THE GASES OF THE BLOOD 61 



According to some authors, the parenchyma of the lungs plays 

 an active part in the giving off of carbon dioxide (in the same 

 manner as gland cells in the secretion). 



The lowest barometric pressure at which respiration of the quiet 

 body can continue undisturbed is about 350 mm Hg. 



The oxygen taken up by the blood favors the giving off 

 of carbon dioxide because by it the carbon dioxide tension 

 is increased, owing to the fact that oxyhaemoglobin is more 

 acid than reduced haemoglobin. 



An adult man inhales in 24 hours about 700 g or 500 

 litres of oxygen and exhales 900 g or 450 litres of carbon 

 dioxide. 



The ratio of the volume of the exhaled carbon dioxide to the 

 volume of the inhaled oxygen is called the respiratory quotient. 

 Concerning its value under various circumstances see Chapter XII. 



Besides the lungs, the skin also throws off carbon dioxide in 

 small amounts (8.4 g per day). 



3. TISSUE RESPIRATION 



This consists of the giving off of oxygen by the blood to 

 the tissues and the taking up of carbon dioxide. This takes 

 place in the systemic capillaries. The giving off of oxygen 

 takes place because the oxygen tension in the blood is 

 greater than that in the tissues. Because of the continual 

 oxygen consumption, the oxygen tension in the tissues is o. 

 The carbon dioxide formed by the combustion of the tissues 

 accumulates to such an extent that its pressure is higher than 

 that of the carbon dioxide in the arterial blood, hence it 

 must pass into the blood. 



The physiological combustion, by which oxygen of the blood is 

 consumed and carbon dioxide produced, does not take place in 

 the blood, but in the tissue. This is based on the following facts: 



i. The extent of the physiological combustion is, up to a certain 

 limit, independent of the amount of blood in the body. After a 

 considerable loss of blood, warm-blooded animals show no change 

 in the amount of oxygen consumed and the carbon dioxide formed, 

 and in cold-blooded animals (frog) the physiological combustions 

 can take place when all their blood has been taken away and 

 replaced by an injected physiological salt solution. 



