FEB 2 3 1925 BRARY ESSENTIALS OF CHEMICAL PHYSIOLOGY INTRODUCTION Chemical Physiology, or Physiological Chemistry, as it is sometimes termed, deals with the chemical composition of the body, and also 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, as 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 its most 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 alterations 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 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 them B selves 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. Living material is called protoplasm, and protoplasm is characterised by its (1) power of movement (seen in amoeboid movement, ciliary movement, muscular movement); (2) its power of assimilation, that is, it is able to take in nutrient material or food, and convert it into protoplasm; (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. The chemical structure of protoplasm can only be investigated after the protoplasm has been killed. The substances it yields are (1) Water; protoplasm is semifluid, and at least three-quarters of its weight, often more, are due to water. (2) Proteids. These are the most constant and abundant of the solids. A proteid or albuminous substance consists of carbon, hydrogen, nitrogen, oxygen, with sulphur and phosphorus in small quantities only. In nuclein, a proteid-like substance formed in the nuclei of cells, phosphorus is more abundant. White of egg is a familiar instance of an albuminous substance or proteid, and the fact (which is also familiar) that this sets on boiling into a solid will serve as a reminder that the greater number of the proteids 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 phosphates 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. We may now proceed to an enumeration of the chemical constituents of the animal body, and group them in a systematic way. 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, silicon, sodium, potassium, calcium, magnesium, lithium, iron, and occasionally manganese, copper, and lead. Of these very few occur in the free state. Oxygen (to a small extent) and nitrogen are found dissolved in the blood; hydrogen is formed by putrefaction in the alimentary canal; particles of carbon breathed in with the air may be found in the tissues of the lungs. With some few exceptions such as these, the elements enumerated above are found combined with one another to form what are called compounds. The compounds, or, as they are generally termed in physiology, the proximate principles, found in the body are divided into (1) Mineral or inorganic compounds. (2) Organic compounds, or compounds of carbon. The inorganic compounds present are water, various acids (such as hydrochloric acid in the gastric juice), ammonia (as in the urine), and numerous salts, such as calcium phosphate in bone, sodium chloride in blood and urine, and many others. The organic compounds are more numerous: they may be subdivided into 1. Various groups of alcohols and organic acids, and their compounds, such as the fats and carbohydrates. 2. Various derivatives of ammonia, amides, amines, urea, &c. 3. Aromatic bodies, or derivatives of benzene. 4. Proteids, the most important of all, and substances allied to proteids like the albuminoids, pigments, and ferments. If we study the chemical composition of foods we find that they also may be subdivided by chemical analysis into proximate principles, and these we may conveniently group as follows: Inorganic Nitrogenous Organic Water. Salts-e.g. chlorides and phosphates of sodium Proteids-e.g. albumin, myosin, casein. creatine. Fats-e.g. butter, fats of adipose tissue. Non-nitrogenous Simple organic bodies-e.g. alcohol, vegetable acids, and salts. |