Circulation of Body Fluids 537 



the aortic bodies of the aortic arch, and in pain endings. Some of the sensory 

 endinas of the carotid sinus are stimulated by pressure in the carotid artery. 

 Impulses go to the vasomotor center in the medulla of the brain, from which 

 vasoconstrictor messages leave over the sympathetics, and vasodilator messages 

 by several pathways, including certain parasympathetics. A delicate balance 

 exists between vasoconstrictors and vasodilators. Parasympathetic discharge 

 causes vasodilatation and a fall in blood pressure, probably associated with 

 liberation of acetylcholine, whereas sympathetic constriction is associated with 

 liberation of adrenin (sympathin) somewhere in the chain of innervation of 

 vessel smooth muscles. Adrenalin"^ causes a rise in blood pressure by its vaso- 

 constrictor action. Numerous long-term factors, such as variations in arterial 

 elasticity, presence of the vasoconstrictor, hypertensin, from the kidneys, 

 and the general state of health, contribute to basal blood pressure levels. 



In general, the resting blood pressure is higher in large animals than in 

 small ones (Table 67); carotid pressure in the horse ranges up to 190 

 mm. Hg. Blood pressure at birth depends on the state of development of 

 a particular species; arterial pressure at birth in the rabbit is 21 mm. Hg, 

 in the cat 25-30 mm. Hg, and in the sheep, an animal more mature at birth, 

 73 mm. Hg.i« 



The circulation is somewhat more sluggish and pressures are lower in 

 cold-blooded vertebrates than in warm-blooded animals. The systolic arterial 

 pressure in the frog is 30 mm. Hg, in gill arteries of the elasmobranch 

 ScylUum 30 mm. Hg, and in gill arteries of the skate Raja 20 mm. Hg. 



The blood pressure is higher in bony fishes than in elasmobranchs. In 

 fishes the blood leaves the heart via the ventral artery, passes through gill 

 capillaries, and then enters the systemic circulation by the dorsal artery. The 

 ratio of pressure in the branchial (ventral) arteries to pressure in the dorsal 

 artery is about 3 to 2 (Table 67). Pressure in the pericardial cavity of fishes 

 is negative."" 



Regulation of blood pressure in fishes contrasts sharply with regulation in 

 tetrapods. The autonomic system of fishes, particularly of elasmobranchs,^^^ 

 is not sharply divided into sympathetic and parasympathetic divisions. The 

 heart of fishes receives only what may be called parasympathetic (vagus),^^ 

 and the peripheral vessels may receive only sympathetic innervation. '^ The 

 blood pressure in the branchial arteries is closely correlated with heart 

 activity; if water flow over the gills of a skate or shark is stopped, the heart 

 stops and blood pressure falls. If the fish is injected with atropine, which 

 blocks cardiac inhibition due to the vagus nerve, cessation of water flow does 

 not affect the heart and blood pressure."^ With each expiration the blood 

 pressure rises momentarily.^-'^ Aff^erent impulses in the vagus, hypobranchial, 

 and lateral line nerves result in cardiac and respiratory inhibition.^-" Increase 

 in pressure in perfused branchial arteries sets up sensory impulses in the 

 branchial nerves, and the sensory discharge occurs when the arteries fill at 

 each heart beat.^^'^ The junction of the third branchial vessel and the 

 ventral aorta is the homologue of the carotid sinus in mammals. Injection 



* Adrenin (equivalent to epinephrine) is the natural compound produced in the body. 

 Adrenalin is a commercial product. Sympathin is the adrenin-like substance apparently 

 hberated at adrenergic nerve endings. 



