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
