Control of Blood Flow

Question 1

Conductance (G) equation

Answer 1

G = 1 / TPR

Question 2

Relationship between CO and conductance

Answer 2

CO = (Pa – CVP) x G

Question 3

What does TPR control

Answer 3

Blood flow and blood pressure.
- Increase in resistance means need to increase pressure difference to keep same flow. (usually the arterial pressure is increased)
• By controlling TPR in difference areas in the body, you can control the distribution of blood in the body.

Question 4

What control TPR?

Answer 4

1. Poiseulle’s law
2. Myogenic response
3. Blood viscosity

Question 5

What causes blood flow?

Answer 5

Pressure drop between arteries & arterioles causes blood flow.

Question 6

Effect of increased and decreased TPR.

Answer 6

• Decrease in TPR = Decreased blood pressure upstream, but greater flow.
• Increase in TPR = Increased blood pressure upstream, but less flow.

Question 7

What causes hypertension and under perfusion?

Answer 7

Hypertension – over constriction of arterioles.

Higher arterial BP but less capillary flow – under perfusion

Question 8

What is postprandial? and what changes happen postprandial sedentary?

Answer 8

• Postprandial – just after a eating.
• Superior mesenteric dilated - Increased flow to intestines.
• Common iliac
constricted - Decreased flow to legs.

Question 9

What changes happen during exercise?

Answer 9

• Superior mesenteric constricted - Decreased flow to intestines.
• Common iliac dilated - Increased flow to legs.

Question 10

What is poiseulle’s law? and what is the equation?

Answer 10

• Describes parameters that govern TPR
• Resistance = 8nL / (pir^4)
• conductance equation flips the RHS upside down.

Question 11

The r4 effect

Answer 11

Small change in r, has a big overall effect on blood flow.

Question 12

Arterioles radius and pressure

Answer 12

• Arterioles have largest pressure drop - by the radius
• Arteriole radius is tightly controlled by
sympathetic nerves providing constant tone – we both dilate and constrict.

Question 13

Why is TPR not controlled by capillaries?

Answer 13

• Less pressure drop across capillaries due to less resistance to blood flow in capillaries
• No sympathetic innervation/smooth muscle in capillaries so cannot alter radius
•Individual capillaries are short (L)
• Less resistance is capillaries because bolus flow reduces viscosity (η)
• Capillaries are arranged in parallel, so have a low total resistance as RTotal = 1/R1 + 1/R2 etc.
In contrast, arterioles are in series
RTotal = R1 + R2 etc so total resistance is greater

Question 14

What is intrinsic and extrinsic?

Answer 14

Intrinsic
Factors entirely within an organ or tissue
(Allow response to local factors)

Extrinsic
Factors outside the organ or tissue.

Question 15

Intrinsic and extrinsic control mechanisms of arteriole radius.

Answer 15

INTRINSIC:

• Local hormones
• Tissue metabolites
• Myogenic properties of muscle
• Endothelial factors (NO)

EXTRINSIC:

• Neural
  eg. sympathetic nervous system
• Hormonal
  eg. adrenaline

Question 16

Bayliss myogenic response

Answer 16

• Expect linear resistance (in blue) for flow vs pressure – with no other factors affecting it, e.g. diameter stays the same.
•True: initially they are linear, and after reaching a certain pressure, they constrict and slow increase in flow with increase in pressure. – automatically.
- Stretching of the muscle causes ion channels to open, which then depolarize, leading to muscle contraction.

Question 17

What does blood flow depend on?

Answer 17

1. Viscosity of blood
2. Vessel diameter
3. Haematocrit

Question 18

What is viscosity a measure of?

Answer 18

Viscosity is a measure of internal friction opposing the separation of the lamina

Question 19

Factors that affect viscosity.

Answer 19

• Haematocrit (45%)
- Polycythaemia (high n) - inc. TPR, inc. BP, dec. BF
- anaemia (low n) - dec. TPR, dec. BP, inc. HR
• Tube diameter
(Fahraeus-Lindqvist effect)
- Blood viscosity falls in narrow tubes
• Red cell deformability
- inc. n, dec. BF, SCA crises
• Velocity of blood
- Slow venous flow in immobile legs – increased viscosity due to partial clotting.

Question 20

Describe veins structure

Answer 20

Thin-walled, collapsible, voluminous vessels
Contain 2/3rd of blood volume
Contractile – contain smooth muscle, innervated by sympathetic nerves
but thinner than arterial muscle & more compliant – so form blood reservoir

Question 21

Volume of blood in veins and contractility

Answer 21

Contraction of vessels – Expels blood into central veins
– Increases venous return/CVP/end-diastolic volume
– Increases stroke volume (Starling’s law)

Question 22

Venous return

Answer 22

Venous pressure high at the feet so pressure for return to heart.
Also helped by thoracic pump and skeletal muscle contraction.
- Stimulation of sympathetic nerves causing venoconstriction shifts blood centrally.
- Increases venous return, CVP & end-diastolic pressure.
- Increased CVP increases preload and so increases stroke volume (Starling’s law)

Question 23

How does Bernoulli’s law explain the blood flow from the feet back to the heart?

Answer 23

Bernoulli theory – mechanical energy of flow is determined by pressure, kinetic, potential energies (ρ = fluid mass)
Energy = Pressure (PV) + kinetic (ρV2/2) + potential (ρgh)
-90 mmHg pressure gradient against flow back to heart from feet
Ejected blood has greater kinetic energy at heart than feet (more velocity, V).
Also, greater potential energy than at heart than feet (more height, h)
Greater kinetic/potential energies overcome pressure gradient to maintain flow
But flow to feet easily compromised – clinically important
Returning blood to heart - no pressure gradient but kinetic energy