The objective of this article is to look at haemostasis viz., the arrest of bleeding from broken blood vessels which involves 3 steps:

(1)   VASCULAR SPASM

• The first step in haemostasis is when a blood vessel is disrupted the smooth muscle surrounding the vessel constricts as a result of a response to injury and sympathetic induced vaso-constriction formation of a platelet plug. Platelets are small cells which aggregate and coagulate at the site of injury to arrest bleeding.
 
(2)  BLOOD COAGULATION OR CLOTTING

CLOT FORMATION
• Platelets do not normally adhere to vessels due to prostacylin
• When vessels are disrupted they expose collagen fibres (afibrous protein), platelets congregate at the site and begin to release the chemicals Adenosine Di-Phosphate (ADP) which cause platelet aggregation 
• Pplatelets also release chemicals called Thromboxin 2A to enhance the blood coagulation. Thromboxin catalyses fibrinogen to fibrin to form a clot only at the site of injury 
• Clot is fully developed in between 3 - 6 minutes.

(3)  CLOT RETRACTION

• Once a clot has formed, platelets contrct within the clot and shrinks the fibrin meshwork pulling the edges of the damages vessels together.
• During clot retraction, the fluid is squeezed from the clot. This fluid is essentially plasma minus the fibrinogen and other clotting precursors and is called serum.
• A clot is a transient dance to stop bleeding until the vessel can be repaired.
• Platelets secrete a chemical partially responsible for the invasion of fibroblasts which form a scar at the site or defect.  
• When the clot is no longer required, clot retraction occurs to prevent haemmorage. The clot is dissolved by plasmin, a fibrolinic enzyme.  
• Phagocytic white blood cells remove debris and products of clot dissolution.  
 
SUMMARY
 
In summary, a VASCULAR SPASM occurs followed by PLATELET AGGREGATION & PLATELET PLUG leading to CLOT FORMATION and finally CLOT RETRACTION.
 
 
FUNCTION OF THE BLOOD
 
The main function of the blood is to carry Oxygen (O2) to sites for Cellular respiration  and Carbon Dioxide (CO2)  to Lungs for expulsion and Wastes to the Kidneys for expulsion. Further to that it also has the function of maintaining the ph of the body maintaining the body temperature. The blood also carries electrolytes in plasma for membrane excitability and  osmotic function. The blood also has other duties viz defending the body against infection, invading disease causing cells and cancer cells.