Anatomy Fisiology Of The Heart

A. The Heart

In the embryo,the heart begin to beat at 4 weeks of age, even before its nerve supply has been estabilished. If a person lives to be 80 years old, your heart continues to beat an average of 100,000 time a day, every day for each of those 80 years. Imagine trying to squeeze a tennis ball 70 times a minute. After few minutes your arm muscles would begin to tire, then imagine increasing your squeezing rate to 120 times a minute. Most of us could not keep that up very long, but that is what the heart does during exercise. So, A healthy human heart can increase its rate and force of contraction to meet the body`s need for more oxygen.

The primary function of the heart is as single pump for pumping the blood through the arteries, capillaries,and veins. As you known the blood transport oxygen,nutrients and has other important function as well, also the heart is the pump that keeps blood circulation properly.

Your heart is positioned obliquely between the lungs in the space of mediastinum. About twothirds of its bulk lies to the left side of the midline of the body. It is shaped like a blunt cone. It Is about the size of a closed fist. It is approximately 5 inches long (12 cm), 3.5 inches wide at its broadest point (9 cm), and 2.5 inches thick (6 cm).

B. Structure Of The Heart

The heart is composed of 3 layers of tissue (see picture 1) :
• Pericardium
• Myocardium
• Endocardium.













(pic : 1)



Pericardium

The pericardium is made up of two sacs. The outer sac consists of fibrous tissue and the inner of a continuous double layer of serous membrane. The outer fibrous sac is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents overdistension of the heart.

The outer layer of the serous membrane is the parietal pericardium or lines the fibrous sac. The inner layer is the visceral pericardium, or epicardium, which is continuous with the parietal pericardium, is adherent to the heart muscle. A similar arrangement of a double membrane
forming a closed space is seen also with the pleura, the membrane enclosing the lungs.

The serous membrane consists of flattened epithelialcells. It is the secretes serous fluid into the space between the visceral and parietal layers which allows smooth movement between them when the heart beats. The space between the parietal and visceral pericardium is only a potential space. In health the two layers are in close association, with only the thin film of serous fluid between them. It is named pericardium serous (abaut 20-50 ml).


Myocardium

The myocardium is composed of specialised cardiac muscle found only in the heart. It is notunder voluntary control but, like skeletal muscle, cross-stripes are seen on microscopic examination. Each fibre (cell) has a nucleus and one or more branches. The ends of the cells and their branches are in very close contact with the ends and branches of adjacent cells.

Microscopically these 'joints', or intercalated discs, can be seen as thicker and darker lines than the ordinary cross-stripes. This arrangement will give heart muscle the appearance of being a sheet of muscle rather than a very large number of individual cells. Because of the end-to-end continuity of the fibres, each one does not need to have a separate nerve supply.When an impulse is initiated it spreads from cell to cell via the branches and intercalated discs over the whole 'sheet' of muscle that causing heart muscle contraction. So the 'sheet' arrangement of the myocardium enables the atria and ventricles to contract in a coordinated and efficient manner.

The myocardium is thickest at the apex and thins out towards the base. This reflects the amount of work each chamber contributes for pumping of the blood. The left ventricle is thickest than left ventricle. The atria and the ventricles are separated by a ring of fibrous tissue that does not conduct electrical impulses. Consequently, when a wave of electrical activity passes over the atrial muscle, it can only spread to the ventricles through the conducting system which bridges the fibrous ring from atria to ventricles.


Endocardium

This forms the lining of the myocardium and the heart valves. It is a thin, smooth and glistening membrane which permits smooth flow of blood inside the heart. It consists of flattened epithelial cells and connected with the endothelium that lines the blood vessels.



C. The Chambers of the Heart

The four chambers of the heart are made of cardiac muscle called the myocardium. The chambers are lined with endocardium, simple squamous epithelium that also covers the valves of the heart and continues into the vessels as their lining (endothelium). The important physical characteristic of the endocardium is not thinness, but rather its smoothness.
This very smooth tissue prevents abnormal blood clotting, because clotting would be initiated by
contact of blood with a rough surface.

The upper chambers of the heart are the right and left atria (or atrium), which have relatively thin walls and are separated by a common wall of myocardium called the interatrial septum. The lower chambers are the right and left ventricles, which have more thicker walls than atrial wall and separated by the interventricular septum. Look at the picture below, the atria receive blood, either from the body or the lungs, and the ventricles pump blood to either the lungs or the
body.(see picture 2)















(pic : 2)



Right Atrium

The two large caval veins return blood from the body to the right atrium. The superior vena cava carries blood from the upper body, and the inferior vena cava carries blood from the lower body and sinus coronary from the heart itself. From the right atrium, blood will flow through the right atrioventricular (AV) valve, or tricuspid valve, into the right ventricle. The tricuspid valve is made of three flaps (or cusps) of endocardium reinforced with connective tissue. The general purpose of all valves in the circulatory system is to prevent backflow of blood during or abaut to systolic occur.

The specific purpose of the tricuspid valve is to prevent backflow of blood from the right ventricle to the right atrium when the right ventricle contracts. As the ventricle contracts, blood is forced behind the three valve flaps, forcing them upward and together to close the valve.


Left Atrium

The left atrium receives blood from the lungs, by way of four pulmonary veins. This blood will then flow into the left ventricle through the left atrioventricular (AV) valve, also called the mitral valve or bicuspid (two flaps) valve. The mitral valve prevents backflow of blood from the left ventricle to the left atrium when the left ventricle contracts.

