How the heart functions as a pump

The objective of this essay is to show how the heart functions as a pump in transporting oxygen to the different parts of the body and how reduction in coronary blood flow can impair the cardiac function. The first part of the essay describes the location, structure, electrical activity within the heart and how the heart transports oxygen throughout the body. The second part describes how reduced coronary blood flow in case of a disease can impair blood flow and its treatment.

The heart forms an integral part of the cardiovascular system whose primary function is the maintenance of hemodynamic and homeostatic functions such as maintenance of body temperature, transport of nutrients to the cells, removal of waste materials, transport of oxygen and hormones. [8,1]

Heart Location

The human heart is like a cone shaped organ composed of four different chambers and is located obliquely across the chest midline with its tip behind the fifth left intercostal space. It weighs on an average between 250-350 grams in adults and is approximately the size of a human fist. [2] An average human heart beats on an average of 75 beats per minute and pumps more than 200 million litres of blood in 80 years. [3]. Although the heart is located in the centre of the chest cavity its beating action is felt on the left side of the chest cavity since the most powerful pumping action of the ventricles of the heart takes place towards the base of the heart which is located in the left side of the chest cavity. [2] The figure below shows the location of the heart in the body.

Fig1: Location of the Heart [8]

Lecture Physiology and Anatomy- Cardiovascular System – Alan Richardson; slide no. 8

The heart is enclosed in a multi-layered sac known as Pericardium which protects the heart by reduction of friction and prevents excessive expansion. Between the different layers of the pericardium (visceral and the parietal layers), the pericardial cavity is present which holds about 5-15 ml of Pericardial Fluid that reduces the friction created due to the movement of the heart. [3]

The heart wall consists of three different layers Epicardium (outer layer), Endocardium (inner layer) and Myocardium (middle layer). The 2picardium and the endocardium are both made of simple squamous epithelial cells and a thin areolar tissue layer. However the myocardium is the thickest amongst all the three layers consisting of the heart muscles and its thickness in each chamber of the heart depends upon the amount of force generated by which chamber during the pumping action. [3] The figure below clearly shows the various layers of the heart wall.

Fig2: Layers of the Heart wall [9]

Structure of the Heart

The heart is divided into two different halves depending upon the kind of blood (deoxygenated or oxygenated) received – right and left halves. The heart consists of four different chambers with an atria and a ventricle on each side. The atria have relatively thinner walls since they only have to pump the blood to much shorter distances than the ventricles. [4].The atria connect to the ventricles by means of atrioventricular valves (tricuspid in the right half, bicuspid in the left half). The atrioventricular valves are connected to the base of the ventricles by chord like structures known as the chordate tendinae that prevent the valves from swinging in the opposite direction and thus prevent the back flow of blood into the atria from the ventricles. [3,5] The two atria are separated from one another by means of a muscular wall known as the interatrial septum. [3] The atria and the ventricles are separated by means of a fibrous connective tissue known as annulus fibrosis, this helps in giving a skeleton for attachment of the muscles of the heart and help in providing the site of placement of the heart valves. [4]

The ventricles are the lower and the larger chambers of the heart. The two ventricles are separated from one another by means of a thick muscular wall known as the interventricular septum. The right ventricle is connected to the pulmonary artery by means of the pulmonary semilunar valve while the left ventricle is connected to the aorta by means of the aortic valve. [3]. On the surface of the heart the heart chambers grooves are marked by fatty layers containing coronary blood vessels these layers are also known as Sulci.[3]

