How do substances get into, out of and around our bodies?The heart

Blood is pumped around the body by the heart. The heart is an organ made of cardiac muscle tissue. Therefore it can contract.

The heart has four chambers - two atria and two ventricles.

Blood enters the heart through the atria. Blood from two vena cavae enters the right atrium. Blood from the pulmonary veins enters the left atrium.The atria fill, followed by the ventricles. Blood is prevented from flowing back into the atria by heart valves.

Blood leaves the heart in the body's main artery - the aorta - from the left side, and the pulmonary artery, from the right.

Double circulation

Humans have a double circulatory system. The heart pumps blood through two circuits:

• the pulmonary circulation
• the systemic circulation

The pulmonary circulation transports blood to the lungs. At the lungs:

• oxygendiffuses into the blood from the alveoli - the blood becomes oxygenated
• carbon dioxide diffuses from the blood into the lungs.

The systemic circulation transports:

• oxygen and nutrients to the body
• carbon dioxide and other wastes away from cells

The systemic circulation is under high pressure - it has to deliver blood to the extremities of the body.

The pulmonary circulation is under lower pressure, because:

• blood is delivered to the lungs only, which are very close to the heart
• the wall of the right ventricle is much thinner than the wall of the left ventricle, meaning it cannot create as much pressure when it contracts
• in a healthy person, this lower pressure is optimum for the diffusion of gases

Circulating blood

Blood passes through the heart twice in one complete circulation of the body.

Following the blood through one complete circulation of the body:

Note that although this is shown as a sequence, the atria contract at the same time. When blood has been delivered to the ventricles and they are full, they also contract at the same time. Part of the blood is being delivered to the body, while the remainder of the blood is being transported to the lungs. This means that oxygen can be picked up from the lungs, while at the same time, blood is also being delivered to the body.

Listen to a podcast about the circulatory system with Dr Alex Lathbridge.

Listen to the full series on BBC Sounds.