Digestive tract variations

Schmidt (1974) in the northern and southern Atlantic varied from 0.8 to 5.4 with an average of 2.5. From these data, extrapolated to other oceans, Schmidt (1974) and Seiler and Schmidt (1974b) estimated a global emission rate of 4Tg/yr. In contrast to CO, the aqueous concentrations of H2 do not undergo diurnal variations. This does not preclude a photochemical production of H2, as was shown for carbon monoxide, but other production mechanisms for hydrogen, such as by bacteria or in the digestive tracts of zooplankton, may be more effective. 

Iodine occurs in traces in most foods and is present mainly as inorganic iodide, in which form it is absorbed from the digestive tract.The richest sources of this element are foods of marine origin, and values as high as 6 g/kg DM have been reported for some seaweeds fishmeal is also a rich source of iodine. The iodine content of land plants is related to the amount of iodine present in the soil, and consequently wide variations occur in similar crops grown in different areas. 

The dietary input of lead has been assessed at approximately 300/tg per day, which is excreted in a dynamic equilibrium, (Kehoe et al., 1933). Of this input, 5-10%, with wide individual variation, is absorbed from the digestive tract (Kehoe, 1960), both passively by diffusion, and actively via a cationic pump (Gruden, 1975). 

Thiessen NW, Snell AM. The variations of intragastric temperature in response to vasodilating agents. Am J Physiol 105 665-673, 1933. Similar but more detailed studies added little Masuda H, Ohara M, Katsura S. Studies in the temperature of the digestive tract. Tohoku JExp Med 57 129-135,1953 Bishton RL. The effect of pilocarpine on gastric blood flow. J Physiol Lond 124 62P-63P, 1954. 

Other nerve structures are also subject to variation. The position in the vertebral column at which the spinal cord terminates varies over three full vertebrae, and no standard terminal position exists. The positions at which various nerves enter the spinal cord likewise vary from individual to individual. About 15 per cent of the population do not have a "direct pyramidal" nerve tract in the spinal cord. Most human beings have 31 pairs of spinal nerves corresponding to 30 vertebrae some, however, have 32 and some 33 pairs, corresponding to 31 and 32 vertebrae, respectively. Most people have two splanchnic nerves (sympathetic nerves to the digestive system), but occasional individuals have three. In some individuals the sciatic nerve is so embedded that it is always fully protected in others, the pathway of the nerve is such that injury is very likely to occur. 

In addition to the toxicokinetic and toxicodynamic differences mentioned above, other aspects of differences between experimental animals and humans include different types of organs and tissues, differences in digestion, and differences in the stmcture of the upper respiratory tract. Furthermore, animal studies are performed in homogenous groups of animals, but the results have to be apphed for the protection of all individuals in a heterogeneous population of humans. In consequence of this, interspecies variation must also be expected. 

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