RESPIRA T10N 



3i 



may still be kept alive in an atmosphere of pure oxygen under 

 pressure, the amount of oxygen dissolved by the plasma at an 

 oxygen pressure of two atmospheres being sufficient to carry on 

 the functions. 



Hyperpncea is the term applied to the slightly increased 

 amplitude and frequency of the respiratory movements, such 

 as occur in gentle exercise, as the immediate result of any 

 commencing defective oxygenation of the blood, or other cause 

 which acts as a stimulus to the respiratory centre (see p. 136). 

 When the stimulus is strong or continued, a further increase in 

 the force and frequency of the respiratory movements takes 

 place, and this condition is known as dyspnoea. The later stage 

 of dyspnoea is characterised by the respiratory movements 

 becoming ' convulsive ' in their activity, and this finale to 

 dyspnoea marks the onset of true asphyxia. 



If the air supply be entirely cut off, asphyxia and death 

 rapidly ensue. Asphyxia has been divided into three stages. 

 In the first the attempts at breathing are laboured and painful, 

 deep and frequent, and all the respiratory muscles, including the 

 supplemental ones, are brought into play. Convulsions occur, 

 and the blood-pressure rises. In the second stage the inspira- 

 tory muscles are less active, the expiratory still powerful, and 

 the convulsions cease. In the third stage the animal lies uncon- 

 scious, occasional violent inspiratory gaspings occur, the mouth 

 is open (even in the horse), the pupils dilated, the pulse barely 

 perceptible or absent. During this stage the blood-pressure 

 rapidly falls. Death occurs in from five to six minutes from 

 the commencement of the first stage. Young animals are less 

 easily asphyxiated than adults for the reason that their tissue 

 respiration is much less. The length of time necessary to drown 

 puppies and kittens is evidence of this, and they may recover 

 even after prolonged immersion. 



Excess of Oxygen. — It has been shown that oxygen above a 

 certain pressure it very poisonous. From 3 to 5 atmospheres of 

 oxygen, corresponding to 15 to 25 atmospheres of air, suffice to 

 kill seeds, hinder the development of eggs, and produce convul- 

 sions in warm - blooded animals. In these latter cases the 

 amount of extra oxygen in the blood is 10 volumes, so that 

 there are 30 instead of 20 in every 100 volumes of blood. This 

 extra oxygen is not carried in the red cells, but in solution by 

 the plasma. All animals are instantly killed by a pressure of 

 50 atmospheres of oxygen. The toxic nature of oxygen at high 

 tensions is unknown. By increasing the amount of oxygen 

 above that contained in air, the blood cannot be made to take 

 up much more oxygen than if the normal amount only were 

 present. A pressure of 10 atmospheres only causes an increase 



