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SGUL - Physiology 1 > Control of Blood Flow > Flashcards

Control of Blood Flow Flashcards

1
Q

What does TPR control?

A

Blood flow and blood pressure
- Increase in resistance means response required to increase pressure to maintain the same flow

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2
Q

What is TPR controlled by?

A
  • Darcy’s and Poiseuilles laws
  • Myogenic response
  • Blood viscosity
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3
Q

What is conductance in relation to resistance?

A

Opposite of resistance

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4
Q

What causes hypertension in the arterioles?

A

Overconstriction of arterioles

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5
Q

What happens when the arterioles dilate?

A
  • Decrease in TPR.
  • Decreased blood pressure upstream but greater flow downstream
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6
Q

What happens when the arterioles constrict?

A
  • Increase in TPR.
  • Increased blood pressure upstream, but less flow downstream
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7
Q

What are the features of blood flow in the common iliac and superior mesenteric arteries when sedentary?

A
  • Superior mesenteric is dilated so increased flow to intestines to improve digestion
  • Common iliac constricted so decreased flow to legs
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8
Q

What are the features of common iliac and superior mesenteric arterioles when exercising?

A
  • Superior mesenteric is constricted so decreased flow to intestines
  • Common iliac dilated so increased flow to legs
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9
Q

What does Poiseuille’s law describe?

A

The parameters that govern TPR

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10
Q

What is resistance related to?

A

Blood viscosity - if increased, resistance will also increase
Radius of vessel -if increased, resistance will decrease
Vessel length - if increased, resistance will also increase
Pressure difference across vessels

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11
Q

What is viscosity?

A

A measure of the internal friction opposing the separation of the lamina

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12
Q

What are four factors in viscosity?

A

Haematocrit, diameter, red cell deformability, velocity of blood

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13
Q

What is Bernoulli’s theory?

A

Flow is determined by pressure, kinetic and potential energies

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14
Q

What does stimulation of sympathetic nerves cause in the CVS?

A
  • Vasoconstriction of veins
  • Increases venous return as CVP and EDP is increased.
  • CVP increases preload so greater stretching
  • Greater force of contraction
  • Causes a greater stroke volume
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15
Q

What effect does cardiac output have on venous return?

A
  • Circulation is a closed system so the heart pushes the blood further into the veins in the direction of the right side of the heart
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16
Q

What effect does breathing have on venous return?

A
  • The pressure in the chest is negative on inhalation
  • Intra-abdominal pressure rises as the diaphragm moves downwards.
  • Causes the venous valves in the pelvic veins to close, and the blood moves into the thorax.
  • On exhalation, the intra-abdominal pressure decreases and the pelvic veins and inferior vena cava refills
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17
Q

What effect does muscle pump have on venous return?

A
  • Deep venous system is embedded in muscles. D
  • Muscle contraction squeezes the veins to push the column of blood in them in the direction of the heart.
  • When the muscle relaxes, the venous valves prevent the retrograde flow of blood towards the capillaries
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18
Q

What effect does venous tone have on venous return?

A
  • The blood in the veins exerts pressure on the veins wall, which generates tension and maintains pressure.
  • Sympathetic vasoconstriction can mobilise more blood back to the heart
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19
Q

What is conductance?

A

How well a vessel conducts flow.
- 1/TPR

20
Q

What is the equation for Darcy’s law using conductance rather than TPR?

A

Flow = Pressure difference x Conductance

21
Q

What is the result of hypertension in arterioles?

A
  • Increased arterial blood pressure
  • Reduced capillary flow - can lead to underperfusion
22
Q

Which factor has the largest effect on blood flow and how is this change brought about?

A
  • Small change in the radius has a massive change on blood flow due to the r4 effect in Pouseuille’s Law.
  • Changes in the radius of blood vessels is brought about through sympathetic activity of vasoconstriction and vasodilation
23
Q

In which blood vessel can someone find the largest pressure drop?

A

Arterioles

24
Q

How is the radius of the arterioles controlled?

A

Through sympathetic nerves providing constant tone of dilation using NO released from endothelium vs constriction using noradrenaline

25
Q

Which vessel does not control TPR?

A

Capillaries.

26
Q

Why do arterioles, rather than capillaries, regulate TPR?

