The Peripheral Circulation Flashcards
Describe capillaries
Exchange vessels
Thin walled and have small diameter
Big surface area: volume ratio
Where are clefts in capillaries?
Between epithelial cells
Where are pores in capillaries?
Across epithelial cells
Explain continuous capillaries
No clefts or pores ex. brain
Clefts only ex. muscles and most other capillaries
Explain fenestrated capillaries
Clefts and pores ex. intestine and kidney for fluid exchange
Explain discontinuous capillaries
Clefts and massive pores ex. liver
What are the benefits of diffusion?
Non-saturable, self-regulating, non-polar substances across phospholipid membrane and polar substances cross by pores and clefts
What is bulk flow determined by?
Starling’s Forces
What are starling’s forces?
Capillary hydrostatic pressure vs ISF hydrostatic pressure
Plasma osmotic pressure vs ISF osmotic pressure
Net filtration pressure = (HC-HIF0 - (πC - πIF)
How much fluid is retained and lost per day?
20l lost
17l is regained
3l drains to lymphatics
How is the 3l of fluid drained in lymphatics?
Drains to lymphatic system to LN then to larger lymphatic vessels and makes way back up to heart where drains to vena cava
Then returned to CVS
What is oedema?
Accumulation of excess fluid
What could oedema be caused from?
Raised CVP
Lymphatic obstruction
Hypoproteinaemia
Increased capillary permeability
How does raised CVP cause oedema?
Raised CVP can be caused by ventricular failure
If left side not pumping blood correctly then blood can accumulate in lungs
Increase in hydrostatic pressure of capillaries leading to oedema
How does hypoproteinaemia cause oedema?
Protein helps build up oncotic pressure and pull water back in so if not enough protein then lose more fluid
What is Darcy’s Law?
Flow = Difference in pressure/ resistance
What can help control flow and redirect blood?
Varying the radius of resistance vessels
What can varying the radius of capillaries help control?
Flow and redirect blood
Control TPR and therefore regulate MAP
What is MAP equal to?
MAP = CO x TPR
What happens if MAP is too low?
Decreased perfusion through vascular bed so brain will notice and can lead to light headedness/ fainting
What happens if MAP is too high?
Wont be any immediate effects
Hypertension if over long period, time increases the risk
What happens if resistance of vascular bed increases?
Increases flow through vascular bed
What happens if TPR is reduced?
Increased flow
Reduces MAP
What are the 2 levels of control over smooth muscle surrounding arterioles?
Local (intrinsic) mechanisms
Central (extrinsic) mechanisms
What are local mechanisms concerned with?
With meeting the selfish needs to each individual tissue
What are the central mechanisms concerned with?
With ensuring the TPR and therefore MAP of whole body stays in right ball park
Explain active hyperaemia - local control
Trigger is increase in local metabolite which can be caused by increase in metabolic activity
Release of paracrine signal - EDFR
Causes arteriolar dilation so increased flow to wash out metabolites
Explain pressure autoregulation - local control
Trigger is a decrease in perfusion pressure which increases MAP and decreases flow
Metabolites accumulate
Triggers release of EDRF ad arterioles dilate and flow is restored
What is the aim of active hyperaemia?
Match blood supply to the metabolic needs of that tissue
What is the aim of pressure autoregulation?
Ensure that a tissue maintains its blood supply despite changes in MAP
Explain reactive hyperaemia - local control
Same mechanism, different trigger
Trigger is occlusion of blood supply which causes increase in blood flow and an extreme version of pressure autoregulation
Explain the injury response
Aids delivery of blood born leukocytes etc. to injured area
C fibre releases AP to trigger substance P which acts on mast cells
Mast cells release histamine which causes arteriolar dilation to increase blood flow and permeability
What is the affect of sympathetic nerves on central controls?
Release Noradrenaline which binds to alpha 1 receptors on smooth muscle
Causes arteriolar constriction so decrease in flow through tissue that tends to increase TPR and MAP
What is the affect of parasympathetic nerves on central controls?
Usually no effects as don’t usually innervate blood vessels
Genitalia and salivary glands are exception and increase flow
What is the affect of adrenaline on central controls - hormonal?
Released from adrenal gland and binds to alpha 1 receptors
Causes arteriolar constriction which increases TPR and MAP as decreased flow
What happens when adrenaline binds to beta 2 receptors in skeletal and cardiac muscle?
Causes arteriolar dilation
Increase in flow so decreased TPR
This is significant during exercise
Describe coronary circulation?
Blood supply is interrupted by systole but still has to cope with decreased demand during exercise
Express many B2 receptors
Shows active hyperaemia
Swamp any sympathetic arteriolar constriction
Why is blood supply interrupted by systole?
When heart contracts an builds up big pressure in ventricles for pushing out blood. Increases in pressure of wall of heart too,
This squishes the blood vessels in heart
What happens if adrenaline binds to B2 receptors in coronary circulation?
Relaxes and dilates arterioles
Does diastole or systole have higher pressure?
Diastole - blood flow is higher
Describe cerebral circulation
Needs to be kept stable
Shows excellent pressure autoregulation so if pressure falls then arterioles dilate and main perfusion
Explain pulmonary circulation
Decreased O2 causes arteriolar constriction which is opposite of other tissues
Ensures blood is directed to best ventilated parts of the lung
Explain renal circulation
Main job is filtration which is dependant on pressure
Filtration rate kept relatively constant during normal fluctuations in MAP due to excellent pressure autoregulation
Protects glomeruli from HBP as causes damage