Blood Flow in the Heart

The deoxygenated blood from the various parts of the body flows into the heart by the pair of vena cava into the right atria. The blood flowing from the upper part of the body relative to the heart is carried by the superior vena cava while the blood flowing from the lower part of the body relative to the heart is carried by the inferior vena cava. [8] The cardiac muscles empty their deoxygenated blood into the right atria by the coronary sinus. The deoxygenated blood is pumped from the right atria into the right ventricles through the right atrioventricular valves (tricuspid valve) upon atrial sytole and ventricular diastole. The blood in the right ventricles is then pumped into the pulmonary artery through the right semilunar valve (pulmonary valve) to the lungs for oxygenation upon ventricular systole. However, during the ventricular systole the semilunar valves do not open unless the pressure generated in the ventricles due to contraction (systole) is sufficient to push open the valves, such contraction is known as isometric contraction. The pulmonary artery bifurcates into two smaller branches the left and the right pulmonary artery (one for each of the lungs). The pulmonary vein from the lungs brings the oxygenated blood from the lungs into the left atria of the heart which then pumps the blood into the left ventricle through the bicuspid valve (mitral valve) during atrial systole and ventricular diastole. The left ventricle pumps the blood to the different parts of the body through the aorta through the aortic valve during ventricular diastole. The heart’s muscles are themselves are supplied by oxygenated blood from the coronary artery branches present on the aortic arch. [3] The figure below shows the various chambers of the heart along with the flow of blood within the heart.

Fig3: Blood Flow within the heart [8]

Lecture Physiology and Anatomy- Cardiovascular System – Alan Richardson, Slide no 12

Blood enters the chambers during the diastole (relaxation) phase and is pumped out during the systole (contraction) phase. As a result, the blood is under a higher pressure in the systolic phase than the diastolic phase. The blood pressure is the pressure exerted by the blood upon the walls of the blood vessels.[5] The blood pressure on the walls of the artery in a healthy individual lies around 80mm Hg for diastole and 120mm Hg for systole. [4] The valves of the heart prevent the back flow of blood and thereby only allow the unidirectional flow of blood. [5]

The circulation of deoxygenated blood to the lungs and oxygenated blood back to the heart is known as pulmonary circulation while the circulation of oxygenated blood to all the parts of the body and deoxygenated blood from the various parts of the body into the heart is known as systemic circulation.[5] The entire process is displayed in the figure below.

Fig4: Systemic and Pulmonary Circulation [10]

Electrical Conduction within the Heart and Heart Beat

The cardiac impulse trigger is generated by the group of specialised cells which together form the sino-atrial node (SA node). The SA node is present in the right atrium near the point of attachment of the superior vena cava. The cells in the SA node generate the impulses spontaneously as they are capable of spontaneous depolarisation, hence they are said to possess automaticity. [6] Due to these spontaneous impulses the SA node forms the atrial pacemaker.

These electrical impulses are spread throughout the walls of the atrium by means of specialised pathways known as the Bachmann’s Bundle, thereby causing the stimulation of the myocardial walls of the atria to contract and push the blood into the ventricles. The wave of electrical excitation travels from the atrial walls via specialised pathways called internodal tracts from the SA node to the Atrioventricular (AV) node.

The AV node is also composed of similar autorhythmic cells as the SA node and is capable of pacing the heart in case the SA node fails in pacing and is located in the right side of the interatrial septum. However the pacing of the AV node is slower than the SA node and it thus provides the critical delay in the electrical conduction system, preventing the simultaneous contraction of both the atria and the ventricles. The distal portion of the AV node is known as the Bundle of His which then divides into the two bundle branches for spreading the electrical excitation to the two ventricles. The bundle branches are present along the interventricular septum and end at the tip of the heart by further differentiating into numerous small fibres known as Purkinje fibres. The Purkinje fibres are responsible for depolarising the individual myocardial cells of the ventricles. Thus causing the ventricles to contract and push the blood into the pulmonary artery or the aorta. [3]

Blood circulation and Transport of Oxygen

The blood vessels and capillaries are the pipes which carry blood throughout the body for metabolic, waste and gaseous transport. The blood vessels include arteries, arterioles, veins and venules.

Arteries carry the oxygenated blood away from the heart with the Aorta being the largest artery. Since the artery carry blood in jerks and under high pressure they are surrounded by smooth muscles which prevent it from collapsing. The resistance to blood pressure is controlled by the autonomic nervous system which controls the width of the artery (lumen) through which the blood passes (vasoconstriction and vasodilation). The arteries further divide into smaller divisions known as arterioles which carry blood to smaller parts of the body. The arterioles are also covered with smooth muscles and like the arteries also resist any changes to the blood pressure. The arterioles further differentiate into smaller blood vessels known as capillaries which possess an extremely thin wall so as to allow the exchange of oxygen with the individual cells and carbon-dioxide from the cells. Apart from the exchange of gases the metabolic exchange of nutrients and wastes are also possible at the capillaries. Several billions of capillaries then join together to form the venules which are smaller blood vessels carrying the deoxygenated blood from the capillaries to the veins. The veins are the formed by the integration of millions of smaller venules and it carries the deoxygenated blood back to the heart. The blood in the veins does not flow under considerable high amounts of pressure and hence the walls of the veins are not as thick as those of the artery. The veins join together to form the two vena cavas. [8]

