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Circulatory System

The blood

The blood is composed of three types of cells suspended in a straw-coloured fluid called plasma. Plasma is over 90% water but also contains a complex mixture of vitamins, proteins, and hormones. The three types of cells in the blood

  • Red blood cells. These account for the majority of the blood cells in the plasma. Their main function is to carry oxygen. This is bound to a protein called haemoglobin within the red cells.
  • White blood cells. The main function of these blood cells is dealing with infection.
  • Platelets. These cells play a vital role in repairing damaged blood vessels. By attaching themselves to the damaged vessel wall they trigger the formation of blood clots. These blood clots prevent further bleeding.

An average 70kg (11stone) person has about 5 litres (8 pints) of blood in their circulation. The circulation is a closed system of blood vessels. The blood is pumped through this system by the heart.

The blood circulation

In order to survive and function properly, every organ in the body needs a continuous supply of oxygen and nutrient-rich blood. The circulatory system is the means by which the body transports nutrients absorbed from the gut and oxygen absorbed from the lungs to the tissues. The circulation also removes the waste products from the tissues. Carbon dioxide is returned to the lungs to be breathed out. Other waste products are taken to the liver and kidneys where they are filtered away.

The circulatory system is a closed network of blood vessels which, in conjunction with the heart allows blood to flow throughout the body. The heart is the pump of the system, moving blood continuously around the body. Blood is the means by which the body delivers oxygen and nutrients to the tissues and carries away the unwanted waste products. 

The heart is effectively two pumps working in series.

  • In the major or systemic circulation: oxygenated and nutrient rich blood is pumped from the left ventricle into the aorta which is the largest artery in the body. From the aorta arise all the major branches carrying blood via arteries, arterioles (smaller arteries) and capillaries (microscopic blood vessels) to the tissues. The arteries are the passageways through which fresh blood is delivered. Waste products from the tissues are taken back to the heart via thinner walled blood vessels called veins. Laid end to end, all these vessels would stretch for about 60,000 miles - enough to encircle the earth more than twice.
  • In the minor or pulmonary circulation: blood flows from the right atrium, to the right ventricle and is then pumped through the vessels of the lungs and drains back to the left atrium for the cycle to continue. Within the lungs, oxygen is added to the blood and carbon dioxide is removed.


Heart problems may be associated with any of the following symptoms:

  • Chest pain
  • Breathlessness
  • Ankle swelling (oedema)
  • Palpitations


The type of symptoms patients may develop are determined by their underlying condition. They can range from just one of the above, a combination, or even all of them.

Chest pain

Chest pain caused by the heart muscle receiving insufficient oxygen is called angina. Angina is a temporary discomfort in the chest that may vary from a mild tightness or discomfort to a severe crushing heaviness. It may spread to the jaw, arms or through to the back and shoulder blades. It is often associated with breathlessness.

The duration of anginal pain is usually brief, occurring on exertion and lasting for only a few minutes or less. You will learn to recognise your own pattern of symptoms: that is when they are likely to occur, how long they will last and the type of discomfort you may feel.

There are many other commoner causes of chest pain such as muscular pains, heartburn or indigestion. 


Breathlessness caused by heart-related problems is usually due to the heart muscle not pumping with its normal efficiency. This causes congestion or a build-up of fluid in the lungs. Breathlessness may be present with symptoms including:

  • Shortness of breath on exertion or at rest.
  • Breathlessness or coughing when laid flat the medical term for this is orthopnea
  • Waking up breathless at night; the medical term for this is paroxysmal nocturnal dyspnoea, usually abbreviated to PND.

Swelling of the ankles and feet (oedema)

When the heart fails to pump with its normal efficiency a backpressure of fluid may build up through the lungs and veins, causing congestion in the tissues. Most commonly this fluid accumulates in the lungs and the legs; the site of fluid accumulation depends upon which chambers of the heart have been affected. Fluid in the lungs interferes with breathing and fluid in the legs causes swelling known as oedema’.


The term palpitation simply means an awareness of the heartbeat. The heart may be beating at a normal rate, slowly, fast, erratically or just missing beats. Abnormal heartbeats are called arrhythmia. The majority of palpitations are a nuisance rather than a danger. Although they can be unpleasant they are usually quite harmless. On occasions they may indicate problems with the heart muscle, valves or arteries. Palpitations may occur on their own or at the same time as other symptoms such as breathlessness, dizziness, blackouts or chest pain.

Blackouts or syncope

Blackout or syncope are the medical terms used for what is commonly known as fainting. They are associated with a sudden but temporary loss of consciousness that recovers quickly, completely and on its own.

Syncope is a symptom of another underlying problem and not a disease itself. The symptoms may take the form of or be associated with:

  • Fainting
  • Near fainting (near or pre-syncope)
  • Dizziness
  • Palpitations