Sympath & Parasymp: vasodilation/vasoconstriction

[babeexphat]

sorry for the long title.

So parasympathetic and sympathetic activity have different effects in different systems. I was wondering what happens to the vasoconstriction/vasodilation in these systems:

Digestive system: Since it is more active in parasymp than sympath, would there be Vasoconstriction in symp and Vasodilation in Parasymp?

Cardiovasc system: For the opposite reason as digestive system, would there be Vasoconstriction in parasympathetic and Vasodilation in Sympathetic?

And I know it's bad to generalize, but it is GENERALLY accepted that sympathetic means more activity (like when a bear attacks you). So is it GENERAL that sympathetic causes vasodilation?

Thanks,

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[jphwki82]

sorry for the long title.

So parasympathetic and sympathetic activity have different effects in different systems. I was wondering what happens to the vasoconstriction/vasodilation in these systems:

Digestive system: Since it is more active in parasymp than sympath, would there be Vasoconstriction in symp and Vasodilation in Parasymp?

Cardiovasc system: For the opposite reason as digestive system, would there be Vasoconstriction in parasympathetic and Vasodilation in Sympathetic?

And I know it's bad to generalize, but it is GENERALLY accepted that sympathetic means more activity (like when a bear attacks you). So is it GENERAL that sympathetic causes vasodilation?

Thanks,

Sympathetic innervation of the vessels will cause vasoconstriction via NE acting on alpha adrenergic receptors. Sympathetics, also, increase the contractility of the heart, and thus cardiac output. Parasympathetic innervation will cause vasodilation. So it depends on the blood vessels and circumstances as to whether it is receiving more sympathetic or parasympathetic tone. Sympathetics are not acting in mass over the whole body. If that were the case, every time you turned off the lights and your pupils dilated, you would ejaculate as a male. Obviously this does not happen. Certain targeted areas will have increased sympathetic tone at a given time. So during the flight or fight response, you will not be actively digesting food. So sympathetics will act on that corresponding tissue to slow down digestion. In general, sympathetics will act on a blood vessel to cause vasoconstriction. It acts on the heart, as I mentioned before. Vasoconstriction will keep venous return at normal so that cardiac output isn't decreasing. Some areas will not have vasoconstriction during a fight or flight response, such as you leg muscles when you are running. They will have higher metabolic needs during that time. Other areas will constrict to sustain venous return.

In general, sympathetics will act on smooth muscle to constrict blood vessels, dilate bronchial airway, decrease GI tract motility, dilate your pupils, cause sweating, increase heart rate, inhibit contraction of the bladder to expel urine, salivation, ejaculation in males and etc. In general, parasympathetics will promote functions that occur at rest, such as digestion, urination, dilate blood vessels, salivation, sexual arousal (i.e. erection in males). Note that for urination, you don't have to be in a fight or flight state to urinate. Sympathetic tone and somatic innervation to the external urethral sphincter will prevent frequent urination. Distension of the bladder will cause increased sympathetic tone to the bladder and with drawl of sympathetic tone, and withdrawl AP to cause contraction in the external urethral sphincter to urinate.

 

[jphwki82]

What type of prep material are you using to study for the MCAT? Some of the physiology concepts seem to be tripping you up a little bit. If you want suggestions on some MCAT prep material, shoot me a PM, and I can help you out with that.

 

[babeexphat]

Thank you for your response! That clears up a whole lot.

One more question though, regarding this :

So during the flight or fight response, you will not be actively digesting food. So sympathetics will act on that corresponding tissue to slow down digestion. In general, sympathetics will act on a blood vessel to cause vasoconstriction.

Although sympathetics will generally act on a blood vessel to cause vasoconstriction, will the blood vessels in the digestive system vasoconstrict as well or vasodilate? ( Because you do not need to actively digest food )

 

[babeexphat]

What type of prep material are you using to study for the MCAT? Some of the physiology concepts seem to be tripping you up a little bit. If you want suggestions on some MCAT prep material, shoot me a PM, and I can help you out with that.

