Some years ago, when Jennifer Pluznick was nearing the end of her training in physiology and sensory systems, she was startled to discover something in the kidneys that seemed weirdly out of place. It was a smell receptor, a protein that would have looked more at home in the nose. Given that the kidneys filter waste into urine and maintain the right salt content in the blood, it was hard to see how a smell receptor could be useful there. Yet as she delved deeper into what the smell receptor was doing, Pluznick came to a surprising conclusion: The kidney receives messages from the gut microbiome, the symbiotic bacteria that live in the intestines.

In the past few years, Pluznick, who is now an associate professor of physiology at Johns Hopkins University, and a small band of like-minded researchers have put together a picture of what the denizens of the gut are telling the kidney. They have found that these communiqués affect blood pressure, such that if the microbes are destroyed, the host suffers. The researchers have uncovered a direct, molecular-level explanation of how the microbiome conspires with the kidneys and the blood vessels to manipulate the flow of blood.

The smell receptor, called Olfr78, was an orphan at first: It had previously been noticed in the sensory tissues of the nose, but no one knew what specific scent or chemical messenger it responded to. Pluznick began by testing various chemical possibilities and eventually narrowed down the candidates to acetate and propionate. These short-chain fatty acid molecules come from the fermentation breakdown of long chains of carbohydrates — what nutritionists call dietary fiber. Humans, mice, rats and other animals cannot digest fiber, but the bacteria that live in their guts can.

As a result, more than 99 percent of the acetate and propionate that floats through the bloodstream is released by bacteria as they feed. “Any host contribution is really minimal,” Pluznick said. Bacteria are therefore the only meaningful source of what activates Olfr78 — which, further experiments showed, is involved in the regulation of blood pressure.

Our bodies must maintain a delicate balance with blood pressure, as with electricity surging through a wire, where too much means an explosion and too little means a power outage. If blood pressure is too low, an organism loses consciousness; if it’s too high, the strain on the heart and blood vessels can be deadly. Because creatures are constantly flooding their blood with nutrients and chemical signals that alter the balance, the control must be dynamic. One of the ways the body exerts this control is with a hormone called renin, which makes blood vessels narrower when the pressure needs to be kept up. Olfr78, Pluznick and her colleagues discovered, helps drive the production of renin.

How did a smell receptor inherit this job? The genes for smell receptors are present in almost every cell of the body. If in the course of evolution these chemical sensors hooked up to the machinery for manufacturing a hormone rather than to a smell neuron, and if that connection proved useful, evolution would have preserved the arrangement, even in parts of the body as far from the nose as the kidneys are.

Olfr78 wasn’t the end of the story, however. While the team was performing these experiments, they realized that another receptor called Gpr41 was getting signals from the gut microbiome as well. In a paper last year, Pluznick’s first graduate student, Niranjana Natarajan, now a postdoctoral fellow at Harvard University, revealed the role of Gpr41, which she found on the inner walls of blood vessels. Like Olfr78, Gpr41 is known to respond to acetate and propionate — but it lowers blood pressure rather than raising it. Moreover, Gpr41 starts to respond at low levels of acetate and propionate, while Olfr78 kicks in only at higher levels.