08 Respiration and Circulation - part 13 - Working mechanism of human heart : Cardiac Cycle

 


08 Respiration and Circulation - part 13 - Working mechanism of human heart : Cardiac Cycle


Working mechanism of human heart : 
Cardiac Cycle :
  • Human heart alternately contracts and relaxes. 
  • Contraction is called systole and relaxation is called diastole. 
  • Atria and ventricles contract alternately. 
  • Consecutive systole and diastole constitutes a single heartbeat or cardiac cycle. 
  • Completed in 0.8 sec. 
  • On an average, 72 beats are completed in one minute in an adult, at rest.
a. Atrial systole (AS):
  • Right atrium receives deoxygenated blood and left atrium receives oxygenated blood. 
  • When both the atria are completely filled with blood, pressure is exerted on the wall. 
  • In response to this pressure, SA node gets excited and generates cardiac impulse.
  • Due to this, cardiac muscles in the atrial wall contract causing atrial systole. 
  • During atrial systole, blood is pumped into ventricles. 
  • Blood is prevented from going back to the veins and coronary sinus by Eustachian and Thebesian valve respectively. 
  • After completing systole the atria go into diastole.
  • In normal conditions, atrial systole is for 0.1 sec. and atrial diastole (AD) is for 0.7 sec.
b. Ventricular systole (VS):
  • The impulse which started from SA node now reaches the AV node and it gets excited. 
  • AV node sends impulses to bundle of His and from bundle of His to Purkinje fibers.
  •  Purkinje fibers spread impulses all over the wall of ventricles.
  • Due to this, ventricular wall contracts causing ventricular systole. 
  • During ventricular systole-  
  1. right ventricle pumps deoxygenated blood into pulmonary trunk and 
  2. left ventricle pumps oxygenated blood into aorta. 
  3. The cuspid valves close both the atrioventricular apertures preventing blood flow into atria (lubb sound is heard).
  • In normal conditions, ventricular systole lasts for 0.3 sec. and ventricular diastole (VD) lasts for 0.5 sec. 
  • During ventricular diastole- 
  1. semilunar valves are closed
  2. preventing backflow of blood from pulmonary trunk and systemic aorta into ventricles (dub sound is heard).
  • For about 0.4 second, both atria and ventricles are in diastole. 
  • When all the chambers of heart are in diastole, this condition is called joint diastole or complete diastole.
  • Thus, duration of one cardiac cycle is 0.8 sec.
  • Right side of heart contains deoxygenated and left contains oxygenated blood. 
  • Total volume of blood pumped during one ventricular systole is called stroke volume (SV) and it is approximately 70 ml.
Cardiac output (CO):
  • Volume of blood pumped out per min. 
  • For a normal adult human being it is calculated as follows :
                         (CO) = SV × HR= 70 × 72
                                  = 5040 ml/min
Regulation of cardiac activity :
  • Human heart is myogenic.
  • Also it is  under dual control, the nervous as well as hormonal. 
  • The nervous control includes the part of autonomic nervous system. 
  • Sympathetic system (with hormone epinephrine as neurotransmitter) increase the rate of heartbeat during emergency.
  • Parasympathetic system (with acetylcholine as neurotransmitter) reduces rate of heartbeat.
Nervous control :
  • Includes the part of the autonomous nervous system- 
  • its cardiovascular center lies in the medulla oblongata. 
  • It controls rate of heart beat in response to inputs from various receptors like - 
  1. proprio-receptors - which monitor the position of limbs and muscles
  2. chemoreceptors - monitoring chemical changes in blood and
  3. baroreceptors - monitoring the stretching of main arteries and veins.
  • Chemical control of the heart rate includes -  
  1. Conditions like hypoxia, acidosis, alkalosis causing decreased cardiac activity
  2. Hormones like epinephrine and norepinephrine enhance the cardiac activity. 
  • Besides, concentration of cations like K+, Ca++ and Na+ have major effect on cardiac activity. 
  • Cardiac activity decreases with the elevated blood level of K+ and Na+

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