Ramchandra: Regulation of Vascular Function Flashcards

1
Q

What is the normal cardiac output at rest?

A

5L/min

Most of the cardiac output goes to the kidneys, liver, muscle and brain.

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2
Q

What is the cardiac output of an individual during exercise? How does it increase?

A

25L/min - increased through increased heart rate as well as stroke volume, the second due to the frank starling law

Most of the cardiac output goes to the skin and muscles. Percentage of CO going to the brain drops but the volume stays the same. Drop caused by the increased CO.

Muscle - Increased O2 for increased ATP generation.

Skin - Increased blood flow to release heat; reducing protein damage.

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3
Q

What is the consequence of constant ultramarathon running?

A

Increased periods of reduced internal organ perfusion leading to internal organ damage.

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4
Q

How does the innervation vary across the vascular tree?

A

Sympathetic Nerves supply all of the vascular beds. However, arteries and arterioles are densely innervated, whereas the veins are more sparsely innervated.

Innervation of the veins increases as the vessel size increases.

Capillary beds receive no direct innervation.

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5
Q

How do blood vessel dimensions change?

A

Smooth Muscle Cells that surround the endothelium of the capillaries contract when stimulated by sympathetic nerves.

The highest proportion of vascular smooth muscle found in small arteries and arterioles.

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6
Q

How does vasoconstriction affect pressure?

A

Vasoconstriction increases the precapillary resistance and therefore blood flow is reduced.

  • However, this also leads to a greater dissipation of energy as blood flows through the small arteries and arterioles.
  • This further leads to an increased pressure gradient and therefore reduced hydrostatic pressure in the capillaries.

Postcapillary constriction can occur to some level but is not as effective as precapillary

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7
Q

What is autoregulation?

A

This is where perfusion pressure is altered to a specific organ and the neural input is ablated. It uses constriction to adjust the flow in order to counteract pressure changes. (eg. if pressure increases then the precapillary vessels constrict allowing less flow thus returning perfusion to its constant)

  • Acute Reduction in Perfusion Pressure = Increase to constant over time.
  • Acute Increase in Perfusion Pressure = Decrease to constant over time.
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8
Q

What is reactive hyperaemia?

A

This is the phenomenon that demonstrates the link between metabolism and blood flow. It occurs after a blood vessel is occluded for a short period. When the occlusion is released, blood flow rises above the pre-occlusion level and this hyperaemia is maintained for a period roughly proportional to the duration of the occlusion.

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9
Q

What is the myogenic hypothesis?

A

Is linked to the understanding/explanation of autoregulation

  • Increased perfusion pressure increases vascular pressures throughout the circulation.
  • Increased transmural pressure leads to vascular distension.
  • Stretch elicits smooth muscle contraction.
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10
Q

What is the metabolic hypothesis of local control?

A

If a given organ is working harder it will demand more flow and cause vasodilation of the arteries supplying it. It does this through release of metabolites that cause vasodilation.

  • For Example: If the heart is asked to work harder (through an increase in heart rate) it will result in coronary vasodilation. In this example, the effects of norepinephrine that binds to alpha receptors causing vasoconstriction is overridden.

.

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11
Q

What are the mediators of the metabolic hypothesis?

A
  • Decreased pH - Especially in exercising muscle - lactic acid
  • Increased potassium - As more ATP is utilised.
  • Adenosine and Adenine Nucleotides.
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12
Q

Where are the effects of the metabolites most strong?

A

The metabolites have their strongest effect in causing vasodilation in the terminal of the pre-capillary vessels but is limited in the post-capillary vessels.

The effects of these metabolites do not explain the coordinated vasodilation throughout pre-capillary distribution circuit

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13
Q

How is NO formed?

A

Shear stress on vascular endothelium - caused by increased blood flow - leads to the catalysation of L-Arginine to citrulline and NO by NO synthase (endothelial form of the enz.)

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14
Q

What are the two other integrative models?

