Composition of the human body
Body composition may be analyzed in various ways. This can be done in terms of the chemical elements present, or by molecular type e.g., water, protein, fats (or lipids), hydroxylapatite (in bones), carbohydrates (such as glycogen and glucose) and DNA. In terms of tissue type, the body may be analyzed into water, fat, connective tissue, muscle, bone, etc. In terms of cell type, the body contains hundreds of different types of cells, but notably, the largest number of cells contained in a human body (though not the largest mass of cells) are not human cells, but bacteria residing in the normal human gastrointestinal tract.
Almost 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Only about 0.85% is composed of another five elements: potassium, sulfur, sodium, chlorine, and magnesium. All 11 are necessary for life. The remaining elements are trace elements, of which more than a dozen are thought on the basis of good evidence to be necessary for life . All of the mass of the trace elements put together (less than 10 grams for a human body) do not add up to the body mass of magnesium, the least common of the 11 non-trace elements.
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 toxics, depending on amount (cadmium, mercury, radioactives). The possible utility and toxicity of a few elements at levels normally found in the body (aluminium) is debated. Functions have been proposed for trace amounts of cadmium and lead, although these are almost certainly toxic in amounts very much larger than normally found in the body. There is evidence that arsenic, an element normally considered a toxin in higher amounts, is essential in ultratrace quantities, in mammals such as rats, hamsters, and goats.
Some elements (silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Bromine is used abundantly by some (though not all) lower organisms, and opportunistically in eosinophils in humans. One study has found bromine to be necessary to collagen IV synthesis in humans. Fluorine is used by a number of plants to manufacture toxins (see that element) but in humans only functions as a local (topical) hardening agent in tooth enamel, and not in an essential biological role.
Elemental composition listEdit
The average 70 kg (150 lb) adult human body contains approximately 7×1027 atoms and contains at least detectable traces of 60 chemical elements. About 29 of these elements are thought to play an active positive role in life and health in humans.
The relative amounts of each element vary by individual, mainly due to differences in the proportion of fat, muscle and bone in their body. Persons with more fat will have a higher proportion of carbon and a lower proportion of most other elements (the proportion of hydrogen will be about the same). The numbers in the table are averages of different numbers reported by different references.
The adult human body averages ~53% water. This varies substantially by age, sex, and adiposity. In a large sample of adults of all ages and both sexes, the figure for water fraction by weight was found to be 48 ±6% for females and 58 ±8% water for males. Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. Because of water content, the human body contains more oxygen by mass than any other element, but more hydrogen by atom-fraction than any element.
The elements listed below as "Essential in humans" are those listed by the (US) Food and Drug Administration as essential nutrients, as well as six additional elements: oxygen, carbon, hydrogen, and nitrogen (the fundamental building blocks of life on Earth), sulfur (essential to all cells) and cobalt (a necessary component of vitamin B12). Elements listed as "Possibly" or "Probably" essential are those cited by the National Research Council (United States) as beneficial to human health and possibly or probably essential.
|Atomic number||Element||Fraction of mass||Mass (kg)||Atomic percent||Essential in humans||Negative effects of excess||Group|
|8||Oxygen||0.65||43||24||Yes (e.g. water, electron acceptor)||Reactive oxygen species||16|
|6||Carbon||0.18||16||12||Yes (organic compounds)||14|
|1||Hydrogen||0.10||7||62||Yes (e.g. water)||1|
|7||Nitrogen||0.03||1.8||1.1||Yes (e.g. DNA and amino acids)||15|
|20||Calcium||0.014||1.0||0.22||Yes (e.g. Calmodulin and Hydroxylapatite in bones)||2|
|15||Phosphorus||0.011||0.78||0.22||Yes (e.g. DNA and phosphorylation)||white allotrope highly toxic||15|
