7.4 Human Circulatory System · Part 5

diastolic pressure. Normal blood pressure in man is about /80mm Hg . Mean arterial pressure is a function of cardiac output and resistance in the arterioles. The primary reflex pathway for homeostatic control of mean arterial pressure is the baroreceptor reflex . The baroreceptor reflex functions every morning when you get out of bed. When you are lying flat the gravitational force is evenly distributed. When you stand up, gravity causes blood to pool in the lower extremities. The decrease in blood pressure upon standing is known as orthostatic hypotension . Orthostatic reflex normally triggers baroreceptor reflex. This results in increased cardiac output and increased peripheral resistance which together increase the mean arterial pressure. . . Electrocardiogram (ECG) An electrocardiogram (ECG) records the electrical activity of the heart over a period of time using electrodes placed on the skin, arms, legs and chest. It records the changes in electrical potential across the heart during one cardiac cycle. The special flap of muscle which initiates the heart beat is called as sinu-auricular node or SA node in the right atrium. It spreads as a wave of contraction in the heart. The waves of the ECG are due to depolarization and not due to contraction of the heart. This wave of depolarisation occurs before the beginning of contraction of the cardiac muscle. A normal ECG shows waves designated as P wave, QRS complex and T wave as shown in Figure . and the stages of the ECG graph are shown in Figure . . FRPSOH[ 9HQWULFXODU GHSRODUL]DWLRQ 9HQWULFXODU UHSRODUL]DWLRQ      $WULDO GHSRODUL]DWLRQ 7LPH V  LQWHUYDO  LQWHUYDO  VHJPHQW Figure . Graph of a normal ECG P Wave (Atrial Depolarisation) It is a small upward wave and indicates the depolarisation of the atria. This is the time taken for the excitation to spread through XI Std Biology-Zoology Chapter- atria from SA node. Contraction of both atria lasts for around . - . sec. PQ Interval (AV Node Delay) It is the onset of P wave to the onset