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The Prince Charles Hospital Health Service District

How Does the Normal Heart Work?

The heart consists of four chambers- the two upper chambers are the Atria; the two lower chambers are the Ventricles. Dividing the Atria and the Ventricles vertically is the Septum. Valves in the heart control the flow of blood during the heart cycle. There are two atrioventricular valves. These are the Mitral and Tricuspid Valve. The Tricuspid Valve allows blood to flow from the Right Atrium to the Right Ventricle; the Mitral Valve allows blood to flow from the Left Atrium to the Left Ventricle. There are also valves at the opening of the great vessels i.e. Pulmonary Artery and the Aorta.

During the heart cycle the Atria contracts simultaneously and this is followed by the Ventricles contracting simultaneously. The right side of the heart deals with deoxygenated (or Venous) blood. This venous blood returns to the Right Atrium from the head and neck via the Superior Vena Cava. Blood returning from the lower body is via the Inferior Vena Cava. (See Diagram)

Key to image:

Ao:	Aorta
IVC:	IVC
LA:	Left Atrium
LV:	Left Ventricle
PA:	Pulmonary Artery
RA:	Right Atrium
RV:	Right Ventricle
SVC:	Superior Vena Cava

The actions of a Normal Heart Beat Cycle: - See diagram:

Right side of the Heart:

• When the atrial pressure is sufficient the Tricuspid Valve opens allowing blood to flow into the Right Ventricle. When the ventricular filling is of sufficient pressure:
  
• The Tricuspid Valve closes
  
• The Pulmonary valve opens
  
• The Right Ventricle pumps venous blood via the Pulmonary Artery to the Right & Left Lung. This is where oxygenation occurs.

Left side of the Heart:

• Oxygenated (Arterial) blood is then returned from the lungs into the Left Atrium of the heart via the Pulmonary Veins located at the Left Atrium.
  
• As the pressure in the Left Atrium rises, the Mitral Valve opens allowing blood to flow into the Left Ventricle.
  
• When the pressure in the Left Ventricle is higher than that of the Left Atrium:
  
  • the Mitral Valve closes,
    
  • the Aortic Valve opens,
    
  • the Left Ventricle contracts pumping arterial blood into the aorta.
    
• The arteries branching off the Aorta distribute arterial blood to the body.

The heart has its own conduction system or pacemaker. In the right atrium a small mass of specialised cells (the Sinoatrial node) controls the heart rate by firing an electrical impulse. This impulse spreads across the Atria to the Atrioventricular node, which is situated at the septum close to the Atrioventricular valves, resulting in contraction of the Atria. The electrical impulse then travels down throughout the Septum and into the Ventricular Wall which results in Ventricular Contraction.