MadSci Network: Biochemistry

Re: How much part of atoms from human body are replaced during a year?

Date: Sun Sep 10 14:44:29 2000
Posted By: Lon Brouse, Faculty, Chemistry, Challenge Charter School
Area of science: Biochemistry
ID: 958384152.Bc

Dear Paul,

This question is very interesting.  We must look to the physiology and 
cellular anatomy of the various tissues of the human body to begin to 
answer it.

Some cells are replaced very rapidly.  The cells lining the small 
intestine are most rapidly replaced and all may be new in just a few 
days.  Other cells, such as the nerves, brain, and spinal cord, are so 
specialized that they have lost the ability to replace themselves and are 
never replaced, even if they are damaged.  This has some good points 
because if you replace brain or nerve cells, you would lose memory and 
reflex arcs.  Bone is made up of organic cells and of inorganic calcium 
hydroxy apatite crystals.  Bone is one of the more slowly replaced tissues 
of the body, but it is replaced.  It may take up to seven years for all of 
the material in your bones to be dissolved and rebuilt.  Remember, you 
cannot replace atoms in a cell without having some effect on the DNA, 
membranes and organelles that make up the cell.

Other atoms are used as food or as the oxidizer to burn the food.  These 
atoms may be recycled very rapidly.  Atoms that are part of the strucutre 
of long-lived cells may reside in your body for your entire lifetime.  
Baby girls are born with all of the egg cells they will ever have (about 2 
million).  Not only that, but these eggs are arresed in their development 
and do mature only one at a time after the onset of puberty.  The atoms of 
these eggs are the same ones the girl/woman was born with and will be the 
same ones she dies with (Except for the five-hundred or so she shed 
through ovulation during her lifetime.)

When cells are replaced, the daughter cells are made up of the nutrients 
in the blood.  The atoms of these nutrients may have come from digested 
food that has been in your body less than 24 hours, or from other tissues 
in your body that are being recycled.  The iron in your hemoglobin, for 
example, is captured by your liver after the red blood cells break from 
being folded inside the very small capillaries in the spleen.  Men 
normally have a several-year supply of iron in their bodies because they 
normally do not lose much blood and because of this recycling process even 
though RBC's only live for about 120 days! These iron atoms may have been 
in your body for years!

An answer to your question is very complex.  There are on the order of 100 
Trillion cells in your body and each is made up of hundreds of trillions 
of atoms.  To trace each one in light of the complexities of new and 
recycled tissues, would require more than a simple answer that could be 
posted on this site.  I would direct you to a good histology book.  
Histology is the study of tissues on the cellular level and such a medical 
textbook goes into great detail about the interactions of these cells.

May I recommend:  Histology, by Arthur W. Ham.  The Latest Edition (mine  
is the 7th,1974).  I do not know of a comparable text in Romanian and 
material of this level of detail is best understood in a person's native 

You may not have access to such books in Romania.  If you need more 
information or would like to discuss this further, please contact me at 

I hope this helps.

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