Exchange and the Lymphatic system Flashcards
Features of capillaries
- Specialised for exchange
- Lots of them: every tissue within 100 micro metre of one.
- Thin walled: presents a small diffusion barrier
- Small diameter: big surface area:volume ratio
Different structures of capillaries
Structure relates to function
- Continuous: no clefts or pores e.g. brain, clefts only e.g. muscle.
- Fenestrated: clefts and pores e.g. intestine.
- Discontinuous: clefts and massive pores e.g. liver.
Features of diffusion in capillaries
- Self-regulating
- Non-saturable
- Non-polar substances across membrane
- Polar substances through clefts/channels
Where would you need a protein carrier for capillaries?
Only in a few places e.g. the capillaries of the brain where the tight junctions of the blood-brain-barrier prevent diffusion of polar (hydrophilic) substances e.g. glucose
What mass exchange is going on in the background of diffusion between the capillaries?
Bulk flow
What is bulk flow?
Movement of water and solutes together due to a pressure gradient.
- This is the second mechanism of capillary exchange.
How do the different types of capillaries affect bulk flow?
- Continuous capillaries (tight structure) reduce bulk flow
- Fenestrated capillaries (perforated structure) increases bulk flow
- Discontinuous capillaries (great intercellular gaps) enable bulk flow.
How does bulk flow occur?
Hydrostatic pressure pushes fluid out through the leaky capillaries.
- That builds up an osmotic (oncotic) pressure which draws fluid back in.
What is the balance of hydrostatic and osmotic pressures in bulk flow known as?
Starling’s forces
What structures comprise the lymphatic system?
- Lymph nodes
- Lymph capillaries
- Pulmonary blood vessels
- Lymphatic vessel
- Valve
- Lymph node
- Systemic blood capillaries
- Lymphatic vessel
- Lymph capillaries
What occurs when the lymphatic system doesn’t work and the excess fluid (lymph) cannot drain into the lymphatic vessels?
This causes Oedema
What is oedema?
The accumulation of excess fluid
Main 3 reasons for oedema
- Obstruction of the lymph vessels
- Raised hydrostatic pressure
- Low osmotic pressure
What are examples of causes of oedema?
- Lymphatic obstruction e.g. due to filariasis, surgery.
- Raised CVP ( central venous pressure) e.g. due to ventricular failure.
- Hypoproteinemia e.g. due to nephrosis, liver failure, nutrition
- Increased capillary permeability: inflammation, e.g. rheumatism.
What law is used to calculate resistance in control of peripheral blood flow?
Poiseuille’s Law
What law is used to calculate flow in control of peripheral blood flow?
Darcy’s Law
How is blood redirected to regions of the body that need it - what closes and opens the taps?
Resistance vessels (arterioles) do it. - Since capillaries don't have any smooth muscle.
What is the fundamental equation for MAP (mean arterial pressure)?
MAP = CO x TPR
How is MAP important?
It provides the driving force that pushes blood through useful places like your brain.
- So you have a system which monitors and regulates it by controlling the state of constriction of your arterioles.
What is the function of intrinsic mechanisms in keeping blood flow to each vascular bed sufficient and mean arterial pressure in the right range.
They are concerned with meeting the selfish needs of each individual tissue.
What is the function of extrinsic mechanisms in keeping blood flow to each vascular bed sufficient and mean arterial pressure in the right range.
They are concerned with ensuring that the total peripheral resistance (and therefore MAP) of the whole body stays in the right ball park.
How do sympathetic nerves (neural) affect extrinsic control (neural)?
- Release noradrenaline
- binds to a1-receptors
- Causes arteriolar constriction
- therefore decreases flow through that tissue, and tends to increase TPR/
How do parasympathetic nerves (neural) affect extrinsic control (neural)?
Usually no effect
- Genetic and salivary glands are the only ones affected by parasympathetic nerves.
Where is adrenaline released from?