A
  • ATP released in microcirculation by red blood cells due to reduced pO2 acts directly on endothelium.
  • Electrically activated vasodilation conducted rapidly upstream via the endothelium.
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15
Q

The diagram on the integrated model of local control…

A
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16
Q

What occurs in postganglionic sympathetic nerves in response to their depolarization?

A
  1. Calcium enters the nerve terminal via calcium channels.
  2. Calcium causes vesicles containing norepinephrine to fuse to the nerve membrane,
  3. Norepinephrine then released out into the synaptic cleft
  4. It then binds to a receptor or difuses into the blood stream
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17
Q

What is tyrosine hydroxylase?

A

This is an enzyme found in all postganglionic sympathetic nerves which act to convert tyrosine into DOPA and then dopamine.

18
Q

What do the postganglionic vesicles contain?

A
  • Norepinephrine
  • ATP
  • Neuropeptide-Y
  • Vasopressin
19
Q

What is the function of the norepinephrine released from the postganglionic sympathetic nerves?

A
  • Binds to the postsynaptic receptors - Resulting in a conformational change of the postsynaptic cell.
  • Binds to receptors on the nerve terminal itself - This acts to regulate the release of norepinephrine.
  • Some washed off into the circulation.
  • Some is metabolised.
20
Q

How do you influence the postsynaptic cell function?

A
  • Increase or decrease the amount of sympathetic drive.

OR

  • Increase or decrease the number of adrenoreceptors in the smooth muscle (postsynaptic cell).
21
Q

What are the two types of adrenoreceptors present in the postsynaptic cell?

A
  • Alpha - 1
  • Alpha - 2
22
Q

What happens when norepinephrine binds to the alpha-1 receptor?

A
  1. Norepinephrine binds the surface receptor.
  2. Activates G-protein q
  3. Activates Phospholipase C
  4. Phospholipase C cleaves PIP2 into DAG and IP3.
  5. DAG activates Protein Kinase C which leads to further protein phosphorylation.
  6. IP3 leads to further release of intracellular Ca2+ .

Overall, causes increased cytoplasmic Ca2+ and therefore vasoconstriction.

23
Q

What occurs when norepinephrine binds to alpha-2 receptors?

A
  1. Gi-Protein inhibits adenylate cyclase.
  2. Leads to decreased cAMP production.
  3. Leads to inactivation of Protein Kinase A.
  4. Less phosphorylation of proteins.
  5. Also, increases the activity outwardly flowing K+ channel which effectively hyperpolarizes the cell leading to vasodilation.

Overall, leads to a decrease in cytoplasmic Ca2+ leading to vasodilation.

24
Q

What are the inhibitory modulators of adrenergic neurotransmission?

A
  • Acetylcholine
  • Adenosine
  • Dopamine
  • Histamine
  • Prostaglandin E1 and E2
25
Q

What are the excitatory modulators of adrenergic neurotransmission?

A
  • Angiotensin II
  • Adrenaline
26
Q

What is the function of Angiotensin II?

A

Binds to angiotensin receptors on the presynaptic nerve terminal causing increased fusion of vesicles to the membrane as well as stopping reuptake of NE leaving it in the cleft or longer. Therefore acts to increase/promote norepinephrine release.

Also has a role on the postsynaptic cell where it binds to alpha-1 receptors leading to vasoconstriction.

27
Q

Why are some individuals with hypertension given angiotensin blockers?

A

Angiotensin Blockers reduce norepinephrine release from the presynaptic terminal and decreases alpha-1 receptor activation on the postsynaptic cells.

Both act to reduce vasoconstriction and therefore lower blood pressure.

28
Q

What are cholinergic nerves?

A

The cholinergic innervation of blood vessels is largely parasympathetic. They lead to an increase in ACh which binds M3 muscarinic receptors in the endothelium of blood vessels, stimulating nitric oxide release and therefore vasodilation.

