Exchange and the lymphatic system Flashcards
How does stuff get across the capillary walls?
Diffusion
Transcytosis
Clefts in between endothelial cells
Junctions
Pores (fenestrated & discontinuous)
What types of capillaries contain clefts and/or pores?
Continuous
- no clefts or pores eg brain
- clefts only eg muscle
Fenestrated
- clefts and pores eg intestine
Discontinuous
- clefts and massive pores eg liver
What 2 forces drive entry and exit of stuff into/out of capillaries?
The Starling forces:
- Hydrostatic
(pushing out of the capillary) - Osmotic (oncotic) pressure
(pushes stuff into the capillary)
Where is there a net outward force out of the capillaries?
Arterial end of the capillary bed
Hydrostatic > Osmotic
Where is there a net inward force into the capillaries?
Venous end of the capillary bed
Osmotic > hydrostatic
What is the overall movement of fluid into and out of capillaries due to Starling forces?
per day:
20L out
17L in
3L drained via lymph system
In terms of starling forces
What causes Oedema?
Hydrostatic»_space; Osmotic
Too much imbalance means overall too much fluid is lost for the lymph system to drain away
Accumulates = oedema
What are the main causes of oedema?
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
How would raised CVP cause oedema?
Abnormally high hydrostatic pressure
Too much fluid shoved out
How would hypoproteinemia and increased capillary permeability cause oedema?
Osmotic pressure too low
Not enough sucky-in force
Too allow the lymphatic system to work properly, blood flow/pressure must be kept at a correct level
What types of mechanisms regulate this?
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
What is the extrinsic mechanism for controlling blood flow?
Neural control:
- Sympathetic nerves
- Hormonal
Describe how the brain can control blood flow to certain areas
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
What other hormones can be released to affect the arteriolar resistance?
Angiotensin II:
- Arteriolar constriction
Vasopressin (= antidiuretic hormone):
- Arteriolar constriction
Atrial natriuretic factor:
- Arteriolar dilation
What are the local mechanisms for altering blood flow
Active (metabolic) hyperaemia
Pressure (flow) autoregulation
Reactive hyperaemia
Injury response (inflammation)
Special areas: coronary, cerebral etc
Describe the process of active hyperaemia
- 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
Describe pressure autoregulation
- 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
Describe reactive hyperaemia
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
The injury response is mediated by mast cells
What do they release to cause the response?
Histamine
Causes vasodilation thus increases blood flow to that area to aid delivery of leukocytes to that area
This is inflammation bois
Why does the coronary circulation have an abundance of B2 receptors?
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
Cerebral circulation must be kept at a very stable pressure
What regulation mechanism is central to this?
Cerebral circulation has a very sensitive pressure autoregulation mechanism
Even small changes in pressure are quickly corrected
What special local control mechanism does the lung posses?
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
What local control mechanism is in the Renal circulation and why?
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
Summarise what active hyperaemia does
Reacts to increased metabolic activity, increasing blood supply to that area to match the metabolic demands of that tissue
Summarise what pressure autoregulation does
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
