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# Poiseuille and blood flow. watch

1. Can anyone explain Poiseuille's Law in terms of blood flow to me please because I don't understand it whatsoever

Thanks!
2. Hi, I will try and recreate the equation for Poiseuille's Law in MS Word Equation editor in a sec, but basically the important parts of the equation are the pressure (in this case the blood pressure in the vessel in question - the p in the equation below is on the numerator and has no power so the flow rate is directly proprtional to the pressure), the diameter of the vessel (the a [radius] is on the numerator with a power of ^4, so the flow rate is directly proprtional to the vessel girth to the power of four - in other words, the diameter has a massive effect), and eta (the viscosity [of the blood in this case]) is on the denominator, so flow is inversely proportional to viscosity [you would expect that as you can guess that if the blood is thicker, it will flow slower as it will be impeded more by the vessel wall).

The vessel diameter will be determined by several factors, most of them working through the autonomic flow to the vessel wall muscle layer (smooth muscle of course); simulation of alpha1 adrenergic receptors [by a sympathetic nerve](usually by norepinephrine, but some drugs might do the same e.g. phenylephrine) will cause vasoconstriction leading to much slower flow rate; and vice versa. [muscle arterioles can be dilated by stimulation of beta2 adrenergic receptors, greatly increasing blood flow - e.g. due to "adrenaline rush" if a lion jumps in through the window - extremely unlikely in UK, but has been known in Kenya where I was born - ).

A complicating point is that vasoconstriction will increase blood pressure, which will tend to do the oppositei.e. increase flow rate, but this happens with generized vasoconstriction less so with localized vascular beds.

If the haematocrit is increased (the proportion of cells in the blood as against liquid (plasma), the blood could be considered to be "thicker", will have a greater viscosity so flow rate will reduce.

Watch this space for equation - give me a few minutes to upload.

Regards,
Mukesh
3. (Original post by macpatelgh)
Hi, I will try and recreate the equation for Poiseuille's Law in MS Word Equation editor in a sec, but basically the important parts of the equation are the pressure (in this case the blood pressure in the vessel in question - the p in the equation below is on the numerator and has no power so the flow rate is directly proprtional to the pressure), the diameter of the vessel (the a [radius] is on the numerator with a power of ^4, so the flow rate is directly proprtional to the vessel girth to the power of four - in other words, the diameter has a massive effect), and eta (the viscosity [of the blood in this case]) is on the denominator, so flow is inversely proportional to viscosity [you would expect that as you can guess that if the blood is thicker, it will flow slower as it will be impeded more by the vessel wall).

The vessel diameter will be determined by several factors, most of them working through the autonomic flow to the vessel wall muscle layer (smooth muscle of course); simulation of alpha1 adrenergic receptors [by a sympathetic nerve](usually by norepinephrine, but some drugs might do the same e.g. phenylephrine) will cause vasoconstriction leading to much slower flow rate; and vice versa. [muscle arterioles can be dilated by stimulation of beta2 adrenergic receptors, greatly increasing blood flow - e.g. due to "adrenaline rush" if a lion jumps in through the window - extremely unlikely in UK, but has been known in Kenya where I was born - ).

A complicating point is that vasoconstriction will increase blood pressure, which will tend to do the oppositei.e. increase flow rate, but this happens with generized vasoconstriction less so with localized vascular beds.

If the haematocrit is increased (the proportion of cells in the blood as against liquid (plasma), the blood could be considered to be "thicker", will have a greater viscosity so flow rate will reduce.

Watch this space for equation - give me a few minutes to upload.

Regards,
Mukesh
Thank you so much, very helpful!

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