Special Circulations: Cerebral and Pulmonary Flashcards
How much of the resting cardiac output does the brain use?
15%. It is the most metabolically active tissue of the body.
How does the Blood Brain Barrier work?
It limits the transport of substances from the circulation into the brain, with some substances like EtOH/O2/CO2 able to cross it.
What are the factors that affect cerebral blood flow?
Autoregulation
Tissue Pressure
Metabolism
Neural Control
What is the normal cerebral perfusion pressure (CPP) of the brain?
80-100 mmHg
How does autoregulation affect cerebral blood flow?
If the CPP falls outside of its normal range, auto regulation will serve to correct these changes immediately.
If the CPP falls, how does auto regulation respond?
Cerebral vasodilation occurs to decrease the resistance in the arteries
If the CPP rises, how does auto regulation respond?
Cerebral vasoconstriction occurs to increase the resistance in the vessels
How does tissue pressure affect cerebral blood flow as CSF pressure increases?
As CSF pressure increases, veins will collapse against the pressure of the CSF but arteries will remain open due to their higher pressure. Autoregulation will dilate arteries to keep blood flow available, but flow will rapidly decrease as CSF pressure approaches arterial pressure.
What is the CPP equal to?
Mean Arterial Pressure - Intracranial Venous Pressure = CPP
What happens as intracranial pressure increases?
Vascular compression will occur
What is the Monro-Kellie Doctrine?
When the volume of one compartment increases, there must be a compensatory change in the volumes of the other compartments.
What is the equation of the Monro-Kelli Doctrine?
Brain Volume + Cerebral Vascular Volume + CSF Volume = Constant Value
What metabolite is the cerebral blood flow especially sensitive to?
CO2
As pH decreases, what happens to blood flow?
Vasodilation will occur and it will increase blood flow by decreasing the resistance.
As pH increases, what happens to the blood flow?
Vasoconstriction will occur and it will decrease blood flow by increasing the resistance.
Do changes in systemic pH affect the cerebral blood flow?
No, because H+ cannot cross the BBB easily.
How are pH changes in the brain occurring?
CO2 can cause pH changes as it can cross the BBB.
What is the effect of increased CO2 in the blood?
Vasodilation will occur and it will increase blood flow by decreasing the resistance.
What is the effect of decreased CO2 in the blood?
Vasoconstriction will occur and it will decrease blood flow by increasing the resistance.
What is the effect hyperventilation on cerebral blood flow?
It will decrease CO2 and raise the pH, therefore causing vasoconstriction leading to dizziness and fainting.
How does O2 affect cerebral blood flow?
Decreases in O2 can cause vasodilation, but it has much weaker effects than changes in CO2.
What is the effect of adenosine on cerebral blood flow?
Adenosine is formed from ATP in cases of ischemia and it causes vasodilation to increase blood flow.
What is the effect of K+ on cerebral blood flow?
K+ stimulates vasodilation and increases cerebral blood flow.
What is the effect of NO on cerebral blood flow?
NO relaxes smooth muscle via cGMP and PKG which increases Pi of myosin light chain kinase and decreases Pi of MLC. This leads to vasodilation.
How does the parasympathetic system affect cerebral blood flow?
Facial nerve carries some nerves to cerebral vessels and causes vasodilation.
How does the sympathetic system affect cerebral blood flow?
Control is weak but it increases vascular resistance via minimal vasoconstriction. The baroreceptor reflex has little effect on cerebral resistance.
What is Cushing’s Response?
High intracranial pressure will decrease cerebral perfusion so the ischemia causes sympathetic nerve activity increase which raises systemic blood pressure. Elevation of the intracranial pressure will activate the parasympathetics in order to decrease the HR.
What are the signs of Cushing’s Response?
High intracranial pressure
High BP
Low HR
What circulations do lungs have?
Pulmonary
Bronchial
How much of the cardiac output does the pulmonary circuit receive?
100%
How much of the cardiac output does the bronchial circuit receive?
1%
Are pulmonary arteries more or less compliant than systemic arteries?
They are 7 times more complaint due to minimal smooth muscle
What is different about pulmonary capillaries?
They have major influence on the vascular resistance and contribute about 40% of it.
Do pulmonary vessels autoregulate?
NO
How does pulmonary circulation respond to exercise?
Resistance decreases as flow and pressure increase as new vessels are recruited.
What are alveolar vessels?
Microvessels located within the alveolar walls.
What are extra-alveolar vessels?
Larger arterial and venous vessels which are not in the alveolar walls.
How does inspiration affect pulmonary circulation?
It causes high lung volume which will distend extra-alveolar vessels and cause a decrease in resistance.
It will compress the alveolar vessels and cause an increase in resistance.
The net effect is a slight increase or no effect on the pulmonary resistance.
How does expiration affect pulmonary circulation?
It causes low lung volume which will constrict extra-alveolar vessels and cause a increase in resistance.
It will distend the alveolar vessels and cause an decrease in resistance.
The net effect is a slight decrease or no effect on the pulmonary resistance.
How does hydrostatic pressure influence pulmonary blood flow?
Gravity affects the pressure as in the upright position, intravascular pressures are higher at the bottom of the lung.
What is the vascular “waterfall” effect?
The height divides the pulmonary vascular system and blood flow into separate zones.
What is Zone 1 of the waterfall effect?
It does not exist under normal conditions.
This zone is at the top of the lungs and alveolar pressure is greater than arterial and venous pressure so that capillaries collapse and no blood flow occurs.
What is Zone 2 of the waterfall effect?
It is in the middle of the lung and it occurs when alveolar pressure is greater than venous pressure and less than arterial pressure. The capillaries are partially collapsed and flow depends on the gradient between the arteries and the alveoli pressure.
What is Zone 3 of the waterfall effect?
Both arterial and venous pressures exceed the alveolar pressure and so the flow depends on an arterial-venous gradient.
Under normal conditions, what does zone 2 comprise?
Upper third of the lungs
