The heart is the most important organ of the blood which keeps the other internal organs alive by supplying blood and oxygen hence making it a life maintaining organ. The objective of this essay is to explain the pumping action of the heart, transportation of oxygen by the cardiovascular system and how reduced blood flow can affect cardiac function. A treatment for this cardiac malfunctioning is also explained. I will begin the essay by explaining the anatomical structure of the heart and then explain how the pumping action of the heart. The second part of the essay will include how the reduction in blood flow can affect the cardiac function and finally a treatment to cure this problem will be explained.
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The heart is a life maintaining organ weighs almost less then a pound, barely the size of the fist and located in the mediastinum of the throracic cavity of human body. The shape of the heart is like a inverted cone which has a blunt tapered end that points to the left hip and the base pointing towards the right shoulder. The heart lies more close to the left that is nearly third of it and rest on the diaphragm in between the two lungs. This is shown in figure 1 which shows the location of the heart.
Fig.1: Anterior view of the heart in the mediastinum
The entire heart is covered by a dense fibrous tissue called the pericardium which comprises of a dense connective tissue called the fibrous pericardium which protects the heart from over stretching due to excessive filling, it also consists of the serous pericardium which forms deeper portion of the heart. The serous layer is further divided into the parietal layer which lines the outer layers the and visceral layer which lines the innermost layers of the heart. A fluid which reduces friction is present in the pericardial cavity that separates these parietal and visceral layers. The below figure (see fig. 2 ) shows the different layers of the the pericardium.
Fig.2: The pericardium and the Heart wall
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cardio&part=A1016&rendertype=figure&id=A1019
As shown in the above figure (see fig.2), the heart wall is composed of three different layers ; epicardium, myocardium and the endocardium. The epicardium which forms superficial layer of the heart wall consists of mainly adipose tissue. The endocardium forms the deeper layer of the heart and is filled with squamous endothelium and aerolar tissues. Between these two layers lies the myocardium which is made up of cardiac muscles that help in the contraction of the heart. Its relatively a thick layer as it helps the heart to perform its normal pumping action that is contraction and expansion of the heart at regular intervals. On the outer surface of the heart there many grooves and layers of fat called the sulci.
The heart is divided into four chambers which has two inferior intermittent pumps which discharge blood out of the heart called the right and the left ventricles and two superior primer pumps called the right and left atria receiving deoxygenated and oxygenated blood from the body and the lungs respectively. The ventricles are separated from each other by a relatively thick muscle called the interventricular septum but atria are separated from each other by relatively a thinner wall called the interatrial septum as it has lighter workload comparing the ventricles. The right ventricle has thicker walls comparing the left ventricle because it has to pump more blood during systemic circulation. The presence of heart valves prevents the back flow blood and hence ensures that blood flows effectively in one direction. There are two types of valves which are the atrioventricular (AV) valves and the semilunar valves. The AV valves consists of the the tricuspid and bicuspid (mitral) valves that are located on the right and left side of the heart between the ventricles and atria respectively. The semilunar valves on the other hand lie on the bases of aorta and the pulmonary artery. These valves consists of the pulmonary valve and the aortic valve. The tricuspid valve has thread like structures that are connected to tendon like cords called the chordae tendinae. The anatomical structure of the heart and the heart valves is shown in figure 3.
Fig.3: The heart and the heart valves
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Blood Flow Through The Heart
The blood flow through the heart is explained by the pulmonary and systemic circulation. Deoxygenated blood is drained into the right atrium by the superior and the inferior vena cava. The pressure in the right atrium increases forcing the tricuspid valve to open and hence draining the entire deoxygenated blood to the right ventricle. The volume of blood in the ventricle increases and the maximum volume of the blood in the right ventricle after the contraction of the right atrium is called end diastolic volume (EDV). EDV is generally about 140ml. As the tricuspid valve closes the pressure in the ventricles increases. During this phase the ventricles contract but the pressure is not enough for the pulmonary valve to open hence resulting in isometric contraction as a result all the heart valves are closed during this phase and the volume in the ventricles remains constant. As the pressure continues to increase comparing the right atrium the blood forces open the pulmonary valve and the deoxygenated blood is pushed into the pulmonary trunk that divides into the pulmonary arteries. After the contraction of the ventricle that is the systole, the amount of blood remaining in the ventricle is called the end systolic volume (ESV). The difference between EDV and ESV gives the stroke volume (SV) that is the blood pumped out of the ventricles during a single heart beat. The pulmonary arteries carries the deoxygenated blood to the right and the left lung for oxygenation. Once the blood is oxygenated it is returned back to the heart by the pulmonary vein. The pulmonary vein empties the oxygenated blood into the left atrium, hence completing the pulmonary circulation and as the pressure in this atrium increases the blood is drained into the left ventricle by forcing open the mitral valve. When the mitral valve is closed the the pressure rises again comparing the left atrium and the blood is pushed into the aorta by opening the aortic valve. This oxygenated blood is transported to various parts of the body to carry out haemodynamic activities ( which includes the exchange of oxygen and carbondioxide with the blood ) . The systemic circulation is completed once the deoxygenated blood is returned back to the right atrium from different parts of the body by the venae caveae.
Fig.4: Pulmonary and Systemic Circulation of the heart
http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f22-1_cardiovascular_sy_c.jpg
During the phase of the first diastole, the ventricular relaxation takes place as a result the semilunar valves are closed and also the AV valves are also closed during this time as a result the volume of blood in the ventricles remains constant, hence this phase is called the isovolumetric relaxation. The diagrammatic explanation of the cardiac cycle is explained in figure 5.
