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The CARDIAC CYCLE

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The CARDIAC CYCLE

The cardiac cycle describes the series of event that occurs during a complete heartbeat- the contraction and the relaxation of both the atria and the ventricle.

  • The atria serve to receive the blood that enters the heart and passes it to the ventricle.
  • One cardiac cycle is completed when the heart fills with blood and the blood is pumped out.
  • The phases of the cardiac cycle are two:
  • The phase during which the heart muscles are relaxed and fill with blood is termed the diastole
  • The other phase where there is a contraction and blood pumps out of the heart to the arteries is termed the systole.
  • As the heart beats, it transports blood to the circulatory system of the body.
  • The cardiac cycle is vital in the proper functioning of the cardiovascular system.
  • The cardiovascular system is made up of the heart and the circulatory system, they transport nutrients and remove toxic substances from the cells in the body.
  • The muscle needed to pump blood throughout the body is also supplied by the cardiac cycle.
The following describes the event that occurs during a cardiac cycle:
  • At the beginning of the diastole period, the atria and ventricles relax and the atrioventricular valves (AV) open.
  • The oxygen-starved blood returning from the circulatory system passes through the vena cavae( the two largest veins in the body) into the right atrium.
  • Simultaneously, the oxygen-starved blood returning from the circulatory system passes through the pulmonary veins and fill the left atrium.
  • The open atrioventricular valves aid the unstrained flow of blood from the atria to the ventricles.
  • The motion from the sinoatrial node (known as the pacemaker of the heart, that coordinates contraction) send waves that trigger both atria to contract.
  • The contraction spurs the right atrium to empty its content into the right ventricle, and at the same time, the left atrium empties its content into the left ventricle.
  • The valve between the right atrium and the right ventricle (tricuspid valve) prevents blood from flowing back into the right atrium.
  • The valve between the left atrium and the left ventricle (mitral valve) prevents blood from flowing back into the left atrium.
  • These processes are called the first and second diastole period.
The Systole
  • During the systole period, the right ventricle fills with blood transported from the right atrium.
  • The left ventricle also fills with blood transported from the left atrium.
  • Fiber branches carrying electrical impulse sends impulses to the ventricles causing them to contract.
  • This causes the atrioventricular valves to close.
  • The semilunar valve (located between the pulmonary and the right ventricle, and between the aorta and the left ventricle) opens.
  • The contraction of the ventricles allows oxygen-starved blood from the right ventricle to be pumped to the pulmonary artery and

oxygenated blood in the left ventricle is pumped to the aorta.

  • The pulmonary valve when closed prevents blood to flow back to the right ventricle.
  • The aortic valve when closed prevents blood to flow back to the left ventricle.
  • The pulmonary artery carries the oxygen-starved blood to the lungs through the pulmonary circuit. The blood picks up oxygen in the lungs where it is then transported back to the left atrium through the veins.
  • The aorta serves the oxygenated blood to all parts of the body
  • Afer its tour through the body, oxygen-starved blood is then returned to the heart through the venae cavae
  • These processes are called the first and the second systole period.