I used Kaplan but scored an 8 and need to improve my score. I am now looking at my Physio book (Silverthorne) and reading from cover to cover to make sure I understand the basics. I am going to do Exam Krackers after.

 

[jphwki82]

Thank you for your response! That clears up a whole lot.

One more question though, regarding this :



Although sympathetics will generally act on a blood vessel to cause vasoconstriction, will the blood vessels in the digestive system vasoconstrict as well or vasodilate? ( Because you do not need to actively digest food )

This would be a fair assumption. So the cardiovascular system will vasoconstrict to the GI tract because there is not digestion going on during the sympathetic response. This will help sustain venous return to the right atrium, and sustain stroke volume and cardiac output. There be vasodilation, which has nothing to do with sympathetic stimulation, to muscle groups that you would be using during the flight or fight response. Those muscles are active and require oxygen and other metabolic needs at that time.

In terms of using a physiology text book, that may be overkill for the MCAT. Subject reviews are a great time saver. If something is really catching you up, then I would crack open the physiology textbook. The MCAT in general, is not going to be a test where you regurgitate straight facts from memory. You are best served by doing as many sample passages and full length practice exams as possible. With their passages, a good chunk of the test will come from your interpretation of the info that they give you in the passage. While having a good biology foundation will help you better understand things, you don't want to get caught up with the chore of memorizing everything. I found the subject review MCAT prep books sufficient enough for the purpose of the MCAT. I used kaplan and EK subject reviews. Some of the material covered on the MCAT will be more basic than what a physiology textbook will cover, especially with autonomics.

 

[brood910]

Sympathetic innervation of the vessels will cause vasoconstriction via NE acting on alpha adrenergic receptors. Sympathetics, also, increase the contractility of the heart, and thus cardiac output. Parasympathetic innervation will cause vasodilation. So it depends on the blood vessels and circumstances as to whether it is receiving more sympathetic or parasympathetic tone. Sympathetics are not acting in mass over the whole body. If that were the case, every time you turned off the lights and your pupils dilated, you would ejaculate as a male. Obviously this does not happen. Certain targeted areas will have increased sympathetic tone at a given time. So during the flight or fight response, you will not be actively digesting food. So sympathetics will act on that corresponding tissue to slow down digestion. In general, sympathetics will act on a blood vessel to cause vasoconstriction. It acts on the heart, as I mentioned before. Vasoconstriction will keep venous return at normal so that cardiac output isn't decreasing. Some areas will not have vasoconstriction during a fight or flight response, such as you leg muscles when you are running. They will have higher metabolic needs during that time. Other areas will constrict to sustain venous return.

In general, sympathetics will act on smooth muscle to constrict blood vessels, dilate bronchial airway, decrease GI tract motility, dilate your pupils, cause sweating, increase heart rate, inhibit contraction of the bladder to expel urine, salivation, ejaculation in males and etc. In general, parasympathetics will promote functions that occur at rest, such as digestion, urination, dilate blood vessels, salivation, sexual arousal (i.e. erection in males). Note that for urination, you don't have to be in a fight or flight state to urinate. Sympathetic tone and somatic innervation to the external urethral sphincter will prevent frequent urination. Distension of the bladder will cause increased sympathetic tone to the bladder and with drawl of sympathetic tone, and withdrawl AP to cause contraction in the external urethral sphincter to urinate.

Bump..

This is confusing as hell.
So, does this mean vasoconstriction = more blood to the organs?
According to EK, parasympathetic system dilates vessels leading to digestive system though..

 

[Cmdr_Shepard]

I used Kaplan but scored an 8 and need to improve my score. I am now looking at my Physio book (Silverthorne) and reading from cover to cover to make sure I understand the basics. I am going to do Exam Krackers after.

