（单词翻译:单击）
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 molecules2 containing hydrogen together, each of which has a covalent bond with another atom. For example, water is a molecule1 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 molecular3 weight. Hydrogen bonds hold protein molecules in shape by linking up various sections. When proteins are heated, these bonds collapse4 and the protein is physically5 altered, even though its chemical composition remains6 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 embryo7 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 tumour8 which may be benign9 if it is well contained, not destructive and does not spread. But it may be malignant10, 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. Enamel11 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.
