Circulatory System

The circulatory system consists of blood vessels and the heart.

Blood vessels

Blood vessels are of three types:

  • Arteries: Arteries are blood vessels which carry blood away from the heart
  • Veins: Veins are blood vessels which carry blood to the heart
  • Capillaries: Capillaries link arteries and veins

Capillaries are microscopic blood vessels.Their walls consist of one layer of cells. Exchange of substances between blood and body tissues takes place through the capillaries.

Blood vessels form a network which transports blood to:

  • The lungs (PulmonaryCirculation) where oxygen is absorbed from the air in the lungs and carbon dioxide is excreted from the blood into the air
  • Cells in all parts of the body (Systemic Circulation)


The heart is a muscular pump which pumps blood to all parts of the body. It maintains blood pressure in lungs and body. The heart is one of the most important organs in the body. If the heart ever stops pumping blood, the body begins to shut down and will die within a short period of time.

The heart muscle is not under voluntary control. It is an involuntary muscle. At rest, the heart contracts 65 to 75 times per minute. During physical exertion, the heart increases greatly to meet the oxygen and nutrient demand of the cells. Heart rate also increases during some emotional states.

Heart rate can be measured by counting the pulse. Pulse is a rhythmical throbbing of the arteries as blood is propelled through them. Pulse can be easily felt in locations where an artery is near the surface and can be pressed against a bone. The wrist is commonly used to measure pulse.