Juxtamedullary versus cortical nephrons
Juxtamedullary nephrons, which sit next to the medulla (the inner part of the kidney), have long loops of Henle (hairpins of tubule) reaching deep into it.
Remember
The length of the loop separates the two nephron types: juxtamedullary nephrons have long loops reaching deep into the medulla and contribute most to the osmotic gradient (a steady rise in salt concentration) that concentrates urine, while cortical nephrons have short loops.
Memory aid
short loop, short reach; long loop, long reach
A short loop reaches only a little way into the medulla, whereas a long loop reaches deep into it.
Nephrons, the microscopic filtering units of the kidney, come in two types, and the names tell you where each type sits.
Cortical nephrons lie nearer the surface, in the outer cortex of the kidney, and the tip of their loop descends only just into the medulla. They are the large majority, about 85 percent of nephrons.
Juxtamedullary nephrons ('juxta' means next to) are the minority, about 15 percent. Their glomeruli sit close to the junction between the cortex and the medulla, and their loop of Henle, the hairpin of tubule that dips into the medulla, is long and extends deep into it.
Nephrons with deeper loops contribute more to the gradient that lets the kidney make concentrated urine. The thick ascending limb pumps salt out without letting water follow, while the descending limb loses water. This arrangement is called countercurrent multiplication: the two limbs carry fluid in opposite directions, and that magnifies the salt difference, so the surrounding tissue becomes saltier the deeper the loop goes. A longer loop spans more of the medulla, so it can build a steeper gradient.
When antidiuretic hormone (ADH, the hormone that lets water leave the collecting duct, the final part of the nephron) is present, water can be pulled out down that gradient, and the urine becomes concentrated. The vasa recta, specialised capillaries that run alongside the loop, keep the gradient in place.
Key facts
- Cortical nephron loop
- short, with a tip that descends only just into the medulla.
- Juxtamedullary nephron loop
- long, extending deep into the medulla.
- Glomeruli
- all glomeruli lie in the cortex, and those of juxtamedullary nephrons are close to the junction between the cortex and the medulla.
- Numbers
- cortical nephrons are about 85 percent of nephrons and juxtamedullary nephrons about 15 percent.
- Limbs of the loop
- the descending limb loses water, while the thick ascending limb pumps salt out without water following.
- ADH
- antidiuretic hormone lets water leave the collecting duct down the gradient, giving concentrated urine.
- Vasa recta
- specialised capillaries that run alongside the loop and protect the gradient.
Exam traps
- A glomerulus lying near the surface, in the outer cortex, marks a cortical nephron rather than a juxtamedullary nephron, the type that sits next to the medulla.
- Blood supply from peritubular capillaries, the capillaries around the tubules of the outer cortex, instead of the vasa recta fits a cortical nephron, whereas a juxtamedullary nephron is accompanied by the vasa recta, long straight capillaries running alongside its loop.
- Cortical nephrons are the more common type, about 85 percent of nephrons, but that does not give them the greatest effect on concentrating urine: numbers do not set concentrating power, and their short loops barely reach the medulla.
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