Structure of the Heart & the cardiac cycle

Structure of the Heart & the cardiac cycle




Structure of the Heart

  • Heart is about the size of a clenched fist and pumps the blood around the body


  • It consists of 4 chambers connected by one way valves.


  • Around 60-80 beats a minute at rest


  • Can reach up to as much as 200 with exercise


  • Heart is a complex organ


  • RIGHT SIDE - Pulmonary circulation (Deoxygenated blood: Heart - Lungs - Heart)


  • LEF SIDE - Systemic Circulation (Oxygenated blood: Heart - body - heart)


Chambers of the Heart

  • The heart consists of 4 chambers


  • 2 on the right side that are linked and 2 on the left side that are linked. Which are divided by a wall of muscle called the septum.


  • The upper chamber is the atrium and the lower chamber is the ventricle.


  • Atria: Thinner walls pushing blood down into ventricles.

  • Ventricles: thicker walls pushing blood out of the heart.

Valves of the Heart

  • Valves keep blood flowing in one direction and prevent the backflow of blood.


There are 4 one way valves in the heart:


Atrioventricular valves           Location: found between the atrium and ventricle

  1. Tricuspid valve (right)

  2. Bicuspid valve (left)



Semilunar Valves       Location: found between the ventricle and blood vessel

  1. Pulmonary semilunar valve (right)

  2. Aortic semilunar valve (left)

The Cardiac Cycle

  1. Atrial systole → 2. Ventricular systole → Diastole


Systole - heart pumping


Diastole - heart filling


Atrial systole - the phase when both atria contracts to force blood into the ventricles through the right and left AV valves.


Ventricular systole - the phase when both ventricles contract to eject blood into the pulmonary artery and aorta for transportation around the circulatory systems


Diastole - the relaxation phase of the cardiac cycle, no contraction takes place and blood enters the atria from the vena cava and pulmonary vein.

Cardiac Conduction System


  • The heart is mainly composed of cardiac muscle known as Myocardium


  • It is myogenic - it has the capacity to generate its own electrical impulses - that pass through the muscular walls, forcing them to contract


  • The conduction system has a set of 5 structures which pass the electrical impulse through the cardiac muscle in a coordinated fashion


  • Cardiac muscle tissue has high resistance to fatigue.

The conduction system process


  1. The SA node starts the sequence by causing the atrial muscles to contract. That's why doctors sometimes call it the anatomical pacemaker.


  1.  Next, the signal travels to the AV node,


  1.  through the bundle of HIS


  1.  down the bundle branches,


  1. and through the Purkinje fibers, causing the ventricles to contract.