Exchange in & control of the Peripheral Circulation

Question 1

features of capillaries

Answer 1

specialised for exchange lots of them thin walled - small diffusion barriersmall diameter (big SA:vol ratio)

Question 2

what are the 3 types of capillaries

Answer 2

1. continuous
2. fenestrated
3. discontinuous

Question 3

continuous capillaries

Answer 3

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

Question 4

fenestrated capillaries

Answer 4

clefts and pores e.g. intestine

Question 5

discontinuous capillaries

Answer 5

clefts and massive pores e.g. liver

Question 6

types of exchange

Answer 6

diffusion (majority of exchange) carrier mediated exchange bulk flow

Question 7

where does exchange occur from capillary to cell

Answer 7

between capillary and ECF, between cell and ECF

Question 8

how does oxygen diffuse

Answer 8

down its conc gradient is lipophilic -no barriers to diffusion

Question 9

features of diffusion

Answer 9

1. self regulating - if the cell starts using more oxygen there is a larger conc grad so more is supplied
2. non-saturable - there is no point where oxygen transport is at its max
3. non-polar substances across membrane
4. polar substances through clefts/channels

Question 10

describe an example of carrier mediated transport

Answer 10

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

Question 11

features of bulk flow

Answer 11

fluid transport driven by hydrostatic and osmotic (oncotic) pressure

Question 12

what is hydrostatic pressure

Answer 12

loss of water as you move down the capillary from arteriole to venule through cleftsbig solutes remain in the capillary

Question 13

what is osmotic pressure

Answer 13

drawing water back in to the more concentrated plasma due to retention of large solutes in the capillary

Question 14

what is the pressure in the arteriole end

Answer 14

~40mmHg

Question 15

what is the pressure in the venule end

Answer 15

~20mmHg

Question 16

what are starlings forces

Answer 16

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

Question 17

what amount of fluid is lost and regained in the capillary network each day

Answer 17

~20L lost and ~17L regained

Question 18

what happens to the remaining fluid that isn’t regained into the capillary network

Answer 18

drains into the lymph capillaries

Question 19

describe the structure of lymph capillaries

Answer 19

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)

Question 20

define oedema

Answer 20

accumulation of XS fluid

Question 21

what are some causes of oedema

Answer 21

1. lymphatic obstruction e.g. due to filariasis, surgery raised CVP e.g. due to ventricular failure
2. hypoproteinemia e.g. due to nephrosis, liver failure, malnutrition
3. increased capillary permeability e.g. inflammation, rheumatism

Question 22

how does filariasis cause oedema

Answer 22

parasitic worm lives in lymph vessels and blocks them

Question 23

how does ventricular failure cause oedema

Answer 23

LV isnt pumpin out the blood that is coming into it

back up of pressure

increased loss of fluid from the veins

Question 24

how does rheumatoid arthritis lead to oedema

Answer 24

1. gaps between walls of capillaries and endothelial cells to allow WBC out
2. other contents of plasma are also drawn out so no build up of osmotic pressure
3. fluid accumulates

Question 25

how does malnourishment lead to oedema

Answer 25

1. lack of protein in diet 2. less plasma protein 3. no osmotic pressure developed 4. water is lost and isn't drawn back inn

Question 26

control of peripheral blood flow

Answer 26

Darcy's law (flow = pressure difference/resistance) and Poiseuille's law varying the radius of vessels is used to control flow and redirect blood

Question 27

how is MAP calculated

Answer 27

flow = pressure difference/resistance MAP - CVP = CO x TPR MAP = CO x TPR CVP is negligible as it is so small

Question 28

what is MAP

Answer 28

mean arterial pressure ~90-95 mmHg

Question 29

what does varying the radius of resistance vessels control

Answer 29

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

Question 30

what happens when the resistance of a vascular bed is reduced

Answer 30

increased flow through that vascular bed

Question 31

if TPR is reduced, what effect does that have on MAP

Answer 31

reduced MAP smaller driving force pushing blood through all arterioles insufficient blood flow to the other regions

Question 32

what is used to keep the blood flow to each vascular bed sufficient and keep MAP in the right range

Answer 32

2 levels of control over the smooth muscle surrounding arterioles intrinsic and extrinsic mechanisms

Question 33

what are intrinsic mechanisms

Answer 33

concerned with meeting the selfish needs of each individual tissue

Question 34

what are extrinsic mechanisms

Answer 34

tend to affect the whole body concerned with ensuring that the TPR (and therefore MAP) of the whole body stays in the right range

Question 35

extrinsic control (neural)

Answer 35

sympathetic nerves : 1. release noradrenaline 2. binds to alpha 1 receptors 3. arteriolar constriction 4. reduced flow through that body region 5. increased TPR parasympathetic nerves : 1. usually no effect

Question 36

what is the smooth muscle around vessels innervated by

Answer 36

heavily innervated by sympathetic post-ganglionic fibres

Question 37

extrinsic control (hormonal - adrenaline )

Answer 37

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

Question 38

extrinsic control (other hormonal controls)

Answer 38

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

Question 39

what are the 4 types of intrinsic control

Answer 39

1. active (metabolic) hyperaemia 2. pressure (flow) auto-regulation 3. reactive hyperaemia 4. the injury response

Question 40

active (metabolic) hyperaemia

Answer 40

- 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

Question 41

what are paracrines

Answer 41

local signalling molecules e.g. EDRF

Question 42

what does EDRF stand for

Answer 42

endothelium derived relaxing factor

Question 43

give 3 examples of metabolites

Answer 43

CO2 H+ K+

Question 44

pressure (flow) autoregulation

Answer 44

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

Question 45

reactive hyperaemia

Answer 45

- 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

Question 46

the injury response

Answer 46

- 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

Question 47

special areas of circulation

Answer 47

coronary circulation cerebral circulation pulmonary circulation renal circulation

Question 48

coronary circulation

Answer 48

- 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

Question 49

cerebral circulation

Answer 49

- 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

Question 50

pulmonary circulation

Answer 50

- 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

Question 51

renal circulation

Answer 51

- 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