Diagram 13.—Scheme of the Circulatory System.
Blood system on the right, lymph system on the left.
As the medium for chemical communication throughout the community of cells, the blood has another all-important and obvious function, viz., that of clearing away the waste products of life. Of these there is, of course, the same quantity as of new material introduced. Carbonic acid gas is discharged into the lungs, but all the nitrogen and most of the other elements in the new combinations which protoplasm has made them assume leave by the kidneys, plus a little water by the skin as sweat and a few items discharged into the last part of the alimentary canal amongst the unabsorbed portions of the food.
In their constituents, blood and lymph resemble one another, being both weak solutions of salts and proteid material; but the blood is distinguished from the lymph by the presence of innumerable extremely minute bodies, which give it its red colour. These corpuscles, to give them their proper name, are the vehicle by which oxygen is transported from the lungs to the tissues. They consist of an envelope of protoplasm filled with a red fluid (hæmoglobin), which combines loosely and easily with oxygen. In shape they are discoid, with a thickened rim and biconcave sides, another device for increasing surface and reducing bulk. ([See Diagram 14.])
Diagram 14.—A Red Blood Corpuscle.
With one more fact we may now conclude the chemical survey of the body. The blood has to pass through certain glands, or it becomes poisoned, and this quite apart from whether the gland secretes healthily or not.
Disease of the thyroid (a ductless gland in the Adam’s apple) causes goitre; of the suprarenal, Addison’s disease; of the pancreas, diabetes. Whether these organs secrete some substance into the blood which counteracts poisons formed in it, or whether they remove injurious elements from it, is not certain, but they are necessary to keep the great means of chemical communication in order.