MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 4. BLOOD SYSTEM

HEMOSTASIS

2. BLOOD COAGULATION SYSTEM

Blood clotting is a protective physiological response that prevents excessive blood loss. Escaped from the Circulatory system, blood coagulates within 3–4 minutes, transforming from a liquid state into a jelly-like clot that seals the vascular lesion. Coagulation is a complex enzymatic process involving platelets and so-called plasma clotting factors. There are thirteen recognized plasma clotting factors, designated by Roman numerals: I, II, III, ..., XIII.

The Blood Coagulation mechanism comprises three distinct stages. The First stage is characterized by the release of tissue and blood thromboplastin. Tissue thromboplastin is released from the membranes of damaged Tissues and vessel walls, whereas blood thromboplastin is formed from the membranes of ruptured platelets and erythrocytes. Successful completion of this stage requires Caⁿ⁺ ions and numerous plasma factors. The first stage culminates in the activation of factor X.

A deficiency in antihemophilic globulin drastically impairs thromboplastin formation, which in turn inhibits blood coagulation. This condition is known as hemophilia, where even a minor finger injury can result in severe blood loss.

In the second stage, the plasma protein prothrombin is converted into the active enzyme Thrombin, driven by the thromboplastin generated in the first stage. Caⁿ⁺ ions are likewise essential for the normal progression of this reaction. Prothrombin is synthesized in the Liver, a process that requires vitamin K.

In the Third Stage, the soluble blood protein fibrinogen is transformed into insoluble fibrin through the action of thrombin produced in the Second Stage. This reaction also requires Caⁿ⁺ ions and plasma factors. Fibrin forms fine threads that weave into a dense meshwork. Driven by thrombosthenin released from damaged erythrocytes, clot retraction (shrinkage) occurs. As a result, the dense clot excludes even blood serum.

Removing fibrinogen from blood prevents it from clotting; such blood is referred to as defibrinated. Blood also loses its clotting ability in the absence of Caⁿ⁺ ions. Coagulation can be prevented using sodium citrate. Blood treated this way is called citrated blood and is widely used for transfusions.