Another function of the atria is the production of a hormone involved in blood pressure maintenance. When the walls of the atria are stretched by increased blood volume or blood pressure, the cells produce atrial natriuretic peptide (ANP), also called atrial natriuretic hormone (ANH). (The ventricles of the heart produce a similar hormone called B-type natriuretic peptide, or BNP, but we will use ANP as the representative cardiac hormone.) ANP decreases the reabsorption of sodium ions by the kidneys, so that more sodium ions are excreted in urine and retained some potassium.


Right Ventricle

When the right ventricle contracts, automaticlly the tricuspid valve will closes and the blood is pumped to the lungs through the pulmonary artery (or trunk). At the junction of this large artery and the right ventricle is the pulmonary semilunar valve (or more simply, pulmonary valve). Its three flaps are forced open when the right ventricle contracts and pumps blood into the pulmonary artery. When the right ventricle relaxes, blood tends to come back, but this fills the valve flaps and closes the pulmonary valve to prevent backflow of blood into the right ventricle. Projecting into the lower part of the right ventricle are columns of myocardium called papillary muscles. Strands of fibrous connective tissue, the chordae tendineae, extend from the papillary muscles to the flaps of the tricuspid valve. When the right ventricle contracts, the papillary muscles also contract and pull on the chordae tendineae to prevent inversion of the tricuspid valve. So If you have ever had your umbrella blown inside out by a strong wind, you will see what would happen if the flaps of the tricuspid valve were not anchored by the chordae tendineae and papillary muscles.


Left Ventricle

The walls of the left ventricle are thicker than those of the right ventricle, which enables the left ventricle to contract more forcefully. The left ventricle pumps blood to the body through the aorta, the largest artery of the body. At the junction of the aorta and the left
ventricle is the aortic semilunar valve (or aortic valve). This valve is opened by the force of contraction of the left ventricle, which also closes the mitral valve.

The aortic valve closes when the left ventricle relaxes, to prevent backflow of blood from the aorta to the left ventricle. When the mitral (left AV) valve closes, it prevents backflow of blood to the left atrium; the flaps of the mitral valve are also anchored by chordae tendineae and papillary muscles. All of the valves are connected to the fibrous skeleton of the heart. This is fibrous connective tissue that anchors the outer edgesof the valve flaps and keeps the valve openings from stretching. It also separates the myocardium of the atria and ventricles and prevents the contraction of the atria from reaching the ventricles except by way of the normal conduction pathway.


D. Artery Coronary

The right and left coronary arteries are the first branches of the ascending aorta, just on the aortic semilunar valve. The two arteries branch into smaller arteries and arterioles, then to capillaries. The coronary capillaries merge to form coronary veins, which empty blood into a large coronary sinus that returns blood to the right atrium. (see picture 3)
The purpose of the coronary vessels is to supply blood to the myocardium itself, because oxygen is essential for normal myocardial contraction. If a coronary artery becomes obstructed, by a blood clot for example, part of the myocardium becomes ischemic, that is the deprived of its blood supply.

Prolonged ischemia will create an infarct, an area of necrotic (dead) tissue. This is a myocardial infarction, commonly called a heart attack.














(pic : 3)



D. Cardiac Cycle

The function of the heart is to maintain a constant circulation of blood throughout the body which needed for metabolism. The heart acts as a pump and its action consists of a series of events known as the cardiac cycle. During each heartbeat, or cardiac cycle, the heart contracts and then relaxes. The period of contraction is called systole and that of relaxation, diastole.

Stages of the cardiac cycle
The normal number of cardiac cycles per minute ranges from 60 to 80. Taking 74 as an example each cycle lasts about 0.8 of a second and consists of:

• atrial systole — contraction of the atria
• ventricular systole — contraction of the ventricles
• complete cardiac diastole — relaxation of the atria and ventricles.

It does not matter at which stage of the cardiac cycle a description starts. For convenience the period when the atria are filling has been chosen. The superior vena cava and the inferior vena cava transport deoxygenated blood into the right atrium at the same time as the four pulmonary veins conveyoxygenated blood into the left atrium. The atrioventricular valves are open and blood flows through to the ventricles.

The SA node triggers a wave of contraction that spreads over the myocardium of both atria, emptying the atria and completing ventricular filling (atrial systole 0.1 s). When the wave of contraction reaches the AV node it is stimulated to make an impulse which fastly or very fast spreading to the ventricular muscle via the AV bundle, the bundle branches and Purkinje fibres. This results in a wave of contraction which sweeps upwards from the apex of the heart and across the walls of both ventricles pumping the blood into the pulmonary artery and the aorta (ventricular
systole 0.3 s). The high pressure generated during ventricular contraction is greater than that in the aorta and forces the atrioventricular valves to close, preventing backflow of blood into the atria

After contraction of the ventricles there is complete cardiac diastole, a period of 0.4 seconds, when atria and ventricles are relaxed. During this time the myocardium
recovers until it is able to contract again, and the atria refill in preparation for the next cycle.
The valves of the heart and of the great vessels open and close according to the pressure within the chambers of the heart.

The AV valves are opened while the ventricular muscle is relaxed during atrial filling and systole. When the ventricles contract there is a gradual increase in the pressure in these chambers, and when it rises above atrial pressure the atrioventricular valves close. When the ventricular
pressure rises above that in the pulmonary artery and in the aorta, the pulmonary and aortic valves open and blood flows into these vessels. When the ventricles relax and the pressure within them falls, the reverse process occurs. First the pulmonary and aortic valvesclose, then the atrioventricular valves open and the cycle begins again. This sequence of opening and closing valves ensures that the blood flows in only one direction.