ESSENTIALS OF CHEMICAL PHYSIOLOGY INTRODUCTION Chemical Physiology or Physiological Chemistry deals with the chemical composition of the body and with the chemical changes it undergoes; it also deals with the composition of the food which enters, and the excretions which leave, the body. When a chemist examines living things he is placed at a disadvantage when compared with an anatomist; for the latter can with the microscope examine cells, organisms, and structures in the living condition. The chemist, on the other hand, cannot at present state anything positive about the chemical structure of living matter, because the reagents he uses will destroy the life of the tissue he is examining. There is, however, no such disadvantage when he examines non-living matter, like food and urine, and it is therefore in the analysis of such substances that chemical physiology has made very important advances, and the knowledge so obtained is of the greatest practical interest to the student and practitioner of medicine. The animal organism is in its earliest embryonic state a single cell; as development progresses it becomes an adherent mass of simple cells. In the later stages various tissues become differentiated from each other by the cells becoming grouped in different ways by alteration in the shape of the cells, by deposition of intercellular matter between the cells, and by chemical changes in the living matter of the cells themselves. Thus in some situations the cells are grouped into the various epithelial linings; in others the B cells become elongated, and form muscular fibres; in the connective tissues we have a preponderating amount of intercellular material, which may become permeated with fibres, or be the seat of the deposition of calcareous salts, as in bone. Instances of chemical changes in the cells themselves are seen on the surface of the body, where the superficial layers of the epidermis become horny (i.e. filled with the chemical substance called keratin); in the mucous salivary glands, where the cells become filled with mucin, which they subsequently extrude; and in adipose tissue, where they become filled with fat. In spite of these changes, the variety of which produces the great complexity of the adult organism, there are many cells which still retain their primitive structure: notable among these are the white corpuscles of the blood. A cell may be defined as a mass of living material containing in its interior a more solid structure called the nucleus. The nucleus exercises a controlling influence over the nutrition and subdivision of the cell. The living substance is usually pervaded with granules: one of these minute particles called the centrosome exercises an attractive influence on the granules and fibrils of the protoplasm in its neighbourhood, and the appearance so produced is called the attraction sphere. The attraction sphere becomes specially prominent, and divides into two when the cell is about to divide; this usually precedes the division of the nucleus. Living material is called protoplasm, and protoplasm is characterised by (1) irritability—that is, the property of responding by some change when subjected to the influence of an external agent or stimulus the most obvious of these changes is movement (amoeboid movement, ciliary movement, muscular movement, &c.); (2) its power of assimilation-that is, it is able to convert into protoplasm the nutrient material or food which is ingested; (3) its power of growth -this is a natural consequence of its power of assimilation; (4) its power of reproduction-this is a variety of growth; and (5) its power to excrete, to give out waste materials, the products of its other activities. Of all the signs of life, those numbered 2 and 5 in the foregoing list are the most essential. Living material is in a continual state of unstable chemical equilibrium, building itself up on the one hand, breaking down on the other; the term used for the sum total of these intra-molecular rearrangements is metabolism. The chemical substances in the protoplasm which are the most important from this point of view are the complex nitrogenous compounds called Proteins.' So far as is at present known, protein material is never absent from living substance, and is never present in anything else than that which is alive or has been formed by the agency of living cells. It may therefore be stated that Protein Metabolism is the most essential characteristic of vitality. The chemical structure of protoplasm can only be investigated after the protoplasm has been killed. The substances it yields are (1) Water; protoplasm is semi-fluid, and at least three-quarters of its weight, often more, are due to water. (2) Proteins. These are the most constant and abundant of the solids. A protein or albuminous substance consists of carbon, hydrogen, nitrogen, oxygen, with sulphur and phosphorus in small quantities only. In nuclein, a protein-like substance obtained from the nuclei of cells, phosphorus is more abundant. The protein obtained in greatest abundance from the cell-protoplasm is nucleo-protein: that is, a compound of protein with varying amounts of nuclein. White of egg is a familiar instance of an albuminous substance or protein, and the fact (which is also familiar) that this sets into a solid on boiling will serve as a reminder that the greater number of the proteins found in nature have a similar tendency to coagulate under the influence of heat and other agencies. (3) Various other substances occur in smaller proportions, the most constant of which are lecithin, a phosphorised fat; cholesterin, a monatomic alcohol: and inorganic salts, especially phos. phates and chlorides of calcium, sodium, and potassium. It will be seen from this rapid survey of the composition of the body how many are the substances which it is necessary we should study; the food from which it is built up is also complex, for animals do not possess, to such an extent as plants do, the power of building up complex from simple materials. The substances out of which the body is built consist of chemical elements and of chemical compounds, or unions of these elements. The elements found in the body are carbon, hydrogen, nitrogen, oxygen, sulphur, phosphorus, fluorine, chlorine, iodine, silicon, sodium, potassium, calcium, magnesium, lithium, iron, and occasionally manganese, copper, and lead. Of these very few occur in the free state. Oxygen and nitrogen (to a small extent) are found dissolved in the blood-plasma; hydrogen is In most English text-books these substances have hitherto been called Proteids. The change to Protein brings English, American, and German usage into harmony. |