The amount and type of chemical varied among different tissues or organs. This depends upon two factors. One is volume of blood flowing through the specific tissue or organ and the other is barriers that slow down the toxicant entrance. Organs that have high blood supply receives the high percentage of toxicants.
The concentration of toxicant in a tissue depends upon the tissue affinity for chemicals. These toxicants accumulate in high concentrations inspite of low flow of blood. For instance adipose tissues which have low flow of blood accumulates the high concentration of lipid soluble toxicants. These toxicants remain for long time in tissues due to their solubility in tissues and low blood flow.
The distribution of toxicants from capillaries to the organs is not even. The structural barriers exist that restrict the entry of toxicant in the organ or tissue. These are e.g. brain, placenta and testes.
BLOOD- BRAIN BARRIER
The main barrier in human body is blood-brain barrier that restricts the toxicants from entering in the brain. The astrocytes cells consisting of small branches acted as barrier between capillary endothelium and neurons of the brain. Lipids in astrocyte cell wall and tight junctions between adjacent endothelial cells restricts the passage of water soluble molecules. This barrier is not totally non penetateable. It slow down the rate of toxicant passing in the brain tissues and allows the transfer of necessary nutrients such as oxygen.
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| Blood Brain Barrier |
PLACENTAL BARRIER
The placental barrier protects the developing fetus from toxicants distributed in maternal circulation. This barrier consist of layers of cells between the maternal and fetus circulatory vessels in the placenta. The water soluble toxicants diffusion are inhibited by lipids present in cell membranes. But the waste, nutrients and gases of fetus can pass through placental barrier. It is not totally impenetrable but slow the rate of transfer of toxicants from mother to fetus.
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