plasma is the liquid component that accounts for about 55% of the volume of blood and in which are suspended the 45% taken up by cellular elements. When the two components are separated by centrifuge, using a test tube sample of blood treated to prevent clotting, the plasma separates as a yellowish layer. Plasma is different from serum — the fluid which is extruded when blood is allowed to clot — because some plasma constituents take part in clot formation. Plasma is about 95% water, with dissolved or suspended substances ranging from simple chemicals to complex protein molecules and fat particles. Its osmolarity (close to 300 milliosmoles/litre) and pH (close to 7.4) are closely controlled; likewise the main anions (chloride, bicarbonate) and cations (sodium, potassium, calcium, magnesium) and the major proteins (albumin and globulins). Nutrients (glucose, fatty acids, amino acids) and the nitrogenous waste product urea, vary more in their concentration, along with meals and metabolic activity. Other substances in much smaller concentration are equally important, such as the hormones, factors required for coagulation, and trace elements. Plasma is continually exchanging its constituents, apart from the large proteins, with all tissue fluids across capillary walls; it is also continually ‘sampled’ and its composition corrected by the kidneys, which partially filter it off from the blood at a rate of 2–3 times the whole 2–3 litres of circulating plasma every hour, process it, and restore it to the circulation — except for about 1% which forms the urine.

Stuart Judge

plasma, in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions (see electron and ion). It is electrically conductive and is affected by magnetic fields. The study of plasma, called plasma physics, is especially important in research efforts to produce a controlled thermonuclear reaction (see nuclear energy). Such a reaction requires extremely high temperatures; it has been computed that a temperature of about 10 million degrees Celsius would be needed to initiate the reaction between deuterium and tritium. By passing a very high electric current through plasma great heat is produced and, simultaneously, an electromagnetic field is created, causing the plasma to withdraw from the walls of its container. The contraction of the plasma, called the pinch effect, prevents the container from being destroyed, but the effect may become unstable too quickly for the fusion reaction. The properties of plasma are distinct from those of the ordinary states of matter, and for this reason many scientists consider plasma a fourth state of matter. Interstellar gases, as well as the matter inside stars, are thought to be in the form of plasma, thus making plasma a common form of matter in the universe. See also condensate.

plas·ma / ˈplazmə/ (also plasm / ˈplazəm/ ) • n. 1. the colorless fluid part of blood, lymph, or milk, in which corpuscles or fat globules are suspended. ∎  this substance taken from donors or donated blood for administering in transfusions. 2. an ionized gas consisting of positive ions and free electrons in proportions resulting in more or less no overall electric charge, typically at low pressures (as in the upper atmosphere and in fluorescent lamps) or at very high temperatures (as in stars and nuclear fusion reactors). ∎  an analogous substance consisting of mobile charged particles (such as a molten salt or the electrons within a metal). 3. a dark green, translucent variety of quartz used in mosaic and for other decorative purposes. 4. another term for cytoplasm or protoplasm. DERIVATIVES: plas·mat·ic / plazˈmatik/ adj. plas·mic / -mik/ adj. ORIGIN: early 18th cent. (in the sense ‘mold, shape’): from late Latin, literally ‘mold,’ from Greek plasma, from plassein ‘to shape.’

plasma (blood plasma) (plaz-mă) n. the straw-coloured fluid in which the blood cells are suspended. It consists of a solution of various inorganic salts of sodium, potassium, calcium, etc., with a high concentration of protein and a variety of trace substances.

plasma In physics, an ionized gas that contains about the same amount of positive and negative ions. Plasma, often described as the fourth state of matter, occurs at extremely high temperatures, as in the interiors of the Sun and other stars and in fusion reactors.

plasma A low-density, high-temperature, completely ionized gas, consisting of free atomic nuclei and free electrons. Overall it is electrically neutral. It is sometimes referred to as the ‘fourth state of matter’.

plasma †form; green variety of quartz XVIII; colourless coagulable liquid of blood XIX; ionized gas XX. — late L. plasma mould, image, f. Gr. plássein fashion, form.

plasma See blood plasma.

plasma See BLOOD PLASMA.

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