|19||Potassium||2.0×10−3||0.14||0.033||Yes (e.g. Na+/K+-ATPase)||1|
|16||Sulfur||2.5×10−3||0.14||0.038||Yes (e.g. Cysteine, Methionine, Biotin, Thiamine)||16|
|11||Sodium||1.5×10−3||0.10||0.037||Yes (e.g. Na+/K+-ATPase)||1|
|17||Chlorine||1.5×10−3||0.095||0.024||Yes (e.g. Cl-transporting ATPase)||17|
|12||Magnesium||500×10−6||0.019||0.0070||Yes (e.g. binding to ATP and other nucleotides)||2|
|26||Iron*||60×10−6||0.0042||0.00067||Yes (e.g. Hemoglobin, Cytochromes)||8|
|9||Fluorine||37×10−6||0.0026||0.0012||Yes (AUS, NZ), No (US, EU), Maybe (WHO)||toxic in high amounts||17|
|30||Zinc||32×10−6||0.0023||0.00031||Yes (e.g. Zinc finger proteins)||12|
|29||Copper||1×10−6||0.000072||0.0000104||Yes (e.g. copper proteins)||11|
|56||Barium||310×10−9||0.000022||0.0000012||No||toxic in higher amounts||2|
|53||Iodine||160×10−9||0.000020||7.5×10−7||Yes (e.g. thyroxine, triiodothyronine)||17|
|34||Selenium||190×10−9||0.000015||4.5×10−8||Yes||toxic in higher amounts||16|
|28||Nickel||140×10−9||0.000015||0.0000015||Probably||toxic in higher amounts||10|
|25||Manganese||170×10−9||0.000012||0.0000015||Yes (e.g. Mn-SOD)||7|
|33||Arsenic||260×10−9||0.000007||8.9×10−8||Possibly||toxic in higher amounts||15|
|3||Lithium||31×10−9||0.000007||0.0000015||Yes (intercorrelated with the functions of several enzymes, hormones and vitamins)||toxic in higher amounts||1|
|42||Molybdenum||130×10−9||0.000005||4.5×10−8||Yes (e.g. the molybdenum oxotransferases, Xanthine oxidase and Sulfite oxidase)||6|
|27||Cobalt||21×10−9||0.000003||3.0×10−7||Yes (cobalamin, B12)||9|
|79||Gold||3×10−9||2×10−7||3.0×10−7||No||uncoated nanoparticles possibly genotoxic||11|
|23||Vanadium||260×10−9||1.1×10−7||1.2×10−8||Possibly (suggested osteo-metabolism (bone) growth factor)||5|
|4||Beryllium||3.6×10−8||4.5×10−8||No||toxic in higher amounts||2|
*Iron = ~3 g in men, ~2.3 g in women
Of the 94 naturally occurring chemical elements, 60 are listed in the table above. Of the remaining 34, it is not known how many occur in the human body.
Most of the elements needed for life are relatively common in the Earth's crust. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is harmful in large amounts. Transferrins can bind aluminium.
Deemed essential trace element by U.S., not by European Union
Suggested function from deprivation effects or active metabolic handling, but no clearly-identified biochemical function in humans
Limited circumstantial evidence for trace benefits or biological action in mammals
No evidence for biological action in mammals, but essential in some lower organisms.
(In the case of lanthanum, the definition of an essential nutrient as being indispensable and irreplaceable is not completely applicable due to the extreme similarity of the lanthanides. Thus Ce, Pr, and Nd may be substituted for La without ill effects for organisms using La, and the smaller Sm, Eu, and Gd may also be similarly substituted but cause slower growth.)
The composition of the human body is expressed in terms of chemicals:
- Proteins – including those of hair, connective tissue, etc.
- Fats (or lipids)
- Hydroxylapatite in bones
- Carbohydrates such as glycogen and glucose
- Dissolved inorganic ions such as sodium, potassium, chloride, bicarbonate, phosphate
- Gases such as oxygen, carbon dioxide, nitrogen oxide, hydrogen, carbon monoxide, acetaldehyde, formaldehyde, methanethiol. These may be dissolved or present in the gases in the lungs or intestines. Ethane and pentane are produced by oxygen free radicals.
- Many other small molecules, such as amino acids, fatty acids, nucleobases, nucleosides, nucleotides, vitamins, cofactors.
- Free radicals such as superoxide, hydroxyl, and hydroperoxyl.
The estimated gross molecular contents of a typical 20-micrometre human cell is as follows:
|Molecule||Percent of Mass||Mol.Weight (daltons)||Molecules||Percent of Molecules|
Body composition can also be expressed in terms of various types of material, such as:
- Bone and teeth
- Nervous tissue (Brain and nerves)
- Connective tissue
- Body fluids (blood, lymph, Urine)
- Contents of digestive tract, including intestinal gas
- Air in lungs
Composition by cell typeEdit
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 (much less by mass or volume). Some of these symbionts are necessary for our health. Those that neither help nor harm humans are called commensal organisms.
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