7<> A TEXTBOOK OF PHYSIOLOGY 



The Colour of vertebrate blood varies from a bright red in the 

 blood of the arteries to a deep purple-red in the veins. It is due to 

 a pigment haemoglobin which is contained within the red corpuscles 

 of the blood and acts as the carrier of oxygen. The blood of inverte- 

 brates and cordates may be either colourless or possess one of a variety 

 of colours. Haemoglobin occurs in solution in the plasma of many of 

 the worms and lower Crustacea. The echinoderms also possess a red 

 pigment in their blood (echinochrom). Some Crustacea have a blue 

 pigment haemocyanin which contains copper and is in solution in 

 the blood. It is blue in the arterial blood, when combined with 

 oxygen, and colourless in venous blood i.e., when deoxygenated. 

 Its combining power for oxygen is only about one-fourth that of 

 haemoglobin. Some worms contain a green pigment (chlorocruorin) ; 

 others a red pigment (haemoerythrin). Certain molluscs and tunicates 

 possess colourless blood and yet have substances in it capable of com- 

 bining with oxygen and transporting it to the tissues (achroglobin). 



The Specific Gravity of the blood of man varies between 1055 and 

 1 060. The red corpuscles have a specific gravity of 1080 and the plasma 

 one of 1030. The specific gravity is usually obtained by the fol- 

 lowing method. By means of a pipette a drop of blood is placed in 

 the middle of a mixture of benzol (sp. gr. 0-88) and chloroform (sp. gr. 

 1-485). The mixture has a specific gravity approaching that of blood. 

 If the blood -drop falls, more chloroform is added; if it rises, more 

 benzol until a condition is obtained when the corpuscle remains 

 suspended in the mixture. The specific gravity of the mixture is 

 now taken with the hydrometer. The operation must be quickly 

 performed (1 to 2 minutes), otherwise the specific gravity of the blood 

 alters owing to diffusion taking place between the blood and the 

 liquids. The specific gravity is influenced by the number of corpuscles 

 and the amount of haemoglobin. It is high in the new-born (1066). 

 It sinks in starvation, after haemorrhage, in anaemias, and in diseases of 

 the kidney, etc. 



SPECIFIC GRAVITY OF BLOOD. 



Man 1056-1061 Rabbit 1042-1062 



Woman 1053-1061 Of serum 1030-1042 



Dog . . . . 1060 Of red corpuscles . . . . 1080-1085 



The specific gravity varies with the age and sex; it is diminished 

 after eating and increased by exercise. It gradually falls during the 

 day and rises during the night. 



Reaction. The reaction of a fluid depends upon the number of 

 free hydrogen (H) ions in it, which give an acid reaction relative to 

 the number of free hydroxyl (OH) ions, which give a basic or alkaline 

 reaction. 



Blood is almost neutral in reaction as determined by the electrical 

 method. Its reaction to litmus is slightly alkaline. This is because 

 litmus acts as a weak acid, and, displacing carbonic acid gas from its 

 combination in the blood (carbonate), combines with the base and turns 