A
  • Low pressure drop due to less resistance to blood flow in capillaries. No large effect on blood pressure
  • Radius remains unaltered as there are no smooth muscles and no sympathetic innervation
  • Individual capillaries remain very short which limits resistance
  • Cannot dilate or constrict due to thin walls needed for nutrient exchange
  • High number of capillaries so high overall cross-sectional area
  • Low resistance in capillaries due to bolus flow which reduces viscosity
27
Q

How is local blood flow to individual tissues/organs controlled?

A

Controlled by changes in radius of arterioles supplying the tissue/organ.

28
Q

What are the two control mechanisms of arteriole radius?

A
  • Intrinsic factors: Factors found within the organ or tissue
  • Extrinsic factors: Factors found outside the organ or tissue (nervous or hormonal control of blood vessels)
29
Q

What is the Bayliss myogenic response? PART 1

A
  • Due to the linear relationship between pressure and blood flow, a large difference in pressure would cause a large difference in blood flow.
  • The response maintains blood flow at the same level despite changing arterial pressures.
  • Initially linear relationship between pressure and flow. Becomes non-linear since high pressures are not considered ideal due to need to alter pressure with changing circumstances
30
Q

What is the Bayliss myogenic response? PART 2

A
  • Increasing distension of vessels makes them constrict and decreasing distension within vessels makes them dilate.
  • Increasing distension means the vessels are more stretched causing ion channels to open, which then depolarize, leading to smooth muscle contraction
  • Reduce blood flow
31
Q

How does increasing haematocrit influence viscosity?

A

Increased haematocrit increases viscosity. This increases TPR and BP and decreases flow.

32
Q

How does decreasing haematocrit influence viscosity?

A

Decreasing haematocrit decreases viscosity. This reduces TRP and BP and increases flow and heart rate

33
Q

How does tube diameter influence viscosity?

A

Blood viscosity is reduced in narrow vessels as cells move to the centre of the vessel to reduce friction. Decreased TPR and therefore flow increases

34
Q

How does red cell deformability influence viscosity?

A

Normally cells are able to deform in order to flow efficiently through vessels. However in some cases (e.g. sickle cell) the RBCs lose their deformability which increases viscosity and reduces flow due to greater TPR.

35
Q

How does blood velocity influence viscosity?

A

Slow venous flow increases viscosity due to partial clotting

36
Q

Why are veins blood reservoirs?

A
  • Contain the majority of blood in the body.
  • Contain a thinner layer of smooth muscles (compared to arteries) which means they are more compliant and contract less.
37
Q

How does Bernoulli’s theory explain arterial blood flow? PART 1

A
  • When standing there is a pressure gradient against the flow from the heart to the feet.
  • This is because pressure is higher in the feet and lower in the heart.
  • However blood is ejected from the heart with greater kinetic energy than that at the feet as it has more velocity.
38
Q

How does Bernoulli’s theory explain arterial blood flow? PART 2

A
  • Also has greater potential energy than the feet as it has a greater height
  • Greater potential and kinetic energies overcome the pressure gradient and maintains blood flow to the feet.
39
Q

What is the mathematical equation for Pouseille’s Law?

A

Conductance (G) = (πr^4)/(8ηL)

r: radius of vessel
η: blood viscosity
L: vessel length

40
Q

What is Poiseuille’s and Darcy’s Law combined?

A

CO = Pa - CVP x (πr^4)/(8ηL)

41
Q

Give examples of times when the Bayliss myogenic response is important

A

Renal, coronary and cerebral circulation

42
Q

Is the Bayliss myogenic response intrinsic or extrinsic?

A

Intrinsic

43
Q

What is the name given to the condition giving a high number of red blood cells?

A

Polycythaemia

44
Q

How does polycythaemia cause a reduced blood flow?

A
  • Greater haematocrit and so viscosity rises
  • TPR and blood pressure rise
  • Blood flow decreases
  • Tissues and organs become underperfused
45
Q

How does anaemia cause increased blood flow?

A
  • Viscosity decreases
  • TPR and blood pressure decrease
  • Triggers baroreceptor reflex which increases heart rate
  • Blood flow increases
46
Q

What is the main symptom of reduced blood flow?

A
  • Ischaemia due to underperfusion

SGUL - Physiology 1 (68 decks)

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