L23 Blood, bleeding & clotting

Question 1

What is blood?

Answer 1

specialised body fluid

contains nutrients

Question 2

Functions of blood

Answer 2

-Delivery of O2 and nutrients-aa, glucose to tissues
• Transport of waste products out of circulation-via kidney (eg uric acid)
• Immune response
• Haemostasis: balance between bleeding and clotting
• Regulation of pH
• Regulation of body temperature

Question 3

What are the components of blood?

Answer 3

• Plasma (liquid component)
• Cellular elements
− Platelets
− Blood cells (Red, white)

Question 4

Plasma

Answer 4

55% of blood
~90% water by volume
complex system containing proteins from a variety of cellular localisations
Intracellular and membrane proteins secreted in plasma as a result of cell lysis and cellular turn-over.
– Considered to be one of the best “reporter systems” by clinicians as it communicates with most parts of the body.
• Also contains nutrients, salts and waste products.

Question 5

Plasma proteins

Answer 5

Most plasma proteins are synthesized in the liver.
• Most disease states tend to be reflected by changes in plasma proteins:
– Troponin levels in plasma are used as an indicator of acute ischemic heart disease
(e.g. increase in concentration indicates heart muscle damage)
– Cancer Antigen (CA)-125 or Prostate Specific Antigen (PSA) as markers of specific cancers.

Question 6

Platelets/thrombocytes

Answer 6

• In charge of blood clotting
• Platelet membrane is the site of interaction with the damaged vessel wall (platelet plug).
• Surface for interaction of coagulation factors
• Anucleate cell fragments (no DNA)
• Source of growth factors
• The number of platelets is important for balance between bleeding and clotting!

Question 7

Vascular Endothelium

Answer 7

Non-thrombogenic
• Central regulator of Haemostasis
• Able to constrict in response to injury (vasoconstriction)
• Proteoglycans on the endothelial surface are able to activate and control coagulation response at the site of injury.

Question 8

Endothelium

At baseline and following injury

Answer 8

• At baseline = antithrombotic
– antiplatelet, anticoagulant & fibrinolytic-any clots formed, lysised quickly properties

• Following injury = prothrombotic

Question 9

Endothelium and Platelets

receptors

Answer 9

1. vWf binds GpIb platelet receptor
   (platelets & endothelium) -> holds platelets to the endothelium
2. Fibrinogen-found in the plasma binds GpIIb-IIIa platelet receptor
   platelet aggregation-binding of platelets together

• Congenital deficiencies in receptors or bridging molecules lead to diseases.

Deficiencies:
Glanzmann thrombasthenia– gpIIB-111a effector not expressed, platelets do not aggregate to each other
Bernard-Soulier syndrome- ineffective gPIb- platelets don’t bind to endothelium, difficult to stop blood lost

Question 10

Haemostasis

Answer 10

Balanced interaction of blood cells, vasculature-endothelium and plasma proteins.

Question 11

Functions of Haemostasis

Answer 11

• Maintain blood in a fluid state whilst circulating throughout the vascular system.
• Impedes loss of blood & disturbance of blood flow.
• Provides repair of injured vasculature & tissue.

• Achieved by thrombus formation (at the site of injury) and subsequent breakdown.

Question 12

Components of Haemostasis

Answer 12

• Blood vessels
• Platelets

• Plasma coagulation factors
(coagulation proteins)
• Plasma coagulation inhibitors: proteins in the plasma that are pro-coagulant and proteins are anti-coagulant
• Fibrinolytic system- system that helps to break down clots

Question 13

Tissue protein

Answer 13

• Membrane protein
– Intracellular

– Trans-membrane

– Extracellular domain

• Mostly located in sub-endothelium and becomes exposed when endothelium is damaged.
• Localized almost in all tissues

– except in joints – where haemophiliacs have most bleeding problems.

