And placental barrier

Describe the roles and functions of the blood-brain and placental barriers. 

Nicotine is well absorbed from the mucous membranes in the oral cavity, gastrointestinal tract, and respiratory system. If tobacco smoke is held in the mouth for 2 seconds, 66 to 77% of the nicotine in the smoke will be absorbed across the oral mucosa. If tobacco smoke is inhaled, approximately 90 to 98% of the nicotine will be absorbed. Nicotine is distributed throughout the body, readily crossing the blood-brain and placental barriers. The liver, kidney, and lung metabolize approximately 80 to 90% of the alkaloid. The kidney rapidly eliminates nicotine and its metabolites. 

Chloramphenicol is completely absorbed after oral administration, bound to plasma protein (approximately 60%) and widely distributed in body. It crosses the blood-brain and placental barrier and shows its presence in CSF, bile and milk. It is conjugated with glucuronic acid in liver and excreted in urine. Small amount is excreted in urine in unchanged form. 

After oral administration, it is absorbed well and distributed to different body tissues and penetrates meninges, caseous masses and placental barrier. It is metabolized in liver to active deacylated metabolite and induces the microsomal enzymes in liver. It is excreted mainly in bile and urine. 

Metallic Mercury. The lipophilic nature of metallic mercury results in its distribution throughout the body. Metallic mercury in solution in the body is highly lipophilic, thereby allowing it to cross blood-brain and placental barriers with ease (Clarkson 1989). Mercury distributes to all tissues and reaches peak levels within 24 hours, except in the brain where peak levels are achieved within 2-3 days (Hursh et al. 1976). The longest retention of mercury after inhalation of mercury vapor occurs in the brain (Takahata et al. 1970). Japanese workers who died 10 years after their last exposure to metallic mercury vapors still had high residual levels of mercury in their brains (Takahata et al. 1970). Autopsies of 3 dentists revealed 0.945-2.110 mg Hg/kg in the renal cortex, compared to 0.021-0.810 mg Hg/kg for unexposed controls (Nylander et al. 1989). 

Metallic and Inorganic Mercury. Data on the distribution of ingested elemental mercury were not located, and data on the ingestion of inorganic mercury are limited. The metallic mercury that is absorbed from an oral exposure is expected to resemble many aspects of the distribution of mercuric salts because metallic mercury is oxidized to mercuric ion in biological fluids, and the resulting distribution reflects that of the mercuric ion. Unlike elemental mercury, however, the amount of divalent mercury that crosses the blood-brain and placental barriers is much lower because of its lower lipid solubility (Clarkson 1989). 

The distribution of mercury in humans and animals appears to be similar. The lipophilic nature of metallic mercury results in its distribution throughout the body in humans (Takahata et al. 1970) and in animals (Berlin and Johansson 1964 Berlin et al. 1966). Distribution of inorganic mercury compounds resembles that of metallic mercury however, human distribution is preferentially to the kidneys, liver, and intestines. Also, levels in the brain are substantially lower, as these compounds have a lower lipophilicity. Distribution of organic mercury compounds is also similar to that of metallic mercury. The ability of methylmercuric compounds to cross the blood-brain and placental barriers enables ready... 

Ethanol is entirely miscible in water and reasonably soluble in lipid. The amphiphilic nature of this small, weakly polar molecule allows it to incorporate readily into the structure of water while at the same time partitioning into hydrophobic environments such as the lipid bilayer. At the concentrations needed to produce a cognitive effect, ethanol will distribute fairly evenly throughout the aqueous and hydrophobic regions of the body, easily crossing the gut, blood-brain and placental barriers. 

Inhalation of tellurium vapor and tellurium hydride causes irritation of the respiratory tract. Exposure of rats, rabbits, and mice to tellurium hexafluoride caused pulmonary edema [20]. In addition to this, elemental tellurium can cross the blood-brain and placental barriers in rats [21]. 

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