Structure of the heart and the cardiac cycle.

The heart is made up of cardiac muscle and this muscle is called the myocardium. The heart is a pump that continuously pumps blood throughout the body. The picture below shows that there are four chambers to the heart. These chambers are:

Right atrium:
This is where deoxygenated blood returns to after it has been round the body. The blood returns via the superior vena cava (blood from the upper body) and the inferior vena cava (blood from the lower body). When the blood has filled the atrium it gets pushed in to the right ventricle via the tricuspid valve. This valve prevents blood from flowing back up into the atrium.

Right Ventricle:
This receives blood from the right atrium. Once the ventricle is full the heart contracts and pushes the the blood through the pulmonary valve and into the pulmonary artery (the pulmonary artery is the only artery in the body that carries deoxygenated blood). The blood is then pumped to the lungs where it is oxygenated before returning to the left atrium via the pulmonary vein.

Left Atrium:

This receives blood from the pulmonary vein. Blood that enters here has been oxygenated in the lungs and is ready to be pumped around the body. Once the atrium is full the blood is pushed down into the left ventricle through the bicuspid valve.

Left Ventricle:

The walls of the left ventricle are three times thicker than the right ventricle. This is because blood has to be pushed around the body and so requires a more forceful contraction. Once the blood has entered from the left atrium the bicuspid valve closes to stop blood flowing backwards. The left ventricle then contracts pushing blood through the aortic valve and into the aorta, where it then travels around the body providing oxygen and nutrients throughout the body before returning to the right atrium via the superior and inferior vena cava and starting the cycle again.

During the cardiac cycle there are two distinct phases. The diastole phase is where the heart is relaxed and fills with blood. The systole phase is where the heart contracts and pushes blood into the arteries.

Diastole phase:

During this phase the heart muscle is relaxed and the atrioventricular valves are open. Deoxygenated blood from the vena cavae fills the right atrium and oxygenated blood from the pulmonary vein fills the left atrium. A nerve impulse from the sinoatrial node causes the atria to contract, pushing the contents into the respective ventricles.

Systole Phase:

During this phase the atrioventricular valves are closed and the semilunar valves open. Nerve impulses from the purkinje fibres cause the ventricles to contract pushing the blood from the ventricles into the pulmonary artery from the right ventricle and into the aorta from the left ventricle.














References:


http://web.buddyproject.org/web019/web019/heart.html



http://biology.about.com/od/anatomy/ss/cardiac_cycle.htm