THE RESPIRATORY SYSTEM. 285 



marked. The rise of pressure begins just after the beginning of 

 inspiration, and continues for a short time during expiration, so that 

 the variations in blood pressure are not quite synchronous with the 

 respiratory movements. This delay in the rise of blood pressure is 

 due to the fact that, when more blood enters the right auricle at the 

 beginning of inspiration, it has to travel through the lungs before it 

 reaches the left ventricle and is expelled into the aorta. Again, at the 

 beginning of expiration there is, for the same reason, a slight delay in 

 the diminution in the amount of blood sent out by the left ventricle. 



In man the alterations in blood pressure produced by the respira- 

 tory movements are of very complex origin, and the effects are not so 

 constant as those just described for animals ; they vary with the type 

 of respiration, a costal inspiration causing a fall, and a diaphragmatic 

 inspiration causing a rise of blood pressure. 



SECTION VII. 

 MUSCULAR EXERCISE. 



The supply of oxygen to the tissues and the removal of carbonic 

 acid are effected by the conjoint action of the respiratory and circulatory 

 systems ; and, in order that the tissues may receive their due supply of 

 oxygen, it is necessary, not only that the blood in its passage through 

 the lungs should become fully saturated, but that an adequate amount 

 of blood should be carried to the tissues. We find, therefore, that the 

 demands of the body as a whole for an increased amount of oxygen are 

 met by alterations in both the respiratory and circulatory systems, and 

 that for this purpose the two systems exert a correlated action, which 

 is very clearly illustrated in muscular exercise. Many of these changes 

 have already been considered, but they may conveniently be summarised 

 here. 



During muscular exercise the contracting muscles require a large 

 amount of oxygen and give off much carbonic acid. By means of 

 increased respiratory movements sufficient oxygen reaches the alveolar 

 air to replace that which passes into the blood, and sufficient carbonic 

 acid is removed from the lungs to keep its percentage in the alveolar 

 air almost unchanged ; and the blood, during exercise, is almost as fully 

 oxygenated as during rest, the haemoglobin being at least 90 per cent. 

 saturated with oxygen. 



The adjustment of the respiratory mechanism to the increased 

 needs of the muscles is brought about (1) by impulses passing from the 

 cerebral cortex to the respiratory centre at the very outset of exercise, 



