Know yourself; love yourself; be true to yourself. These old adages have been batted around throughout the years by a whole cross section of artists, entertainers and philosophical types, from Jean-Paul Sartre ("We only become what we are by the radical and deep-seated refusal of that which others have made of us") and Bob Dylan ("If you try to be anyone but yourself, you will fail") to Katharine Hepburn ("If you always do what interests you at least one person is pleased").
But how do you get to know yourself when your body is constantly changing? Whether it's shedding skin or growing new hairs, the human body is in constant flux.
According to research, our bodies replace many of their nearly 30 trillion human cellsregularly. About 330 billion of those cells are replaced every day — that's about 1 percent of all our body's cells. Other cells, like the tiny ones in our gut, renew within a week.
You might have hoped your new cells would be the key to a longer life. Unfortunately, it's a little more complicated. For example, some of our body's cells, like those in our brain, heart and eyes, are with us our entire lives.
In the early 2000s, Swedish researchers from Karolinska Institute studied body tissue renewal by measuring levels of a radioactive material called carbon-14. There was a sudden increase of carbon-14 into the air during the 1950s and '60s when nuclear weapons were tested aboveground until the Limited Test Ban Treaty was signed in 1963. Carbon-14 is breathed in by plants, which humans and animals eat every day, and is part of our DNA.
But unlike other atoms and molecules that are constantly changing, a person's DNA remains the same from the day of a cell's birth — which occurs when a parent cell divides — throughout its life span. When a cell divides, in other words, the DNA incorporated in the new cell includes a certain level of carbon-14 that corresponds to the level of the material in the air around us at the time. This serves as a timestamp of sorts, by which researchers can determine when the cell was created based on the level of carbon-14 in its DNA.
What the researchers found is that the average age of all cells in the human body is seven to 10 years. That doesn't mean the cells replace themselves every seven to 10 years. At the individual level, cell renewal happens at different rates in the body. Cells in our colon, for instance, are replaced every three to five days, but our muscle and fat cells can take up to 70 years to renew.
If our bodies are constantly being renewed with brand-spanking-new cells, why do we grow old? Shouldn't this influx of new cells be like a shot of Botox? When it comes to aging, it appears that the secret lies not in our cells but, more specifically, in the cellular DNA inside the body.
Cancer and Cell Renewal
Doctors and scientists think that various cancers grow in the human body when cancerous cells self-replenish through division. But one of the most common forms of treatment, chemotherapy, works by wiping out a wide range of cells indiscriminately, without focusing particularly on those that are the source of the cancer. By learning how and when cells self-renew, researchers hope to be able to pinpoint cancer originators and block those cells from duplicating without interfering with other healthy cells.
The Life Span of a Cell
As we've mentioned, cells in the body renew at varying rates. Just how long certain cells last depends on how much work they're tasked with doing. Red blood cells, for example, have an average life span of about 120 days because of their arduous journey through the circulatory system, carting oxygen to tissues throughout the body.
Skin: The epidermis sees a fair amount of wear and tear, thanks to its role as the body's outermost layer of protection. These skin cells rejuvenate every two to four weeks.
Hair: The body's natural fuzz has a life span of about six years for women and three years for men.
Liver: The liver is the human body's detoxifier, purifying a wide variety of contaminants from our systems. It's aided in the process by a constant blood supply and remains largely immune to damage from these toxins by renewing itself with new cells every 150 to 500 days.
Stomach and intestines: Cells that line the surface of the stomach and intestines have a difficult, short life. Constantly battered by corrosives like stomach acids, they typically last only up to five days.
Bones: Cells in the skeletal system regenerate almost constantly, but the complete process takes a full 10 years. The renewal process slows down as we age, so our bones get thinner.
Despite all this regeneration and the fact that the average age of all our cells is about seven years, the truth is that we still get old and die because of mutations that appear in our DNA as our cells replicate over time. So if you think you're going to live forever, you'll will have to find the Fountain of Youth.
Still, there are cells that never leave us and may aid the aging process, or at least the body's breakdown over time. While the eye's cornea can regenerate itself in as little as one day, the lens and other areas don't change. Similarly, neurons in the cerebral cortex — the brain's outside layer that governs memory, thought, language, attention and consciousness — stay with us from birth to death. Because they aren't replaced, the loss of these cells over time can cause maladies like dementia.
The good news is that other areas of the brain, like the olfactory bulb that helps us smell and the hippocampus that helps us learn, can and do rejuvenate.
So get out there and show off that big ol' brain. It's one asset that won't last forever.
How Many Human Cells FAQ
Are your cells replaced every seven years?
Rhe body replaces cell types every seven to 10 years with the exception of neurons in the cerebral cortex, which stay with us from birth to death. The most recurring cell changes occur in the skin, bones, liver, stomach and intestines.
Why does the liver regenerate quickly?
The liver's primary function is detoxification. To ensure that it continues to carry out this function efficiently, it regenerates every 150 to 500 days and remains immune to most toxins that can damage it.
What cells are replaced frequently?
The most frequently replaced cells in the human body are the cells that line the stomach walls and intestine. They typically last around five days before regeneration. Skin cells are replaced every two to four weeks.
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