Check us out at http://www.tutorvista.com//videos Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream, for instance. Digestion is a form of catabolism: a break-down of larger food molecules to smaller ones. In mammals, food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the esophagus into the stomach, where acid both kills most contaminating microorganisms and begins mechanical break down of some food (e.g., denaturation of protein), and chemical alteration of some. After some time (typically an hour or two in humans, 46 hours in dogs, somewhat shorter duration in house cats, ...), the results go through the small intestine, through the large intestine, and are excreted during defecation. Other organisms use different mechanisms to digest food. Digestive systems take many forms. There is a fundamental distinction between internal and external digestion. External digestion was the first to evolve, and most fungi still rely on it. In this process, enzymes are secreted into the environment surrounding the organism, where they break down an organic material, and some of the products diffuse back to the organism. Later, animals evolved by rolling into a tube and acquiring internal digestion, which is more efficient because more of the breakdown products can be captured, and the chemical environment can be more efficiently controlled. Some organisms, including nearly all spiders, simply secrete biotoxins and digestive chemicals (eg, [enzymes]) into the extracellular environment prior to ingestion of the consequent "soup". In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through a tube, or through several specialized organs aimed at making the absorption of nutrients more efficient. Absorption :Skin absorption is a route by which substances can enter the body through the skin. Along with inhalation, ingestion and injection, dermal absorption is a route of exposure for toxic substances and route of administration for medication. Absorption of substances through the skin depends on a number of factors, the most important of which are concentration, duration of contact, *solubility of medication*, and physical condition of the skin and part of the body exposed. Certain substances called carriers can be used to greatly increase the amount of other substances that is able to penetrate the skin. Dimethyl sulfoxide (DMSO) is a carrier that is frequently used to transport medication through the skin. This allows treatment to be localized, unlike with ingestion. Alongside that, certain medications seem to be more effective (or are more efficient) using this route of administration, while it still remains clear that others are not
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