Nooooo. Don't use your physiology text. The MCAT physio is way less complex and you're likely wasting time by trying to read that thing cover to cover. Best thing to do is passages and read review books. TPR, or EK.

 

[Oso]

Nooooo. Don't use your physiology text. The MCAT physio is way less complex and you're likely wasting time by trying to read that thing cover to cover. Best thing to do is passages and read review books. TPR, or EK.

Agree. I would only use class notes/ textbook if you're really confused on a particular topic and would like some more background info. But definitely don't use this as your main study material, use MCAT review books.

 

[brood910]

Agree. I would only use class notes/ textbook if you're really confused on a particular topic and would like some more background info. But definitely don't use this as your main study material, use MCAT review books.

This thread is 2 years old. Sorry. I bumped it to ask a similar question.

 

[Oso]

This thread is 2 years old. Sorry. I bumped it to ask a similar question.

ooops! So what's your question exactly? Sympathetic innervation will cause vasoconstriction of vessels to GI organs and stuff you DON'T need during a sympathetic (fight or flight) response (in addition to increasing heart rate, etc.). Parasympathetic will cause vasodilation of these vessels, so lots of blood to GI organs during rest and digest.

 

[brood910]

ooops! So what's your question exactly? Sympathetic innervation will cause vasoconstriction of vessels to GI organs and stuff you DON'T need during a sympathetic (fight or flight) response (in addition to increasing heart rate, etc.). Parasympathetic will cause vasodilation of these vessels, so lots of blood to GI organs during rest and digest.

That clears it up. Thanks!

 

[steelersfan1243]

Sorry to bump this thread, but I have another similar question:

Which of the following would not be observed in a patient with reduced intravascular volume status?
A. Decrease urine volume
B. Dry mucous membranes
C. Vasoconstriction
D. Decreased urine osmolarity

The answer is D, but I put down C. If the SNS vasodilates to vessels to increase the blood volume reaching skeletal muscle, why would vasoconstriction occur with someone who has low blood volume? What is the fine line I am missing? The answer is D, which in hindsight makes sense, I just need to know why not C.

 

[sc4s2cg]

I haven't looked at osmolarity in a long, long time - a decreased urine osmolarity means that the urine is more concentrated?

Either way, to answer your question, the initial reaction of the SNS is to vasodilate so it can decrease the afterload on the heart and thus hopefully increase cardiac output. However if we wait a bit, it will begin to go into 'emergency mode' and cause vasoconstriction. Remember that there is an ideal blood pressure that needed so that nutrients can go from blood vessel to cell. The reasoning for the increased constriction is that some crucial organs, like the kidneys, liver, and the brain, need a constant flow of blood/nutrients/oxygen. So we vasoconstrict to get what little blood is circulating to the organs faster, so these organs can have the close to the same volume of blood that it used to have before whatever caused the decreased intravascular volume.

If the kidney gets 5L of blood at 120/80 BP, but now the blood volume is less so the BP is 60/40 and now the kidney only gets 2.5...vasoconstriction attempts to raise the BP back up to 120/80 regardless the blood volume. The ultimate effect is that what little blood is squirting around in there, with a higher blood pressure the blood will be flowing at 2x the speed. Which means that yes, there is less blood, but the kidney can get that 5L of circulating blood faster.

That is the reasoning I was taught. Disclaimer: I am not 100% clear on whether vasodilation or vasoconstriction happens first. Regardless, the same reasoning would apply. Check out this link if you are interested in the autoregulation process.

 

[steelersfan1243]

I haven't looked at osmolarity in a long, long time - a decreased urine osmolarity means that the urine is more concentrated?

Either way, to answer your question, the initial reaction of the SNS is to vasodilate so it can decrease the afterload on the heart and thus hopefully increase cardiac output. However if we wait a bit, it will begin to go into 'emergency mode' and cause vasoconstriction. Remember that there is an ideal blood pressure that needed so that nutrients can go from blood vessel to cell. The reasoning for the increased constriction is that some crucial organs, like the kidneys, liver, and the brain, need a constant flow of blood/nutrients/oxygen. So we vasoconstrict to get what little blood is circulating to the organs faster, so these organs can have the close to the same volume of blood that it used to have before whatever caused the decreased intravascular volume.

