280 



ESSENTIALS OF PHYSIOLOGY. 



Provided that the flow of blood to an organ is sufficient to supply 

 the amount of oxygen which it needs, a further increase in the flow of 

 blood through it does not increase the consumption of oxygen. For 

 instance, the rate of blood flow through the submaxillary gland may 

 be increased tenfold without any rise taking place in the oxygen 

 consumption of the gland. The oxygen consumption rises, however, 

 whenever the functional activity of a tissue becomes greater, and in the 

 case of skeletal muscle it may be increased tenfold during and just 

 after muscular contraction. A similar rise occurs when the heart does 

 more work, or when the secretory activity of the submaxillary and other 

 glands is evoked. 



OXYGEN CONSUMED BY THE SUBMAXILLARY GLAND. 



The increased consumption of oxygen occurs not only during the 

 period of secretion or of muscular work, but for some time after this is 



60 100 150 200 



FIG. 113. Ordinate = volume of oxygen used in c.c. per minute ; abscissa = time in 

 seconds; upper signal = duration of flow of saliva; lower signal = duration of 

 chorda stimulation. Blackened area represents the oxygen used by the salivary 

 gland. ( From Barcroft, Respiratory Function of the Blood. ) 



over (fig. 113); and the oxygen is apparently used mainly -in the 

 carrying out of chemical changes, whereby the gland or muscle stores 

 up potential energy and is restored to its previous condition. Every 

 increase in the functional activity of an organ is also accompanied by 

 dilatation of its blood-vessels, which, as already mentioned (p. 236), is 



