Blood is always regarded as being of special significance and has rightly been seen as the very essence of life from ancient times. But it was not until the circulation of blood was well described by English physician William Harvey (1578-1657) that any real knowledge was attained. Although the development of the microscope later enabled scientists to study and count blood cells and to identify different types, many of the functions of the blood have become understood only this century and mysteries still remain for research to solve. So what do we know about blood and its role in the body?

Blood is a fascinating and extremely complex substance. It is a suspension of cells in a fluid called plasma. Blood is constantly flowing round in a closed system of vessels, the cardiovascular system of blood circulation. In this way it comes into contact with the environment of every cell in the body, the tissue fluid.

Transport function

The transport of nutrients to every body cell is one of the most important functions the blood performs; in addition to oxygen, other essential nutrients, such as glucose (sugar), are delivered to the tissues. In exchange, the waste products of metabolism are collected and carried to the liver, kidneys and lungs where they can be dealt with. Another essential role is that blood carries hormones such as insulin”’ and Cortisol.


Blood is also an important part of the immune system, for the body’s self-defence. White cells called lymphocytes and a family of proteins, the immunoglobulins, fight against infection. The blood acts as a vehicle to carry this vital army of cells from the bone marrow and lymph nodes, where they are manufactured, to the tissues where they are needed. Blood also has its own form of defence. To prevent, or at least reduce, blood loss following injury, it has a clotting system. This works via a complex interaction between platelets and a group of proteins known as clotting factors which between them act to block a leak in the circulation.

Maintaining constant composition

The cells of the body are embedded in the so-called interstitial or tissue fluid, the ‘internal environment’ of the cells. For a cell to perform its function adequately, the composition of this fluid should remain constant. This is an important function of the blood, because there is a close contact between blood and tissue fluid.

Because of its water content, blood has a large capacity to conduct heat. At places of heat production – for example in muscles or in the liver – the blood quickly takes up this heat. On its way through the body it reaches places with a lower temperature, which are then warmed up again. Excess heat can be yielded to the environment, when blood flows through the skin. To keep the composition of tissue fluids at an optimum, various chemicals have to be present in the blood in balanced amounts. This holds true for carbohydrates, fats, proteins and minerals – such as sodium or potassium. It also holds true for the amount of acid substances in the blood.

The control of the blood’s acidity or alkalinity, called the pH, is essential for the functioning of all cells. Normal metabolism produces an excess of acid and maintenance of body pH at, or around, the body’s normal value of 7.4 depends on adequate compensation, or buffering. Excess acid products are excreted via the lungs as carbon dioxide or from the kidneys. The blood not only provides for transport of these acidic substances to the lungs and kidneys but also contains three important buffers: bicarbonate ion, haemoglobin and plasma proteins. Bicarbonate is the principal buffer. Under the influence of an enzyme called carbonic anhydrase it combines with the acid hydrogen ions to form a very weak acid, carbonic acid. In this way the acidity of the blood hardly changes at all when comparatively strong acids are released into it.

Volume of blood

A normal adult has about 4.5 litres (8 pints) of circulating blood compared with only a few hundred millil-itres in a newborn baby. Sudden loss of more than a third of the body’s blood reduces the supply of blood to the heart’s muscle sufficiently to cause a heart attack; less severe blood loss may lead to a state of shock with symptoms of cold extremities, rapid pulse and low blood pressure. The restoration of blood volume in someone who has suffered a severe haemorrhage can be a life-or-death emergency. The victim is given a blood transfusion or, if this is not possible, a transfusion of plasma or saline solution.

Blood contents

Every millilitre of blood contains many millions of blood cells. The relative proportions of the cells and the amount of liquid plasma can be seen when a tube of blood, with an additive to prevent it clotting, is allowed to stand. Different layers separate out with a deep layer of red cells covered by a thin cream-coloured layer of white cells, then platelets above this and topped by the clear yellow plasma which makes up 55 per cent of the total volume. If blood is allowed to stand without an anti-clotting additive, after about 30 minutes a fluid exudes from a mass of blood that is the clot. This is the serum, which is essentially plasma without the clotting chemicals. Plasma contains a complex mixture of chemicals, chiefly proteins, with different origins and many different functions. Albumin is the most common. It is produced by the liver and is important in maintain- ing the balance of water between the inside and outside of blood vessels. The concentration of albumin in the plasma also reflects the general state of nutrition and the manufacturing capabilities of the liver. Another protein group, the globulins, are a diverse family. Of prime importance are the immunoglobulins or antibodies. These are produced in response to an infection and can play an important role in the development of allergies or of certain inflammatory disorders such as rheumatoid arthritis. The globulins also include the transport proteins, for example for iron, copper or hormones. Other proteins found in plasma include hormones, various enzymes and blood-clotting chemicals called clotting factors. The plasma water is not water as we know it but a precise mixture of chemicals dissolved in water. The relative concentrations of sodium and potassium are kept constant by a number of complex controlling mechanisms. Similarly, the concentration of nutrients, such as glucose, and metabolites, such as urea, are monitored and closely adjusted by the body’s regulating systems.

Blood and capillaries

Nutrients must be able to leave the blood and enter the tissues and for this to occur there are small holes between the cells in the walls of the capillaries (tiny blood vessels). The pores are extremely small, only six millionths of a millimetre across, but allow small molecules such as water, salt and glucose to pass relatively freely. Larger molecules such as albumin and the other plasma proteins are too large to pass through these pores. Their presence in large numbers is largely confined to within capillary walls where they create a force called osmotic pressure. This pressure depends on the concentration of the large molecules; in their absence water would pour freely from blood into the tissues and blood pressure could not be maintained. The osmotic pressure draws the water back from the tissues and this process repeats many times during the flow of blood through the capillaries. It results in a proper balance of nutrients and metabolites between the blood and body tissues.