Peripheral circulation Flashcards
Describe the structure of the capillaries
Lots of them
-every tissue within 100m of one
Thin-walled
-presents a small diffusion barrier
Small diameter
-big surface area:volume ratio
What is a continuous capillary?
no clefts or pores eg brain
clefts only eg muscle
What is a fenestrated capillary?
clefts and pores eg intestine
What is a discontinuous
clefts and massive pores eg liver
Describe 4 factors that defines diffusion?
self-regulating
non-saturable
non-polar substances across membrane
polar substances through clefts/channels
What is an example of carrier mediated transport?
glucose transporter
Describe bulk flow?
Pressure inside the arteriole is higher than the venule. High hydrostatic pressure which pushes the water out and cant take the solutes out. Therefore the concentration of solutes increases and goes down a different gradient which makes up the osmotic pressure
What 2 factors determine the hydrostatic pressure?
Capillary hydrostatic pressure vs ISF hydrostatic pressure
How many litres lost on average everyday and regained?
20L lost and 17 Regained
Where is the extra 3L regained from?
Lymph system
Define Oedema
accumulation of excess fluid
What are 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
Which factors in poseuilles law can affect peripheral blood flow?
viscosity and length and radius
What can be used to control the TPR and therefore regulate MAP?
Varying the radius of resistance vessels
MAP = CO x TPR ( total preipheral ressitance)
What does arteriolar radius affect?
Flow through the vascular beds and the mean arterial pressure
What are the two levels of control over smooth muscle surrounding arterioles that help keep mean arterial pressure in right range and blood flow sufficient?
Intrinsic mechanisms and extrinsic mechanisms
Describe the control aim behind intrinsic mechanisms
concerned with meeting the selfish needs of each individual tissue
Describe the control aim behind extrinsic mechanisms
concerned with ensuring that the total peripheral resistance (and therefore MAP) of the whole body stays in the right ball park
What does extrinsic control involve in neural?
Sympathetic nerves release noradrenaline binds to alpha-1-receptors causes arteriolar constriction therefore decrease flow through that tissue, and tends to increase TPR
Parasympathetic nerves
usually no effect on smooth muscle surrounding the peripheral arterioles
What does extrinsic control involve in hormonal?
Adrenaline released from adrenal medulla binds to alpha-1-receptors causes arteriolar constriction therefore decerases flow through that tissue, and tends to increases TPR
but in some tissues, eg skeletal and cardiac muscle, it also activates beta2-receptors
- causes arteriolar dilation
- therefore increase flow through that tissue, and tends to decrease TPR
Other hormones include:
Angiotensin II
Vasopressin (= antidiuretic hormone)
Atrial natriuretic factor
What does the local intrinsic control of active hyperaemia involve?
increase metabolic activity causes increase conc of metabolites
triggers release of EDRF/NO (?) (= paracrines)
causes arteriolar dilation
increases flow to wash out metabolites
an adaptation to match blood supply to the metabolic needs of that tissue
What does the local intrinsic control of pressure autoregulation involve?
Same mechanisms as the active hyperaemia
but different initial cause
decrease MAP causes decrease flow
metabolites accumulate
triggers release of EDRF/NO (?)
arterioles dilate and flow is restored to normal
(or it could be myogenic)
an adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
What does the local intrinsic control of reactive hyperaemia involve?
- occlusion of blood supply causes a subsequent increase in blood flow
- an extreme version of pressure autoregulation
What does the local intrinsic control of injury response involve?
When you scratch – you get a small red mark which represents increase in blood flow
C- fibres respond to damage
Reach a terminal to cause release of P and then that activates mast cells to release histamine
Arteriolar dilation allow increasing blood flow and permeability
- aids delivery of blood born leucocytes etc to injured area
What happens in the cornoary circulation?
Cuts off blood supply everytime it contracts due to high pressure build up
blood supply is interrupted by systole
but still has to cope with increased demand during exercise
shows excellent active hyperaemia
expresses many beta2-receptors
these swamp any sympathetic arteriolar constriction
Whats involved in cerebral circulation?
needs to be kept stable, whatever
shows excellent pressure autoregulation
What occurs when O2 decreases in pulmonary circulation?
decrease O2 causes arteriolar constriction
ie the opposite response to most tissues
ensures that blood is directed to the best ventilated parts of the lung
What is the main function of renal circulation?
main function is filtration which depends on pressure
What is the effect of changing MAP in renal circulation?
changes in MAP would have big effects on blood volume
Does renal circulation have good or bad pressure autoregulation?
Good!
