The heart is arguably the most important organ in a person's body. Indeed, it is the first organ to start working - the human heart starts beating 21 days after conception. Without the heart transporting blood around the body, tissues and cells would be unable to work and the human body would be unable to sustain itself.
Anatomy of the Heart
The heart is divided into 2 parts: Atria and Ventricles. You have an atrium and a ventricle on either side of your body - hence the average person would have a total of 4 chambers comprising of 2 atria and 2 ventricles. Without going into detail, you may think of the atria as 'receiving chambers': they receive blood from parts of the body. The ventricles however play the important role of pumping blood out of the heart to the rest of the body.
Ventricles are essentially a Pump
The heart transports blood around the body - this is a simple fact. Blood carries oxygen, which is required by almost all the cells in our bodies. How does it do this?
The Right Ventricle
First of all, blood has to go to the lungs where it receives oxygen from inhaled air. It is the function of the right ventricle to pump the oxygen-poor blood into our pulmonary circuit (to the lungs), where carbon dioxide diffuses out of the blood and oxygen diffuses in. The oxygen rich blood then travels down the pulmonary artery into the left atrium. Valves open and the left atrium contracts, causing blood to move into the left ventricle.
The Left Ventricle
The Left Ventricle is much thicker than the right because it has to pump blood to the rest of the body - and let's face it, your feet are a lot further away from your heart than your lungs are. It also has to overcome the higher pressure of the aorta (the artery leading out of the heart to the rest of the body).
Relaxation of the Ventricles
The moment the ventricles contract and send blood to the lungs/body, they start to relax. This makes it easier for blood to enter the ventricles from the atria as they do not have to work against the a high pressure. This period is known as the diastolic (filling) period, and after the ventricles are filled with blood they contract once more to repeat the above sequence.
The Cardiac Cycle
The atria passively receives blood from parts of the body (oxygen-poor blood) as well as the lungs (oxygen-rich blood). Valves between the atria and ventricles open, allowing blood to flow into the ventricles (ventricular filling). Near the end of this period, the atria contract slightly to ensure that all blood enters the ventricles. The ventricles contract and the valves snap shut to prevent blood from flowing back into the atria. Instead, blood flows out through blood vessels from the right ventricle to the lungs and from the left ventricle to other parts of the body. This sequence of events occur at an average of 70 times per minute, giving rise to a pulse.