32.1 Introduction

Blood is a fluid connective tissue and consists of blood cells and various factors suspended in the plasma. It is responsible for circulation of oxygen, nutrients throughout the body and removal of toxic metabolites by transporting them to the excretory system. If blood is allowed to stand in a glass tube it coagulates and if anticoagulants like heparin or chelating agents like ethylenediaminetetraacetic acid (EDTA) is added, it prevents blood coagulation and if kept still for half an hour red blood cells (RBC) form rouleaux (stack of red blood cells) and settle down leaving a clear plasma. On centrifuging the blood at low speed (around 2000 rpm) clear zone of separation between the RBC and the plasma can be seen as a white buffy coat that consists of WBC's. Contact activation explains the reason behind the coagulation of blood when it is taken from the body and put in a glass tube. The glass surface acts just similar to the collagen and induces similar activation of factor XII and aggregation of thrombocytes (platelet) as a damaged vessel surface. A silicone coating if applied on the glass tube prolongs clotting since its smooth texture reduces the activity of the glass surface.

In this lecture initially, we shall understand the importance and mechanism of blood clotting followed by disorders related to blood and the available remedies.

32.2 Process of blood clotting:

A simplified schematic below depicts the process of blood clotting. During any injury if a blood vessel ruptures then the blood gets exposed to the subendothelial tissue where the plasma enzyme activation cascade begins to function. The endothelium has a unique feature of allowing proper fluidity at normal condition but when it encounters injury, it starts the coagulation process (mediated by various factors and secretions of the endothelial cell) in order to contain the injury and prevent any further loss of blood.

Normally, endothelial cells are not involved in the synthesis of tissue factors but when they encounter cytokines or interact with monocytes they begin to synthesize the tissue factor. The tissue factors are glycosylated intrinsic membrane proteins, which come in contact with blood only when there is any injury. They are located on the adventitious side of the endothelial cells hence do not come in contact with blood under normal condition. There are two classical pathways of blood clotting extrinsic and intrinsic. Since, the tissue factor is an extrinsic protein thus, the clotting initiated by tissue factor falls under the extrinsic pathway. All the blood clotting factors are present in the plasma hence coagulation due to the internal components of plasma falls under the intrinsic pathway.