Nephrons of human kidneys separate and recycle vital ions, carbohydrates, proteins, and water using selectively‑permeable membranes lining tubes that pass through regions of varying salt content

Industrial chemical processes often involve resource intensive separation steps. While most biochemical synthetic methods themselves do not need separation steps, other physiological processes do. One example is the filtering of blood which removes potentially harmful organic and inorganic materials. Water, as well as useful minerals, , and s are separated from the harmful materials and recycled back into the blood stream. If the water and useful compounds were excreted along with the harmful compounds, the organism would have to consume gallons of water and many grams of minerals on a constant, daily basis to recover the loss.

The filtration aspect of the can be thought of as a collection of hundreds of thousands to over a million independent assembly lines carrying out the same intricate task. The assembly line here is the nephron, a tubular structure with specialized regions along its length. The great quantity of useful water that is excreted is reabsorbed back into the blood as the nephron first descends into the deeper tissues of the kidney. This part of the nephron is the descending limb of its “loop of Henle.” The cells lining the descending limb are permeable to water and impermeable to ions. As the limb descends through progressively saltier (hypertonic) regions of the kidney, water in the relatively un-salty (hypotonic) filtrate passively flows out of the nephron’s lumen (interior space) and into the surrounding salty tissues via osmosis. The concentrated filtrate then travels up the ascending limb of the loop of Henle, which ascends back towards the surface of the kidney through progressively less salty (increasingly hypotonic) interstitial regions. In contrast with the descending limb, the cells that line the lumen of the ascending limb are permeable to ions and impermeable to water. Useful ions in the filtrate are reabsorbed as they flow down their concentration gradient into the surrounding kidney tissues through protein channels, while water is excluded.

Further along the nephron, protein pumps actively transport useful ions and substances between the lumen and the kidney cells and vice versa. Active transport requires an energy input by depleting molecules. More water is also absorbed, so that by the time the filtrate reaches the bladder as fully formed urine, it contains only one percent of the volume of the early filtrate.

Image: Gray's Anatomy /
Image: Emily Harrington / Copyright © - All rights reserved

Nephron showing movement of ions, water, glucose, and proteins from the tubules to the bloodstream. Created by Emily Harrington. This work shall be and remain at all times the shared property of Emily Harrington and the Biomimicry Institute. Emily Harrington grants to the Biomimicry Institute a non‑terminating, nonexclusive, non‑limiting right to use the materials for educational purposes. Any use by outside parties requires permission from Emily Harrington and can be requested from [email protected] or through the Biomimicry Institute.

Last Updated June 26, 2020