The transfer of oxygen from the blood into the cells at the capillaries is explained by the process of diffusion. Diffusion is the process of movement of particles from their region of higher concentration to a region of lower concentration. Thus in the capillaries the oxygenated blood has a higher concentration of oxygen than that present outside the capillaries in the surrounding cells. At the same time there is higher concentration of carbon-dioxide in the surrounding cells than the oxygenated blood in the capillaries. Hence the oxygen from the blood in the capillaries diffuses out into the surrounding cells while the carbon-dioxide from the surrounding cells diffuses into the capillaries.

Thus the oxygenated blood from the lungs passes into the heart which pumps it into the aorta which divides into the arteries which further divides into arterioles and then capillaries. The capillaries then exchange the oxygen with the cells and take carbon-dioxide from the cells and rejoin to form the venules which then form the veins which return the deoxygenated blood back to the heart. Thus the heart acts a pump in the entire cardiovascular system which transports the oxygen to the different parts of the body and carbon-dioxide from the different parts of the body. The figure below shows the overview of the cardiovascular system.

Fig5: The Cardiovascular System [11]

Reduced Coronary Blood Flow and Coronary Artery Disease

The heart needs to perform all the time in the body and can never relax, hence the cardiac muscles have a high demand for oxygen and have very limited capacity for anaerobic respiration. [7]

The chest pain which is felt in the patient due to the obstruction of the blood flow in the coronary arteries is known as Angina Pectoris. This deposition of the plaque and lipid layers within the coronary blood vessels thereby causing the hardening and narrowing of the blood vessels is known as Atherosclerosis. Due to the obstruction the cardiac cells are deprived of oxygen and start anaerobic fermentation resulting in the formation of lactic acid. The lactic acid formation in the heart stimulates the pain receptors present in the heart. [3] Depending upon the type of plaque formation in the coronary blood vessel the angina might be termed as stable or unstable. [8]

Thus with the reduced coronary blood flow the cardiac output of the heart is severely impaired since the muscles of the heart are deprived of oxygen and nutrients resulting in tissue death or myocardial infarction. Hence the heart is not able to pump properly and thus has a reduced cardiac output. Myocardial Infarction causes severe pain and can even cause death to the patient. [6]

The blood flow to the target cells can be increased by vasodilation and thereby allowing more blood to flow through them. This can be done by using organic nitrate medications which release nitric oxide (NO) into the blood stream. Medications known as beta blockers (β) which also cause of the coronary artery vasodilation can also help in the treatment of the condition in the same manner.

Apart from medications surgically also the condition of reduced coronary blood flow can be treated by coronary bypass surgery where the atherosclerotic narrowing of the coronary artery is bypassed by a blood vessel which is grafted from any other part of the patients body. There also is the possibility of performing other angioplasty operations such as balloon angioplasty, etc. [7]

The therapeutic goals in treating stable angina are to improve the coronary blood flow to the target cells and reduce the cardiac oxygen demand. While in the treatment of unstable angina steps are taken to prevent the occurrence of myocardial infarction

Conclusion

The heart acts as a muscular pump which pumps blood throughout the lifecycle beating at an average of 72 beats a minute and pumping 200 million litres of blood in 80 years. [3] The cardiovascular system consists of several different components, the pump (heart), an extensive piping network (blood vessels and capillaries) and finally a working fluid (blood). The heart receives deoxygenated blood from all over the body pumps it to the lungs for oxygenation and receives the oxygenated blood from the lungs and pumps them to the different parts of the body. The piping network includes arteries, arterioles, veins, venules and capillaries. The capillaries are the site of gaseous exchange where the exchange takes place by diffusion. Reduced coronary blood flow impairs the cardiac output by starving the cardiac cells of oxygen and nutrients carried by the blood.