Blood and Circulation


Blood is a mixture of liquid and a number of solids. The liquid element is called plasma, a transparent yellow fluid. The solids include red and white blood cells, and platelets. These are suspended in the plasma. The composition of blood is:

  • Red blood cells – carry oxygen from your lungs to all parts of your body and carry carbon dioxide from other parts of the body to your lungs so that it can be exhaled.
  • White blood cells – help fight infections by attacking bacteria, viruses, and germs that invade the body.
  • Platelets – help your body form clots to stop bleeding.
  • Plasma – carries all the above throughout the body.

The Circulatory System

Blood is continually circulated through the lungs and body. The circulatory system consists of the heart, the arteries, the capillaries, and the veins.

The Heart

The heart is a muscular organ, which acts as a double pump. It lies in the chest behind the sternum and ribs, between the two lungs and immediately above the diaphragm.

The heart is divided into a left and right side, separated by a thin wall. Each side is further divided into an upper collecting chamber and a lower thicker muscular pumping chamber. The pressure of the blood rises from the heart pumping it out to all parts of the body.

diagram of heart

The Blood Vessels

The arteries are the stronger of the blood vessels. They take the blood away from the heart. The arteries continue to divide, becoming smaller and smaller until they become capillaries. The capillaries are very small blood vessels., consisting only of a a thin layer of cells, which allow the exchange of fluids and gases to and from the cells of the tissues. The veins are formed by joining u the capillaries and they take blood back to the heart. The smaller veins unite to form larger veins until they end in two main veins, which enter the right collecting chamber of the heart.

  • Arteries take blood away from the heart.
  • Capillaries allow the exchange of fluids and gases to and from the cells of the tissues.
  • Veins take blood back to the heart.



With each contraction of the heart, blood is forced from it through the circulatory system and with each relaxation of the heart, blood pours into the collecting chambers, which then contract to fill the pumping chambers. The amount of blood present in the circulatory system of an adult is approximately 6 litres (10 pints). When a large vessel is cut and blood escapes, the blood volume falls, the circulation fails and the vital organs no longer receive adequate supplies of oxygen.


Each contraction of the heart results in a pulse of blood. The pulse rate for a normal adult at rest is 60 – 80 per minute, the average being 72. Infants and young children have a much higher normal resting rate between 90 and 140 per minute. The pulse beat may be detected at a number of locations, for example at the radial artery about 1 cm in from the thumb side of the wrist or, more easily, at the carotid artery between the Adam’s apple and the major neck muscle.