Exchange and the lymphatic system Flashcards
Describe the gross structure of capillaries
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
Describe the different ultrastructures of capillaries
Structure is related to function
Continuous
Fenestrated
Discontinuous
What is a continuous ultrastructure? What are some examples?
No clefts or channels e.g. brain
Clefts only e.g. muscle
What is a fenestrated ultrastructure? What are some examples?
Clefts and channels
e.g. intestine
What is a discontinuous ultrastructure? What are some examples?
Clefts and massive channels
e.g. liver
What are the different exchange methods between capillaries and cells?
Diffusion
Carrier-mediated transport
Bulk flow
What are the features of diffusion?
Self-regulating Non-saturable Non-polar substances across membrane Polar substances through clefts/channels Vast majority of exchange happens through diffusion, down the concentration
How is diffusion self regulated?
If a cell has too much oxygen, due to concentration gradient it will diffuse back
The more oxygen you need, the more you get
What are the features of carrier-mediated transport?
Glucose transporter in brain, polar things e.g. potassium
What are the features of bulk flow?
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
What is the significance of the blood-brain barrier in the ultrastructure of capillaries?
- 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
What does blood clotting involve?
- Formation of a platelet plug. Endothelium breaks, this exposes the circulating platelets to come over and create a platelet plug
- Formation of fibrin clot. Fibrin is signalled by platelets to come over from fibrinogen -> thrombin -> fibrin
What are the anti-clotting mechanisms of the endothelium?
- Stops blood contracting collagen
- Produces prostacyclin and NO
- Produces tissue factor pathway inhibitor (TFPI)
- Expresses thrombomodulin
- Expresses heparin
- Secretes tissue plasminogen activator
In anti-clotting, what does stopping the blood contracting collagen result in?
No platelet aggregation
In anti-clotting, what does producing prostacyclin and NO result in?
Inhibition of platelet aggregation
In anti-clotting, what does the production of TFPI do?
Stops thrombin production
In anti-clotting, what does the expression of thrombomodulin do?
Binds to thrombin and inactivates it
In anti-clotting, what does the expression of heparin do?
Also inactivates thrombin
In anti-clotting, what does the secretion of tissue plasminogen activator do?
Plasminogen turns into plasmin and digests clot, opposite of thrombin
What are Starling forces?
Capillary hydrostatic pressure vs. ISF hydrostatic pressure
Plasma osmotic pressure vs ISF osmotic pressure
Net filtration pressure = (Pc – PIF) – (πp¬ – πIF)
How do Starling forces relate to the lymphatic system?
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
What is oedema?
Accumulation of excess fluid
Fluid flow: capillaries, extracellular space, lymph capillaries
What can cause oedema?
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
How does Kwashiorkor relate to Starling forces?
Decreased protein in capillaries, increased hydrostatic pressure but there are no proteins so there’s no osmotic pressure, no return of fluid
