540 



Comparative Animal Physiology 



TABLE 67 (continued) 

 BLOOD PRESSURE IN VARIOUS ANIMALS 



of adrenalin causes a prolonged pressor effect (rise in blood pressure), which 

 may last as long as 1 to 21/2 hours. ^°^' ^'^^ However, the branchial vessels 

 dilate to adrenaUn, as do mammalian coronaries, in contrast to visceral 

 vessels which constrict in response to adrenalin. ^i'^ Embryologically the 

 coronaries are derived from the gill arch vessels. Injection of acetylcholine 

 inhibits the heart of skates but causes a rise in blood pressure; atropine 

 antagonizes the cardiac effect but only slightly reduces the pressor action. ^^" 

 The pressor action of acetylcholine is not the result of stimulation of sym- 

 pathetics and adrenals, since direct observations of skate arteries showed 

 that acetylcholine caused them to constrict.^ In tetrapods acetylcholine con- 

 stricts some vessels (e.g., the pulmonaries), but causes dilatation of most 

 vessels; the dilating effect is antagonized by atropine. After atropinization, 

 acetylcholine may raise the blood pressure of mammals by stimulation of 

 the adrenals and of sympathetic ganglia. In fishes, then, the blood pressure 

 falls when the heart is stopped by the vagus; acetylcholine constricts most 

 arteries and raises blood pressure in fish, whereas in mammals it dilates most 

 vessels. Adrenalin constricts somatic vessels in both mammals and fish and 

 dilates coronaries in mammals and branchials in fish. Elucidation of the 

 details of the striking differences in innervation and drug effects in fish 

 and mammals should aid in understanding the site of action of acetylchohne 

 and adrenalin. 



In cyclostomes no innervation to the heart has been found, and destruc- 

 tion of the spinal cord failed to alter heart beat in Petromyzon}'^-* There 

 is probably no distinct autonomic system in cyclostomes. 



Invertebrates of large size and high activity, and possessing a closed circu- 

 latory system, show blood pressures comparable to pressures in vertebrates. 

 In large cephalopod molluscs pressures in the cephalic artery of 47.6-60 

 mm. Mg (maximum 80 mm. Hg) have been recorded, and in gill vessels 

 5.4-6.1 mm. Hg'"'' (Fable 67). Mechanisms of regulation arc unknown. 



Pressure in Open Circulatory Systems. One of the most striking differences 



