please help!understanding resistance:decreasing or increasing arterial pressure?

[ketap]

hello, i am new here and really glad to know that there is a forum for discussion for talking about medical issues..
btw, my quick question is...i am quiet confuse about the effects of resistance on arterial pressure..

i have read some books: Ganong's physiology, Guyton's medical physiology, Lauralee sherwood's physiology books. and i found that :

1. in one part, all of the books said that the high resistance (high viscosity, long vessel length and small radius of the vessel) would decrease the arterial pressure ,so that there would be a pressure gradient to allow the fluid to flow..(because of this reason, i understand why there is a drop in MAP in the arterioles)
2. but i found in the other part, that the arteries vasoconstriction ( which means that the vessels radius is becoming smaller) will elevate the MAP..

i am quiet confuse understanding this contradictions

..please someone help me to understand...
warm regards,

Ketap
note: sorry for my bad english, hope you all understand my question...

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[amberisma]

R = p*L/A

This is a physics equation, not sure if it was used in Physio, but it's because stating that the larger the area of the vessel, the less resistance there is. Less resistance means a lower pressure. The same also happens if you shorten the length of the vessel. P is the resistivity constant.

You could technically use P = F/A if you're wondering about the effect of area. Decrease the area (vasoconstriction), increase in the pressure. Increase the area (vasodilation), decrease the pressure.

 

[Charles_Carmichael]

If a blood vessel constricted, the downstream pressure would decrease since the increased resistance dissipates more energy (and pressure can be thought of as potential energy). The pressure upstream of the constricted site increases though. I like to think of it in terms of renal function: constriction of the efferent arteriole increases upstream hydrostatic pressure and thus, increases GFR; on the other hand, constriction of the afferent arteriole decreases downstream pressure and thus, decreases GFR.

MAP increases due to an increase in peripheral resistance. Think of the equation dP = Q x R, where dP is the mean arterial pressure, Q is the cardiac output (or blood flow), and R is the peripheral resistance. Constriction increases resistance but cardiac output is pretty constant (at resting conditions) at about 5 liters/min. By looking at the equation, you can see that if cardiac output is constant and peripheral resistance increases, then the blood pressure has to increase also. Hope this helps.

 

[ketap]

hi, thx for the answers and concern Kaushik and Amberisma🙂
i do now understand how the resistance increase the arterial pressure, but i want to ask you again :
i am still confuse...how about the 1 st part of the book?

1. in one part, all of the books said that the high resistance (high viscosity, long vessel length and small radius of the vessel) would decrease the arterial pressure ,so that there would be a pressure gradient to allow the fluid to flow..(because of this reason, i understand why there is a drop in MAP in the arterioles)

it said that the resistance would decrease the pressure so there would be a pressure gradient to let the blood to flow. 😕
i am still confuse about this contradiction..😕
please help..thx u🙂

 

[rocuronium]

The resistance in the tube causes the downstream pressure to decrease. So in this example the pressure at B would be less than the pressure at A.

(A) -> (B)
===R===

It is true that a pressure differential must exist from end to end for flow to occur. Don't confuse this pressure differential with the one created by the resistance, however.

Think of a tube like this:

------> flow
A =========== B

In order for flow to occur as shown, the pressure at A must be greater than the pressure at B. As we add a resistance, this fact doesn't change:

------> flow
A=====R=====B

In the example above, A is still at a higher pressure than B, but the resistance causes the pressure at B to be even lower.

So basically, what I'm trying to say is that the resistance in the tube causes the pressure to drop, but that pressure drop isn't what causes the flow through the tube. The flow is due to the difference in pressure on either end of the tube.

Does that help clear things up? I hope my simple diagrams helped! 🙂

 

[ketap]

hi, gleek: 🙂 thx a lot for the respond, but i am sorry,i think u misunderstood what i am asking about..i do understand that principle, but what i am asking for is..why this principle is against the 2nd principle (see my first post)
i am still confuse about this...😕
please help 🙁 thx u

 

[Charles_Carmichael]

hi, gleek: 🙂 thx a lot for the respond, but i am sorry,i think u misunderstood what i am asking about..i do understand that principle, but what i am asking for is..why this principle is against the 2nd principle (see my first post)
i am still confuse about this...😕
please help 🙁 thx u

I think you're confusing what those two points are saying. There's no contradiction there. Systemic vasoconstriction will increase the mean arterial pressure. In the first point, the decrease in pressure is greater because the resistance is greater; the first point is talking about downstream pressure and the dissipation of pressure along the length of a vessel with increased resistance. Resistance dissipates energy (think of pressure as energy). If it helps, think of an object sliding across a table that has friction; the kinetic energy (similar to pressure, but pressure is potential energy) of the object is being dissipated by the friction (ie. resistance). So, the object slows down and comes to a stop because it loses energy.

Similarly, as blood is traveling through a vessel and the resistance increases, more pressure is dissipated per unit length of the vessel. So, downstream of the area where resistance is increased, there will be a decrease in pressure compared to when there was no resistance previously. This was well illustrated by gleek in his/her post. That's all that statement is saying; it's not saying that the systemic pressure would decrease. Hope this helps.