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1 6 ESSENTIALS OF PHYSIOLOGY. 



of the dissolved substance. The lowering of the freezing-point below 

 C. is expressed by the letter A. 



When a gram-molecule of any substance is dissolved in 1 litre 

 of water the freezing-point is lowered by I'ST C., and its osmotic 

 pressure = 17,000 mm. Hg. The osmotic pressure of a substance in 

 solution can be calculated from the formula 



Osmotic pressure = - x 17,000 mm. Hg. 



1 '87 0. 



Solutions which have the same osmotic pressure are said to be 

 isotonic, and the tissues in mammals are isotonic with a solution con- 

 taining 0*9 per cent, sodium chloride in water ; this is known as 

 normal saline solution. When- the tissues are immersed in stronger, 

 i.e. hyper tonic, salt solution, water passes from the tissue into the salt 

 solution by osmosis ; when the salt solution has a lower osmotic 

 pressure than the tissues, it is hypotonic, and the tissues take up' water 

 from the solution. In an isotonic solution, the tissues neither take up 

 nor lose water. 



Both osmosis and diffusion take place in the body, but the 

 membranes are not completely impermeable to substances such as 

 sugar or salt, so that osmosis is soon brought to an end by the passage 

 of these substances through the membrane. 



Colloids. The term colloid was originally applied to all substances, 

 such as starch and proteins, which would not form crystals or pass 

 through an animal membrane, in contra-distinction to easily crystallis- 

 able bodies, such as sugar, which diffuse rapidly through animal 

 membranes and are called crystalloids. 



It is now known that the colloidal form is a state in which sub- 

 stances exhibit certain characteristic features, and that a very large 

 number of substances, including metals, may exist in the colloidal form. 

 Further, some colloids, such as haemoglobin, are crystallisable. 



Most colloids consist of very large molecules or aggregates of 

 molecules, and their characters depend largely on the fact that they 

 do not form true solutions in water or other solvents, but that their 

 pseudo-solutions consist of particles, suspended in a very dilute 

 solution of the colloid. Owing to the size of the particles, colloidal 

 suspensions do not follow the laws of true solution, and they exert only 

 a very small osmotic pressure. 



If an electrolyte, such as sodium chloride, is added to a colloidal 

 suspension, the salt concentrates at the surface of each colloidal 

 particle, this being called adsorption. Its occurrence can be readily 

 demonstrated by dipping strips of blotting paper, which is colloidal, 



