusps form the aortic valve that guards the opening of the aorta.

Through the pulmonary circulation, blood flows from the heart in the direction of right to the left side continuously. However, it is important to note that both the atria contract simultaneously, followed by both ventricles contracting simultaneously.

Atria has a thinner muscle layer compared to ventricles. According to the workload they have, this becomes normal. Atrioventricular valves push blood from the atria to the ventricles due to gravity while ventricles actively pump blood from the lungs throughout the body. An ascending pulmonary appendage leaves the right ventricle from the upper part while ascending aortic branches leave the left ventricle.

Sinoatrial node (SA node)
Atrium right contains a small number of specialized cells near the opening to the superior vena cava. It is the electrical instability of the sinoatrial cells that generate these regular impulses. They discharge (depolarize) regularly as a result of this instability, usually 60 to 80 times a minute. Almost immediately after repolarization (depolarisation) occurs, they will discharge again due to their instability, thereby setting the heart rate. The SA node of the heart normally sets the heart's rate due to its rapid discharge, making it known as the pacemaker. The responsibility for atrial contraction comes under the SA node.

Atrioventricular node (AV node)
There is the presence of atrioventricular valves behind these dense masses of neuromuscular cells. Atrioventricular nodes transmit electrical signals to the ventricles from the atria. A delay occurs here; the signal passes from the blood vessels into the ventricles after 0.1 of a second. By letting the atria contract before the ventricles start, the atria can finish contracting before they start contracting. Additionally, the AV node also performs the function of a pacemaker, taking over when the SA node fails, or when impulses from the atria cannot be transmitted. This node indeed has a slower intrinsic firing rate (40-60 bpm) than the SA node.

Atrioventricular bundle (AV bundle or bundle of His)
The responsibility for initiating specialized fibers is taken by these nodes. A fibrous ring separates atria and ventricles, and the AV bundle crosses through it before dissolving into right and left bundle branches, just above the flap of the ventricular septum. To pump blood into the lungs and aorta, electric impulses travel between the bundle of AV, bundle branches, and Purkinje fibers into the ventricular apex. The signals then traverse upward and outward, stimulating ventricular contraction.

Nerve supply to the heart
Furthermore, the heart is influenced by autonomic nerves depending on where they originate in the medulla oblongata (sympathetic and parasympathetic). In addition to the SA and AV nodes, the vagus nerves (parasympathetic) supply muscle tissues in the atrium. As a result of parasympathetic stimulation, the heartbeat is slowed and less force is generated. In addition to myocardial atria and ventricles, the sympathetic nerves supply the SA and AV nodes. As a response to sympathetic stimulation, both the rate and force of the heartbeat increase.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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