Afferent vs efferent arteriole: which one do you squeeze?

A hose and a filter

In the video at 0:00

Squeeze the end of a garden hose and the pressure inside the hose goes up. The kidney's filter runs on pressure too. Each glomerulus sits between two tiny tubes, one bringing blood in and one taking it out, and the question is which one to squeeze to filter more.

Fluid is only filtered out of the glomerulus because the blood pressure inside it pushes fluid through the filtration membrane into the capsule around it.

Afferent and efferent arterioles

In the video at 0:24

An arteriole is a tiny artery. Blood arrives at the glomerulus through the afferent arteriole (afferent means arriving) and leaves through the efferent arteriole (efferent means exiting). A quick way to remember it: afferent = arriving, efferent = exiting.

Both arterioles have smooth muscle in their walls, so each can narrow (constrict) or widen (dilate). That is what lets the body tune the pressure inside the glomerulus.

Why a small pressure change matters

In the video at 0:36

The pressure pushing fluid out is only a little bigger than the pressures pushing back. In the book's numbers, net filtration pressure = glomerular blood hydrostatic pressure − (capsular hydrostatic pressure + blood colloid osmotic pressure) = 55 − (15 + 30) = about 10 mm Hg.

Because the net pressure is so small, a very small change in pressure makes a big change in how much fluid is filtered per minute. That amount per minute is the glomerular filtration rate (GFR). The book says the same about the afferent side: only a 10 mm Hg differential across the glomerulus is needed for a normal GFR, so very small changes in afferent arterial pressure significantly increase or decrease GFR.

Squeeze the entrance (afferent)

In the video at 0:49

Narrow the afferent arteriole and less blood gets into the glomerulus. The pressure in the glomerulus drops and filtering goes down. Open the afferent arteriole wider and the opposite happens: more blood gets in, pressure rises and GFR rises.

The book gives a real case. Under stress, sympathetic nerve activity increases and directly constricts the afferent arterioles (the norepinephrine effect), and the adrenal medulla adds a general vasoconstriction through epinephrine. Blood flow through the kidneys falls, less filtration occurs, and blood is redirected to organs with more immediate needs. The juxtaglomerular apparatus lesson shows the kidney narrowing its own afferent arteriole when too much salt reaches the macula densa (tubuloglomerular feedback).

The afferent arteriole also works on its own: in the myogenic mechanism, smooth muscle in its wall contracts when blood pressure stretches it and relaxes when pressure drops, so filtration changes very little across a wide range of blood pressures.

Squeeze the exit (efferent)

In the video at 1:10

Constrict the efferent arteriole and blood can't leave the glomerulus as easily, so it backs up inside. The pressure in the glomerulus goes up and filtering goes up. That is the hose: squeeze the exit and the pressure behind it rises.

So to filter more, squeeze the exit (efferent) or open the entrance (afferent). To filter less, squeeze the entrance.

Using both at once

In the video at 1:25

The body can act on both arterioles together. Dilating the afferent arteriole while constricting the efferent arteriole lets more blood in and holds it back, which keeps the pressure in the glomerulus up even when blood pressure changes (A&P 2e 25.6).

Back to the question

In the video at 1:57

Which one do you squeeze to filter more? The exit, the efferent arteriole. Squeeze the exit to raise the pressure; squeeze the entrance, the afferent arteriole, to lower it. That is the whole trick.

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