304 HUMAN PHYSIOLOGY 



from the lymph, because of the very low pressure of oxygen 

 in the protoplasm and because the protoplasm has an affinity 

 for oxygen as for other nourishing substances. 



(2) As a result of this the oxygen pressure in the lymph 

 falls and becomes lower than the oxygen pressure of the 

 blood plasma, and thus the gas passes from the blood 

 through the capillary walls to the lymph. How far this 

 is simply the result of mechanical diffusion, and how far it 

 is carried on by the vital action of the endothelium of the 

 capillaries, we do not know. 



(3) As a result of the withdrawal of oxygen from the 

 plasma, the partial pressure round the erythrocytes is 

 diminished, and the blood being at a high temperature, a 

 dissociation of oxyhsemoglobin takes place, and the oxygen 

 passes out into the plasma, leaving reduced haemoglobin in 

 the erythrocytes. 



2nd. Passage of Carbon Dioxide from Tissues to Blood. 



The tissues are constantly producing carbon dioxide. In 

 the blood the carbon dioxide is combined with the soda of 

 the plasma and is thus at a low tension. Hence there is a 

 constant passage of carbonic acid from the tissues to the 

 blood. 



III. Extent of Respiratory Interchange. 



The extent of the respiratory interchange in the lungs is 

 governed by the extent of the internal respiratory changes 

 i.e. by the activity of the tissues. Every factor which 

 increases the activity of the metabolic changes in the tissues 

 increases the intake of oxygen and output of carbon dioxide 

 by the lungs. 



Under average conditions a normal man of 65 kgs. 

 excretes, in 24 hours, about 432 litres (about 850 grms.) of 

 C0 2 , or 230 grms. of C, and takes up 540 litres of (about 

 770 grms.). That is, in one hour, on an average, he excretes 

 20 litres of C0 2 and absorbs 22 litres of 0. The rate of 

 exchange per unit of weight is more active in the child than 

 in the adult. 



1. Muscular Work. Since muscle is the most abundant 

 and active tissue of the body, muscular work more than 

 anything else increases the respiratory changes (see p. 71). 



