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Absorption follows either pulmonary or cutaneous exposure or oral ingestion.
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Other modes of entry are subcutaneous, intramuscular, intraperitoneal or intravenous administration.
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Absorption also occurs via mammary gland, uterus and eye.
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Transport of xenobiotic from the site of entry through the cell membrane depends on various transport mechanisms like simple diffusion, filtration, facilitated diffusion, active transport, pinocytosis and phagocytosis.
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Toxic effects may be local, but the poison must be dissolved and absorbed to some extent to affect the cell.
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The primary factor affecting absorption is solubility.
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Insoluble salts and ionized compounds are poorly absorbed, while lipid-soluble substances are generally readily absorbed, even through intact skin. For example, barium is toxic due to its absorption, but barium sulphate is used for intestinal contrast radiography because of low absorption.
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Pulmonary absorption is important in the case of gases, materials that can be volatilized at elevated temperatures, finely divided powders or dusts and the small droplets associated with the use of aerosols and spraying operations. Pulmonary absorption also plays a major role in painters and workers at pertol sumps, cement industry and flour mills .
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All the soluble substances are rapidly absorbed via the highly vascular pulmonary mucous membrane.
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Species differences in susceptibility to volatile toxic agents exist. E.g. Birds are vulnerable to poisonous fumes produced by overheating polytetrafluroethylene coated cooking vessels.
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Cutaneous absorption may be significant with materials presented either in oily solutions or as emulsions and with some chemicals such as nicotine.
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DMSO increases the rate and degree of absorption.
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Powders are generally not absorbed via the skin in terrestrial animals with chlorinated hydrocarbons as an exception.
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Skin abrasions increase the rate of cutaneous absorption.
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Toxic agents enter the portal circulation via the alimentary tract mucous membrane.
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Small intestine is the important site of absorption while large intestine is also important in non-ruminants.
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Absorption from the stomach in dogs and rumen and reticulum in the ruminant are also important sites of absorption.
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Hydrochloric acid in the stomach can increase the solubility of certain poisons and thereby increase the absorption.
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Presence of feed in the stomach will dilute the toxins and thereby delay the intoxication or protect the animal against harmful effects. This will be greater in the ruminants.
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However, this is not true always. ANTU, which is a rodenticide, will be vomited out if taken in an empty stomach, but recent feeding prevents vomiting and thereby increases absorption.
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Constipation increases toxicity while diarrhoea decreases absorption.
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In general, solubility increases absorption of poisons.
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With subcutaneous, intramuscular, intraperitoneal or intravenous administration, intoxication can be produced by considerably smaller quantities than oral ingestion.
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