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Storage Sites
Storage of toxicants in body tissues sometimes occurs. Initially, when a toxicant enters the blood plasma, it may be bound to plasma proteins. This is a form of storage since the toxicant, while bound to the protein, does not contribute to the chemical's toxic potential. Albumin is the most abundant plasma protein that binds toxicants. Normally, the toxicant is only bound to the albumen for a relatively short time.
The primary sites for toxicant storage are adipose tissue, bone, liver and kidneys. Lipid-soluble toxicants are often stored in adipose tissues. Adipose tissue is located in several areas of the body but mainly in subcutaneous tissue. Lipid-soluble toxicants can be deposited along with triglycerides in adipose tissues. The lipids are in a continual exchange with blood and thus the toxicant may be mobilized into the blood for further distribution and elimination, or redeposited in other adipose tissue cells.
Another major site for storage is bone. Bone is composed of proteins and the mineral salt hydroxyapatite. Bone contains a sparse blood supply but is a live organ. During the normal processes that form bone, calcium and hydroxyl ions are incorporated into the hydroxyapatite-calcium matrix. Several chemicals, primarily elements, follow the same kinetics as calcium and hydroxyl ions and therefore can be substituted for them in the bone matrix. For example, strontium (Sr) or lead (Pb) may be substituted for calcium (Ca), and fluoride (F-) may be substituted for hydroxyl (OH-) ions. Bone is continually being remodeled under normal conditions. Calcium and other minerals are continually being resorbed and replaced, on the average about every 10 years. Thus, any toxicants stored in the matrix will eventually be released to reenter the circulatory system.
The liver is a storage site for some toxicants. It has a large blood flow and its hepatocytes (i.e., liver cells) contain proteins that bind to some chemicals, including toxicants. As with the liver, the kidneys have a high blood flow, which preferentially exposes these organs to toxicants in high concentrations. Storage in the kidneys is associated primarily with the cells of the nephron (the functional unit for urine formation).
 
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