Why is the Afferent wider the Efferent arteriole in the kidney? Watch

Bubblybabybling
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It doesnt make sense because in my ocr book it says 'the difference in diameters ensures that the blood in the capillaries in the glomerus is under increases pressure' but isnt it the opposite because if the afferant is wider then surely it cant have a high pressure, a narrower one will put it under pressure.
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hknobody
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Ultrafiltration means "filtration with pressure", the pressure is to force fluid out of the blood into the nephron. Afferent is wider so that pressure is maintained in the glomerulus.
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littleangel9914
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(Original post by Bubblybabybling)
It doesnt make sense because in my ocr book it says 'the difference in diameters ensures that the blood in the capillaries in the glomerus is under increases pressure' but isnt it the opposite because if the afferant is wider then surely it cant have a high pressure, a narrower one will put it under pressure.
Well as the afferent is wider it means more blood can enter increasing pressure. The glomeruli are small so this large volume of blood is forced into them causing pressure to increase/be maintained. The fact that the efferent is smaller means that at any one time more blood enters than leaves the kidney meaning a large blood volume hence pressure to be maintained.
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N97
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(Original post by Bubblybabybling)
It doesnt make sense because in my ocr book it says 'the difference in diameters ensures that the blood in the capillaries in the glomerus is under increases pressure' but isnt it the opposite because if the afferant is wider then surely it cant have a high pressure, a narrower one will put it under pressure.
If the afferent arteriole is wider than the efferent arteriole, the rate of blood entering the glomerulus will be higher than the the rate of blood leaving the glomerulus. Therefore with more blood going in than going out, there is a high filtration pressure within the glomerular capillaries

Hope this helped
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Lordbtew123
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The afferent arteriole is much wider than the efferent. Hence, the speed of blood is slower. The slower the speed, there will be a build up of blood in the glomerulus, like a traffic jam. This traffic jam ensures that there is a difference in water concentration gradient, hence there can be the process of ultrafiltration. I suppose this is the most logical explanation to me..... Hope this helps!!!
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2007PSanHa
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Think about the blood as people running through a tunnel. The afferent arteriole is wide so the people can run through easily. The efferent arteriole is narrow so the people have to run through more slowly, thus, causing a traffic jam. This keeps the pressure high in the glomerulus for ultrafiltration.

Some extra information which might help you to understand (but I'm presuming you are A-level so I doubt this will be on your syllabus):
-If the renal artery constricts the rate of filtration increases.
-If the afferent arteriole constricts the rate of filtration decreases.
-If the efferent arteriole constricts the rate of filtration increases.

Hope I helped!
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