- Adrenaline is from adrenal medulla
How does Adrenaline affect extrinsic control (hormonal)?
- Binds to a1-receptors
- causes arteriolar constriction
- therefore decrease in flow through that tissue, and tends to increase TPR.
- but in some tissues, e.g. skeletal and cardiac muscle, it also activates b2-receptors.
- causes arteriolar dilation.
- Therefore increase in flow through that tissue, and tends to decrease TPR.
What is the most important effect of the alpha 1 effect occurring in most regions of the body?
Increasing total peripheral resistance (TPR)
What is the most important effect of the beta 2 effect?
- The B2 effect only occurs in couple of places, so its most important effect is to redirect blood flow to the heart and skeletal muscle during the fight or flight reaction.
- However, beta 2 has more receptors compared to alpha 1.
Effect of Angiotensin II in extrinsic control (hormonal)
- Produced in response to low blood volume.
- Causes arteriolar constriction
- therefore increases TPR
Effect of Vasopressin (=antidiuretic hormone) in extrinsic control (hormonal)
- Released in response to low blood volume
- Causes arteriolar constriction
- Therefore increase TPR
Effect of Atrial natriuretic factor in extrinsic control (hormonal)
- Released in response to high blood volume.
- Causes arteriolar dilation
- therefore decreases TPR
Names of local (intrinsic) controls
- Active (metabolic) hyperaemia
- Pressure (flow) autoregulation
- Reactive hyperaemia
- The injury response
What is active hyperaemia?
Active hyperemia is the increase in organ blood flow (hyperemia) that is associated with increased metabolic activity of an organ or tissue.
- e.g. the increase in blood flow that accompanies muscle contraction, which is also called exercise or functional hyperemia in skeletal muscle.
What occurs in active hyperaemia?
- Increased metabolic activity causes increase in concentration of metabolites
- This triggers release of EDFR/NO which causes arteriolar dilation
- Causes increased flow to wash out metabolites
- An adaptation to match blood supply to the metabolic needs of that tissue.
What is EDRF in active local controls?
Endothelium derived relaxing factor (EDRF)
- Doesn’t have to be the same relaxing factor for every part of the body.
What is pressure (flow) autoregulation?
Autoregulation is a manifestation of local blood flow regulation. - - It is defined as the intrinsic ability of an organ to maintain a constant blood flow despite changes in perfusion pressure.
What occurs in pressure (flow) autoregulation?
- Decrease in MAP causes decrease in flow
- then metabolites accumulate
- This triggers release of EDRF/NO
- arterioles dilate and flow is restored to normal (or it could be myogenic - originating in muscle tissue)
- an adaption to ensure that a tissue maintains its blood supply despite changes in MAP occurs
What occurs in Reactive hyperaemia?
- Occlusion of blood supply causes a subsequent increase in blood flow.
- this is an extreme version of pressure autoregulation.
What occurs in the injury response?
- Substance P is released which acts on mast cells
- Mast cells release histamine causing smooth muscle and arterioles to relax.
- Arteriolar dilation causes increased blood flow and increased permeability.
- Increased redness in injury is due to increased blood flow.
Special areas of blood circulation
- Coronary circulation
- Cerebral circulation
- Pulmonary circulation
- Renal circulation
Features of coronary circulation?
- Blood supply is interrupted by systole
- but still has to cope with increased demand during exercise
- shows excellent active hyperaemia
- expresses many b2-receptors
- these swamp any sympathetic arteriolar constriction
Features of cerebral circulation
- Needs to be kept stable
- Shows excellent pressure autoregulation
Features of pulmonary circulation
- Decrease in O2 causes arteriolar constriction
- This is the opposite to what happens to most tissues.
- Ensures that blood is directed to the best ventilated parts of the lung
Features of renal circulation
- Main function is filtration which depends on pressure.
- Changes in MAP would have big effects on blood volume
- Shows excellent pressure autoregulation
