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    Habitat - a location which has a suitable environment for an organism to live in. Caves are natural habitats for bats, trees for birds; oral surfaces and crevices, for some bacteria.

    Haemostasis - the prevention of blood loss through a damaged vessel wall. There are three main mechanisms, vasoconstriction, formation of a platelet plug and blood clotting.

    Heparin sulphate - a glycosaminoglycan which is unusual in that it is stored inside the cell (mast cells) surrounding the liver. Heparin prevents blood clotting.

    Herbivorous- an animal whose diet consists of grass, leaves, roots or other plant matter.

    Heterodont- a dentition in which some of the teeth have different shapes and special functions. Form "hetero" = different) see also homodont.

    Hierarchy - an order of power between individuals. A ranking of most dominant to least dominant.

    Hippocampus - part of the limbic system, it seems to provide a spatial map, useful in the event of a sudden need to escape from an unpleasant sensory experience.

    Hypothalamus - this small body of nerve cells controls the activity of the pituitary gland, the source of several hormones which control the activity of other hormones, including ACTH which in turn controls the level of glucocorticoid secretion. The hypothalamus also has powerful connections with the other members of the limbic system, from which nerve pathways descend to control nerves in the spinal cord. The influence of the limbic system on the hypothalamus explains the raised levels of glucocorticoids in response to emotional stress. This bridge with the peripheral nervous system, provides a link between the emotional state of a person, as influenced by the activity of the limbic system, and the excitability of neurones in the spinal cord, to in-coming pain impulses.

    Histamine - is a product of the amino acid, histidene and is released by damaged cells. Histamine causes an increase in capillary permeability and vasodilation, two vascular events which are the first stages of inflammation. Histamine is also a neurotransmitters substance, released at nerve synapses mostly in the hypothalamus.

    Homeostasis - control of an organism抯 internal environment. Water content, temperature, acid-base balance, level of oxygen and carbon dioxide, adequate supply of energy are some of the many factors in the organism which require monitoring and control. A common form of control is feedback.

    Homodont - a dentition in which all the teeth are the same shape (from "home" = same) see also heterodont.

    Hyaluronic acid - the largest glycosaminoglycan known, it plays an important role of restricting the flow of water in tissues, particularly in synovial fluid where it acts as a lubricant..

    Hydrogen bonds - a weak force holding two molecules containing hydrogen together, each of which has a covalent bond with another atom. For example, water is a molecule made up of two hydrogen atoms attached covalently to an oxygen atom. The hydrogen proton is however not completely balanced and is still attracted to the oxygen atom of a neighbouring molecule. Molecules of water are thus held together by hydrogen bonds, which accounts for the unusually high boiling point of water considering its low molecular weight. Hydrogen bonds hold protein molecules in shape by linking up various sections. When proteins are heated, these bonds collapse and the protein is physically altered, even though its chemical composition remains unchanged. When an egg is heated the white part rapidly gels, indicating a change in the shape of the protein. The process is not reversible.. so an egg cannot be uncooked.

    Hydroxyapatite - one of the apatites which is the main salt of bone and teeth.

    Hydroxyl ion- a negatively charged ion of hydrogen and oxygen written as OH-.

    Hyperplasia - an increase in the size of an organ due to an increase in the numbers of cells. The developing embryo increases in size due to cell division. The cells of some tissues retain the ability to divide throughout life, like the epithelium and connective tissues but muscle and nerve cells lose their ability to divide soon after birth. When hyperplasia is uncontrolled it produces a tumour which may be benign if it is well contained, not destructive and does not spread. But it may be malignant, destroys normal tissue, and spread all over the body.

    Hypertrophy - an increase in the size of an organ due to an increase in the size of each cell. Muscles increase in size due to hypertrophy.

    Hypoplasia - reduced formation of a tissue during development. Enamel hypoplasia may be recognised as pits and depressions in the enamel and may be cause by fluorosis.

    Hypotonic - a comparison between two solutions, indicating that one has a lower osmotic pressure, or is less salty than the other.

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