Exchange in & control of the Peripheral Circulation Flashcards
features of capillaries
specialised for exchange lots of them thin walled - small diffusion barriersmall diameter (big SA:vol ratio)
what are the 3 types of capillaries
- continuous
- fenestrated
- discontinuous
continuous capillaries
no clefts (between cells) or pores (within cells)
e.g. brain
for the blood brain barrier, protect the brain from blood K conc
clefts only
e.g. muscleallows some exchange
fenestrated capillaries
clefts and pores e.g. intestine
discontinuous capillaries
clefts and massive pores e.g. liver
types of exchange
diffusion (majority of exchange) carrier mediated exchange bulk flow
where does exchange occur from capillary to cell
between capillary and ECF, between cell and ECF
how does oxygen diffuse
down its conc gradient is lipophilic -no barriers to diffusion
features of diffusion
- self regulating - if the cell starts using more oxygen there is a larger conc grad so more is supplied
- non-saturable - there is no point where oxygen transport is at its max
- non-polar substances across membrane
- polar substances through clefts/channels
describe an example of carrier mediated transport
glucose transportin the brain - glucose is trapped within the capillaries but is highly needed by the brain so a protein transporter is required to move the glucose across
features of bulk flow
fluid transport driven by hydrostatic and osmotic (oncotic) pressure
what is hydrostatic pressure
loss of water as you move down the capillary from arteriole to venule through cleftsbig solutes remain in the capillary
what is osmotic pressure
drawing water back in to the more concentrated plasma due to retention of large solutes in the capillary
what is the pressure in the arteriole end
~40mmHg
what is the pressure in the venule end
~20mmHg
what are starlings forces
NOT THE SAME AS STARLING’S LAW (preload on the heart) capillary hydrostatic pressure vs ISF hydrostatic pressure (determines movement of water out)
plasma osmotic pressure vs ISF osmotic pressure (determines movement of water in)net filtration pressure varies between capillary beds
what amount of fluid is lost and regained in the capillary network each day
~20L lost and ~17L regained
what happens to the remaining fluid that isn’t regained into the capillary network
drains into the lymph capillaries
describe the structure of lymph capillaries
same as blood capillaries except they are blind ended valves prevent backflow of fluid
fluid drains into the low pressure heart of the systemic circulation (vena cava)
define oedema
accumulation of XS fluid
what are some causes of oedema
- lymphatic obstruction e.g. due to filariasis, surgery raised CVP e.g. due to ventricular failure
- hypoproteinemia e.g. due to nephrosis, liver failure, malnutrition
- increased capillary permeability e.g. inflammation, rheumatism
how does filariasis cause oedema
parasitic worm lives in lymph vessels and blocks them
how does ventricular failure cause oedema
LV isnt pumpin out the blood that is coming into it
back up of pressure
increased loss of fluid from the veins
how does rheumatoid arthritis lead to oedema
- gaps between walls of capillaries and endothelial cells to allow WBC out
- other contents of plasma are also drawn out so no build up of osmotic pressure
- fluid accumulates
how does malnourishment lead to oedema
- lack of protein in diet
- less plasma protein
- no osmotic pressure developed
- water is lost and isn’t drawn back inn
control of peripheral blood flow
Darcy’s law (flow = pressure difference/resistance) and Poiseuille’s law varying the radius of vessels is used to control flow and redirect blood
how is MAP calculated
flow = pressure difference/resistance
MAP - CVP = CO x TPR
MAP = CO x TPR
CVP is negligible as it is so small
what is MAP
mean arterial pressure ~90-95 mmHg
what does varying the radius of resistance vessels control
TPR and therefore regulates MAP
MAP is the driving force pushing blood through arterioles, continuous flow out of arterioles
arteriolar radius affects flow through individual vascular beds and MAP
what happens when the resistance of a vascular bed is reduced
increased flow through that vascular bed
if TPR is reduced, what effect does that have on MAP
reduced MAP
smaller driving force pushing blood through all arterioles
