CIRCULAR l)lSTURi;Ai\n:s. Ill 



may be evident when the functional activit)- cif the heart is 

 varied. 



Vascular Variations. — The amount of Idood passing into or 

 out of a given organ is determined l)y tlie cahl)er of the blood 

 vessels, provided the heart action and general blr.od pressure 

 remain normal. \'ariations in the caliber (if nnmial blood ves- 

 sels depend primarily upon tlie rcspi.nsc of the vessel muscu- 

 lature to vaso-motor stimulation. Illdud pressure is dependent 

 upon the elasticity of the arteries and the force of the heart. In 

 general, pathologic vascular variations are the result of, first, 

 disturbed arterial elasticity; second, the maintainance of the 

 normal caliber of the vessels and ; third, the nermeal)ility of the 

 vascular walls. 



Arteriosclerosis is a condition in whicli the elasticitv of tlie 

 \-essel walls is lessened or destroyed. In the production of 

 arteriosclerosis there is vascular dilatation succeeded hv suljcu- 

 dothelial fibrous formation which continues until the lumen of 

 the dilated vessel is reduced to its normal size. The hvper- 

 plastic fibrous tissue mav later i)ccome calcified. Sclerotic 

 arteries are thick, stiff and nmielastic. Sclerosis is most com- 

 mon in arteries although it occurs in veins. 



The \ascnlar caliber may ])c diminished by muscular con- 

 traction or by hj'pertrophied vessel walls. In animals affected 

 with chronic nephritis there is contraction of the svstemic 

 arteries resulting in compensatory cardiac hypertrophv. Arterial 

 constriction is also common in the peripheral vessels of animals 

 aft'ected with carbon dioxide poisoning. Local diminution of 

 vascular caliber mav be produced bv parietal thrombi. The 

 ^•ascular caliber may be increased bv ]iaralysis of tlie vaso- 

 motor nerves a condition which is sometimes obser^'ed in ani- 

 mals that have received injuries in the cervical region. 



Increased permeability of vessel walls nsualh- results from 

 insufificient nutriment to the vascular structures and occurs 

 most frequentlv in small vessels, i. e., capillaries and venules. 

 Increased permeability usually accom]>anies \'enous hyperemia 

 although it may exist independent of variations in the quantity 

 of blood. Thus oedema is common in hydremic individuals. 



Variations in Quantity and Quality of Blood. — The quantity 

 of blood in a part is determined by the caliber of the supplying 

 vessel and by the blood pressure, .\cute general anemia re- 

 sults in a diminished blood pressure which, if not corrected in 

 a short time, terminates fatally. Chronic general anemia is 

 accompanied by a slightly diminished blood pressure and a re- 

 tarded blood current. 



