Before explaining what Haemostasis is, we need to put the question into context. Pathology has many disciplines - Haematology, Biochemistry, Virology, Bacteriology, Molecular Biology, etc...

- Our field of activity (haemostasis) is included in the area covered by Haematology
- the science of blood and its diseases. In order to better understand our activity, a simple understanding of blood is required.

BLOOD

              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 a multiplicity of 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 defences against micro-organisms: bacteria, parasites and viruses. This defence 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).

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

The 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 defence against infections (act as 'scavengers'), while the lymphocytes play a role in the cellular supports of immunity (the more specific defence mechanism).

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

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

Its physiochemical properties are remarkably constant, especially its pH (the degree of acidity, which is maintained at pH7.42) and the concentration of various inorganic elements (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 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. It 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 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 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?

HAEMOSTASIS

Haemostasis is the body's normal physiological response for the prevention and stopping of bleeding/haemorrhage. It results in the blocking of any vascular breach. Generally speaking, it helps ensure blood fluidity and blood vessel integrity. Abnormalities in haemostasis can result in bleeding (haemorrhage) or blood clots (thrombosis).

Haemostasis consists of:

• Primary Haemostasis with:
              • Local vascular contraction (to reduce blood flow to the injury site)
              • platelet plug formation

• Clotting of the plasma (secondary haemostasis), involving interaction between numerous factors and inhibitors.
• Fibrinolysis - process for removing the clot once blood vessel integrity has been restored.

When there is a breach in a blood vessel, the first priority (primary haemostasis) is to "plug" this breach. The main players in the blood are the platelets and Fibrinogen: these react together and block the breach by the formation of a platelet plug.

Brêche = Breach
Epanchement sanguin = Bleed/haemorrhage
Aggrégation plaquettaire = platelet aggregation
Adhésion plaquettaire = platelet adhesion
Activation de la coagulation = activation of coagulation
Fibrine = fibrin

Figure 1: Formation of the clot. (Source: Introduction à l'étude de l'hémostase et de la thrombose - B. Boneu, J-P. Cazenave - 1997) Bleeding at the site of the vascular injury is stopped by the formation of an extravascular clot. An injury causes a transitory vasoconstriction of the small blood vessels reducing blood flow. Platelets stick to the sub-endothelial tissue at the site of injury and finally they aggregate together. Initiation of coagulation leads to the formation of fibrin, which helps stabilise this clot and stops bleeding.

Following this first step, the formation of a clot (coagulation) stops any further bleeding (secondary haemostasis). This process consists of a series of chemical reactions involving various plasma components. To date 10 major coagulation factors are known to be involved in this process. These complex interactions lead to the transformation of a soluble protein, the fibrinogen, into an insoluble protein, the fibrin, which forms the frame of the clot Wound healing finally closes the bridge and fibrinolysis dissolves the clot.

Abnormalities in Coagulation

Coagulation problems are diagnosed by laboratory examination of the blood coagulation process, either by using a global test (clotting time), analytical test (looking specifically at the different components of coagulation) or by a 'blood count' (the amount of haemoglobin and the number of white cells, red cells and platelets per cubic millimetre of blood).

Some of these tests can also be used to measure the effect of any anticoagulant treatment that is given to treat or reduce the risk of thrombosis. Diagnostica Stago's activity is to develop and design the equipment and the different test methods that are used in the laboratory to measure and allow better understanding of coagulation/haemostasis.

CONCLUSION

Haemostasis can be compared to a balance

A careful equilibrium between coagulation factors (activators) and anti-coagulation factors (inhibitors) maintains blood fluidity. Any upset in this equilibrium will upset this balance resulting in either:
              - Thrombosis: the formation of a clot or
              - Haemorrhage: bleeding

VISUALISATION OF THROMBOSIS

Venous thrombosis is a common disease and it is estimated that 159 people in every 100,000 are affected each year. Clots in veins are the most frequent, but the major risk is when these break and block blood flow in the lungs (pulmonary embolism). Because of the danger associated with this type of disease, a number of experimental models have been created to study the effect of antithrombotic drugs. The following model system has been used to visualise the formation of a thrombus by the haematology department at the faculty of Pharmacy in Bordeaux, France.

Click on the picture to display the formation of the clot

In this experimental model of thrombosis a laser creates a vascular lesion in small veins or arterioles (diameter between 15 to 25µm). Blood flow can be observed using reverse phase microscopy and recorded.

In this model you can see that immediately after the lesion is created a small clot (thrombus) is formed. This is rapidly washed away in the blood flow and this cycle repeats many times. Eventually a large clot forms reducing the blood flow. In certain cases the thrombus can occlude (block) the vessel, stopping blood flow and can result in tissue damage.

               - If you wish to see the film of clot formation please click on the figure.
              - To obtain the bibliography on this experimental model please go to
               http://www.u-bordeaux2.fr/hemato/p05.html.