If the kidney gets 5L of blood at 120/80 BP, but now the blood volume is less so the BP is 60/40 and now the kidney only gets 2.5...vasoconstriction attempts to raise the BP back up to 120/80 regardless the blood volume. The ultimate effect is that what little blood is squirting around in there, with a higher blood pressure the blood will be flowing at 2x the speed. Which means that yes, there is less blood, but the kidney can get that 5L of circulating blood faster.

That is the reasoning I was taught. Disclaimer: I am not 100% clear on whether vasodilation or vasoconstriction happens first. Regardless, the same reasoning would apply. Check out this link if you are interested in the autoregulation process.

Thank you for your thorough response! However, to add just one point of clarity, does the SNS act not to increase blood volume to skeletal muscle when it vasodilates? Is its purpose of vasodilation to compensate the "stress" on the heart? How is the heart "stressed" through the SNS though? But regardless, why does the SNS cause vasoconstriction to our digestive system, in my Kaplan book it states it is to reduce blood flow to "unnecessary" organs, is there another detail I am missing?

 

[sc4s2cg]

For simplicity's sake I'll go ahead an answer your questions in the order given:

1. Well yes. Vasodilation is supposed to increase blood volume. However if, for whatever reason, that does not happen then the next step is vasoconstriction. To make sure that the correct BP is maintained at all costs.
2. Vasodilation has many effects, the 'goal' is to alleviate the problem that caused the low blood volume in the first place. If the heart is having trouble beating, then vasodilation will help it. If a blood vessel is clogged, vasodilation will help it. And so on.
3. Remember that we are talking abnormalities (the question you posted assumed that the patient has low blood volume). Something bad happened that caused low blood volume, and one of the ways the body compensated is by vasodilating. At the same time the heart will start pumping faster. Try to keep your pathologies and normal physiology separate or all this can get confusing pretty quickly.
4. So here we're talking normal function. SNS is your fight or flight system. If you see a lion your heart will race, vessels near skeletal muscle will vasodilate, mind focuses on one task, breathing intensifies, muscles tense, and so on. What you don't want is the feeling that you gotta number 2 while you're running away! The body in this case makes sure that your key organs get the blood that their now increased demand requires, and it does so by limiting flow to currently-nonessential organs.

Hope this helps, let me know if I missed something or if I can clarify further!

 

[deleted647690]

Sympathetic innervation of the vessels will cause vasoconstriction via NE acting on alpha adrenergic receptors. Sympathetics, also, increase the contractility of the heart, and thus cardiac output. Parasympathetic innervation will cause vasodilation. So it depends on the blood vessels and circumstances as to whether it is receiving more sympathetic or parasympathetic tone. Sympathetics are not acting in mass over the whole body. If that were the case, every time you turned off the lights and your pupils dilated, you would ejaculate as a male. Obviously this does not happen. Certain targeted areas will have increased sympathetic tone at a given time. So during the flight or fight response, you will not be actively digesting food. So sympathetics will act on that corresponding tissue to slow down digestion. In general, sympathetics will act on a blood vessel to cause vasoconstriction. It acts on the heart, as I mentioned before. Vasoconstriction will keep venous return at normal so that cardiac output isn't decreasing. Some areas will not have vasoconstriction during a fight or flight response, such as you leg muscles when you are running. They will have higher metabolic needs during that time. Other areas will constrict to sustain venous return.

