In many organisms, including man, the mechanical respiration and the circulation of the blood are 'regulated' so as to correspond to the demand of the moment.

In man, the mechanical breathing is essential to life, and it is one of the old tests for death to see whether these movements have ceased completely.

The affinity of blood or pure haemoglobins for oxygen is a complex phenomenon, depending upon a number of conditions, the most important of which are temperature and hydrogen ion concentration.

In many cases of inflammation, the vascular changes develop slowly and long after the application of the stimulus which is responsible for the inflammatory reaction.

The circulatory system of man and the vertebrate animals can be considered as made up of a small number of organs or subordinate systems, which are easy to recognize anatomically, and the functions of which are on the whole quite distinct.

It is no doubt technically possible to study metabolism and respiration of fishes during swimming at a constant rate, and of certain insects and birds during flight, and to obtain information similar to that obtained on man during work on a bicycle ergometer or a treadmill.