Not a filter? New study changes everything we know about our kidneys!

‘Everyone hears that kidneys filter blood, but conceptually that is incorrect. What we showed is that kidney cells are pumps, not filters, and they are generating forces.’

BALTIMORE — It’s common knowledge that the kidneys filter blood moving through our bodies, right? Wrong! A new study finds that kidney cells don’t actually filter blood — they pump it.

A team from Johns Hopkins University explains that human kidneys are basically a complex network of tubes which process approximately 190 quarts of blood each day. Along the surface of each of these tubes are epithelial cells which move blood through the kidneys and circulate it back through the rest of the body. However, these cells are immobile, making it a mystery as to how they generate the force necessary to actually do their job.

“Fundamental physical laws say that you need forces to move things. In this case, the cells are not moving, but they are moving fluid. The question then becomes how do they do this?” says Sean Sun, a professor in the Whiting School of Engineering’s Department of Mechanical Engineering, in a university release.

To find out, the team created a device which measures mechanical forces generated by either healthy or diseased kidney cells. The researchers recreated the environment within a real kidney using a micro-fluidic kidney pump (MFKP). Specifically, the device has two microchannels with kidney epithelial cells separating them. While the cells pass fluid between the channels, the device records the fluid pressure these cells generate in real time.

Kidney cells act like a household water pump

The study shows kidney epithelial cells behave just like mechanical fluid pumps. Moreover, they generate their own pressure gradient. Researchers compare the cells’ behavior to a water pump in someone’s house.

Until now, the common belief has been that the kidneys act like a filter for the body, with something else in the human body providing the pressure to move fluid through these organs. The new study, however, reveals that kidneys generate their own pressure — providing important new insight into how the human body actually works.

“Everyone hears that kidneys filter blood, but conceptually that is incorrect. What we showed is that kidney cells are pumps, not filters, and they are generating forces,” Sun says.

Kidney disease changes this pumping behavior

In a collaboration with researchers from the University of Maryland, the team also examined how the kidneys really work if a patient has autosomal dominant polycystic kidney disease. ADPKD is a common, hereditary disorder which causes fluid-filled cysts to grow in the kidneys. This also causes the kidney to enlarge, meaning urine cannot drain out to the bladder.

Researchers found that in patients with ADPKD, the kidney cells pump fluid in the opposite direction in comparison to healthy cells. This causes the pressure to change in the kidney tubes, leading to changes in their shape and function.

They also tested how the FDA-approved drug Tolvaptan affects ADPKD cells using the micro-fluidic kidney pump. Results show the drug successfully delays the progression of the disease. Study authors say their new device could become an effective screening tool for future kidney disease drugs.

The study is published in the journal Nature Communications.

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