Blood can carry out its transportation function only if it can circulate in the human body. The organ responsible for generating the pressure to pump the blood through the vessels is the muscular heart. The heart is a dark red cone-shaped muscular organ found in the thoracic cavity. It is the size of a clenched fist and weighs from 350 to 450 grams in an average adult. The heart is located between the lungs with its apex slightly oriented to the left. The heart has four chambers: two upper thin-walled atria and two lower thick-walled ventricles. The septum separates the right chambers from the left chambers.
The valves in the heart ensure that blood flows in one direction. Heart disease is the number one killer in many countries in the world. An important preventive measure is to educate people about heart fitness and a healthy lifestyle. The valve between the left atrium and left ventricle is the bicuspid valve. The valve between the right atrium and right ventricle is the tricuspid valve. The valves at the base of the aorta and pulmonary artery are the semilunar valves. The heart mainly made up of myogenic cardiac muscles. The heart muscles contract and relax automatically throughout life and are not controlled by the nervous system.The heart functions like two pumps with different pressure system, the right pump forces deoxygenated blood to the lungs. The left pump forces oxygenated blood to other parts of the body.
The valves in the heart ensure that blood flows in one direction. Heart disease is the number one killer in many countries in the world. An important preventive measure is to educate people about heart fitness and a healthy lifestyle. The valve between the left atrium and left ventricle is the bicuspid valve. The valve between the right atrium and right ventricle is the tricuspid valve. The valves at the base of the aorta and pulmonary artery are the semilunar valves. The heart mainly made up of myogenic cardiac muscles. The heart muscles contract and relax automatically throughout life and are not controlled by the nervous system.The heart functions like two pumps with different pressure system, the right pump forces deoxygenated blood to the lungs. The left pump forces oxygenated blood to other parts of the body.
The sino-atrial node (SAN) is a group of specialised cells located in the right atrial wall, near the entrance of the superior vena cava. It acts like a pacemaker which initiates the heartbeat. The SA node generates a wave of excitatory impulses which spread to the two artria, causing them to contract simultaneously. Blood is then forced from the atria into the ventricles.The second node, the atro-ventricular node (AVN), lying at the base of the right atrium, is then stimulated. Impulses from the AV node are conducted by specialised muscle fibres called bundle of His and Purkinje fibres to the ventricular walls. This causes the contraction of both ventricles to pump the blood out of the heart.
The right ventricle pumps the blood into the pulmonary artery, which forces the blood to the lungs. The left ventricle, which is thicker and more muscular than the right ventricle, generates greater pressure to pump blood through the aorta to the other arteries in the body.
The cardiac cycle is the series of events that occur during one complete heartbeat. It includes the contraction (systole) and relaxation (diastole) of both the atria and the ventricles. The average heart rate is about 72 heartbeats per minute.
The pumping of the heart generates sufficient force to move the blood through the arteries, arterioles and capillaries. However, when the blood reaches the veins, the pressure produced by the heart is insufficient to force it back into the heart. The blood on the veins also has to flow against gravitational pull. How does the blood in the veins flow back to the heart? When the body moves, the skeletal muscles around the veins contract and press on the veins. The blood pressure increases, forces open the valves and pushes the blood towards the heart. The valves in the veins prevent the blood from flowing backwards.
The sino-atrial node can initiate the heartbeat on its own. However, the heart rate may be modified by certain external factors. The sympathetic nerve carrying impulses to the heart can increase the heart rate and the parasympathetic nerve can slow it down. When a person is excited, an increase in the secretion of the hormone, adrenaline, causes the heart to beat faster. An increase in the partial pressure of carbon dioxide in the blood also increases the heart rate. Heart rate also increases when body temperature is elevated. The human heart is a very hard-working organ. It works non-stop day and night. The average heart beats about 72 times per minute. In an average lifetime, the human heart beats more than two and a half billion times. In total it pumps 34 million litres of blood throughout the circulatory system in the body! (1 billion = 1 000 000 000).
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