Exchange & Lymphatics Flashcards
What 3 gross characteristics make capillaries so good at exchange?
- Large surface area to volume ratio
- Very thin walled (some are fenestrated or discontinuous)
- Every piece of tissue is within 1mm of a capillary
How does exchange occur in continuous capillaries?
Leaky junctions between endothelial cells.
Some, particularly in muscle, have clefts between cells but no channels.
What makes a fenestrated capillary?
They have both clefts between cells and channels through cells
Where are fenestrated capillary found?
Places specialised for exchange like the intestine where theres a lot of fluid being exchanged
What makes a discontinuous capillary?
One with clefts and massive channels
Where are discontinuous capillaries found?
The liver where large proteins need to enter the blood
Why is blood prone to clot within capillaries?
The flow is very slow and in some cases discontinuous
How does the endothelium prevent platelets aggregating and starting a clot?
It prevents the blood making contact with the collagen underlayer. Normal clotting occurs when the endothelium is broken and the platelets make contact with the collagen basement membrane
How does the endothelium producing prostacyclin and NO prevent clotting?
Both prostacyclin and NO inhibit platelet aggregation
What is TFPI and why does the endothelium produce it?
Tissue Factor Pathway Inhibitor
It stops Thrombin production so fibrinogen isnt converted to fibrin and no fibrin clot forms
Why does the endothelium express heparin and thrombomodulin?
Both thrombomodulin and Heparin bind to and deactivate thrombin, preventing fibrin clots
What is the benefit of the capillaries secreting t-PA?
tissue plasminogen activator
It converts plasminogen to plasming which digests any clots that form.
What are the 3 main methods of exchange?
Diffusion
Carrier-Mediated Transport
Bulk Flow
What does it mean to say diffusion across capillaries is ‘non-saturable’?
Since theres no proteins involved it cant become saturated and so only the gradient determines max rate.
How do polar vs non-polar substances pass diffuse?
Polar diffuse through clefts & channels
Non-polar substances pass the membranes of cells
Name a type of carrier-mediated transporter?
A glucose transporter
What type of forces determines bulk flow?
Starling forces
What are the various starling forces determining bulk flow?
Inward: ISF hydrostatic pressure & Plasma osmotic pressure
Outward: Plasma Hydrostatic Pressure & ISF osmotic pressure
How do we calculate the net filtration pressure?
Net Filtration Pressure = (Hc - Hisf) - (Oc - Oisf)
Where H is hydrostatic and O is osmotic
How much fluid leaves & enters capilaries a daY?
20L leaves them but only 17L returns
The other 3 L is drained by lymphatic capillaries
Define Oedema?
Accumulation of excess fluid in a tissue
name a load of oedema causes?
Lymphatic Obstruction - surgey, filariasis etc
Raised CVP - Ventricular Failure etc
Hypoproteinaemia - nephrosis, liver failure, poor nutrition
Increased capillary permeability -Inflammation e.g. rheumatism.
