6/ exchange and the lymphatic system

Question 1

what is particular about the structure of capillaries?

Answer 1

specialised for exchange, lots of them (every tissue within 100 um of one other), thin-walled (presents a small diffusion barrier), small diameter (big surface area : volume ratio)

Question 2

what type of junctions do continuous capillaries have?

Answer 2

leaky junctions + transcytosis vesicles

Question 3

what type of junctions do fenestrated capillaries have?

Answer 3

large pores (or fenestrations) + transcytosis vesicles

Question 4

where do you find continuous ultrastructure?

Answer 4

• no clefts or channels: brain

- clefts only: muscle

Question 5

where do you find fenestrated ultrastructure?

Answer 5

• clefts and channels: intestine

Question 6

where do you find discontinuous ultrastructure?

Answer 6

• clefts and massive channels: liver

Question 7

what does clotting involve?

Answer 7

• formation of a platelet plug (due to contact between blood and collagen)
• formation of a fibrin clot (around the platelet plug)

Question 8

what role does thrombin play in the clotting mecanism?

Answer 8

thrombin turns fibrinogen to fibrin

Question 9

what are the anti-clotting mechanisms of the endothelium?

Answer 9

• stops blood contacting collagen (no platelet aggregation)
• produces prostacyclin and NO (both inhibit platelet aggregation)
• produces tissue factor pathway inhibitor (TFPI) (stops thrombin production)
• expresses thrombomodulin (binds thrombin & inactivates it)
• expresses heparin (also inactivates thrombin)
• secretes tissue plasminogen activator (t-PA) (plasminogen-> plasmin & digests clot

Question 10

how does O2 cross the membrane? what is specific about this type of transportation?

Answer 10

• diffusion (self-regulating, non-saturable, non-polar substances across membrane, polar substances through clefts/ channels)
• carrier-mediated transport (e.g: glucose transporter)

Question 11

which forces affect bulk flow? what is the resultant force? what’s the outcome of this on plasma volume variation across the body?

Answer 11

capillary hydrostatic pressure vs ISF hydrostatic pressure and plasma osmotic pressure vs ISF osmotic pressure- resultant force varies between capillary beds, but overall about 20L is lost and 17L is regained each day

Question 12

how is the remaining 3L retrieved?

Answer 12

lymph capillaries surround pulmonary and systemic exchange/ capillary regions, retrieve plasma and places back into venous circulation

Question 13

what are the causes of oedema? (= accumulation of fluid)

Answer 13

• lymphatic obstruction (eg: due to filariasis, surgery)
• raised CVP (eg: due to ventricular failure) (LOOK UP)
• hypoproteinemia (eg: due to nephrosis, liver failure, nutrition)
• increased capillary permeability (inflammation, e.g: rheumatism)