In general, sympathetics will act on smooth muscle to constrict blood vessels, dilate bronchial airway, decrease GI tract motility, dilate your pupils, cause sweating, increase heart rate, inhibit contraction of the bladder to expel urine, salivation, ejaculation in males and etc. In general, parasympathetics will promote functions that occur at rest, such as digestion, urination, dilate blood vessels, salivation, sexual arousal (i.e. erection in males). Note that for urination, you don't have to be in a fight or flight state to urinate. Sympathetic tone and somatic innervation to the external urethral sphincter will prevent frequent urination. Distension of the bladder will cause increased sympathetic tone to the bladder and with drawl of sympathetic tone, and withdrawl AP to cause contraction in the external urethral sphincter to urinate.

Sorry to bump this, but could someone help me?

I thought that it was generally accepted that sympathetic innervation causes vasoconstriction, while parasympathetic causes vasodilation. However, my textbook says that alpha 1 adrenergic receptors cause vasodilation, while beta 2 adrenergic receptors cause vasoconstriction.
Adrenergic receptors are implicated in the sympathetic response. So how does parasympathetic cause vasodilation if parasympathetic has nothing to do with these receptors?

 

[aldol16]

I thought that it was generally accepted that sympathetic innervation causes vasoconstriction, while parasympathetic causes vasodilation. However, my textbook says that alpha 1 adrenergic receptors cause vasodilation, while beta 2 adrenergic receptors cause vasoconstriction.
Adrenergic receptors are implicated in the sympathetic response. So how does parasympathetic cause vasodilation if parasympathetic has nothing to do with these receptors?

The parasympathetic system uses acetylcholine as the neurotransmitter, which binds to cholinergic receptors to effect downstream effects. It's not as simple as vasoconstriction vs. vasodilation. The effects of each system depends on what tissue you're talking about. In the sympathetic response, you want to be in "fight-or-flight" mode, so you want to redirect your blood flow and oxygen to the tissues that will help you do that - so your muscles will be vasodilated while your digestive tract will be vasoconstricted. You can apply this logic to other body systems.

 

[deleted647690]

Sympathetic innervation of the vessels will cause vasoconstriction via NE acting on alpha adrenergic receptors. Sympathetics, also, increase the contractility of the heart, and thus cardiac output. Parasympathetic innervation will cause vasodilation. So it depends on the blood vessels and circumstances as to whether it is receiving more sympathetic or parasympathetic tone. Sympathetics are not acting in mass over the whole body. If that were the case, every time you turned off the lights and your pupils dilated, you would ejaculate as a male. Obviously this does not happen. Certain targeted areas will have increased sympathetic tone at a given time. So during the flight or fight response, you will not be actively digesting food. So sympathetics will act on that corresponding tissue to slow down digestion. In general, sympathetics will act on a blood vessel to cause vasoconstriction. It acts on the heart, as I mentioned before. Vasoconstriction will keep venous return at normal so that cardiac output isn't decreasing. Some areas will not have vasoconstriction during a fight or flight response, such as you leg muscles when you are running. They will have higher metabolic needs during that time. Other areas will constrict to sustain venous return.

In general, sympathetics will act on smooth muscle to constrict blood vessels, dilate bronchial airway, decrease GI tract motility, dilate your pupils, cause sweating, increase heart rate, inhibit contraction of the bladder to expel urine, salivation, ejaculation in males and etc. In general, parasympathetics will promote functions that occur at rest, such as digestion, urination, dilate blood vessels, salivation, sexual arousal (i.e. erection in males). Note that for urination, you don't have to be in a fight or flight state to urinate. Sympathetic tone and somatic innervation to the external urethral sphincter will prevent frequent urination. Distension of the bladder will cause increased sympathetic tone to the bladder and with drawl of sympathetic tone, and withdrawl AP to cause contraction in the external urethral sphincter to urinate.

I believe parasympathetic does NOT innervate the blood vessels. Vasodilation by parasympathetic only occurs via NO release in localized vascular tissues, such as the penis. This does not apply to general blood vasculature.
Sympathetic causes vasoconstriction via alpha 1 of the GI, GU, kidney, skin, brain, etc., while causing vasodilation via beta 2 receptors in the vascular beds of the skeletal muscle.