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Afferent Arteriole: Definition & Function

Instructor: Victoria Leo

Victoria teaches college, authors books, has a therapy practice and masters degrees in anthropology and psychology.

An afferent arteriole connects the renal artery to the glomerular capillary network in your kidney's nephron, starting the filtering process. It also takes action that controls blood pressure.

What is an afferent arteriole?

I live on a normal residential street here in the Puget Sound area of Washington, but there is an eight-lane interstate highway a mile away. To get to my house, you exit one of the limited places on the interstate, then drive on a four-lane connector street to my little two-lane street. In your body, the arteries are the interstates, the connector 'busy' streets are the arterioles and the mesh of little residential streets and cul-de-sacs are the capillary beds.

I've always loved history as well as science, which is why I talked my parents into letting me learn Latin instead of a more useful modern language. Being able to read Julius Caesar's memoirs didn't help me get into college, but it sure is helpful today in understanding biology.

Consider the afferent arteriole. Afferent always means a road IN, while efferent always means the way out. So this arteriole is a connector between the (interstate highway) _renal artery_ and the (residential street) capillaries of a nephron's glomerulus. This is where filtration occurs in your kidney.

Afferent Arterioles Are Active

But the afferent arterioles are much more interactive and exciting than their connector role suggests; they play an active role in regulating your blood pressure. Think about the structure of the individual nephron: blood is filtered from the capillary network into the glomerular capsule; this filtrate travels through the proximal tubule to the distal tubule and then to be collected into the ureter. Proximal is another word Julius Caesar would understand; it means 'close.' Distal comes from the same Latin root as 'distant' so it's always the structure farthest away.

If the flow of filtate through the distal tubule slows down or the concentration of sodium ions in the filtrate crosses a specific threshold value, cells in the distal tubule - at the far end of the nephron, remember? - release prostaglandins. The prostaglandin hormones are carried through the local blood supply back to the afferent arterioles.

Hormone Release

Here's where it really gets interesting: When the afferent arterioles see an upsurge of prostaglandins, they release renin, which you will remember is the first step in the renin-angiotensin-aldosterone cascade. Now you understand how those seemingly-simple connectors can control blood pressure all over your body! The hormone aldosterone increases your blood pressure and signals all the nephrons in both your kidneys to reabsorb more sodium ions back into the bloodstream. Those cells in the distal tubule can also make the afferent arterioles constrict by releasing nitric oxide. By squeezing down into a smaller pathway, the afferent arterioles increase their local blood pressure and in the capillaries that they feed into.

This all makes perfect sense. The distal tubule cells see the filtrate at the end of the process and use nitric oxide and prostaglandins to give the cells at the start of the process - in the afferent arterioles - feedback on what is needed to improve kidney function.

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