Circulatory System

The mammal circulatory system consists of the heart, blood and the blood vessels. There are different types of blood vessels, each type with its own functions, for example, arteries carry blood from the heart to the rest of the body and veins return blood from the body organs back to the heart.

Arteries carry blood from the heart to other body organs and as the blood enters an organ, the arteries branch into smaller vessels called arterioles, which then supply the capillaries.

The capillaries allow the exchange of materials such as oxygen
and glucose, carbon dioxide and other wastes with the body cells.

As the blood leave an organ, the capillaries supply the blood to the venules with then combine to form larger vessels called veins.



The heart pumps blood to all the major organs of the body. It consists of four chambers, two thin-walled atria (singular atrium) at the top and two thick-walled ventricles below.

Structure of the heart

Structure of the heart

The left ventricle muscles are thicker than those of the right ventricle in order to develop enough pressure to pump blood to the rest of the body.

The atria are connected to the ventricles by valves (biscupid valve on the left and triscupid valve on the right). These valves prevent backflow of the blood.

The two sides of the heart are separated by a wall of muscled called a septum to prevent the mixing of oxygenated blood on the left side and deoxygenated blood on the right side.

Blood circulation

The vena cava carries deoxygenated blood from the body into the
right atrium. The blood passes from right atrium to the right ventricle, and the right ventricle pumps it out into the pulmonary artery. The pulmonary artery carries the deoxygenated blood to the lungs.

The blood is oxygenated in the lungs and carried back to the heart through the pulmonary vein. The pulmonary vein passes the blood into the left atrium. The blood passes from left atrium to the left ventricle, and the left ventricle then pumps it out into the aorta, this time to the rest of the body.

Mammal circulatory system

This way, the blood is pumped twice (first to the lungs, then to the body) before returning back to circulation. This is the reason why the heart is called a double action pump. The advantage of double action is the building up of pressure before the blood is pumped a long distance.

Blood vessels

Arteries and veins


Arteries are wide vessels which carry blood from the heart to the organs of the body. Except for the pulmonary arteries, all arteries carry oxygenated blood.

Arteries have the following properties:

  • carry blood from the heart
  • have a narrow lumen
  • thicker walls to withstand the high blood pressure
  • elastic walls to increase blood capacity when necessary
  • no valves because backflow is prevented by blood pressure


Veins return blood from body organs to the heart. The blood pressure in veins is steady and is less than that in the arteries.

Veins have the following properties:

  • are wider but their walls are thinner, less elastic and less muscular than those of the arteries because they carry blood at low pressure
  • have valves to prevent backflow of the blood
  • have a wider lumen
  • carry blood to the heart


Capillaries are tiny vessels with walls only one cell thick. They are permeable which means they allow certain substances to pass through their walls. No living cell in the body is far from a capillary supplying oxygen and food.



Blood consists of red blood cells, white blood cells and platelets all floating in a liquid called plasma.

Red blood cells


Red blood cells are disc shaped and have no nuclei. They contain a red pigment called haemoglobin, a protein combined with iron.
Haemoglobin absorbs oxygen to form oxyhaemoglobin. Oxyhaemoglobin carries oxygen only to release it in places where the oxygen concentration is low. This is how red blood cells carry oxygen from the lungs to the body tissues.

White blood cells

White blood cell

White blood cells have different shapes and sizes but all have a nucleus. They produce antibodies and also help in fighting infections by engulfing pathogens.


Platelets are pieces of special blood cells that help to clot the blood at wounds and so help to stop the bleeding and also to close the wound against infection.


Plasma is the liquid part of the blood. It is mainly water with a large number of substances dissolved in it, such as salts, amino acids, glucose, lipids (fats), carbon dioxide, urea and hormones.

The table below shows the role of transport by the blood system

oxygenlungsbody organs
carbon dioxidebody organslungs
hormonesglandstarget organs
digested foodintestinewhole body
heatabdomen and muscleswhole body