Tissue factor absent in joints

Question 14

Thrombin (Flla)

Answer 14

• “MASTER REGULATOR” of hemostasis
• Converts Fibrinogen to Fibrin
• Activates FXIII to form cross-links between fibrin monomers.
• Activation of platelets, neutrophils, leukocytes, endothelium
• Activates FV and FVIII in a positive feedback mechanism to activate more clotting proteins to produce more fibrin
• Thrombin Involved in Negative feedback when fibrin

Question 15

Clot

Answer 15

• Relatively stable structure (hours/ days)
• Mechanically (impact of flow) & Chemically protected from enzymes that can lyse it

– Limits blood loss in case of vessel injury

– Protects against invasion by infective agents
– Prevents formation of emboli – occlude blood vessels in other areas

Question 16

Clot formation

1) Initiation

Answer 16

• Haemostasis starts with the interaction between TF and FVIIa on the surface of subendothelial cells.

• Injury of vessels wall leads to contact between blood and subendothelial cells.

Tissue factor (TF) is exposed and binds to FVIIa or FVII which
is subsequently converted to FVIIa.
• The complex between TF and FVIIa activates FIX and FX.
• FXa binds to FVa on the cell surface.

Question 17

Clotting

2) Amplification

Answer 17

The small amount of thrombin generated during the amplification phase activates platelets locally on whose surface the subsequent reactions take place.

• The FXa/FVa complex converts small amounts of prothrombin into Thrombin.
• The small amount of thrombin generated activates FVIII, FV, FXI and platelets locally. FXIa converts FIX to FIXa.
• Activated platelets bind FVa, FVIIIa and FIXa.

Question 18

Clotting

3) Propagation

Answer 18

• The resulting thrombin burst results in the formation of a stable clot.

• The FVIIIa/FIXa complex activates FX on the surfaces of activated platelets.
• FXa in association with FVa converts large amounts of prothrombin into thrombin creating a “thrombin burst”.
• The “thrombin burst” leads to the formation of a stable fibrin clot.

Question 19

Plasma Inhibitors

Answer 19

Protease inhibitors 
Inhibition of activated coagulation factors 
-Antithrombin AT 
-A2 macroglobulin 
-TFPI: tissue factor pathway inhibitor  
-Heparin cofactor 2 

Protein C pathway
inactivation of activated cofactors
Protein C and S

Question 20

Fibrinolysis WHY

Answer 20

Lysis of fibrin clots.

• Once a clot is formed and the wound is sealed/healed, there is a risk of reduced blood flow in the affected areas.

• Can lead to necrosis of surrounding tissue.
-Turbulence of the blood flow
• Clot must be dissolved!

Question 21

Fibrinolysis HOW

Answer 21

Series of proteolytic reactions
Generates PLASMIN= dissolves the clot
Plasmin main protein in Fibrinolysis
Unable to dissolve clot physiologically without plasmin, need to use other mechanisms- eg drugs

Question 22

Cause of haemorrhage

Answer 22

Disorders of platelet number (thrombocytopaenia)
• Decreased production (e.g. Folic acid deficiency, Leukemia)
• Increased destruction (e.g. Idiopathic ? Antibodies against platelets)
• Drug induced (e.g. Heparin-patients develop antibodies to platelets )

• Disorders of platelet function

– von Willebrand’s disease (hereditary deficiency of a factor required for platelet adhesion)
– Drug induced (e.g. Aspirin)
– Uremia (renal failure, platelets present but not clotting)

Haemorrhage-Plasma coagulation factors
• Disorders of protein concentration (i.e. deficiency of one or more plasma protein)
• Vitamin K deficiency causes reduced function of coagulation proteins II, VII, IX and X

(bleeding/ haemorrhagic disease of the neonate)

• Drug-induced alteration in coagulation protein activity (i.e. heparin, warfarin as anticoagulants).

Question 23

Types of Haemorrhage

Answer 23

A- most common, deficiency of blood clotting factor 8

B- blood clotting factor 9