Circulation 8 Flashcards
Brain mass vs. blood demand
2% total body weight, but 15% of resting cardiac output
What is the brain’s main energy source?
Aerobic metabolism of glucose (needs a relatively large amount of oxygen and glucose)
What distance of capillaries does the brain contain?
400 miles
Blood brain barrier is due to what four things?
Endothelial cell tight junctions, basement membranes, neuroglial processes, metabolic enzymes
What types of substances can cross the BBB?
Lipid soluble substances such as oxygen, carbon dioxide, ethanol, and steroids, as well as those with transporters
What is the molecular weight cutoff for the BBB?
> 500 daltons (which includes most drugs)
Five factors affecting cerebral blood flow
- Autoregulation
- Tissue pressure (Monro-Kellie doctrine)
- Metabolism
- Autonomic nervous system
- Cushing’s response
Autoregulation of brain bloodflow
Maintains a consistent blood flow to the brain, but flow rates in differing areas of the brain will be dependent upon which part of the brain is active
CPP during normal states
80-100 mmHg
What happens with fluctuations in CPP?
If it falls - cerebral vasodilation
If it rises - cerebral vasoconstriction
Consequences of cranial rigidity on cerebral blood flow
Elevations in intracranial pressure will cause vascular compression, resulting in ischemia
Calculation of CPP
MAP minus intracranial venous pressure
Monro-Kellie doctrine equation
Brain volume + cerebral vascular volume + cerbrospinal fluid volume = constant
Monro-Kellie Doctrine
When the volume of one compartment increases, there must be a corresponding compensatory decrease in the volume of the other compartments
To what metabolite is cerebral blood flow very sensitive?
Arterial PCO2 (indirectly, pH)
Metabolic pH changes and blood flow
Decreased pH causes vasodilation and an increase in blood flow
Increased pH causes vasoconstriction and a decrease in blood flow
What effect does change in blood pH have on cerebral circulation?
Very little; H+ does not easily cross the BBB
Indirect effect of change of blood pH on cerebral blood flow
pH will change the amount of CO2 in the blood, which CAN cross the BBB and will rapidly change the amount of blood flow in the brain
How are patients with high intracranial pressures treated differently?
They are artificially ventilated at high rates to lower PCO2 levels, which vasoconstricts cerebral blood vessels and thereby decreases cerebral edema
For what is O2 in cerebral flow mostly used?
It may have some play in cerebral vasodilation at decreased levels, but it mostly helps with metabolism changes during fMRIs
When is adenosine formed?
In response to ischemia, hypoxia, hypotension, electrical stimulation of the brain, and seizures
Timeframe of adenosine intervention
In situations that either reduce O2 supply or increase O2 demand, adenosine is formed within 5 seconds and remains elevated throughout event
Mechanism of action of adenosine
Acts on purinergic receptors on vascular smooth muscle cells to activate ATP-sensitive potassium current, causing hyperpolarization, turning off calcium current and lowering intracellular calcium
Mechanism of action of potassium ions
EC potassium concentrations increase in response to electrical stimulation of the brain and during seizures, hyperpolarizing and vasodilating vessels by stimulating Na/K-ATPase pump
Parasympathetic cerebral control
A division of the facial nerve carries parasympathetic innervation to some cerebral vessels to cause vasodilation
Sympathetic cerebral control
Exerts minimal vasoconstriction to increase cerebral vascular resistance
Cushing response
An elevation of intracranial pressure can cause a decrease in cerebral perfusion
What does cerebral ischmia stimulate?
Vasomotor centers in the medulla that increase sympathetic nerve activity, elevating systemic blood pressure
Percentage of blood received: pulmonary vs bronchial circulation
100% - pulmonary
1% - bronchial
Compliance in pulmonary vessels
7 times more compliant than systemic arteries due to minimal smooth muscle
Mean pressure gradient in the lungs
6 mmHg
What is different about the capillaries in the lungs than in the systemic circuit?
They have a major influence on vascular resistance: represent about 40% of resistance
How is flow regulated in the pulmonary circuit?
Recruitment of new vessels; they do NOT autoregulate
Effect of inspiration on the extra-alveolar vessels
Negative intrapleural pressure distends the vessels, decreasing resistance
Effect of inspiration on the alveolar microvessels
Inflation of alveoli compresses and elongates the vessels, increasing resistance
Net effect o inspiration
Slight increase or no change in pulmonary peripheral resistance
Hydrostatic pressure in the lungs
Gravity and anatomy’s influence on distribution of pulmonary blood flow
A change in 1 cm of height is equivalent to a change in hydrostatic pressure of __ mmHg.
0.74
Vascular “waterfall” effect
In an upright person, height divides the pulmonary vascular system and blood flow into 3 “zones”
Zone 1
Alveolar pressure exceeds both arterial and venous pressures, causing capillaries to collapse
DOES NOT EXIST UNDER NORMAL CONDITIONS
Conditions under which zone 1 can be met
During hypotension or positive pressure mechanical ventilation
Zone 2
Alveolar pressure exceeds venous pressure, but not arterial pressure, causing partial collapse of capillaries
Conditions of zone 2
Upper third of the lung in a normal person
Zone 3
Both arterial and venous pressures are greater than the alveolar pressure; flow determined by that gradient
Conditions of zone 3
Primarily in the bottom third of the lung
