Mechanism of Blood Clotting
| Home |Chapter: HAP - Haemopoietic System
Blood clotting (coagulation) is the process by which liquid blood turns into a gel-like clot to prevent blood loss after an injury. It is an essential part of hemostasis, which stops bleeding and allows healing to begin.
BLOOD
CLOTTING
Blood clotting (coagulation) is the process
by which liquid blood turns into a gel-like clot to prevent blood loss
after an injury. It is an essential part of hemostasis, which stops
bleeding and allows healing to begin.
Blood normally contains several inactive
clotting factors (procoagulants). When a blood vessel is damaged, these factors
become activated, leading to clot formation.
MECHANISM OF BLOOD CLOTTING
Blood clotting involves three major
mechanisms:
1. Vasoconstriction
- When a
blood vessel is injured, the smooth muscles in its walls constrict.
- This
reduces blood flow and slows blood loss.
- Platelets
stick to the damaged site, making the surface sticky.
2. Platelet Plug Formation
Platelets form a temporary “plug” at the
injury site by three steps:
|
STEP |
DESCRIPTION |
|
Platelet adhesion |
Platelets stick to exposed collagen of
damaged vessel |
|
Platelet release reaction |
Platelets release chemicals such as ADP
and serotonin |
|
Platelet aggregation |
More platelets stick together, forming a
plug |
This plug closes small injuries
temporarily.
3. Blood Coagulation (Clotting)
- Blood
becomes a semi-solid clot due to conversion of soluble fibrinogen into
insoluble fibrin.
- Fibrin
threads form a mesh that traps RBCs and seals the wound.
STAGES OF
BLOOD CLOTTING
Stage 1: Formation of Prothrombin Activator
This is the most important initiating step.
Prothrombin activator is formed by two pathways:
|
PATHWAY |
TRIGGER |
WHERE IT OCCURS |
|
Intrinsic pathway |
Contact of blood with a foreign surface
inside the body |
Begins within blood |
|
Extrinsic pathway |
Release of tissue thromboplastin from
damaged tissues |
Begins outside blood |
Both pathways require calcium ions (Ca²⁺)
and clotting factors.
Clotting Factors
|
FACTOR NUMBER |
NAME |
|
I |
Fibrinogen |
|
II |
Prothrombin |
|
III |
Tissue thromboplastin |
|
IV |
Calcium ions |
|
V |
Labile factor |
|
VII |
Stable factor |
|
VIII |
Antihemophilic factor |
|
IX |
Plasma thromboplastin component |
|
X |
Stuart–Prower factor |
|
XI |
Plasma thromboplastin antecedent |
|
XII |
Hageman factor |
|
XIII |
Fibrin-stabilizing factor |
Stage 2: Conversion of Prothrombin into
Thrombin
- Prothrombin
activator formed in Stage 1 converts prothrombin → thrombin.
- Thrombin
is the key enzyme in blood clotting.
Once thrombin is formed, clot formation
becomes rapid and almost irreversible.
Stage 3: Conversion of Fibrinogen into
Fibrin
- Thrombin
converts fibrinogen (soluble) into fibrin (insoluble).
- Fibrin
strands form a mesh to trap blood cells and form a solid clot.
- Factor
XIII (fibrin stabilizing factor) strengthens and tightens the clot.
