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Y Physiology I > Week 08 Lect 3 - Coronary/Cerebral Blood Flow > Flashcards

Week 08 Lect 3 - Coronary/Cerebral Blood Flow Flashcards

1
Q

What is the heart’s mass?

As a percentage of body weight?

A

About 300 g

About 0.4%

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2
Q

What is the heart’s oxygen consumption rate or VO2?

A

About 30 ml/min

so about 1/8 the body’s total 250 ml/min VO2

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3
Q

What is the capillary density of heart tissue?

A

3,000 - 4,000 capillaries/mm3

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4
Q

What is the oxygen extraction percentage of heart tissue?

And AVDO2 (ml/l) ?

A

60-80%

(only as high as 80% in case of exercise)

about 120 ml/l AVDO2

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5
Q

What is the relationship between coronary blood flow and oxygen consumption?

A

it is approximately linear

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6
Q

What fact about coronary arterial circulation makes coronary blockages particularly dangerous?

A

cornary arteries have no collaterals

- so flow loss due to occlusion can be injurious to cardiac tissue

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7
Q

How does the speed of occlusion make a difference in the outcome of coronary arterial blockages?

A

abrupt occlusion - ischemia and tissue death

slow occlusion - collaterals can develop from occluded arteries to maintain flow

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8
Q

How do systole/diastole of the heart effect coronary blood circulation?

A

Coronary vessel resistance increases due to squeezing action of heart contractions:

Systole: R increases, Q decreases

Diastole: R decreases, Q increases

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9
Q

How does a graph of aortic pressure and left coronary blood flow rate vs. time look?

A

flow is lowest during isovolumetric contraction (red area, 0 ml/min) and highest during isovolumetric relaxation (green area, >350 ml/min)

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10
Q

During what part of the cardiac cycle does coronary blood flow reach its lowest levels?

Why?

A

During isovolumetric contraction of the left ventricle

  • contracting heart tissue compresses vessels, increasing their resistance
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11
Q

During what part of the cardiac cycle does coronary blood flow reach its highest level?

Why?

A

During isovolumetric relaxation of the left ventricle…

because the relaxed heart tissue no longer compresses the vessels, so resistance decreases

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12
Q

How does the coronary flow-decreasing effect of systolic muscle contraction differ in different parts of the heart wall?

What is the clinical signficance of this?

A

interstitial pressure is higher on the endocardial side than on the epicardial side

  • endocardial-side vessels are more constricted during systole
  • endocardial vessels are more susceptible to damage from heart attack
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13
Q

How do systole and diastole times change with increased heart rate?

A

At normal HR of 75 bpm, sys:dias ratio is about 1:2

with increased HR, both sys and dias times decrease, but diastolic decreases more than systolic such that at high heart rates the ratio is closer to 1:1

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14
Q

What is the approximate maximum increase in coronary blood flow during exercise?

A

exercise can induce a maximum of about 5x normal coronary flow

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15
Q

How does autoregulation in coronary vessels change as heart activity increases?

How does a graph of this look?

A
  • at first, the autoregulation curve simply shifts upward indicating increased flow at the same pressures
  • at very high workloads (leading to greater vasodilation) the autoregulatory range decreases and eventually the curve becomes linear
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16
Q

What metabolite level changes lead to metabolic regulation of coronary blood flow?

A
  • increased CO2
  • decreased O2
  • increased [K+]
  • increased adenosine
  • increased lactate
  • decreased pH
17
Q

What is the effect of α-1 adrenergic receptor activation in coronary blood vessels?

And β-2 adrenergic receptors?

How significant are the effects of these receptors on coronary blood flow?

A

α-1 - constriction

β-2 - dilation

  • effects are insignificant compared to metabolic regulation (except in some ischemic diseases)
18
Q

How does sympathetic innervation indirectly affect coronary blood flow?

