320 VETERINARY PHYSIOLOGY 



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 ery throcytes is diminished, and 

 the blood being at a high temperature, a dissociation of 

 oxy haemoglobin takes place, and the oxygen passes out into 

 the plasma, leaving reduced haemoglobin in the erythrocytes. 



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



The tissues are constantly producing carbon dioxide, so that 

 it is at a high tension in them about 60 mm. Hg. In the 

 blood the carbon dioxide is combined with the sodium and is 

 thus at a low tension. Hence there is a constant passage of 

 carbon dioxide from the tissues to the blood. 



C. 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 the output of carbon dioxide 

 by the lungs. 



It may be studied by enclosing an animal or a man in an 

 air-tight case or chamber through which air of known 

 composition is passed in measured quantity and analysed as 

 it is withdrawn. This may be done by making the air pass 

 through vessels containing sulphuric acid to absorb the water 

 and caustic potash to fix the C0 2 , and absorbing the oxygen 

 by pyrogallate of soda. 



In the resting horse the average amount of oxygen con- 

 sumed is about 4 c.cms. per minute per kilo, of body weight, 

 and the amount of carbon dioxide given off about 3 -5 c.cms. 



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. 67). 



2. Food. The taking of food at once sets up active changes 

 in the digestive apparatus. The muscular mechanism is set 

 in action and the various glands secrete. As a result of these 