insufficient blood flow to the other regions
what is used to keep the blood flow to each vascular bed sufficient and keep MAP in the right range
2 levels of control over the smooth muscle surrounding arterioles
intrinsic and extrinsic mechanisms
what are intrinsic mechanisms
concerned with meeting the selfish needs of each individual tissue
what are extrinsic mechanisms
tend to affect the whole body concerned with ensuring that the TPR (and therefore MAP) of the whole body stays in the right range
extrinsic control (neural)
sympathetic nerves :
- release noradrenaline
- binds to alpha 1 receptors
- arteriolar constriction
- reduced flow through that body region
- increased TPR
parasympathetic nerves :
- usually no effect
what is the smooth muscle around vessels innervated by
heavily innervated by sympathetic post-ganglionic fibres
extrinsic control (hormonal - adrenaline )
adrenaline :
- released from adrenal medulla when sympathetic nerves are activated
- binds to alpha 1 receptors
- arteriolar constriction
- reduced flow through that tissue
- increased TPR
- in some tissues (skeletal and cardiac muscle) it also activates beta 2 receptors, arteriolar dilation (2y messengers coupled to receptors), increased flow through that tissue and reduced TPR redirects blood to these muscles where it is needed
extrinsic control (other hormonal controls)
angiotensin II :
- produced in response to low blood vol
- arteriolar constriction
- increased TPR vasopressin (antidiuretic hormone) - released in response to low blood vol
- arteriolar constriction
- increased TPR
atrial natriuretic factor :
- released in response to high blood vol
- arteriolar dilation
- reduced TPR
what are the 4 types of intrinsic control
- active (metabolic) hyperaemia
- pressure (flow) auto-regulation
- reactive hyperaemia
- the injury response
active (metabolic) hyperaemia
- increased blood flow in response to increased metabolism
- increased conc of metabolites triggers the release of EDRF/NO by endothelium causes arteriolar dilation
- increased flow to wash out metabolites
- an adaptation to match blood supply to the metabolic needs of that tissue
what are paracrines
local signalling molecules e.g. EDRF
what does EDRF stand for
endothelium derived relaxing factor
give 3 examples of metabolites
CO2
H+
K+
pressure (flow) autoregulation
sudden event which leads to sudden decreased MAP, reduced flow and accumulation of metabolites
triggers release of EDRF/NO
arterioles dilate and flow is restored to normal (or it could be myogenic - when smooth muscle is stretched it tends to contract which can contribute to the same effect)
an adaptation to ensure that a tissue maintains its blood supply despite changes in MAP
reactive hyperaemia
- occlusion of blood supply causes a subsequent increase in blood flow
- an extreme version of pressure auto-regulation
- accumulation of metabolites when flow is cut off, local arteriolar dilation
- when pressure is released there is a sudden flow of blood through dilated arterioles
the injury response
- aids delivery of blood born leucocytes etc to injured area
- increased C fibre
- substance P acts on mast cells
- histamine released by mast cells
- arteriolar dilation, increased blood flow + increased permeability
special areas of circulation
coronary circulation
cerebral circulation
pulmonary circulation
renal circulation
coronary circulation
- majority of blood flow to coronary arteries is during diastole
- blood supply is interrupted by systole
- still has to cope with increased demand during exercise
- shows excellent active hyperaemia
- expresses many beta 2 receptors
- these swamp any sympathetic arteriolar constriction
cerebral circulation
- always needs to be kept stable
- shows excellent pressure auto-regulation
- if MAP decreases, the arterioles in the brain dilate enough to maintain enough blood flow to the brain
pulmonary circulation
- reduced oxygen causes arteriolar constriction (the opposite response to most tissues)
- ensures that the blood is directed to the best ventilated parts of the lung
- prevents V/Q mismatch
renal circulation
- main function is filtration which depends on pressure
- changes in MAP would have big effects on blood volume
- shows excellent pressure auto-regulation
- if MAP falls, arterioles dilate and vice versa to maintain filtration at a constant level
