The Heart and Circulatory System in Relation 

 to Aquatic and Aerial Respiration 



It is already clear that the structure and function of the cardio- 

 vascular system are very closely linked to the respiratory mechan- 

 isms of an animal. Nowhere is this so clearly shown as in the 

 structure of the heart and aortic arches of vertebrates, which 

 show progressive modifications as we ascend the series from 

 fish to mammal. Many aspects of the physiology of the lower 

 vertebrates remain to be investigated, but sufficient is known to 

 suggest that these structural changes are associated with modifi- 

 cations in function which lead to a more efficient supply of 

 oxygen to the tissues. The general course of evolution has been 

 from a gill-breathing form with a single circulation to the birds 

 and mammals with their efficient lungs and double circulations. 

 The variety of types of heart and circulation which has persisted 

 in living forms gives us a few glimpses of the way in which this 

 functional system has changed during vertebrate evolution. 

 Coupled with the changes in the heart and aortic arches will have 

 gone changes in the respiratory mechanisms and the nervous 

 control ofboth the cardio-vascular and respiratory system. 



The primitive chordate circulatory system consisted of a 

 ventral heart behind the branchial region which led anteriorly 

 into the ventral aorta. Passing round the pharynx from the 

 ventral aorta were a number of vessels which, after forming a 

 capillary network in the gills, joined a lateral dorsal aorta on 

 each side and these led posteriorly into the median dorsal aorta 

 running backwards along the whole length of the animal. The 

 organs of the body were supplied with blood from this main 

 vessel and after passage through the capillaries the blood 



