Physiology 5 Flashcards
What proportion of whole blood comprises plasma?
55%
What proportion of whole blood comprises platelets?
0.5%
What proportion of whole blood comprises erythrocytes?
45%
Describe the structure and activation of the vitamin K dependent coagulation factors
Active enzyme site and long tail of glutatamic acid molecules (prior to activation)
Vitamin K acts a cofactor in converting the glutamic acid to gamma-carboxyglutamic acid (which is negatively charged)
This charged tail binds to the platelet surface through electrostatic Ca2+ ion bridges and thus become haemostatically active.
Which are the Vit K-dependent coagulation factors?
FII, FVII, FIX, FX
Where is tissue factor found?
On the surface of smooth muscle cells and fibroblasts surrounding blood vessels
What is von Willebrand Factor? (vWF)
Long, string-like protein produced by endothelial cells and platelets.
Mostly bound to collagen in subendothelial layer but some released into plasma.
In low-shear conditions vWF is curled up and cannot bind to platelets.
In conditions of bleeding shear stress is increased and vWF stretches out, exposing binding sites which spontaneously bind to inactivated platelets
How does a breach in vessel walls lead to haemostasis?
- vWF unravels, attaching to subendothelial collagen and inactivated platelets loosely bind to the vWF through glycoprotein GP1b
- Factor VII in the plasma is activated by combining with tissue factor, initiating the coagulation cascade
How does FVII initiate the coagulation cascade?
- FVII binds to inactive FX, cleaving it to form FXa
2. FXa cleaves FII (prothrombin) to form FIIa (thrombin)
What follows initial activation of FIIa?
- Platelet activation
- FV ->FVa
- FVIII -> FVIIIa
- FXI -> FXIa (which then activates FIX -> FIXa)
- FXIII -> FXIIIa
Describe the relevant features of inactive platelets
- Neutral phospholipid membrane
- Inactivated surface GPIIbIIIa
- Alpha granules containing vWF, FV and fibrinogen
- Dense bodies containing ADP and Ca2+
How do platelets become activated?
By the presence of:
- FIIa (thrombin)
- Collagen
- ADP release by other platelets
- GP1b binding to vWF
How do platelets change when activated?
- Degranulation of alpha granules and dense bodies
- Translocation of phospholipids to outside of platelet creating negatively charged membrane
- Activation of GPIIbIIIa
How does platelet aggregation occur following activation?
- Tight binding of platelets to vWF
- Binding of platelets to each other via GPIIbIIIa, vWF and fibrinogen crosslinking
How does the ‘second phase’ of the coagulation cascade occur?
- FXIa activates FIXa
- FVIIIa stabilises FIXa so that the active site can convert FX -> FXa in large quantities
- FV stabilises FXa so that active site can cleave FII -> FIIa in large quantities (‘thrombin burst’)
- FIIa cleaves fibrinogen to fibrin
What is the function of coagulation factor XIII?
FXIIIa cross-links fibrin strands, stabilising the clot and platelet plug
Define and outline the Type I hypersensitivity response
Type I: Immediate IgE-mediated hypersensitivity causing mast cell and basophil degranulation
What preformed mediators are released by mast cells?
- Chemoattractants: NCF, ECF-A
- Activators: Histamine (vasodilatation and vascular permeability), Tryptase (activates C3), Kininogenase (-> kinins -> vasodilatation)
- Spasmogens: Histamine (bronchial SM contraction, mucus secretion)
What newly formed mediators are released by mast cells?
- Chemoattractants: LTB4
- Activators: Platelet activating factor (PAF) -> microthrombi
- Spasmogens: PGD2, LTC4. LTD4
What are the clinical effects of Type I hypersensitivity reactions?
Eyes: conjunctivitis Nasopharynx: Rhinorrhoea, rhinitis Lungs: Bronchospasm CV: Vasodilatation/shock Skin: Urticaria/eczema GI: Gastroenteritis
Define and outline the Type II hypersensitivity response
Type II: Cytotoxic hypersensitivity reactions mediated by IgG and IgM Abs activating three possible mechanisms of cytotoxic response:
- Complement activation via classical pathway
- Phagocytosis
- Antibody-dependent cell-mediated cytotoxicity (ADCC) via NK cells and cytotoxic T lymphocytes