Blood, the viscous liquid in arteries and veins, is rapidly circulated by the heart.

Blood has a complex composition and supplies every tissue with nutrients while performing a multiplicity of functions.

Blood flows through a series of very small vessels (the capillary net between the arterial and the venous circulation) which 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 foreign organisms. This defense mechanism uses a multitude of different components found in blood - antibodies and different types of white cells (e.g. neutrophils, monocytes, lymphocytes, eosinophils and basophils).

Every beat of the heart sends about half its volume of blood towards the lungs. There, the carbon dioxide is removed 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 also known as erythrocytes. White cells or white corpuscles are also known as leukocytes (many different types exist: neutrophils, monocytes, lymphocytes, eosinophils, basophils).

Red Cells (Erythrocytes) contain the hemoglobin. 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 specific mechanisms of immunity.

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

Plasma is a yellow liquid that contains variable quantities of other components including nutritional materials and waste products.

The physiochemical properties of plasma are remarkably constant, especially its pH (the degree of acidity, which is maintained at pH7.42) as well as the concentration of various inorganic substances (especially sodium, potassium, chlorine, phosphates,...)

  • The nutritional materials in the plasma are composed of sugars (mainly glucose), fats (cholesterol, triglycerides, fatty acids), amino acids (building blocks 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 hemoglobin as a result of the destruction of red cells by the macrophages (a type of monocyte).
  • There are numerous different proteins in the plasma.
    • All the coagulation factors including fibrinogen which, upon activation by thrombin, polymerizes to form the fibrin clot
    • Albumin, which is quantitatively the most abundant protein in plasma in healthy individuals, playing 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).

Blood Analysis

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

There are 3 common types of blood analysis:

  • Hematological
  • 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 certain cases, when only a few drops of blood are needed, it can be collected after pricking a fingertip. The results of the tests obtained are compared to standard norms that may vary according to the age and sex of the patient, but also by the method used by the laboratory performing the analysis.

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