228 TEXT-BOOK OF PHYSIOLOGY. 



by ice, it separates in the course of a few hours into three distinct layers in 

 accordance with their specific gravities. The lower layer is dark red and 

 consists of the red corpuscles; the middle layer is grayish in color and consists 

 of the white corpuscles; the upper layer is clear and transparent and consists 

 of the plasma. The red corpuscles occupy almost one-half, the white one- 

 fortieth, the plasma a trifle more than one-half of the height of the entire 

 blood-column, which indicates approximately the different volumes of each. 

 The same result can be obtained with human blood by the use of the centri- 

 fuge or hematocrit. 



PHYSICAL PROPERTIES OF BLOOD. 



1. Color. Within the blood-vessels two kinds of blood are distinguished 

 the arterial, the color of which is a bright scarlet, and the venous, the color 

 of which is a dark bluish-red or purple. The cause of the color as well as 

 the difference in color is the presence in the red corpuscles of a coloring- 

 matter, hemoglobin, in different degrees of combination with oxygen. The 

 intensity of the color in either kind of blood is dependent on the thickness of 

 the blood-stream, for in the finest capillaries, as seen under the microscope, 

 there is an almost total absence of color. As the arterial blood passes into 

 and through the systemic capillaries, the hemoglobin yields up a portion of 

 its oxygen to the tissues and changes in color, though the change is not 

 appreciable by the eye. On passing into the veins, however, the blood- 

 stream soon presents its characteristic dark bluish-red color, which deepens 

 as it approaches the lungs. On passing into and through the capillary 

 vessels of the lungs the hemoglobin absorbs a new volume of oxygen, changes 

 in color, and on emerging from the lungs the blood presents its characteristic 

 scarlet color. 



2. Opacity. Owing to the fact that the corpuscles have a refracting 

 power different from the plasma, the blood, even in thin layers, is opaque. 

 The repeated refractions and reflections which light undergoes in passing 

 through plasma and corpuscles is attended by such a dissipation that it is 

 impossible to see printed matter through it. That the opacity is due to the 

 shape of the corpuscles rather than to their contained coloring- matter is 

 evident from the fact that when the hemoglobin is caused to separate from 

 the corpuscles by the addition of chemic reagents, the blood, though it 

 deepens in color, at once becomes transparent. 



3. Odor. When freshly drawn the blood possesses a peculiar charac- 

 teristic odor which has been attributed to the presence of a volatile fatty acid 

 in combination with an alkaline base. The intensity of the odor may be 

 increased by the addition of concentrated sulphuric acid, by means of which 

 the volatile acid is set free. 



4. Specific Gravity. The specific gravity of blood lies within the limits 

 of 1.051 and 1.059, averaging in man 1.056 and in woman 1.053. Normally, 

 variations from these values are only temporary and are connected with 

 variations in physiologic processes. The specific gravity is diminished by 

 the ingestion of liquids and abstinence from solid food. It is increased by 

 abstinence from liquids, by the ingestion of dry food, and by the elimination 

 of large quantities of water by the lungs, skin, and kidneys. 



Inasmuch as the specific gravity of the blood varies from the normal in 



