Heart
Heart is located in the thoracic cavity, in between two lungs, slightly tilted to the left. It is derived from the mesoderm and has the size of clenched fist.
Heart is protected by a double walled membranes called pericardium. The pericardium consists of two layers, and outer parietal pericardium and an inner visceral pericardium attached to the heart. A space called pericardial cavity is present between the two layers which is filled with the with a fluid called pericardial fluid. The pericardium protects the heart from shock and mechanical injuries.
Our heart is divided into four chambers, to relatively smaller upper chamber called atria and two large lower chamber called ventricles. The wall of ventricles are much thicker than that of the atria. The right and left atria are separated by a thin muscular wall called the inter atrial septum where as the right and left ventricles are separated by thick walled called inter ventricular septum.
A thick fibrous tissue called the atrioventricular septum separate the atrium and ventricle of the same side. However, both of the atrioventricular septal are provided with an opening through which the two chambers of the same side are connected.
The opening between the atria and ventricles are guarded by atrioventricular valves(AV). The AV wale between right atrium and right ventricle has three flaps or cusps and is therefore called the tricuspid wale. The AV valve between the left atrium and the last ventricles has two flaps or cusps and is thus called the bicuspid wale on mitral wale.
Special fibrous cord called the chordae tendinae are attached to the flaps of the bicuspid and tricuspid valves at one end and their other ends are attached to the ventricular wall with the special muscles, papillary muscles. The chordae tendinae prevent the bicuspid and tricuspid valves from collapsing back into the atria during powerful ventricular contraction.
Three semilunar valves are found at the point where the pulmonary artery and aorta leave the heart. These valves prevent blood from getting back into the ventricles.
The right atrium receive deoxygenated blood through the coronary sinus and two large veins called Venae cavae. The left atrium receive oxygenated blood from the lungs through two pairs of pulmonary veins.
CONDUCTING SYSTEM IN HUMAN HEART
The entire heart is made up of cardiac muscles. A specialized cardiac musculature called the nodal tissue is also distributed in the heart. A patch of this tissue called the sinoatrial node(SAN) is present in the upper right corner of the right atrium. Another mass of tissue this tissue called the atrioventricular node(AVN) is present in the lower left corner of the right atrium, close to the atrioventricular septum.
A bundle of nodal fiber, that is atrioventricular bundle continues from the AVN which passes to the atrioventricular septa to emerge on the top of the inter ventricular septum and immediately divides into right and left bundle.
These branches give rise to minute fibers called Purkinje fibers throughout the ventricular musculature of respective side. The purkinje fiber along with right and left bundles are known as bundle of His.
The nodal musculature has the ability to generate action potential without any external stimuli, that is it is auto-excitable. Action potential is a short lasting event in which the electrical membrane potential of a cell rapidly rises and falls. All the all the heart muscle cells have the ability to generate the electrical impulses that trigger cardiac contraction, the SAN initiate it, simply leave because it generates the maximum number of action potential, that is 70-75 per minute, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore it is called the pacemaker of heart. Our heart normally beat 72-75 times in a minute this is called heart rate.
CARDIAC CYCLE
The cardiac cycles refers to the repeating pattern of contraction and relaxation of the heart. The phases of contraction is called as systole, and the phase of relaxation is called diastole.
How does the heart function? Let us take a look. To begin with, all the four chamber of heart are in relaxed state, that is they are in joint diastole. Blood from pulmonary veins and Venae cavae, fill the left and right atria ,respectively. the build up of pressure that result, causes AV valves to open and blood to flow from right atria to the ventricles. At this stage the semilunar valves are closed. SAN generates an action potential which stimulate both the atria to undergo simultaneous contraction known as atrial systole. It result in increase of blood flow into ventricle by about 30%.
The action potential generated by the SAN is conducted to the ventricular side by the AVN and AV bundle from where the bundle of his transmit it through the entire ventricular musculation. This causes contraction of ventricular muscles known as ventricular systole. The atria now undergo relaxation call atrial diastole which coincide with ventricular systole. As the ventricular begins their contraction, the intraventricular pressure rises, causing the closer of tricuspid and bicuspid valve due to attempted black-flow of blood in the atria.
When the pressure in the left and right ventricles become greater than the pressure in aorta and the pulmonary artery respectively, the semilunar valves are forced open. Opening of semilunar valves, guarding the pulmonary artery and aorta allows the blood in ventricles to flow through these vessel into the circulatory pathway. Now the ventricular relax that is ventricular diastole occurs and ventricular pressure falls causing the closer of semilunar waves. It prevent the back-flow of blood into the ventricles.
The ventricular pressure declines further and the AV valves are pushed open due to the pressure in the atria exerted by the blood which was being emptied into them by the veins. Once again the blood moves freely into the ventricles.
The ventricles and atria are again in joint diastole as earlier. Soon a new action potential is generated by SAN and the even described above are repeated in the sequence and the process continues.
This sequential event which is cyclically repeated the heart constitute the cardiac cycle which consist of systole and diastole of both the atria and ventricles.
Our heartbeat 72 times per minute, that is 72 cardiac cycles are performed per minute. Now, is 72 cardiac cycle se performed in 60 seconds then one cardiac cycle would occur in 0.8 second.
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