Blood and Bleeding

Question 1

what are the structural features of a red blood cell and how do these assist its function? (2)

Answer 1

• biconcave disc - increase surface area and provide flexibility in capillaries with smaller diameter than that of the cell
• no nucleus - more space for Hb

Question 2

what are the 3 requirements of a red blood cell?

Answer 2

• pass repeatedly through micro circulation where diameter may be smaller than that of RBC
• maintain Hb in a reduced (ferrous) state
• maintain osmotic equilibrium despite high protein concentration

Question 3

what are the main cytoskeleton proteins in RBCs?

Answer 3

actin and spectrin

Question 4

what is the structure of a RBC? (3)

Answer 4

• protein with quaternary structure - 4 polypeptide chains: 2 alpha and 2 beta
• each chain has its own haem group
• each haem ring has a ferrous (Fe 2+) atom at its centre

Question 5

describe how the confirmation of Hb changes (3)

Answer 5

• Hb can bind to a max of 4 O2 atoms - shape changes as more O2 atoms bind
• no O2 - taut with fewer binding sites available
• 1 O2 binds - more relaxed shape, therefore increased O2 affinity

Question 6

describe the Bohr shift using P50 (4)

Answer 6

• P50 is the ppO2 at which 50% of Hb is saturated
• during high energy usage (eg. exercise), increased CO2 conc. lowers blood pH
• blood lowers O2 affinity to increase unloading to supply respiring tissues
• O2 dissociation curve shifts to right (decreased O2 saturation at a given ppO2)

Question 7

how is normal blood distributed when a sample is centrifuged?

Answer 7

• largest top layer of plasma (~55%)
• very think buffs coat containing leukocytes
• bottom haematocrit layer containing RBCs and platelets (~45%)

Question 8

what is the difference between plasma and serum?

Answer 8

• plasma contains fibrinogen + components in preparation coagulation
• serum is what remains after components are used in coagulation

Question 9

from what cell are platelets derived and how are they formed? what is this cell’s lineage?

Answer 9

• megakaryocyte - platelets bed off from cell surface

* myeloid lineage

Question 10

what is the structure of a platelet? (3)

Answer 10

• small and disc-shaped
• no nucleus
• contain granules involved in clot formation

Question 11

what is the sequence of primary haemostasis? (4)

Answer 11

1 - endothelium of a blood vessel is breached, so cells release endothelin to cause vasoconstriction
2 - von Willerbrand factor is exposed and collagen forms to promote platelet adherence and activation
3 - platelets attach, change shape and release granules
4 - allows platelets to aggregate and form a primary platelet plug

Question 12

what is the principle behind the coagulation cascade?

Answer 12

circulating tissues are activated to form proteases to catalyse the next reaction

Question 14

what are the differences between coagulation in vivo and in vitro? (3)

Answer 14

• in vitro - tissue factor has a different role
• 2 separate pathways that are not integrated
• thrombin does not have an amplifying effect

Question 15

describe the coagulation pathway in vitro (6)

Answer 15

1 - -ve charged surface (eg. glass beads) added to solution
2 - activates intrinsic cascade culminating in activation of IX (Xmas)
3 - extrinsic pathway involves the activation of VII using tissue factor
4 - pathways converge as both VIIa and IXa are required to activate X (Stuart)
5 - (common pathway) Xa catalyses prothrombin to thrombin
6 - thrombin then catalyses the conversion of soluble fibrinogen into fibrin to form clot

Question 16

what is the basic coagulation sequence of clotting in vivo? (6)

Answer 16

1 - tissue factor exposed after vascular damage
2 - activates VII, which activates IX (Xmas factor)
3 - IXa activates X (Stuart factor)
4 - Xa catalyses prothrombin to thrombin
5 - thrombin catalyses fibrinogen to fibrin to form clot
6 - thrombin also enhances cascade processes further up (+ve feedback loop amplifying the effect of tissue factor)

Question 17

describe how a fibrin clot is formed (3)

Answer 17

1 - fibrinogen (soluble) is catalysed by thrombin -> peptidolysis
2 - fibrin is insoluble and begins to form a soft clot with lattice structure
3 - hard clot develops as fibrin cross-links form

Question 18

why is fibrinolysis necessary?

Answer 18

after endothelium of blood vessel is restored, thrombus must be removed to restore normal blood flow

Question 19

describe the process of fibrinolysis (3)

Answer 19

1 - endothelial cells release plasminogen activator
2 - converts plasminogen to plasmin
3 - plasmin digests both fibrin and fibrinogen to break down the clot

Question 20

what is the broad definition of shock?

Answer 20

a clinical syndrome where tissue perfusion (fluid in organs) and hence oxygenation is inadequate to maintain normal metabolic function

Question 21

what are the 5 main types of shock?

Answer 21

1 - hypovolaemic: haemorrhagic, burns, severe dehydration
2 - cardiogenic: pump failure
3 - septic: fluid redistribution due to leaking capillaries + vasodilation (usually in response to severe infection)
4 - spinal / neuro: loss of proper vascular tone
5 - anaphylactic: allergic reaction resulting in a combination of the above

Question 22

why does shock lead to increased heart rate an breathing rate?

Answer 22

hypoperfusion -> not enough blood is being pumped around the body, therefore body compensates

Question 23

why does blood pressure only drop in class III and IV shock?

Answer 23

HR and BR try to maintain BP as long as possible

Question 24

what is the ABCDE of advanced trauma life support?

Answer 24

Airway w/ cervical spine control
Breathing with oxygenation
Circulation with haemorrhage control
Disability - assessment with Glasgow Coma Scale
Exposure (eg. cover with warm blankets etc.)

Question 25

how much blood is lost in the classifications I - IV of hypovolaemic shock?

Answer 25

I: <750ml
II: 750-1500ml
III: 1500-2000ml
IV: >2000ml

Question 26

what response does each colour warrant in the Manchester Triage System? give examples of each category

Answer 26

red: see immediately - airway compromise, unresponsive, shock
amber: within 10 mins - very low perforation, very low oxygen saturation, acute onset after injury
yellow: within 1 hour - low perforations, low oxygen saturation, chest pain while breathing
green: within 2 hours - wheeze, chest infection, recent problem
blue (non-urgent) - none of the above

Question 27

how are the 2 coagulation pathways assessed?

Answer 27

extrinsic - prothrombin time (PT)

intrinsic - activated partial thromboplastin time (APTT) assay

Question 28

what effect does heparin have on the APTT?

Answer 28

no coagulation because heparin is an anti-coagulant

Question 29

what is the effect of removing Ca ions from solution in the APTT?

Answer 29

no coagulation as calcium is an essential component of coagulation

Question 30

what effect does benzamidine have on the APTT?

Answer 30

time for clot to form increases because benzamidine acts as a protease inhibitor