- 1) Blood Circulatory System in Humans
- 2) Pumping Organ- The Heart
- 3) Structure and Function
- 4) The Cardiac Cycle
- 5) Mechanism of Heart Excitation and Contraction
- 6) Electrocardiogram
- 7) Artificial pacemaker
- 8) Facts about Human Heart
Blood Circulatory System in Humans
The circulatory system of humans has 3 basic components.
- (A) Circulating fluid – the blood.
- (B) The pumping organ – the heart.
- (C) The blood vessels, arteries, capillaries, and veins.
In this article, we will discuss the pumping organ in human bodies – The Heart.
Pumping Organ- The Heart
The human heart is an organ that pumps blood throughout the body by means of the circulatory system, supplying oxygen and nutrients to the tissues and removing carbon dioxide and other wastes. The heart of humans lies in the chest cavity. The heart is enclosed in a double membranous sac – the pericardial cavity, which includes the pericardial fluid. Pericardium protects the heart, prevents it from overextension.
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Structure and Function
The wall of the heart is composed of three layers.
- (i) Epicardium
- (ii) Myocardium
- (iii) Endocardium
Myocardium of the heart is made up of a unique type of muscles, the heart muscles.
These muscles consist of myofibrils, and myofilaments of myosin and actin. Their structural plan resembles those in skeletal muscle fibres, and their mechanism of contraction is basically the same, except that they are branched cells, in which the succeeding cells are separated by junctions called intercalated discs. The heart contracts automatically with rhythmicity, under the control of the autonomic nervous system of the body.
There are 4 chambers of the heart: two upper thin-walled atria, and 2 lower thick-walled ventricles. The human heart functions as a double pump and is accountable for lung and systemic flow. The total separation of deoxygenated blood (Right side) and oxygenated blood (left side), in the heart, is kept. The right atrium receives deoxygenated blood via venae cavae from the body. The blood is passed on to the right ventricle through the tricuspid valve (called so because it has 3 flaps).
These flaps are connected with fibrous cables called chordae tendineae, to the papillary muscles which are extensions of the wall of the right ventricle. When the right ventricle contracts, the blood is passed to the pulmonary trunk, which carries blood by means of left and right pulmonary arteries, to the lungs.
At the base of the pulmonary trunk, semilunar valves exist. After oxygenation in the lungs, the blood is brought by pulmonary veins to the left atrium, which passes this blood by means of a bicuspid valve (called so because it has two flaps) to the left ventricle. The flaps of the bicuspid valve are similarly attached through chordae tendineae, to the papillary muscles of the wall of the left ventricle. When the left ventricle contracts, it presses the blood through the aorta to all parts of the body (other than lungs).
At the base of the aorta semilunar valves are also present. The valves of the heart control the direction of the flow of blood. The wall of the left ventricle is thicker (about 3 times) than that of the right ventricle. At the base of the aorta, the first pair of arteries, the coronary arteries, arise and supply blood to the heart.
The aorta forms an arch, and prior to descending down offers 3 branches providing blood to the head, arms and shoulders. The aorta descends down in the chest cavity. It gives numerous little branches to the chest wall and then gives it to the stomach. Here it offers branches, which supply blood to different parts of the alimentary canal, kidneys, and the lower abdominal area.
The aorta Bifurcates into iliac arteries, each of which results in supply blood to each leg. The blood from the upper part of the body is collected by different veins, which join to form superior vena cava; which passes its blood to the right atrium.
Two Iliac veins are formed by veins which gather blood from legs and join to from the inferior vena cava. It gets renal vein from each kidney; and hepatic vein from the liver, before it goes into the right atrium. The liver receives a hepatic portal vein which is formed by numerous veins gathering deoxygenated blood with absorbed food from different parts of the alimentary canal.
The Cardiac Cycle
It is the sequence of events which happen during the completion of one heartbeat.
Heartbeat involves 3 unique phases which are as follows:
Relaxation phase – diastole
The deoxygenated blood goes into the right atrium through the vena cava, and oxygenated blood enters the left atrium through pulmonary veins. The walls of the atria and that of ventricles are relaxed. As the atria are filled with blood, they become distended and have more pressure than the ventricles. This relaxed period of heart chambers is called diastole.
Atria Contract – atrial systole
The muscles of atria simultaneously contract, when the atria are filled and distended with blood, this is called atrial systole. The blood goes through the tricuspid and bicuspid valves, into the two ventricles which are relaxed.
Ventricles contract – ventricular systole
When the ventricles get blood from atria, both ventricles contract at the same time, and the blood is pumped to pulmonary arteries and aorta. The tricuspid and bicuspid valves close, and ‘lubb’ sound is made. Ventricular systole ends, and ventricles relax at the same time semilunar valves at the base of the lung artery and aorta close simultaneously, and ‘dubb’ noise is made. (Lubb, dubb can be heard with the help of a stethoscope).
One complete heartbeat includes one systole and one diastole and lasts for about 0.8 seconds. In one’s life, the heart contracts about 2.5 to 3 billion times, without stopping.
Mechanism of Heart Excitation and Contraction
The heartbeat cycle starts when the Sino-atrial node (Pacemaker) at the upper end of the right atrium sends electrical impulses to the atrial muscles and triggering both atria to contract. The Sino-atrial node consists of a small number of diffusely oriented cardiac fibres, possessing few myofibrils; and a couple of nerve endings from the autonomic nerve system.
Impulses from the node travel to the musculature of the atrium and to an atrioventricular node. From it, an atrioventricular bundle of muscle fibres propagates the regulative impulses through excitable fibres in the interventricular septum, to the myocardium of the ventricles. There is a hold-up of around 0.15 second in conductance from the S-A node to A-V node, permitting atrial systole to be finished prior to ventricular systole begins.
As the cardiac impulse passes through the heart, electrical currents spread out into the tissues surrounding the heart, and a small percentage of these spread all the way on the surface of the body. If electrodes are put on the skin on opposite sides of the heart, electrical potentials produced by these currents can be taped.
This recording is called electrocardiogram which is taken by an electrocardiograph (E.C.G.) device. It helps to diagnose the irregularities in the rhythmicity and conduction system of the heart which might be corrected by the use of an artificial pacemaker.
The pacemaker is accountable for starting the impulses which activate the heartbeat rate. If there is some block in the flow of the electrical impulses, or if the impulses initiated by S.A. node are weak; it may cause the death of the individual. So, an artificial pacemaker, which is battery operated producing electrical stimulus is utilized. For example, if A-V pathway is blocked, the electrodes of the artificial pacemaker are connected to the ventricle.
Then this pacemaker supplies continued rhythmic impulses that take control of the control of the ventricles.
Facts about Human Heart
- A human heart is approximately the size of a large fist.
- The heart weighs between about 280 to 340 grams in males and 230 to 280 grams in females.
- The heart beats about 100,000 times daily (about 3 billion beats in a lifetime).
- An adult heart beats about 60 to 80 times per minute.
- Newborns’ hearts beat about 70 to 190 beats per minute which is faster than the adult heart.
- The heart pumps about 5.7 litres of blood throughout the body.