When the cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded the recording is known as an electrocardiogram (ECG).
• The instrument used to record these changes is called electrocardiograph. Waller (1887) first recorded the electrocardiogram but Einthoven (1903) studied electrocardiogram in detail, therefore, he got Nobel Prize in 1924 for the discovery of ECG. He is also considered “Father of the Electrocardiography”.
• When electrocardiogram of a person is to be recorded, four leads (metal electrodes) are attached in the arms and legs. It is done after cleaning and putting a special jelly, which improves electrical conduction. With the help of a rubber suction cup, an additional electrode is placed on the chest. Now the electrocardiograph is switched on which detects and amplifies the electrical current of the heart and transmits to the recording pen. The latter draws a wavy line that is called the deflection waves or the electrocardiogram.
• A normal electrocardiogram (ECG) is composed of a P wave, a QRS wave (complex) and a T wave. The letters are arbitrarily selected and do not stand for any particular words.
• The P wave is a small upward wave that indicates the depolarisation of the atria (atrial contraction). It is caused by the activation of SA node.
• The QRS wave (complex) begins after a fraction of second of the P wave. It begins as a small downward deflection (Q) and continues as large upright (R) and triangular wave, ending as downward wave (S) at its base. It represents ventricular depolarisation (ventricular contraction).
• The T wave is dome-shaped which indicates ventricular repolarisation (ventricular relaxation).
• Normal P-R interval is 0.12 to 0.2 second. Normal QRS complex duration is 0.12 second. Normal Q-T interval is 0.4 second.
Clinical significance of Electrocardiogram
• Any deviation from the normal Electrocardiogram shape indicates a possible abnormality or disease and can be analyzed using Electrocardiogram interpretation.
• Enlargement of the P wave indicates enlargement of the atria. During atherosclerotic heart disease and rheumatic fever, the P-R interval is lengthened. This is due to the inflammation of atria and AV node.
• The enlarged Q and R waves indicate a myocardial infarction (heart attack). The S-T segment is elevated in acute myocardial infarction and depressed when the heart muscle receives insufficient oxygen.
• T wave is flat when the heart muscles receive insufficient oxygen as in atherosclerotic heart disease. It may be elevated when the body’s potassium level is increased.
• Blood pressure is defined as the force or pressure which the blood exerts on the walls of the artery in which it is contained. The arterial blood pressure is the result of the discharge of the blood from the left ventricle into the already full aorta.
• When the left ventricle contracts pushing the blood into the aorta, the pressure produced is known as systolic blood pressure (120 mm Hg). When the complete diastole occurs and the heart is resting, the pressure within the blood vessels is called as the diastolic blood pressure (80 mm Hg).
• The blood pressure is expressed as BP = 120/80 mm Hg. The difference between the systolic and diastolic pressure is called pulse pressure.
• Its normal value is 40 mm Hg but becomes more in case of hypertension. Pulse pressure is lower in case of children.
• Both systolic and diastolic blood pressures are readily measured in human beings with the use of a device called sphygmomanometer, in terms of height in millimeters of a column of mercury.
• In human there are two circuits of blood circulation for greater efficiency and to completely prevent the mixing of oxygenated and deoxygenated blood. Usually it is called double blood circulation.
• Double blood circulation is the passage of same blood twice in the heart through separate pathways for completing one cycle. It consists of pulmonary circulation and systemic circulation.
• Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. Deoxygenated blood is pumped out of the right ventricle of the heart into the pulmonary trunk, then to the pulmonary arteries and into the lungs via pulmonary veins, oxygenated blood is then drained into the left atrium. Blood is then circulated in the systemic blood circulation which supplies all parts of the body except the lungs. It consists of the aorta and all its branches, carrying oxygenated blood to the tissues, and all the veins draining deoxygenated blood into the vena cava.
• The advantage of double circulation is that the blood can be sent to the lungs to pick up oxygen and then returned to the heart to be pumped again before travelling around the body. The blood therefore is pumped through the capillary bed (which slows it down and reduces its pressure) then receives another pump before it enters another capillary bed. Double circulatory systems are therefore high pressure system. In this there is no mixing of the oxygen rich blood and oxygen poor blood in the heart.
Blood Circulation through special regions
(i) Coronary circulation : The flow of oxygenated blood from the ascending aorta to the heart muscle and the return of deoxygenated blood from the heart muscle to the right atrium is called coronary (cardiac) circulation. The right and left coronary arteries arise from the ascending aorta which supply oxygenated blood to the heart muscle (myocardium). The coronary veins bring deoxygenated blood to the coronary sinus. The latter carries deoxygenated blood to the right atrium.
(ii) Portal circulation : A vein which collects blood from one organ by one set of capillaries and distributes blood to some other organ by another set of capillaries instead of sending it to the heart is called a portal vein. Portal vein together with the small veins by which it receives blood and the capillaries by which it supplies blood to a specific organ forms a portal system. A portal system is named after the organ to which it carries blood.
(iii) Hepatic portal system : It includes a single large vein, the hepatic portal vein. This vein receives a number of small veins from the different regions of the digestive tract and its associated structures. At its other end, it bifurcates into right and left branches, which enter the corresponding liver lobes and break up into capillaries to supply the blood. Blood from the liver is carried by a pair of hepatic veins to the inferior vena cava.
(iv) Hypophysial portal system : It is a minor portal system in the pituitary body. A hypophysial portal vein collects blood from the hypothalamus of the brain and distributes it to the anterior lobe of the pituitary body. This portal system enables the hormones from the hypothalamus to reach the anterior pituitary lobe.
Pressures in different parts of the blood circulation system
• The heart pumps blood continually into the aorta, hence, the mean pressure in the aorta is high, averaging about 100 mmHg. Also, because heart pumping is pulsatile, the arterial pressure alternates between a systolic pressure level of 120 mmHg and a diastolic pressure level of 80 mmHg.
• As the blood flows through systemic circulation, its mean pressure falls progressively to about 0 mm Hg by the time it reaches the termination of the venae cavae where they empty into the right atrium of the heart.
• The pressure in the systemic capillaries varies from as high as 35 mmHg near the arteriolar ends to as low as 10 mmHg near the venous ends, but their average “functional” pressure is about 17 mmHg, a pressure low enough that little of the plasma leaks through the minute pores of the capillary walls, even though nutrients can diffuse easily through these same pores to the outlying tissue cells.
• In the pulmonary arteries, the pressure is pulsatile, just as in the aorta, but the pressure level is far less pulmonary artery systolic pressure averages about 25 mm Hg and diastolic pressure 8 mm Hg, with a mean pulmonary arterial pressure of only 16 mm Hg.The mean pulmonary capillary pressure averages only 7 mm Hg.
• Yet the total blood flow through the lungs each minute is the same as through the systemic circulation. The low pressures of the pulmonary system are in accord with the needs of the lungs, because all that is required is to expose the blood in the pulmonary capillaries to oxygen and other gases in the pulmonary alveoli.