antidepressant
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ACE inhibitor, reduce the vasoconstrictive effect of angiotensin II and thus decrease afterload.(Goljan)
i thought vasodilation will cause incrase blood flow to heart, then how does it reduce afterload?
anyone can explain?
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| digant
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ace inhibitors act on both arteries and veins.
so by acting on veins(vasodilation)---> it decrease preload(blood flow to heart)..
and by acting on arteries, it decrease afterload...
so simple....
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| antidepressant
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digant thank, but Goljan said , ace inhibitor decrease the vasoconstirctive action of peripheral resistant arterole, and thus decrease afterload, how is that possible, because if there is vasodilation, more blood is supposed to go to the heart, right? if more blood goes to heart, how is afterload reduced?
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| new_n_lost
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the preload is the degree to which the myocardium is stretched before it contracts and the afterload is the resistance against which blood is expelled.
By dilating the arterioles and arteries the force required to pump the blood from the heart into the aorta is called after load, is greatly reduced and hence causing less work for the heart. Dilation of arteries and veins results in a bit of pooling effect
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| antidepressant
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thanks nnl, thanks for clearing this.it totally makes sense,. got it now, about afterload,
veins have pooling effect, but arteries dont, right?
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| antidepressant
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Another Quiery, want to make sure, i got it right: about HTN
In hypertensive patients, they have too much Na in their body, which increases water in ECF, increases plasma volume, increases SV, heart has to work again extra volume, leads to HTN.
also, Na goes in smooth muscle of peripheral resistant arteriole, Na opens channel for Calicum, increased Ca in smooth muscle of arteriole, vasoconsctriction, increased TPR( TPR= viscosity/r4), increased TPR keeps more blood in arteriole during diastole when heart is filling up with blood, increased diastolic pressure. (goljan)
the quiery is in my underlined part, does it mean, there is not enough blood to pump, so, heart try to pump out all the blood it has, increased SV and increased BP?
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| macintosh
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When the total peripheral resistance of the circulatory system increases, blood is displaced from the vessels towards the heart. This increases the diastolic pressure.
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| antidepressant
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thanks macintosh, but doesnt it mean if you vasodilate the peripheral resistant arteriole, you decrease TPR, more blood returns to heart ( as in septic shock, high output failure)?
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| macintosh
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The prepheral resistance arteriole serves to control the overall resistance of the cicrculatory system to the flow of blood. these vessels have precapillary shincters the tone of which is maintained by stimulation from the adrenergic system. As precapillary sphicters relax more arterioles are opened and facilaitate more blood to flow to the capillaries. This in effect reduces the resistance of the circulation and the afterload is decreased. (Please remeber that afterload is the lod against which the heart contracts. This load is dictated by the relative relaxation or contriction onf the sphincters in the arterioles).
On the other hand the venous system primarily serves the function of storing blood. In certain conditions as blood loss the overall tone of the venous system increases. This reduces their capacity to hold blood and in essence the blood that was previously stored int he veins is displaced towards the heart thus increasing the preload to the heart.
In septic shock the veins are dilated and thus store more blood than normally, therefore causing shock.
Remember:
arteries---->resistance---->afterload
veins---->capacitance----->preload
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| macintosh
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Again if I missed answering your question, if more precapillary sphincters open more blood reaches the capillaries and more exchange of nutrients occur at the tissue level. (Such a condition occurs when a muscle is excercised thereby increasing the metabolites in the tissues) It should not have a significant effect on the preload.
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| antidepressant
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antidepressant wrote:
if you vasodilate the peripheral resistant arteriole, you decrease TPR, more blood returns to heart
thanks, macintosh, i understand afterload and preload( as for now), so there is no confusion about Goljan, is there?
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| blacko
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a sample way to see it is to consider after load as the peripheric resistance (the resistance against the cardiac ejection) and preload as the end diastolic volume.
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| antidepressant
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thank you, 
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