(ORDO NEWS) — By this time tomorrow, every drop of blood in your body will have passed through your kidneys dozens of times. With each pass, waste-laden water is removed to form urine, and fresh, purified blood is returned back into the circulation.
We can think of this vital task as a kind of forced filtering, which is carried out under the pressure of our heartbeat. But according to a new study co-authored by Johns Hopkins University mechanical engineer Sean Sun, that description isn’t as accurate as previously thought.
“Everyone has heard that the kidneys filter the blood, but that’s not conceptually true,” Sun says.
“We have shown that kidney cells are pumps, not filters, and they generate forces.”
It is no coincidence that we missed this unusual mechanical activity. Anatomists knew about the structure of the kidney and its role in the production of urine from blood as early as the 17th century.
The ability of this organ to combine the passive physics of osmosis with the active movement of various chemicals to maintain the balance of salts, wastes and water in our body has also been studied in detail both inside and outside the body.
However, each kidney is made up of miles of channels and tubules that fit in a space no larger than your fist, which can lead to strange plumbing deep in the body.
Research has shown that the cells lining these tubules can sense and even respond to changes in hydrostatic pressure; however, it is not clear how, or even if, these changes can be repelled in any way.
Figuring out how fluids move through these little tubes isn’t easy either. Any experiment to study the hydraulics within individual tubules would require some pretty impressive technology to weed out parasitic forces.
This is exactly what Sun and his colleagues from the United States came up with. Their microfluidic renal pump (MFKP) consists of patterned blocks and porous membranes capable of containing a culture of cells lining the renal tubules.
After the cells had engrafted and were subjected to a series of electrical resistance and permeability tests, the researchers measured pressure changes in the tissue in response to the syringe fluid.
They noticed that fluid movement near the cells was reduced in line with the increase in hydraulic pressure, which was greater at one end of the tissue than at the other. This is exactly what we would expect if the tubules acted as a pump.
A close examination of the proteins that the cells produced showed that small changes in the pressure of the fluids entering the tissues changed the location of the ion channels and the structures that support them, changing their shape and function.
For most of us, this means that the fluids that pass from the blood into the tubular network of the kidneys move partly under the mechanical guidance of the cells themselves, adding a subtle new layer of work that can help explain a range of kidney disorders.
To see how this behavior plays out in less functional kidneys, the researchers used cells taken from people with a kidney disorder called autosomal dominant polycystic kidney disease, or ADPKD.
In this disease, due to the way the cells lining the kidney tubules change shape, cysts usually form, deforming the tissue and increasing the risk of kidney stones and urinary tract infections.
But, according to the results of the team’s work, that’s not all. The researchers observed how the cells pump water in the opposite direction, while the pressure gradient changed from one end to the other.
When the FDA-approved drug tolvaptan for the treatment of ADPD was applied to the cells, the pressure gradient leveled off, suggesting that the drug acts to reduce tissue stress and thereby slow the rate of cyst formation.
With this in mind, it is conceivable that other tissues may also have their own versions of a mechanical pumping system that regulates fluid pressure as they see fit. Sun and his team intend to modify their device to test other tissues and organs.
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