The circulatory system has the function of transporting nutrients, gases and wastes between the cells of the body and the digestive system, respiratory system and excretory system. It also carries hormones for internal communication and co-ordination, and white blood cells for fighting disease, as well as assisting in maintaining body temperature.

The main components of the circulatory system are the blood, blood vessels and the heart.

Blood
Blood is made of a watery yellow fluid called plasma that carries dissolved nutrients, hormones and proteins. The other components of blood are the cells.

• Red blood cells are made in the bone marrow at a rate of more than 1 million per second! They carry the respiratory pigment haemoglobin, an iron-based molecule which combines with oxygen in the lungs to form the unstable bright red compound oxyhaemoglobin. This gives up its oxygen to the cells where the oxygen concentration is low.
  
  Each circulating red blood cell has a biconcave (donut) shape and has lost its nucleus allowing it to carry more haemoglobin. Without the nucleus, red blood cells can only survive a few weeks which is why they must be continuously replaced.
  
  
• White blood cells also made in the bone marrow engulf and destroy foreign substances invading the body such as disease-causing organisms (bacteria and viruses). They are larger, have a nuleus and are less numerous than red blood cells.
  
  
• Platelets are cell fragments necessary for blood clotting.
  

Blood vessels Blood vessels carry the blood around the body in a continuous network of about 95,000 km in length. There are three main types of blood vessels as well as lymph vessels. Arteries carry blood at high pressure away from the heart. Arteries have thick muscular walls to maintain this pressure. The largest artery, the aorta, branches into smaller arteries, and then smaller arterioles, to service all parts of the body. They all carry bright red oxygenated blood except for the pulmonary artery which carries deoxygenated blood to the lungs.

• Capillaries join the arteries to the veins. Their narrow diameter, enormous surface area and thin walls allow optimum transfer of substances between the blood and the body's cells (see Systems, transport and exchange). Passage through these numerous tiny vessels reduces the blood pressure considerably.
  
  
• Veins return the blood from the capillaries to the heart at low pressure. One-way valves in the veins of the legs prevent back-flow when the blood is travelling upwards against gravity. The walls of veins are thinner and less elastic than artery walls. Movement of the surrounding skeletal muscles helps push the blood along the veins.
  
  
• Lymph vessels return fluid and proteins that have leaked out of the capillaries back to the bloodstream. They rejoin the main circulatory system near its entry to the right side of the heart.

The heart
The heart is a rhythmically contracting double pump made of cardiac muscle. It is about the size of a closed fist, and is protected by the ribs.

The atria (auricles) receive the inflowing blood, and the larger, more muscular ventricles pump the blood away.

The right side of the heart pumps deoxygenated ("used") blood through the pulmonary circuit to the lungs where it picks up oxygen, and carbon dioxide is released. The blood is then returned to the left side of the heart which is sufficiently muscular and powerful to pump the blood through the systemic circuit to all tissues of the body, including the kidneys, for waste removal, and the liver for blood sugar regulation. Normal heart rate is approximately 70 beats per minute, but this increases during exercise to ensure sufficient supply of oxygen and sugar to the harder-working cells.