Does the human body replace itself every 7 years?

The cells that you have today are different from the cells that you had yesterday. Your body's cells are constantly replicating because they age and become damaged.

Every seven years or so, your cells have been so productive that your body has replaced every part of itself. You are a brand new collection of cells, inside and out, after seven years of cellular replication.

Is that correct? It's not quite right. Some cells in your body are completely replaced in a matter of months, but others are the same as when you were born.

We haven't cloned a human yet.

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Most of the skin and gut can be replaced in a few months, according to an email from a principal researcher. Bergmann and his colleagues reported in the journal Cell Systems on June 15 that cells in the liver are regenerated at a slower rate. The authors found that most of the cells in the body are replaced within three years.

The cells in other organs and systems are not as fast as they could be.

Bergmann said that the human heart renews at a low rate with only 40% of all cardiomyocytes exchanging throughout life. According to the New York Times, Skeletal cells need 10 years to duplicate a skeleton.

Cell renewal can be done in the brain at a slower pace. According to a study in Cell, some neurons in the hippocampus are regenerated at a rate of 1% annually. Bergmann said that other types of neurons stay with a person for the rest of their lives. He said that the distinct cell populations that can rejuvenation are not completely replaced.

Why don't we stay young if parts of our body, like our skin, gut and liver, are renewed every few years?

Bergmann said that we feel older as we get older because of our age. The biological age of a person's cells reflects how they respond to time. Changes in the replicating cells can lead to the aging of the organs. Mistakes are made as cells replicate. The life of the cell can be affected by the presence or absence of certain genes.

Even if the cells in parts of our bodies are relatively new, our aging, much-copied DNA makes us feel the weight of all those years that have passed.

The article was first published on Live Science in 2011.