RESPIRATION 171 



First of all, it may be due to a secretion of oxygen by the 

 capillary wall. We shall put that aside because there is no 

 evidence in favour of it and because there are several purely 

 physico-chemical possibilities. 



Secondly, it may be due to an increase in the difference of 

 oxygen tension between the blood and the tissue the result 

 either of a rise in oxygen tension in the blood or a fall in 

 oxygen tension in the tissues. 



The blood coming by the arterioles is always fully saturated 

 with oxygen, so that apart from breathing a gas mixture 

 containing more oxygen than does atmospheric air the tension 

 of oxygen cannot be increased in arterial blood. 



In the capillaries, however, the blood becomes partially 

 reduced so that the oxygen tension is less in the capillaries. 

 By increasing the blood flow the venous blood coming from the 

 tissues is less reduced than usual, so that the mean capillary 

 oxygen tension is higher than before the increased blood flow. 

 During activity the blood flow is increased and the venous 

 blood less reduced, but this is not the sole cause of the increased 

 oxygen intake because mere dilation of blood vessels does not 

 cause an increased intake of oxygen. 



During activity there is an increased output of carbon 

 dioxide and an increased carbon dioxide tension causes a rise 

 in oxygen tension of a solution of oxyhsemoglobin. However, 

 this does not initiate the increased intake of oxygen because oxy- 

 gen intake occurs before the increased output of carbon dioxide. 



The rise of oxygen tension by vaso-dilation and by increase 

 of carbon dioxide tension both help to increase the oxygen 

 intake, but as pointed out above they cannot be considered to 

 be the cause of the increased oxygen intake. 



Before we can consider whether a fall in oxygen tension in 

 the tissues is accountable for the increased oxygen intake we 

 must know that there is an oxygen pressure in the tissues. 



It is stated that if a tissue is placed in a vacuum no 

 oxygen is given off, therefore there can be no oxygen tension 

 in the tissue. It may be that any free oxygen is used before 

 the tissue can be placed in the vacuum. 



Methylene blue when injected into the circulation is reduced 

 by many tissues to a colourless compound. This is believed 

 to be possible only when there is complete absence of oxygen. 

 The reduction of methylene blue is due to addition of two 

 hydrogen atoms, thus the reaction is that water is split up, 

 oxygen being taken by the tissues and hydrogen by methylene 

 blue. This reaction requires further investigation before it 



