534 Comparative Animal Physiology 



blood is measured. Red blood cells containing radioactive iron or phos- 

 phorus, or protein with radio-iodine, have also been utilized in determining 

 blood volume. These various methods consistently give somewhat different 

 values of estimated blood volume, and much has been written explaining 

 these differences. More striking, however, is their general agreement. In 

 Table 66 are given some blood volume measurements made by bleeding and 

 hemoglobin estimation, and published in 1854 by Welcker, and beside 

 them a few values obtained by modern methods. 



The blood volume in mammals comprises normally 7 to 10 per cent of 

 the body weight. The lymph volume is not known in any animal, but the 

 total extracellular space as indicated by such substances as thiocyanate and 

 by radioactive sodium, materials which are taken up very little by tissue cells, 

 is 20 to 30 per cent of the body weight of mammals. Birds and amphibians 

 have blood volumes similar to those of mammals. In elasmobranchs the 

 blood volume is less (about 5 per cent of body weight), whereas teleosts 

 have the least blood (1.5 to 3 per cent). 



In animals with open circulation the blood volume should be essentially 

 the same as the total extracellular volume in animals with a closed circula- 

 tion. Such data as are available are given in Table 66. In Camharus both 

 the T-1824 space and the thiocyanate space are 25 per cent of body weight. ^^^^ 

 The blood volume as given by hemoglobin determinations is of the same 

 order (30 per cent) in Arenicola and Planorhis; the blood system in Arenicola 

 is closed, and it is difficult to account for this large volume. The blood 

 volume of larval insects is probably much larger than that of adult 

 insects. ^*'^- -^^ Thiocyanate and T-1824 volumes are also similar in fresh-water 

 mussels but are low (9-9.5 per cent of body volume), possibly because of 

 by-passing some sinuses. ^^**" In many invertebrate animals the specific gravity 

 of the body differs from that of the blood, hence blood volumes are better 

 expressed as fraction of body volume. 



In circulatory systems a small blood volume is more efficient than a large 

 blood volume, in that the same blood is re-used more frequently in transport; 

 on this basis the circulation of bony fish is more efficient than that of crus- 

 taceans. Measurements by modern methods of the partition of body fluids 

 in animals with different types of circulatory systems should add materially 

 to our understanding of the maintenance of fluid balance. 



Pressure in Closed Circulatory Systems: Measurement and Regulation. 

 In any closed tubular system containing a pump, a head of hydrostatic 

 pressure is developed at the pump and the pressure declines with frictional 

 loss in the tubes, particularly if the tubes are distensible. The principles of 

 blood flow, hemodynamics, have been extensively studied and are discussed 

 in detail in textbooks of mammalian physiology.^-*' In man the pressure 

 in a large artery at the time of heart contraction (systole) is about 120 mm. Hg, 

 and at the time of heart relaxation (diastole) is about 80 mm. Hg, often 

 written 120/80. The difference, or pulse pressure, decreases as the blood 

 proceeds to smaller vessels, and the pressure falls off rapidly in the arterioles. 

 Velocity of flow is slow in the vast capillary bed, then in the veins velocity 

 increases slightly although pressure continues to decrease. Blood pressure 

 is normally determined by the peripheral resistance, the head of pressure 