29
Q

In the presence of vascular damage, how does ACh function?

A

In the presence of vascular damage, the ACh will bind directly to the muscarinic receptors in the vascular smooth muscle causing vasoconstriction.

30
Q

What occurs when noradrenaline is given to a patient?

A

Leads to vasoconstriction and therefore decreased…

  • Skeletal Muscle Flow
  • Skin Flow
  • Kidney Flow
  • Splanchnic Flow
31
Q

Where does noradrenaline not act?

A
  • Cerebral Vessels - Unable to cross BBB.
  • Coronary Vessels.
32
Q

What occurs if adrenaline is given to a patient?

A

Leads to increased…

  • Skeletal Muscle Flow
  • Splanchnic Flow

Leads to decreased…

  • Skin Flow
  • Kidney Flow
33
Q

What do the different outcomes caused upon infusion of noradrenaline vs adrenaline tell us about them?

A

Adrenaline has the same affinity for alpha-1 and alpha-2 receptors as noradrenaline but there is a change in alpha-1 adrenoreceptors expression in the skeletal muscle and splanchnic organs. (where for adrenaline the splanchnic and Sk. muscle vasodilates )

or

That the affinity is different for the two.

34
Q

Is beta-2 receptor outcomes similar to alpha-1 or alpha-2 receptors?

A

Beta-2 receptor action has similar outcomes to alpha-2 actions. This is because it ultimately results in the production of protein kinase A and therefore vasodilation.

35
Q

What are the differences in affinity for alpha and beta-2 receptors?

A
36
Q

What are the effects of an increased sympathetic drive on an individual at rest in comparison to during exercise?

A

At rest, an increased sympathetic drive can have profound effects on the blood flow - and thus cardiac output - to the skeletal muscle. However, during exercise, this increase in sympathetic drive does not have nearly the same effect due to it being overriden by metabolic vasodilation (pirmarily seen in skeletal muscles with high metabolite production)

37
Q

What causes the profound vasodilation during exercise?

A
  • Increased Circulating Levels of Adrenaline.
  • Histamine release causes relaxation of pre-capillary vessels and inhibits sympathetic neurotransmission.
  • Increased perivascular concentrations of vasodilator metabolites act to relax smooth muscle and inhibit sympathetic adrenergic neurotransmission.
38
Q

What are the effects of vascular function on the skin’s circulation?

A

The affinity of alpha-receptors in the skin for noradrenaline decrease with a rise in local temperature and vice versa. As well as this, there is greater sympathetic cholinergic innervation in the skin. This cholinergic innervation activates the sweat glands leading to release of the enzyme kallikrein, which activates the kinin cascade.

The bradykinin and lysylbradykinin produced acts to dilate the precapillary vessels, constrict postcapillary vessel and increase capillary permeability.

  • We essentiall will never not be able to supply the skins demands and so the flow is rather determined by heat through the sympathetic system than by metabolic control
39
Q

What is the relationship between pressure and flow in cerebral circulation?

A

Has to maintain a constant flow so expresses autoregulation between the perfusion pressures of 60 - 180 mmHg.

  • This isn’t completely accurate as there are small changes witnessed in acute pressure changes. This is important to remember in our clinical career.
40
Q

What regulates cerebral circulation?

A

Cerebral circulation is regulated by a number of local factors. This includes perivascular pH, pCO2, pO2 and (K+) which are all important vasodilator metabolites.

  • PaCO2 < 40 mmHg = Vasoconstriction
  • PaCO2 > 40 mmHg = Vasodilation
  • PaO2 < 50 mmHg = Vasodilation

The response of Bflow to Partial Pressure of oxygen is far less steep compared to the response to changes on CO2 PP

41
Q

What determines blood flow to the heart?

A

Blood Flow to the heart nearly purely under the control of metabolic vasodilation. As a result, as the heart works harder (increased HR) you get vasodilation due to an increase in the release of metabolites.