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And, yes, it is one of those slightly trick questions designed to get you thinking! So, if you’re ready, here we go. 
Allowing for a little natural reticence (!), we all know exactly how old we are. Well, more accurately, we know how many years – and in some cases days, weeks and months (!) – since the day we were born. Despite our bodies having changed in many ways since that time – not least in “growing up” and then later in “ageing” – we see them as solid and permanent. 
However – and this is what we were hinting at with the title of this week’s blog post – is this really the case? That our bodies are as constant as we like to think they are. The simple answer is no. 
While it may not be immediately obvious, our bodies are in a constant state of change and renewal. And we’re not talking about the myriad of natural processes going on each day, without any conscious effort on our behalf, to keep us functioning. Our software, if you like. Rather, we’re talking about our hardware, our bodies themselves. 
Like everything else in life, the various building blocks of our bodies wear out. True, a lot of maintenance goes on before that but, ultimately, some replacement is needed. Just what and when varies a huge amount, depending on the part involved and the amount of wear and tear it’s been exposed to. Here are a few simple examples to get you thinking: 
Unsurprisingly, the cells lining the digestive tract – the Stomach and Intestines – have a very short and demanding life. Not least from it being a highly corrosive environment, thanks to high levels of acid in the Stomach. This is needed both to facilitate digestion, as well as acting as a first line of defence, neutralising any outside threat which may enter the body along with our food. So, do you want to guess how long they last? The answer will probably surprise you. About 5 days before being broken down and replaced. 
Cells found in the Liver have a longer life, although still a demanding one, with the Liver acting as the body’s waste disposal, recycling and storage centre. Any guesses? From about 150 to 500 days depending on the type of cell. 
Red blood cells also have a demanding life, carrying oxygen around the body and carbon dioxide back to the Lungs. Their life span is similar to those of cells found in the Liver, around 150 to 160 days. 
White blood cells, part of our Immune System, can last longer depending on how much work they get (!); anything up to a year. However, as part of the immune response results in the “death” of many White blood cells, many do not survive their first encounter. 
How about the Epidermis, the outer layer of the Skin? Providing protection to the body, it gets a fair amount of wear and tear, with these cells lasting 2 to 4 weeks. 
There are many more examples we could give but are sure you get the picture. Simply by considering how demanding their particular environment is AND how much work they have to do; you can make a fairly accurate guess on how long a particular type of cell is likely to last before being replaced. 
Now let’s go to the other extreme. 
Have you ever wondered how long your hair would continue growing for and last, if it wasn’t cut, shaved or otherwise removed?!? Well, it’s about 6 years for women and 3 years for men. 
What about your bones? Here there’s less certainty. While it’s clear that the entire Skeletal System is renewed on an ongoing process, it’s thought to take between 10 and 15 years for the entire system to be replaced. This process slows down as we get older, with bones becoming less dense – in other words more brittle – as we age. However, as always, there are many ways we can reduce and slow down this process. 
Similarly, the Skeletal muscles – the Muscles that move our Bones – are also thought to have a longer life span of 10 to 15 years. And here’s something that may surprise you. Despite the huge amount of work they do, muscles found in the Heart – Cardiac Muscle – is replaced at a much slower rate than Skeletal muscle. About 1% per year, meaning that many of these cells aren’t replaced at all during our lives. Quite why this is, no one is sure… 
While the vast majority of our cells are replaced many times, there are some cells that stay with us throughout our lives. One example are the nerve cells – the Neurons – in the outer layer of the Brain, the Cerebral Cortex. This is the part of the Brain that governs memory, thought, language, attention and consciousness. As these cells aren’t replaced, it’s easy to see why their loss over time can cause many of the problems seen in the elderly. Poor concentration, problems in expressing themselves and the like. However, the good news is that the body’s natural resilience can step in to help fill the gap with another part of the Brain, the Hippocampus, being able to help us relearn some of these lost skills. 
A couple of other examples of cells that are with us for life are those in the Lens of the Eye and the Oocytes – female egg cells – found in the Ovaries. 
So where are we going with this, just in case you haven't already guessed?!? 
Well, once again, it highlights the importance of the things we do every day. How little changes – positive and less so (!) – can have an impact on our health and quality of life many years later, long after we’ve forgotten about them. 
At the same time, how much difference doing the “right things” is as we get older. Making sure we get the best possible nutrition and hydration to provide the necessary raw materials. Doing regular exercise, particularly weight bearing, to help strengthen our Bones and Muscles. In other words, all the things we talk about regularly in this blog… 
Before we go, here are a couple of questions that regularly come up when we mention this topic. 
The first is about scars. If the outer layer of the Skin is replaced every 2 to 4 weeks, how come scars never seem to completely vanish? 
Well, the answer is simple and all to do with the way the body repairs external damage from cuts. The Skin is naturally full of collagen, a protein that helps keep it plump and taut. When the Skin is cut, collagen is used to make a patch over the wound, resulting in a raised area exactly the same shape and size as the original cut. Over time, as the cells regrow, some of the collagen is reabsorbed but the Skin never quite goes back to its original condition. This is why scars will fade over time but never completely disappear. 
The other one is about tattoos. Why they only fade a little over time but don’t completely vanish. Unlike scars, which affect the outer layer of the Skin, tattoos go much deeper; into the layer below, the Dermis. Added to this, a tattoo isn’t a cut but an injection, with ink being forced deep into the Skin and into cells which aren’t replaced nearly so often. This is why tattoos remain relatively fresh longer term. While some of the ink does dissipate over time, the tattoo still remains, although exposure to too much bright sunlight can play its part too. 
As always, the choice is yours. 
Photograph by unknown author 
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