Week 08 Lect 3 - Coronary/Cerebral Blood Flow Flashcards
What is the heart’s mass?
As a percentage of body weight?
About 300 g
About 0.4%
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
What is the capillary density of heart tissue?
3,000 - 4,000 capillaries/mm3
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
What is the relationship between coronary blood flow and oxygen consumption?
it is approximately linear
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
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
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
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)
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
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
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
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
What is the approximate maximum increase in coronary blood flow during exercise?
exercise can induce a maximum of about 5x normal coronary flow
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
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?
- Perfusion Pressure (Part - Pven)
- Resistance
How is perfusion pressure calculated?
ΔP = Part - Pven
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 = Part - PIC
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…
ΔPcerebral = 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 = Part - Pintracranial)
- 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?
- Metabolic - usual factors… metabolites -> dilation
- Astrocytes - ^ neuronal activity –> astrocyte Ca2+ signal –> vessel dilation
- 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 PCO2 results in linear increase in cerebral blood flow
What is the normal PO2 in the blood?
How do changes in this affect cerebral blood flow?
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
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
