Exchange and the lymphatic system

Question 1

How does stuff get across the capillary walls?

Answer 1

Diffusion

Transcytosis

Clefts in between endothelial cells

Junctions

Pores (fenestrated & discontinuous)

Question 2

What types of capillaries contain clefts and/or pores?

Answer 2

Continuous

• no clefts or pores eg brain
• clefts only eg muscle

Fenestrated
- clefts and pores eg intestine

Discontinuous
- clefts and massive pores eg liver

Question 3

What 2 forces drive entry and exit of stuff into/out of capillaries?

Answer 3

The Starling forces:

• Hydrostatic
  (pushing out of the capillary)
• Osmotic (oncotic) pressure
  (pushes stuff into the capillary)

Question 4

Where is there a net outward force out of the capillaries?

Answer 4

Arterial end of the capillary bed

Hydrostatic > Osmotic

Question 5

Where is there a net inward force into the capillaries?

Answer 5

Venous end of the capillary bed

Osmotic > hydrostatic

Question 6

What is the overall movement of fluid into and out of capillaries due to Starling forces?

Answer 6

per day:

20L out
17L in

3L drained via lymph system

Question 7

In terms of starling forces

What causes Oedema?

Answer 7

Hydrostatic&raquo_space; Osmotic

Too much imbalance means overall too much fluid is lost for the lymph system to drain away

Accumulates = oedema

Question 8

What are the main causes of oedema?

Answer 8

Lymphatic obstruction
eg due to filariasis, surgery

Raised CVP
eg due to ventricular failure

Hypoproteinemia
eg due to nephrosis, liver failure, nutrition

Increased capillary permeability
inflammation, eg rheumatism

Question 9

How would raised CVP cause oedema?

Answer 9

Abnormally high hydrostatic pressure

Too much fluid shoved out

Question 10

How would hypoproteinemia and increased capillary permeability cause oedema?

Answer 10

Osmotic pressure too low

Not enough sucky-in force

Question 11

Too allow the lymphatic system to work properly, blood flow/pressure must be kept at a correct level

What types of mechanisms regulate this?

Answer 11

Smooth muscle around arterioles:

1) Intrinsic mechanisms - concerned with meeting the selfish needs of each individual tissue
2) Extrinsic mechanisms – concerned with ensuring that the total peripheral resistance (and therefore MAP) of the whole body stays in the right ball park

Question 12

What is the extrinsic mechanism for controlling blood flow?

Answer 12

Neural control:

• Sympathetic nerves
• Hormonal

Question 13

Describe how the brain can control blood flow to certain areas

Answer 13

Sympathetic:

• Release NA which binds to Alpha 1
• Smooth muscle constriction round arteriole

Adrenaline:
- Adrenal medulla into the blood
- Can bind to Alpha 1 and cause constriction
- OR can bind to B2 and cause dilation (skeletal, cardiac)
(redirection of blood for fight or flight)

Other hormones

Question 14

What other hormones can be released to affect the arteriolar resistance?

Answer 14

Angiotensin II:
- Arteriolar constriction

Vasopressin (= antidiuretic hormone):
- Arteriolar constriction

Atrial natriuretic factor:
- Arteriolar dilation

Question 15

What are the local mechanisms for altering blood flow

Answer 15

Active (metabolic) hyperaemia

Pressure (flow) autoregulation

Reactive hyperaemia

Injury response (inflammation)

Special areas: coronary, cerebral etc

Question 16

Describe the process of active hyperaemia

Answer 16

• Increased metabolic activity causes increase in metabolites in the blood (CO2, H+, K+)
• This triggers release of EDRF / NO messengers which cause arteriolar dilation
• Increase in blood supply and flushes out metabolites so osmotic pressure remains normal

Question 17

Describe pressure autoregulation

Answer 17

• Decrease in arteriolar pressure causes slower blood flow at capillaries
• Metabolites accumulate
• Triggers release of EDRF / NO (paracrines)
• These cause arteriolar dilation thus flow increases and metabolites flushed away

Question 18

Describe reactive hyperaemia

Answer 18

If blood supply to an area is occluded (blocked) then there is a subsequent increase in blood flow to that area

Extreme form of pressure autoregulation

Question 19

The injury response is mediated by mast cells

What do they release to cause the response?

Answer 19

Histamine

Causes vasodilation thus increases blood flow to that area to aid delivery of leukocytes to that area

This is inflammation bois

Question 20

Why does the coronary circulation have an abundance of B2 receptors?

Answer 20

Blood supply is interrupted by systole

But still has to cope with increased demand during exercise

Shows excellent active hyperaemia

The large amount of B2 receptors swamp any sympathetic arteriolar constriction

Question 21

Cerebral circulation must be kept at a very stable pressure

What regulation mechanism is central to this?

Answer 21

Cerebral circulation has a very sensitive pressure autoregulation mechanism

Even small changes in pressure are quickly corrected

Question 22

What special local control mechanism does the lung posses?

Answer 22

Drop in O2 (due to poor ventilation) causes arteriolar constriction to decrease perfusion of that area and send the blood away to a better ventilated area

This is the opposite response to other tissues

SHUNT

Question 23

What local control mechanism is in the Renal circulation and why?

Answer 23

Pressure autoregulation (similar to brain)

Must maintain high pressure so that it can filter out stuff:
- Changes to arteriolar pressure would have large effect n blood volume

Question 24

Summarise what active hyperaemia does

Answer 24

Reacts to increased metabolic activity, increasing blood supply to that area to match the metabolic demands of that tissue

Question 25

Summarise what pressure autoregulation does

Answer 25

A reaction to changes in local blood pressure, increasing/decreasing the blood flow to ensure the tissue maintains the correct blood supply despite changes in MAP