Lecture 8 – Control of blood flow

Question 1

What controls TPR? (4)

Answer 1

Myogenic response
Blood viscosity
Role of arterioles vs capillaries
Pouseuilles law

Question 2

Can pressure explain blood flow in all situations?

Answer 2

Bernoulli’s theory

Question 3

What does TPR control? (2)

Answer 3

Blood flow and pressure

Question 4

Vasodilation and TPR (3)

Answer 4

decrease TPR
decrease blood pressure upstream
increase flow

Question 5

Vasoconstriction and TPR (3)

Answer 5

increase TPR
increase blood pressure upstream
decrease flow

Question 6

Changes in blood flow into changes in need (2,2,1)

Answer 6

Sedentary- inactive - e.g. after a meal
Superior mesenteric dilated - increase flow to intestines
Common iliac constricted - decreased flow to legs

Exercise opposite
Superior mesenteric constricted - decrease flow to intestines
Common iliac dilated - increased flow to lungs

Brain and kidney always perfused

Question 7

Poiseuille’s law (3)

Answer 7

TPR = 8nL/Pie x r4.
Viscosity and length (n and L) will increase resistance.
Change in r can have a major effect on resistance.
Conductance is the opp of resistance, so you can inverse equation above and work out conductance.

Question 8

Conductance (5)

Answer 8

TPR = Pie x r4/ 8nL
Conductance is the opp of resistance, so you can inverse equation above and work out conductance.
Darcy and Poiseullies law can be put together to work out CO/Flow = Pressure gradient x Conductance
CO = Pa - CVP x TPR => Darcys law but you insert above equation.

If radius increase, conductance increases, flow increases.
High density or length decrease in conductance.

Question 9

The r4 effect ()

Answer 9

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Question 10

Arterioles and TPR (3)

Answer 10

Arterioles (resistance vessels) have the largest pressure drops 40-50 mmHg.
Long vessels have SM so they can contract/relax.
The radius is tightly controlled by sympathetic nerves providing a constant tone - can dilate/constrict.

Question 11

Why is TPR not controlled by capillaries? (4)

Answer 11

Controlled by arterioles.
Radius doesn’t change, do not have SM so can’t constrict/dilate.
Bolus - continuous/smooth flow, low viscosity.
Pressure gradient is small, arterioles have muscle and squeeze.
Capillaries are arranged in parallel. low total resistance. Arterioles are in series.

Question 12

TPR controlled by 3 main parameters (3)

Answer 12

Radius, r4.
Pressure difference across vessels (P1-P2 or Pa - CVP).
Length.

Question 13

Control mechanisms of arteriol radius (4)

Answer 13

Intrinsic
Factors within an organ or tissue.
Allows response to local factors.

Extrinsic
Factors outside the organ or tissue.
Nervous and hormonal control of BV.

Question 14

Control mechanisms of arteriol radius - Extrinsic examples (2)

Answer 14

o Local Hormones = bee sting inflammatory response and the release of mediators e.g. TNF-alpha which will cause vasodilation.
o Extrinsic Hormones = adrenaline (released from the adrenal gland and nervous system this will have effects on many parts of the body.

Question 15

Bayliss myogenic response (3)

Answer 15

If vessels were fixed in diameter there would be a narrow range of good blood flow.
Muscle stretches, ion channels open, depolarise –> muscle contraction, vessels dilate and blood flow increases.
At high pressure/increase, distension muscle will be stretched but it will contract to reduce blood flow.

Question 16

Blood flow depends on (3)

Answer 16

Blood viscosity
Vessel diameter
Haematocrit - the ratio of the volume of red blood cells to the total volume of blood.

Question 17

Viscosity - define

Answer 17

The measure of internal friction opposing the separation of the lamina.
When the blood is more viscous, there is more friction.

Question 18

Viscosity factors (3)

Answer 18

Haematocrit
Polycythaemia - high n, increase in TPR
Anaemia - low n, decrease in TPR

Diameter
Fahraeus-Lindqvist effect
n decreases in narrow tubes, cells move to centre, decrease in TPR

Red cells deformability
Increase n, increase BF

Velocity of blood
Slow venous flow in immobile legs - increased viscosity due to partial clotting.

Question 19

Veins (7)

Answer 19

2/3 of blood volume in veins.
o Thin-walled, collapsible, voluminous vessels.
o Contractile - contain smooth muscle, innervated by sympathetic nerves but thinner than arterial muscle and more compliant - so form blood reservoir.
o Increases stroke volume (Starling’s law) as CVP increases preload.
• One-way valves in the veins and muscles where the veins lie around will squeeze the blood and mobilise it back to the heart.
• Breathing - diaphragm will move down to reduce the pressure in the lungs and increase the pressure in the abdomen and squeeze the veins known as the thoracic pump.
At low pressure veins will collapse, high pressure they distend.