Week 08 Lect 3 - Coronary/Cerebral Blood Flow Flashcards by Ben Dahlberg

What is the heart’s mass?

As a percentage of body weight?

About 300 g

About 0.4%

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

About 30 ml/min

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

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What is the capillary density of heart tissue?

3,000 - 4,000 capillaries/mm³

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What is the oxygen extraction percentage of heart tissue?

And AVDO₂ (ml/l) ?

60-80%

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

about 120 ml/l AVDO₂

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

it is approximately linear

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

cornary arteries have no collaterals

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

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

abrupt occlusion - ischemia and tissue death

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

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

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

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

Why?

During isovolumetric contraction of the left ventricle

contracting heart tissue compresses vessels, increasing their resistance

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

Why?

During isovolumetric relaxation of the left ventricle…

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

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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?

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

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

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

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

How does a graph of this look?

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

increased CO₂
decreased O₂
increased [K⁺]
increased adenosine
increased lactate
decreased pH

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?

α-1 - constriction

β-2 - dilation

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

How does sympathetic innervation indirectly affect coronary blood flow?

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

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

Via muscarinic receptors on endothelial cells

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

What is the average mass of the brain?

As a percent of total body mass?

1400 g

2% body weight

What is the average flow through cerebral circulation per minute?

As a percentage of cardiac output?

Q = 750 - 850 ml/min

about 15% of cardiac output

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

Internal Carotids = 2x 330 ml/min

Vertebral Arteries = 2x 75 ml/min

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?

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

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

And as a percentage of the body’s total?

about 50 ml/min

about 20% of total (250 ml/min)

After closure of cerebral arteries... how long until loss of consciousness? how long until irreversible damage?

**within 5 seconds** consciousness can be lost **after 5 minutes** irreversible damage occurs

What are the volumes of the "intracranial compartments"? brain, blood, CSF, ISF

Brain - **1400 ml** Blood - **150 ml** CSF - **150 ml** ISF - **75 ml**

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

1. **Perfusion Pressure** (P_art - P_ven) 2. **Resistance**

How is perfusion pressure calculated?

**ΔP = P_art - P_ven** arterial pressure minus venous pressure

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

Venous pressure can be replaced with **_intracranial pressure_** because of the pressure on the vessels within the subarachnoid space ΔP = P_art - P_IC

What is the approximate value of normal intracranial pressure? And of normal cerebral perfusion pressure?

Intracranial pressure = **_~10 mmHg_** cerebral arterial pressure is about 90 mmHg so... ΔP_cerebral = 90 mmHg - 10 mmHg = **_~80 mmHg_** perfusion pressure

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

the normal autoregulatory range: **50-150 mmHg** via resistance vessels, just like in the rest of the body

What effect does an increase in intracranial pressure have on cerebral blood flow? How? And what conditions can cause this?

results in a decrease in _perfusion pressure_ (because ΔP = P_art - P_intracranial) - this leads to a _decrease in flow_ - _intracranial bleeding_ or _tumors_ can cause this

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

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

What is the normal partial pressure of CO₂ in the blood? What happens to cerebral blood flow when this changes?

**40 mmHg** - increased P_CO2 results in linear increase in cerebral blood flow

What is the normal P_O2 in the blood? How do changes in this affect cerebral blood flow?

**100 mmHg** - decreases in P_O2 result in _exponential_ increases in blood flow - however, increases in P_O2 do not greatly increase flow, as seen in the graph below with a horizontal asymptote around the normal flow rate

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

it _widens the autoregulation curve_, increasing the range of pressures across which flow can be maintained