Regulation of BP I Flashcards
What is the eqn for MAP?
(SBP +2*DBP)/3 or
pulse pressure/3 + DBP
What is pulse pressure?
SBP-DBP
Is pulse pressure higher in muscular arteries (e.g femoral artery) downstream of ascending aorta or in the ascending aorta itself?
downstream, due to decreased compliance as compared to aorta (an elastic artery).
If pulse pressure is higher downstream of the ascending aorta, how can blood flow?
MAP in the femoral artery is lower than in aorta, thus allowing blood to flow from aorta to peripheral arteries, via Ohm’s Law.
Why does pulse pressure increase in the aorta with increasing age?
The aorta becomes less compliant, although this does not significantly change MAP
What are the main systems that regulate arterial pressure?
1) rapidly responding systems (5 sec to 1 min)
2) Less rapidly responding systems (1-30 min)
3) Slowly responding systems (days to months)
What is the main effect of rapidly responding systems?
these systems are designed to buffer changes in pressure, not set new ones
What players are involved in rapid response to AP?
- baroreceptors
- chemoreceptors
- regulation of SV by atrial pressure (after load)
- cerebral ischemia-induced response
What nerves are involved in baroreceptor regulation of arterial BP?
baroreceptors on the aortic arch connect to the vagus nerve which transmits to the brain, and Hering’s nerve from the carotid sinus near the bifurcation of the internal and external carotids connect to the glossopharyngeal nerve to the brain
The baroreceptor is aka?
carotid sinus
The chemoreceptor is aka?
carotid body
Once increased arterial pressure is sensed by baroreceptors, where does it transmit?
the effector is the nucleus tractus solitarius in the medulla of the brain as well as the cardiovascular system
What does the nucleus tracts solitarius do in response?
The response of the effector to information that indicates increased arterial pressure is to decrease sympathetic and increase parasympathetic activity.
What is feedback gain?
the strength of the feedback employed by the nucleus tracts solitarius.
Describe feedback gain.
let’s suppose that there is a 10 mm Hg increase in arterial pressure. After the system is allowed to operate, the 10 mm increase is reduced by 5 mm. Thus, the correction is 5 mm and the “error” (amount that could not be corrected) is also 5 mm, giving a gain of 1 (5 divided by 5). Had the 10 mm increase been reduced to 1 mm, then the correction would have been 9 mm and the error 1 mm and the gain 9. Thus, it is possible to calculate the gain or “strength” of the homeostatic feedback system.
This is important to allow a comparison of different mechanisms that are involved in regulation of blood pressure.
What information do baroreceptors convey to the CNS?
- MAP
- pulse pressure
- HR
How is info about MAP conveyed?
action potentials that increase in quantity with rising MAP convey the signal
Describe the relationship between isolated carotid sinus pressure (x) and systemic arterial pressure (y).
reverse sigmoid shape
As we decrease the pressure on the sinus from the set point (the sensed “normal” pressure), the baroreceptor system responds by increasing systemic arterial pressure. This is effected by increasing sympathetic activity and decreasing parasympathetic activity. As a result, inotropy (cardiac contractility) and chronotropy (heart rate) both increase, causing an increase of cardiac output.
Simultaneously, veins constrict and become less compliant, further increasing cardiac output via effects on the vascular function curve.
Finally, resistance arterioles constrict under the influence of increased sympathetic activity, increasing TPR. These events, taken together, increase blood pressure to compensate for the “sensed” decrease of pressure at the carotid baroreceptor. It is also important to note that the steepest portion of the curve is at the set point, that is the sensitivity of the system is greatest when defending against deviations from the arterial pressure defined as “normal”. Also important to note is the fact that the baroreceptors can buffer against both increases and decreases of blood pressure.
Pulse pressure also regulates arterial pressure via the baroreceptor system independent of MAP. How?
the higher the pulse pressure, the lower the systemic arterial pressure, indicating that the baroreceptor system responds to pulse pressure.
The baroreceptor system also responds to the rate of pulses i.e. the heart rate, independently of other variables. How?
The number of nerve action potentials is integrated over time and the greater the number of action potentials (the greater the heart rate), the more the NTS will tend to reduce sympathetic activity and increase parasympathetic activity and vice versa for decreased pulse rate.
How do we know that baroreceptors don’t set pressures, only buffer them?
When a dog’s carotid sinus was denervated, the following happened:
1) the mean arterial pressure remained the same
2) the variability of blood pressure was much greater than in the normal dog with intact baroreceptor activity.
Baroreceptor monitoring is an acute mechanism of regulating BP. How do we know this?
Almost immediately following increased BP, the baroreceptor system will take measures to oppose this increase, by decreasing sympathetic and increasing parasympathetic activities such that within minutes, most of the increase has been able to be reversed. However, after a day or so, the pressure returns to the high value if the cause persists.
This indicates that the baroreceptor plays no long term role in setting the value of blood pressure and that it “resets” or “adapts” to chronically altered pressures.
Baroreceptors exist in both carotid and aortic forms. Which one is more effective?
carotid- can buffer BP ranging from 50-200 mm Hg
aortic- can buffer BP ranging from ~70-200 mm Hg
but a take-home is that they are both very effective in the short term and can buffer decreases and increases in BP
Reduction of MAP below ___ activates chemoreceptors.
80 mm Hg. Thus, chemoreceptors only function to buffer severe HYPOtension