Fig.5: The Cardiac Cycle
http://academic.kellogg.cc.mi.us/herbrandsonc/bio201_McKinley/f22-11_cardiac_cycle_c.jpg
Cardiac Conduction System
In this system the pumping action of the heart is synchronised by the electrical activity of the heart. Electrical signals are generated by the sinoatrial (SA) node which is the bodies natural pacemaker. This node generates pulses that propagate throughout the right atrium and through the Bachmann’s bundle hence stimulating both the atria. These pulses travel from SA node the to the atrioventricular (AV) node through certain paths known as internodal tracts. The AV node acts as a gatekeeper and prevents all the pulses to travel from the atria to the ventricles, hence causing some delay in the excitation. From the AV node the signals travel through the Purkinje fibres that divides itself into right and left and excites both the ventricles. This process repeats and the contraction of the heart takes place.
Transportation of Oxygen by Cardiovascular System
The cardiovascular system is a dense network of arteries, veins, capillaries etc. which is involved in the transportation of blood gases to and from the various parts of the body. In this part I will talking about how the cardiovascular system transports oxygen to different parts of the body. The oxygenated blood which is pumped from the left ventricle is transported by the aorta. The aorta is the largest artery of the human body which is made up of several layers of the elastin fibers and covered by smooth muscle. Blood flows in the arteries with high pressures hence these arteries expand (vasodilation) and contract (vasoconstriction) thus helping to regulate blood pressure. The aorta bifurcates into various different arteries smaller in size carrying oxygenated blood to different parts of the body. These arteries further divide into arterioles whose diameter is much smaller comparing the arteries and are less elastic. These arterioles are made up of thick layer of smooth muscles and are controlled by the autonomic nervous system that control their diameter. Oxygenated blood now passes from the arterioles to the capillaries which are the functional unit of the cardiovascular system. Capillaries are responsible for the exchange of blood gases and other nutrients between different tissues and blood through the process of diffusion. As diffusion is the process by which gases or fluids flow from higher to lower concentration therefore at the capillary level the concentration of oxygen is more in the capillaries and on the other hand the concentration of carbondioxide is more in the tissue than in the capillaries therefore the diffusion of these gases takes place. Oxygen is diffused into the tissues and carbondioxide on the other hand is diffused into the tissues. This is how oxygen is transported to different parts of the body. This deoxygenated blood is collected from the venules which are connected to the capillaries. Theses venules group together to form veins. Hence this deoxygenated blood is returned back to the heart for oxygenation and the entire process is repeated. The entire process of the transportation of oxygen by the cardiovascular system is explained in figure 6
Fig 6: Blood flow and capillary exchange of oxygen
http://www.patient.co.uk/health/Blood-General-Overview.htm
Effects Of Reduced Coronary Blood Flow And Its Treatment
Reduced coronary blood flow results in a condition known as ischemia where the myocardiac tissue is deprived of oxygen due to inadequate blood flow. The inadequacy is caused by formation of localised plaques of lipids that protrude within the artery causing a reduction in blood flow. As a result of reduced blood flow, there is a reduction in the level of oxygen which is required to carry out metabolic activities of the tissues. In ischemia the anaerobic respiration of the tissue results in the formation of lactic acid which leads to sever pain in the chest region. This pain is called angina pectoris. Angina pectoris is classified into two types; stable and unstable. Stable angina which causes pain in the chest region may arise from some physical activity ( running, jogging etc.). During these activities the need for oxygen is more to carry out metabolic activities in a faster rate compared to the supply of oxygen as a result anaerobic respiration of the tissues takes place and hence results in the formation of lactic acids which causes this pain. In unstable angina there is a persistent pain in the chest regions this is because of the accumulation of platelets on the ruptured plaques which leads to the blockage of the arteries and hence causing immense reduction in blood flow. Persistent unstable angina may cause myocardial infraction which ultimately may lead to sudden death. There are many ways to treat angina it may be either surgical or non surgical ways. Surgical methods include coronary balloon angioplasty where a catheter is passed into an artery with a balloon at the tip of the catheter. The balloon is made to expand causing the artery to expand and hence breaking of the plaque of lipids in the artery. Another types of surgical method is the coronary bypass surgery where the blocked artery is completely removed and replaced with a new one taken from different parts of the body. Non surgical methods include taking drugs that cause vasodilation such as nitroglycerin which converts into nitric oxide and hence dilates the coronary artery causing an increase in blood flow. Beta adrenergic blocking receptors are used to reduce the heart rate and therefore causing a reduction in cardiac output. Other techniques that involve the use of calcium channel blockers that are used for vasodilation and hence helping the flow of blood.
Conclusion
The heart is a an electromechanical pump which carries out its action through pulmonary and systemic circulation. The pumping action is well synchronised such that there is proper blood flow within the heart and outside to carry out haemodynamic activities. The transportation of blood is carried by a system consisting various networks that are spread throughout the body known as the cardiovascular system. This system helps in the transportation of oxygen to different tissues in order to carry out different metabolic activities. The exchange of oxygen between tissues and the cardiovascular system takes place through a process called diffusion. The effects of reduced blood are many and can lead to a condition known as ischemia. Angina pectoris is one of the main problems that can cause due to reduced blood flow and hence causing a sever pain in the chest region and on sever reduction in blood flow it may even lead to myocardial infraction which ultimately leads to death. There are numerous treatments for curing this particular problem, it may be either through surgical methods which includes cardiopulmonary bypass and angioplasty or using drugs that increase vasodilation and decrease cardiac output. These drugs include beta receptors, calcium channel blockers and nitroglycerin.
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