Composition of the human body

The composition of the human body can be looked at from several different points of view.

By mass, human cells consist of 65–90% water (H₂O). Oxygen therefore contributes a majority of a human body's mass. Almost 99% of the mass of the human body is made up of the six elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.

About 0.85% is composed of only five elements: potassium, sulfur, sodium, chlorine, and magnesium. All are necessary to life. The remaining elements are trace elements, of which more than a dozen are thought to be necessary for life, or play an active role in health (e.g., fluorine, which hardens dental enamel but seems to have no other function).

Note that not all elements which are found in the human body in trace quantities play a role in life. Some of these elements are thought to be simple bystander contaminants without function (examples: caesium, titanium), while many others are thought to be active toxins, depending on amount (cadmium, mercury, radioactives). The possible utility and toxicity of a few elements at levels normally found in the body (aluminum) is debated. Trace amounts of cadmium and lead have had functions suggested, but are almost certainly toxic in amounts normally found in the body. There is evidence that one element normally thought a toxin (arsenic) is essential in ultratrace quantities, even in mammals. Some elements that are clearly used in lower organisms and plants (arsenic, silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Two halogens used abundantly by lower organisms (fluorine and bromine) are presently known to be used by mammals only opportunistically. However, a general rule is that elements found in active biochemical use in lower organisms are often eventually found to be used in some way by higher organisms.^{[citation needed]}

[edit] Elemental composition

The average 70 kg adult human body contains approximately 6.7 x 10²⁷ atoms and is "composed of" 60 chemical elements. In this sense, "composed of" means that a trace of the element has been identified in the body. However, at the finest resolution, most objects on Earth (including the human body) contain measureable contaminating amounts of all of the 88 chemical elements which are detectable in nearly any soil on Earth. The number of elements thought to play an active positive role in life and augmentation of health in humans and other mammals, is about 24 or 25.^[1]

The relative amounts of each element vary by individual. The numbers in the table are averages of different numbers reported by different references.

The human body is ~70% water, and water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent.

*Iron = ~3 g in men, ~2.3 g in women

The elements needed for life are relatively common in the Earth's crust, and conversely most of the common elements are necessary for life. An exception is aluminium, which is the third most common element in the Earth's crust (after oxygen and silicon), but seems to serve no function in living cells. Rather, it is harmful in large amounts.^{[citation needed]} Transferrins can bind aluminium.^[8]

Periodic table highlighting dietary elements^[9]

[edit] Composition by molecule type

The composition can also be expressed in terms of chemicals, such as:

• Water
• Proteins – including those of hair, connective tissue, etc.
• Fats (or lipids)
• Apatite in bones
• Carbohydrates such as glycogen and glucose
• DNA
• Dissolved inorganic ions such as sodium, potassium, chloride, bicarbonate, phosphate
• Gases such as oxygen, carbon dioxide, nitrogen oxide, hydrogen, carbon monoxide, methanethiol. These may be dissolved or present in the gases in the lungs or intestines. Ethane and pentane are produced by oxygen free radicals.^[10]
• Many other small molecules, such as amino acids, fatty acids, nucleobases, nucleosides, nucleotides, vitamins, cofactors.
• Free radicals such as superoxide, hydroxyl, and hydroperoxyl.

The composition of the human body can be viewed on an atomic and molecular scale as shown in this article.

The estimated gross molecular contents of a typical 20-micrometre human cell is as follows:^[11]

Water: Obviously the amount of water is highly dependent on the level of hydration. DNA: A human cell also contains mitochondrial DNA. Sperm cells contain less mitochondrial DNA than other cells. A mammalian red blood cell contains no nucleus and thus no DNA.

[edit] Materials and tissues

Body composition can also be expressed in terms of various types of material, such as:

• Muscle
• Fat
• Bone and teeth
• Brain and nerves
• Connective tissue
• Blood – 7% of body weight.
• Lymph
• Contents of digestive tract, including intestinal gas
• Urine
• Air in lungs

[edit] Composition by cell type

There are many species of bacteria and other microorganisms that live on or inside the healthy human body. In fact, 90% of the cells in (or on) a human body are microbes, by number^[12][13] (much less by mass or volume). Some of these symbionts are necessary for our health. Those that neither help nor harm us are called commensal organisms.

[edit] See also

• Hydrostatic weighing
• Dietary mineral
• Composition of blood
• List of human blood components
• Body composition
• Abundance of elements in Earth's crust
• Abundance of the chemical elements

[edit] References

1. ^ Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al.. Baltimore : Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI: [1]
2. ^ Thomas J. Glover, comp., Pocket Ref, 3rd ed. (Littleton: Sequoia, 2003), p. 324 (LCCN 2002-91021), which in turn cites Geigy Scientific Tables, Ciba-Geigy Limited, Basel, Switzerland, 1984.
3. ^ Chang, Raymond (2007). Chemistry, Ninth Edition. McGraw-Hill. pp. 52. ISBN 0-07-110595-6. 
4. ^ Distribution of elements in the human body (by weight) Retrieved on 2007-12-06
5. ^ Zumdahl, Steven S. and Susan A. (2000). Chemistry, Fifth Edition. Houghton Mifflin Company. pp. 894. ISBN 0-395-98581-1. )
6. ^ J. Emsley, The Element, 3rd ed., Oxford: Clarendon Press, 1998.
7. ^ Neilsen, cited, [2]
8. ^ Mizutani, K.; Mikami, B.; Aibara, S.; Hirose, M. (2005). "Structure of aluminium-bound ovotransferrin at 2.15 Å resolution". Acta Crystallographica Section D Biological Crystallography 61 (12): 1636. doi:10.1107/S090744490503266X.  edit
9. ^ Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al.. Baltimore : Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI: [3]
10. ^ Douglas Fox, "The speed of life", New Scientist, No 2419, 1 November 2003.
11. ^ Freitas Jr., Robert A. (1999). Nanomedicine,. Landes Bioscience. Tables 3–1 & 3–2. ISBN 1570596808. http://www.foresight.org/Nanomedicine/Ch03_1.html. 
12. ^ Glausiusz, Josie. "Your Body Is a Planet". http://discovermagazine.com/2007/jun/your-body-is-a-planet. Retrieved 2007-09-16. 
13. ^ Wenner, Melinda. "Humans Carry More Bacterial Cells than Human Ones". http://www.scientificamerican.com/article.cfm?id=strange-but-true-humans-carry-more-bacterial-cells-than-human-ones. Retrieved 2010-10-09.