A
  • Sympathetic innervation raises the heart rate
  • Heart contractility increases
  • Metabolism + metabolite creation increases
  • Higher [metabolites] stimulates vasodilation
  • Flow increases
19
Q

Via what path does the parasympathetic nervous system affect coronary blood flow?

A

Via muscarinic receptors on endothelial cells

  • stimulation leads to Ca++ signal > NO production > dilation > increased flow
20
Q

What is the average mass of the brain?

As a percent of total body mass?

A

1400 g

2% body weight

21
Q

What is the average flow through cerebral circulation per minute?

As a percentage of cardiac output?

A

Q = 750 - 850 ml/min

about 15% of cardiac output

22
Q

How much of the total cerebral flow is contributed by each of the main arteries supplying it?

A

Internal Carotids = 2x 330 ml/min

Vertebral Arteries = 2x 75 ml/min

23
Q

How much blood flows to grey matter vs. white matter?

And what cerebral structure’s density within a region of the brain determines flow in that area?

A

80% to grey matter, 20% to white matter

  • regional flow in the brain is highly dependent on number of synapses in that area… not cell count
24
Q

What is the oxygen consumption (VO2) of the brain?

And as a percentage of the body’s total?

A

about 50 ml/min

about 20% of total (250 ml/min)

25
Q

After closure of cerebral arteries…

how long until loss of consciousness?

how long until irreversible damage?

A

within 5 seconds consciousness can be lost

after 5 minutes irreversible damage occurs

26
Q

What are the volumes of the “intracranial compartments”?

brain, blood, CSF, ISF

A

Brain - 1400 ml

Blood - 150 ml

CSF - 150 ml

ISF - 75 ml

27
Q

What two basic physiological factors does cerebral blood flow depend on?

A
  1. Perfusion Pressure (Part - Pven)
  2. Resistance
28
Q

How is perfusion pressure calculated?

A

ΔP = Part - Pven

arterial pressure minus venous pressure

29
Q

How is calculation of perfusion pressure different in cerebral blood flow?

A

Venous pressure can be replaced with intracranial pressure because of the pressure on the vessels within the subarachnoid space

ΔP = Part - PIC

30
Q

What is the approximate value of normal intracranial pressure?

And of normal cerebral perfusion pressure?

A

Intracranial pressure = ~10 mmHg

cerebral arterial pressure is about 90 mmHg so…

ΔPcerebral = 90 mmHg - 10 mmHg

= ~80 mmHg perfusion pressure

31
Q

Within what range is the blood pressure kept in the cerebral circulation?

And how?

A

the normal autoregulatory range: 50-150 mmHg

via resistance vessels, just like in the rest of the body

32
Q

What effect does an increase in intracranial pressure have on cerebral blood flow?

How?

And what conditions can cause this?

A

results in a decrease in perfusion pressure

(because ΔP = Part - Pintracranial)

  • this leads to a decrease in flow
  • intracranial bleeding or tumors can cause this
33
Q

What 3 kinds of local blood flow control are present in cerebral circulation?

A
  1. Metabolic - usual factors… metabolites -> dilation
  2. Astrocytes - ^ neuronal activity –> astrocyte Ca2+ signal –> vessel dilation
  3. Neuronal Control - neurons release NO directly to dilate vessels
34
Q

What is the normal partial pressure of CO2 in the blood?

What happens to cerebral blood flow when this changes?

A

40 mmHg

  • increased PCO2 results in linear increase in cerebral blood flow
35
Q

What is the normal PO2 in the blood?

How do changes in this affect cerebral blood flow?

A

100 mmHg

  • decreases in PO2 result in exponential increases in blood flow
  • however, increases in PO2 do not greatly increase flow, as seen in the graph below with a horizontal asymptote around the normal flow rate
36
Q

What is the effect of sympathetic innervation on autoregulation in cerebral arteries?

A

it widens the autoregulation curve, increasing the range of pressures across which flow can be maintained

Y Physiology I (17 decks)

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