Blood, the red viscous liquid in arteries and veins, is rapidly circulated by a pump: the heart.

Blood has a complex composition. It irrigates every tissue and performs various functions.

The flow through a series of very small vessels (the capillary net between the arterial and the venous circulation) allows the transport of gases (oxygen and carbon dioxide), nutritive materials (carbohydrates, fats and proteins) and all the elements that are necessary for our defenses against micro-organisms: bacteria, parasites and viruses. This defense mechanism uses a multitude of different components found in blood - antibodies and different types of white cells (e.g. neutrophils, monocytes, lymphocytes and eosinophils).

Every beat of the heart sends about half its volume of blood towards the lungs. There, the carbon dioxide is removed and ejected in the expired air, while at the same time oxygen is absorbed and bound in the red cells. The other half volume of blood is sent via the aorta (arterial circulation) to the various tissues. It returns to the heart via the vena cava (venous circulation).

Blood Composition

About half the volume of blood is composed of cells (red cells, white cells and platelets) while the other half is liquid, the plasma. Red cells or red corpuscles are scientifically known as erythrocytes. White cells or white corpuscles are scientifically known as leukocytes (many different types exist: neutrophils, monocytes, lymphocytes and eosinophils).

Red Cells (Erythrocytes) contain the haemoglobin. Their fundamental role is to carry the oxygen from the lungs to the tissues.

White Cells (Leukocytes) consist of a number of different cell types. The main types are the neutrophils, monocytes and lymphocytes. The neutrophils and monocytes (including macrophages) play an essential role in the body's non-specific defense against infections (act as 'scavengers'), while the lymphocytes play a role in the cellular supports of immunity (the more specific defense mechanism).

Platelets are small circulating cells that work together with the coagulation factors (proteins in the plasma) and play an essential role in prevention of blood loss.

Plasma is a yellow liquid, composed mainly of lightly salted water (0.9%) that contains variable quantities of other components including nutritional materials and waste products.

Its physiochemical properties are remarkably constant, especially the pH (the degree of acidity, which is maintained at pH7.42) and the concentration of various inorganic elements (mainly sodium, potassium, chlorine, phosphates...)

  • The nutritional materials in the plasma are composed of sugars (mainly glucose), fats (cholesterol, triglycerides and fatty acids), amino acids (building bricks of proteins), mineral salts and vitamins.
  • Waste products mainly consist of urea and bilirubin. Urea is the final product of the degradation of nitrogenous substances, whereas bilirubin comes from the haemoglobin as a result of the destruction of red cells by the macrophages (a type of monocyte).
  • There are numerous different proteins in the plasma. These include:
    • all the coagulation factors including fibrinogen - which coagulates (clots) to form the fibrin clot (for information a plasma whose fibrin has been removed is called serum);
    • albumin, which is quantitatively the most abundant protein in plasma in healthy individuals and plays an essential role in transporting hormones and vitamins;
    • various other proteins including hormones and some growth factors. These are the chemical messengers carried by blood that regulate the production of the various cells of the different body components (e.g. erythopoietin which stimulates the synthesis of the red cells by the bone marrow).

The larger proteins are involved in a process called oncotic pressure, which helps to keep the blood volume constant.

Blood Analysis

Many diseases cause changes in blood composition that can be measured in a blood sample.

There are 3 common types of blood analyses:

  • Haematological
  • Biochemical
  • Microbiological

The first part of any analysis involves collecting a blood sample. After putting a pressure cuff on the arm above the planned puncture point, blood is collected from a vein in the bend of the arm via a needle. In some cases, when only a few drops of blood are needed, it can be collected by pricking a fingertip. The results of the tests are compared to standard norms that may vary according to the age and sex of the patient, but also to the method used by the laboratory performing the analysis.

Now let's get to the heart of the matter: what is Haemostasis ?