Exchange and the Lymphatic System

Question 1

Describe the gross structure of capillaries

Answer 1

Single layer of flattened endothelial cells on a basal lamina
Most cells with 0.1mm (100um)
Thin walled (small diffusion barrier
Small diameter: large surface area:volume ratio

Question 2

Describe the three ultrastructures of capillaries

Answer 2

Continuous:
In brain - no clefts or channels
In muscle - clefts only

Fenestrated:
Clefts and channels - intestine, kindness, gut with lots of water

Discontinuous:
Cleft and massive channels - liver for protein transport

Question 3

What does clotting involve?

Answer 3

Formation of a platelet plug
Formation of a fibrin clot

A tear in the endothelium is detected by the BM, activating platelet to stick and form a plug

Fibrinogen is converted to fibrin (by thrombin) and seals the hole

Question 4

What are the anti-clotting mechanisms of the endothelium?

Answer 4

Stops blood contacting collagen
Produced prostacyclin and NO
Produces tissue factor pathway inhibitor (TFPI)
Expresses thrombomodulin
Expresses heparin
Secretes tissue plasminogen activator (t-PA)

Question 5

Stops blood contacting collagen

Answer 5

No platelet aggregation

Question 6

Produced prostacyclin and NO

Answer 6

Inhibit platelet aggregation

Question 7

Produces tissue factor pathway inhibitor (TFPI)

Answer 7

Stops thrombin production

Question 8

Expresses thrombomodulin

Answer 8

Binds to thrombin and inactivates it

Question 9

Expresses heparin

Answer 9

Inactivates thrombin

Question 10

Secretes tissue plasminogen activator (t-PA)

Answer 10

Plasminogen -> plasmin and digests clot

Question 11

Describe features of diffusion

Answer 11

Oxygen diffuses from capillary -> ECF -> cell (vice versa)

Self-regulating: if more O2 required, conc gradient increase and more O2 delivered
Non-saturable
Non-polar substance are permeable
Polar substance through clefts/channels

Question 12

What is bulk flow?

Answer 12

The mass movement of fluid as the result of hydrostatic or osmotic pressure gradients

Question 13

Explain hydrostatic pressure

Answer 13

Pushes fluid out of capillaries and the hydrostatic pressure on the ISF in low
The pressure gradient decreases from arteries to venues so less fluid pushed out in venous end

Question 14

What carrier-mediated transport?

Answer 14

Glucose transporter in blood brain barrier - exception

Question 15

Explain osmotic (oncotic) pressure

Answer 15

Large molecules that cannot fit through the channels remain in the plasma - i.e. proteins - which increases the concentration gradient and pushes fluid into the vessel

Gradient increases from arteries to veins

Question 16

Explain starlings forces

Answer 16

Capillary hydrostatic pressure vs ISF hydrostatic pressure
Plasma osmotic pressure vs ISF osmotic pressure

Net filtration: difference between hydrostatic pressure and osmotic pressure (-ve = net absorption and +ve = net filtration)

Overall 20L lost and 17L regained daily

Question 17

Describe process of lymphatic system

Answer 17

Remaining 3L that is not regained in starling forces drain into blind ended lymph capillaries, through the nodes and then drains back into low pressure end of vena cava

Question 18

What happens if fluid cannot be drained by lymphatic capillaries?

Answer 18

Leads to oedema

Question 19

How can low protein in diet lead to swelling?

Answer 19

Low protein = less protein in plasma = less water drawn into plasma via oncotic pressure causing swelling (oedema)

Question 20

Causes of oedema

Answer 20

Lymphatic obstruction (due to filariasis, surgery)
Raised CVP (due to ventricular failure)
Hypoproteinaemia (due to nephrosis (loss of protein via kidneys, liver failure, nutrition)
Increased capillary permeability (inflammation)