1) Blood is carried to the atria of the heart.
- Vena cava carries deoxygenated blood from the body tissues to the right atrium.
- Pulmonary vein carried oxygenated blood from the lungs to the left atrium.
2) As the atria fill, the pressure increases.
3) The atrioventricular valves open, causing the blood to flow into the ventricles.
4) Atrial systol occurs, and the atria contract, ensuring any remaining blood is forced into the ventricles.
5) Pressure in the atria then decreases as it goes into diastol (relaxes).