The Heart: How it works

• The heart has four main chambers; the left ventricle, the right ventricle, the left atrium and the right atrium
• The left atrium receives oxygenated blood from the pulmonary vein (lungs)
• It then passes through the atrioventricular valves into the left ventricle.
• Blood from the left ventricle passes through the semi-lunar valves then leaves through the aorta to the body
• The right atrium receives deoxygenated blood from the superior and inferior vena cava (head and body)
• It passes through the AV valves into the right ventricle.
• From the rigt ventricle, the blood passes through the semi-lunar valves and through the pulmonary artery to the lungs

(Then back through the pulmonary vein)

• Thicker left ventricle: The left ventricle wall is more musular than the right ventricle wall because it needs to be able to contract more powerfully to pump blood all the way around the body.
• Thicker Ventricles: The ventricles need to be more muscular than the atria as they need to push blood out o the heart rather than into the next chamber.
• Valves: The atrioventricular and semi-lunar valves prevent the blood flowing back into the atria (AV) and the ventricles (SL) when the chambers have contracted
• Cords: The AV valves have cords (cordae tendinae) which generate tension that stops the AV valves from being forces into the atria when the ventricles contract

The cardiac cycle has three main stages: atrial systole, ventricular systole and diastole

1. Atrial Systole: The ventricles are relaxed. The atria contract which reduces the volume of the atria and increases the pressure in them. As the pressure is now higher in the atria than in the ventricles, the AV valves are forced to open to allow blood through to the ventricles.
2. Ventricular Systole: Blood from the atria is flowing into the ventricles. The ventricles contract when all the blood is in them, again increasing pressure and reducing volume. The higher pressure in the ventricles causes the AV valves to close (this makes the first noise) preventing backflow. The pressure is now higher than the aorta and pulmonary artery, which orces the SL valves open, so blood is forced into them.
3. Diastole: Both the atria and the ventricles are relaxed. The SL valves close (the second noise). Blood comes back to the heart and refills the atria, so the pressure is increasing and the volume is decreasing. Some blood is able to flow passively from the atria to the ventricles (without atrial systole). 

The heart is myogenic which means it is able to contract and relax without receivig any signals from the brain or nerves.