Exchange and the lymphatic system

Question 1

Describe the gross structure of capillaries

Answer 1

Specialised for exchange
Lots of them: every tissue within 100 microns of one
Thin walled: present a small diffusion barrier, only one cell thick
Small diameter: big surface area:volume ratio

Question 2

Describe the different ultrastructures of capillaries

Answer 2

Structure is related to function
Continuous
Fenestrated
Discontinuous

Question 3

What is a continuous ultrastructure? What are some examples?

Answer 3

No clefts or channels e.g. brain

Clefts only e.g. muscle

Question 4

What is a fenestrated ultrastructure? What are some examples?

Answer 4

Clefts and channels

e.g. intestine

Question 5

What is a discontinuous ultrastructure? What are some examples?

Answer 5

Clefts and massive channels

e.g. liver

Question 6

What are the different exchange methods between capillaries and cells?

Answer 6

Diffusion
Carrier-mediated transport
Bulk flow

Question 7

What are the features of diffusion?

Answer 7

Self-regulating
Non-saturable
Non-polar substances across membrane
Polar substances through clefts/channels
Vast majority of exchange happens through diffusion, down the concentration

Question 8

How is diffusion self regulated?

Answer 8

If a cell has too much oxygen, due to concentration gradient it will diffuse back
The more oxygen you need, the more you get

Question 9

What are the features of carrier-mediated transport?

Answer 9

Glucose transporter in brain, polar things e.g. potassium

Question 10

What are the features of bulk flow?

Answer 10

Hydrostatic pressure pushes water out through holes of capillaries
Causes an increased concentration of proteins as you’re losing water -> osmotic pressure goes up -> water re-enters due to amount of oncotic pressure

Question 11

What is the significance of the blood-brain barrier in the ultrastructure of capillaries?

Answer 11

• It is continuous: no clefts but there are channels between the cells
• Stops potassium leaking across the capillaries
• There is a glucose transporter in the brain

Question 12

What does blood clotting involve?

Answer 12

1. Formation of a platelet plug. Endothelium breaks, this exposes the circulating platelets to come over and create a platelet plug
2. Formation of fibrin clot. Fibrin is signalled by platelets to come over from fibrinogen -> thrombin -> fibrin

Question 13

What are the anti-clotting mechanisms of the endothelium?

Answer 13

• Stops blood contracting collagen
• Produces prostacyclin and NO
• Produces tissue factor pathway inhibitor (TFPI)
• Expresses thrombomodulin
• Expresses heparin
• Secretes tissue plasminogen activator

Question 14

In anti-clotting, what does stopping the blood contracting collagen result in?

Answer 14

No platelet aggregation

Question 15

In anti-clotting, what does producing prostacyclin and NO result in?

Answer 15

Inhibition of platelet aggregation

Question 16

In anti-clotting, what does the production of TFPI do?

Answer 16

Stops thrombin production

Question 17

In anti-clotting, what does the expression of thrombomodulin do?

Answer 17

Binds to thrombin and inactivates it

Question 18

In anti-clotting, what does the expression of heparin do?

Answer 18

Also inactivates thrombin

Question 19

In anti-clotting, what does the secretion of tissue plasminogen activator do?

Answer 19

Plasminogen turns into plasmin and digests clot, opposite of thrombin

Question 20

What are Starling forces?

Answer 20

Capillary hydrostatic pressure vs. ISF hydrostatic pressure
Plasma osmotic pressure vs ISF osmotic pressure
Net filtration pressure = (Pc – PIF) – (πp¬ – πIF)

Question 21

How do Starling forces relate to the lymphatic system?

Answer 21

The forces vary between capillary beds
Overall in the body ~20L is lost and ~17L is regained each day
Remaining 3L goes into the lymphatic system

Question 22

What is oedema?

Answer 22

Accumulation of excess fluid

Fluid flow: capillaries, extracellular space, lymph capillaries

Question 23

What can cause oedema?

Answer 23

Lymphatic obstruction -> due to filariasis, surgery elephantitis
Raised central venous pressure -> e.g. due to ventricular failure
Hypoproteinaemia -> e.g. due to nephrosis, liver failure, nutrition -> Kwashiokor
- Increased capillary permeability -> inflammation e.g. rheumatism

Question 24

How does Kwashiorkor relate to Starling forces?

Answer 24

Decreased protein in capillaries, increased hydrostatic pressure but there are no proteins so there’s no osmotic pressure, no return of fluid