SECTION VIII. 



Respiration. 



It has been seen that through absorption the products of digestion 

 enter into the blood and are carried by means of the circulation to the 

 various organs and tissues of the bod}'. In spite, however, of the con- 

 stant addition of these substances to the blood its general composition 

 remains almost uniform, for the income from the alimentary tract is 

 balanced by the outgo through the different tissues. The blood in its 

 passage through the capillaries of the body nevertheless undergoes 

 great alterations in its composition. It gives up to the tissues the 

 substances brought from the alimentary tract which are destined to 

 nourish the different tissues of the body. Every function of a cell is 

 accompanied by chemical change in its composition, the resulting prod- 

 ucts in the majority of cases being no longer of use to the economy. 

 The blood is charged to remove these substances from the tissues. 



Again, it has been found that the vital functions of every cell 

 necessitated a supply of oxygen. It is the function of the blood to act 

 as carrier of this gas and to remove as well the gaseous products of cell 

 decomposition. We thus find that the blood acts not only as an organ 

 of nutrition, but as an organ of excretion ; in fact, as a common carrier 

 of the several substances essential to cell life, and the carrier of the 

 deleterious substances resulting from the breaking down of cells, and it 

 bears them to different parts of the body where special organs are set 

 aside for their removal. The removal of these deleterious substances 

 constitutes excretion, the mode of removal depending upon the nature 

 of the substances to be eliminated. 



One of the most striking changes which occur in the blood in 

 passing through active organs is the yielding up of ox3'gen and the 

 overloading of the blood with carbon dioxide. It has been stated that 

 the difference between the arterial and venous blood is that the 

 former contains an excess of oxygen, the latter an excess of carbon 

 dioxide, the oxygen of the arterial blood being derived from the atmos- 

 phere and entering into composition with the haemoglobin as the blood 

 passes through the capillaries of the lungs. The carbon dioxide is 

 derived from the breaking down of the cell-constituents, and is likewise 

 removed from the blood while passing through the pulmonary capillaries. 

 The process by which these gases enter and leave the blood is almost 



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