In this lesson, we’ll review blood flow and the pressure differences that allow physiological processes in the kidneys to occur. We’ll also define what an arteriole is and why it is referred to as an efferent arteriole in the urinary system.

Blood Flow

Before we define what an efferent arteriole is, it’s important to first discuss the differences between an artery and vein and review why these differences exist. An artery is a high pressure blood vessel in the circulatory system that carries blood to an organ and away from the heart. Arteries deliver oxygen and nutrient rich blood from the heart to the rest of the body.

A vein is responsible for carrying blood back to the heart. Veins aren’t regulated by our nervous system, meaning that blood pressure (like the kind that your doctor measures and that varies with disease and age) has nothing to do with veins.

So, what is the point of the circulatory system? Why do our arteries bring blood from our hearts to organs like our kidneys or our liver? Why do veins bring the blood back to the heart? It’s a delivery system, of course! Arteries and veins are responsible for bringing our organs and tissues the nutrients they need from the blood to function (like oxygen).

A Matter of Pressure

So, is it really that simple? Are you really just bringing blood to an organ, essentially dumping it there, and then bringing it back once everything is good and stripped from the blood and given to the tissues – kind of like dumping water on a potted plant from a huge bucket, letting the water absorb into the soil, and then pouring out the excess once you’ve realized you didn’t need all that water?

While that is a great way to picture it, keep in mind that instead of that bucket, human anatomy would be like pointing the strongest garden hose you can find at a potted plant – and then adding even more pressure by applying your thumb over the end of the hose. The dirt from the plant would be everywhere because of the hose pressure! Arteries and veins exist to eliminate this problem; blood flow in the arteries slows down once it reaches the target tissue, and the pressure in the veins is significantly slower when bringing the blood back to the heart.

Think of putting your thumb over the end of the hose, squirting the hose with some fierce pressure, and gradually lifting your thumb to return hose pressure back to normal. This is exactly how blood flow in an artery works; except as the pressure decreases the artery is called something different. An arteriole is the branching off of an artery with significantly lower blood pressures, and they are the portions of arteries that deliver blood directly to the tissue(s).

Efferent Arteriole

Now that we have reviewed blood flow, it’s time to define what an efferent arteriole is and review its function in the blood flow process. An efferent arteriole is an arteriole that delivers blood away from the capillaries of the kidney. The term ‘efferent’ is used to describe this specific arteriole in this highly specific location in the body: the kidney. Blood enters the capillaries of the kidney (known as the glomerulus) via the afferent arteriole, and nutrient exchange occurs here. From there, the blood is pumped into the efferent arteriole, and the efferent arteriole removes the blood from the kidney.

A great way to remember the order blood flows (to and from) is to consider the alphabet: A comes before E. The afferent arteriole (A before E) delivers blood to the kidney, and the efferent arteriole (E after A) takes blood away from the kidney.

The letter E can also stand for exception. The efferent arteriole has a special name because it is an exception to what we just covered: They have small diameters (which increase pressure) and they’re taking blood away from capillaries, yet they’re still considered arterioles.

These blood vessels are a part of the urinary tract of humans (as well as other organisms) because the kidneys are responsible for creating and maintaining urine output. Therefore, you can consider the efferent arteriole important in regulating kidney function and maintaining kidney homeostasis.

Another important thing to consider is pressure differences across the entire filtration system in the kidney. The efferent arteriole (which carries blood away from the glomerulus) is smaller than the afferent arteriole. Think back to the hose and your thumb: by using your thumb to generate force and increase pressure, you’re essentially making the hole in which the water squirts out smaller and backing up the water that would normally just flow out of the hose. The efferent arteriole is smaller for that exact reason: it backs up flow and increases pressure in the glomerulus. By doing so, the glomerular capillaries are able to filter nutrients from the blood more efficiently.

Lesson Summary

An arteriole is an artery that has branched off from a bigger, more predominant artery. They deliver blood to and from capillary networks. Arterioles are typically larger in diameter than arteries, which slows blood flow and decreases pressure and allows for gas and nutrient exchange to occur.

In the kidney, the efferent arteriole takes blood away from the network of capillaries in the kidney (known as the glomerulus). Unlike a normal arteriole, the efferent arteriole is an arteriole with a small diameter, which backs up flow and increases pressure behind it, allowing the glomerular capillaries to properly and efficiently filter the blood that enters the kidney.