Regional Circulations Flashcards
Why do organs have different amounts of blood flowing through them and why do they differ in their ability to increase the amount of blood flow
Different organs have differing amounts of blood through them as they provide differing amount of resistance to blood flow through them. Organs that provide high resistance like skeletal muscles have less blood flow
The 2 reasons as to why the organs would differ in their capacity to increase blood flow is:
- Number of beta 2 receptors in the organs
- Extent of sympathetic nervous sytem innervation in those organs
Local influences
Organs can regulate their own blood flow by themselves. Blood flow through an organ is determined by the metabolic rate or rate of oxygen consumption. High metabolic rate allows the organs to dilate by releasing vasodilator metabolites
What are the 3 organs that are most significant for local control
What does this mean for these organs
Brain, heart and kidneys
There is very little sympathetic nerve innervation in these organs so vasodilator metabolites play a key role in these organs
What is the formula for O2 uptake
O2 uptake = Blood flow x difference in O2 content between arterioles and veins
If kindeys can increase blood flow by only 16% does that mean they can increase O2 consumption by only 16%
No, they increase their capacity to uptake O2, which was given by the last equation, the veins as a result would have a less O2 content
Precapillary sphincters open and more blood flow through the capillaries take place
What does the ability to extract oxygen depend upon and what is it independent upon
Precapillary sphincters that regulate the number of perfused capillaries
This is independent upon blood flow
What is a fundamental difference in the heart and kidneys when they endeavor to increase O2 consumption
Heart does not have precapillary sphincters so it cannot increase O2 uptake. The only way heart increases O2 consumption is by increasing blood flow.
On the other hand kidneys cannot increase blood flow and hence they increase O2 uptake by opening more precapillary sphincters.
In other organs it is a combination of both, heart and kidneys are the extreme
Autoregulation
Organs that regulate their own blood flow like brain, heart and kidneys can regulate their blood flow through them regardless of MAP. There is a range of autoregulation.
Autoregulation happens primarily independent of MAP in these organs
What is the mechanism of autoregulation
- Vasodilators
- Myogenic response
What is myogenic response
This experiment shows that once pressure is increased in an artery in the brain, the diameter of the artery initially goes up but then it goes down becuase the stretching causing increasing AP which causes the Ca gates to open up which causes influx of Ca and constraction of the vascular smooth muscles
Ca is pumped out during dilation
What determines levels of vasodilators and how does this explain irreversible shock
Level of vasodilators depends upon their rate of production and the rate at which it is taken away by blood.
In shock blood pressure stays low for a certain time and it then just tanks resulting in irreversible shock. This happens becuase blood supply has been low enough to stop the rate of transport of vasodilators which causes a massive decrease in TPR causing irreversible shock
What are some of the examples of vasodilators
- Increased levels of lactic acid, protons and CO2
- Adenosine levels rise when ATP formation is impaired (when you take off the last phosphate from AMP it forms adenosine). Adenosine is a potent vasodilator
- K ions play an important role regulation of blood flow. There are 2 mechanism (i) when AP are fired some K leaks out of the cell causing local hyperkalemia which causes reduced Ca influx into the smooth muscles (know why) and this causes them to relax (ii) low ATP levels causes special K-ATP channels to open up causing local hyperkalemia
What does all vasodilators do
Decrease Ca levels in vascular smooth muscles causing them to relax
Active Hyperemia
What happens to pulse pressure during exercise
It increases since there is an increase in inotropic state and there is a decrease in afterload so SV increases dramatically causing the pulse pressure to increase
Is there increased sympathetic firing to the skeletal muscles arterioles during exercise
Yes there is, more NE is released but it is not effective as vasodilators are more potent.
Same thing happens in distributive shock, there is an increased sympathetic firing to the skeletal muscles but it is not effective as histamine and other vasoconstrictors are more effective
What causes reactive hyperemia
Increase in the concentration of vasodilators
Ischemia vs reactive hyperemia
Focal active hyperemia in the brain
When we are performing a certain function, the part of the brain that is involved increases AP firing, causing local hyperkalemia which increases blood flow in that part of the brain
Explain the dilemma of cereberal blood flow
Cereberal blood flow only starts to increase when the partial pressure of O2 drops below 50%. This is not a fault as owing to the non linear curve of O2 content and pO2 as O2 content gets only from 20 to 15mm with a 50% reduction in partial pressure
What are cereberal arterioles supersensitive to
To an increase in pCO2
Why is the brain so sensitive to CO2
Neurons are supersensitive to pH
When does maximum flow in the left and right coronary artery occur
Maximum in the left coronary artery occurs during diastole as contraction of the left ventricle causes the artery to be compressed.
In the right ventricle maximum occurs during systole
What is the absolute critical determinant of O2 consumption in the heart
Inotropic state
