Exchange & Lymphatics Flashcards

1
Q

What 3 gross characteristics make capillaries so good at exchange?

A
  • Large surface area to volume ratio
  • Very thin walled (some are fenestrated or discontinuous)
  • Every piece of tissue is within 1mm of a capillary
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2
Q

How does exchange occur in continuous capillaries?

A

Leaky junctions between endothelial cells.
Some, particularly in muscle, have clefts between cells but no channels.

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3
Q

What makes a fenestrated capillary?

A

They have both clefts between cells and channels through cells

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4
Q

Where are fenestrated capillary found?

A

Places specialised for exchange like the intestine where theres a lot of fluid being exchanged

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5
Q

What makes a discontinuous capillary?

A

One with clefts and massive channels

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6
Q

Where are discontinuous capillaries found?

A

The liver where large proteins need to enter the blood

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7
Q

Why is blood prone to clot within capillaries?

A

The flow is very slow and in some cases discontinuous

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8
Q

How does the endothelium prevent platelets aggregating and starting a clot?

A

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

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9
Q

How does the endothelium producing prostacyclin and NO prevent clotting?

A

Both prostacyclin and NO inhibit platelet aggregation

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10
Q

What is TFPI and why does the endothelium produce it?

A

Tissue Factor Pathway Inhibitor

It stops Thrombin production so fibrinogen isnt converted to fibrin and no fibrin clot forms

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11
Q

Why does the endothelium express heparin and thrombomodulin?

A

Both thrombomodulin and Heparin bind to and deactivate thrombin, preventing fibrin clots

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12
Q

What is the benefit of the capillaries secreting t-PA?

A

tissue plasminogen activator
It converts plasminogen to plasming which digests any clots that form.

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13
Q

What are the 3 main methods of exchange?

A

Diffusion
Carrier-Mediated Transport
Bulk Flow

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14
Q

What does it mean to say diffusion across capillaries is ‘non-saturable’?

A

Since theres no proteins involved it cant become saturated and so only the gradient determines max rate.

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15
Q

How do polar vs non-polar substances pass diffuse?

A

Polar diffuse through clefts & channels
Non-polar substances pass the membranes of cells

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16
Q

Name a type of carrier-mediated transporter?

A

A glucose transporter

17
Q

What type of forces determines bulk flow?

A

Starling forces

18
Q

What are the various starling forces determining bulk flow?

A

Inward: ISF hydrostatic pressure & Plasma osmotic pressure
Outward: Plasma Hydrostatic Pressure & ISF osmotic pressure

19
Q

How do we calculate the net filtration pressure?

A

Net Filtration Pressure = (Hc - Hisf) - (Oc - Oisf)

Where H is hydrostatic and O is osmotic

20
Q

How much fluid leaves & enters capilaries a daY?

A

20L leaves them but only 17L returns
The other 3 L is drained by lymphatic capillaries

21
Q

Define Oedema?

A

Accumulation of excess fluid in a tissue

22
Q

name a load of oedema causes?

A

Lymphatic Obstruction - surgey, filariasis etc
Raised CVP - Ventricular Failure etc
Hypoproteinaemia - nephrosis, liver failure, poor nutrition
Increased capillary permeability -Inflammation e.g. rheumatism.