THE FLESH IS WEAK

Got a bad back? Too many colds? Blame the human body and its terrible design

IF a plumber’s installation of your pipes was as scattershot as our internal anatomy, you’d give him a scathing one-star review on Yelp.

Nathan Lents’ new book, “Human Errors: A Panorama of our Glitches, from Pointless Bones to Broken Genes” (Houghton Mifflin Harcourt), out now, points out how so many human weaknesses — including head colds, bad backs and poor eyesight — are a result of our body’s inherently illogical design.

The reason humans get so many colds and sinus infections compared to animals, who get them rarely, is that our sinuses were installed virtually upside-down.

When working properly, our sinuses drain mucus into the stomach with the help of tiny hair-like structures called cilia. It would make sense, then, for the mucus to drain downward. But our two largest sinus cavities are forced to drain upward, leading mucus to collect in our sinuses with much greater frequency, leading the average person to suffer two to five head colds per year.

“One of the important drainage- collection pipes is installed near the top of the largest pair of cavities, the maxillary sinuses, located underneath the upper cheeks,” Lents writes. “Putting the drainage-collection point high within these sinuses is not a good idea because of this pesky thing called gravity.”

The lower part of our breathing apparatus is no better. Incredibly, humans are built to breathe and eat through the same narrow tube, a shockingly ineffective piece of design. “Nearly five thousand Americans choked to death in 2014, the majority of them choking on food,” Lents writes. “If we had separate openings for air and food, this would never happen.”

Our poorly constructed breathing apparatus also ensures that much of the air we’re breathing at any time is old.

“Every breath you take, you’re mixing stale air with fresh air,” Lents tells The Post. “Every breath [includes] air that was already in the lungs, because we only have one route in and out of the lungs. Birds and other creatures have two-way traffic, so every breath is purely fresh air.”

The human eye is also seem- ingly designed for failure, as nearand farsightedness occur because our eyeballs are misshapen.

“The eyeball is often too long or too short,” Lents says. “It’s too long more often than it’s too short. That makes you nearsighted, and there’s really no good [evolutionary] reason for that.”

If it’s not discouraging enough that humans are poorly constructed for breathing and seeing, it turns out we’re also not perfectly designed for standing upright.

“Most of the joints and bones have evolved reasonably well to stand upright, but we never really finished the job of evolving to an upright posture,” says Lents.

Evidence for this lies in the shape of our spine.

“The curve of the spine is Sshaped. We had to go from a Jshaped spine as we stood upright, [and] it would have been easier to just straighten it out,” he says. “We have lower-back pain so much more than any other ape in part because we have this really sharp curve down at the bottom of our spine.”

And if you’ve ever been sidelined with a bulging disk, blame incomplete evolution for that as well.

“The disks in between the vertebra of the spine are more likely to bulge forward because we evolved away from a design where the backbone was horizontal,” Lents explains. When our spines were horizontal, gravity pulled our vertebrae downward.

A tear of an ACL ligament, a common sports injury, is also the result of poor evolutionary design.

When we were quadrupeds, the strain of running and jumping was spread over four limbs. But when we evolved to bipedalism, the muscles of just two had to handle the same strain, and they spread their work to our leg bones. The ACL, which connects our femur (thighbone) to our tibia (shinbone), is not always strong enough to carry that weight.

For now, technology can sometimes repair evolutionary failures on an individual basis — Lents gives the example of LASIK surgery, which reshapes part of the eyeball.

But as we learn more about scientific advances, it’s possible these problems could eventually be fixed society-wide at the genetic level.

“We don’t have the first clue about what genes to tweak yet,” Lents says.

“But if we were to fix [one of these problems] genetically, you would pass that fix on, and we might be able to do this to an embryo. We absolutely have the ability to tweak the genetics of a human embryo at a very early stage. It’s forbidden by law right now, but we do have the ability.”

We never really finished the job of evolving to an upright posture . . . the spine is S-shaped. — author Nathan Lents