If every cell in your body is replaced roughly in 7 years, why do the affects of smoking and other bad habits still affect people who quit?
In: Biology
Because it is your current cells that replicate themselves to create the new cells.
So, what do you think is gonna do better in that replication. A bunch of perfectly healthy cells. Or a bunch of kinda fucked up cells?
Like for smokers and your lungs. You have very tiny sacks in your lungs called Alveoli. These microscopic sacs are where the air and your blood intersect separated by a really thin filter, allowing the oxygen and CO2 to transfer across that membrane.
For smokers, it’s possible to damage these little sensitive sacs to the point that they never recover. You kill enough of their cells that they can’t regrow properly.
That being said, you do make a GREAT point. Because a lot of people think “why should I quit smoking, I already ruined my lungs there isn’t a point.” WHEN THERE IS A POINT. People who quit smoking actually record a SIGNIFICANT amount of their king capacity/function. They will never be 100% perfect. But being 95% is way better than being 60%.
So yes, our bodies are remarkably good at healing, and recovering is possible.
All your new cells are created by existing cells. Cells that are very healthy can create a lot of very healthy cells to replace dying cells. Cells that are damaged by poison don’t work as well, so they can’t make very good replacement cells. This damage stacks up over many generations of cells, where each generation can’t work as well and also can’t make good new cells to replace them.
Somewhat relevant to your question, if you quit smoking by the age of 40 you reduce your risk of dying from a smoking-related illness by 90%. Source: a seminar by Richard Peto quite a few years ago.
The premise of your question is a bit off because the 7 year thing is a myth. A nerve cell may never be replaced. A red blood cell lives for just a couple months. Just as two examples
Not every cell gets replaced 7 years, if accurate at all, is an average. Some cells may never be replaced (nervous system), some are replaced often (skin, blood, hair). The cells that are replaced are replicated from your existing cells so damage accumulates over time.
First, those “7 years” are just an average between “every couple of days” and “never” someone took at one time that got misquoted over and over again. And it wasn’t even very useful to begin with, given the wide range between different types of cells.
Second, cancer is what happens when making a new cell goes wrong. So, cells being replaced is actually a bad thing. For example, the brain, which rarely replaces cells, is pretty cool with radiation, whereas your stomach and colon lining will go cancerous with even small amounts of it.
Third, plenty of the damage in the lung is not cell-based. Just like you keep scars on your skin forever, damaged structures in the lungs will not regrow either. There are very few places in the human body where macroscopic damage will fix itself. Your tongue and gums can do it to some extent.
Fourth, some of the aftereffects are from substances that remain in the body or from changes they caused that will persist. For example, a stunted growth (as can happen when kids smoke) will not fix itself by giving a grownup a growth boost when they stop smoking.
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