Circulation of Body Fluids 533 



(Fig. 196, E). The primary body cavity persists in internal transport in 

 the form of intercellular spaces, in animals with closed circulatory systems. 



CoELOMic INTERNAL TRANSPORT. The coelom is a special, mesodermally 

 lined organ, more or less extensive in such groups as echinoderms, annelids, 

 sipunculoids, ectoprocts, and chordates. In the blood-sucking (Gnathob- 

 dellid) leeches the coelom is reduced to a system of tubes which contain a 

 pigmented fluid. Also in some polychaetes and echinoderms the coelomic 

 fluid contains corpuscles with respiratory pigments. The coelom is reduced, 

 in those molluscs with hemococlic circulation, to the pericardial cavity and 

 the lumen of gonads and kidneys; the coelom is reduced in arthropods 

 (Crustacea particularly) to cavities of gonads and kidneys. In vertebrates 

 the coelom persists as peritoneal, pericardial, and pleural cavities. 



Blood vascular system. A closed system of tubes, often with one or 

 more pumps, is found in oligochaetes (Fig. 196, D), many polychaetes, 

 rhynchobdellid leeches, phoronids, brachiopods, nemerteans, cephalopod 

 molluscs, holothurians, sipunculoids, and vertebrates. The blood and tissue 

 fluids are separate, although a certain amount of exchange occurs between 

 them. The blood comes into intimate association with the tissues, either by 

 capillaries or by closed sinuses. 



Lymphatic channels. In the vertebrates (Fig. 196, F) the intercellular 

 space (primary body cavity) is connected with the blood vascular system 

 through lymph channels. These converge on veins and form a lined net- 

 work which may be as extensive as the capillary bed. In some animals 

 (amphibians and teleosts), there may be lymph hearts. 



In the above classification each system builds on the preceding ones. There 

 may be numerous fluid compartments in one animal. For example, in ver- 

 tebrates the fluid spaces are: cardiovascular system, lymphatics, intercellular 

 spaces, and coelom. The distinction between "open" and "closed" vascular 

 systems is relative rather than absolute. Those animals in which the blood 

 passes from arteries to veins predominantly by way of small lined vessels 

 can be considered to have closed vascular systems. In general, the efficiency 

 of a closed system is greater with respect to velocity of blood flow, economy 

 of blood volume, and maintenance of blood pressure. 



Blood Volume. In animals with a closed circulatory system the volume 

 of circulating fluid is relatively fixed. If the fluid volume increases, the 

 pressure in the system rises; if the fluid volume is diminished the pressure 

 falls. Determinations of blood volume are made by measuring the dilution 

 of a known quantity of some material which is added and which remains 

 confined to the blood stream. Early investigators measured the concentra- 

 tion of hemoglobin in a small sample of blood; they then drained out the 

 blood from the animal, extracted the hemoglobin from the viscera, and 

 calculated the blood volume. Another method for calculating blood volume 

 is by using carbon monoxide, which can be given in a small known amount. 

 On the assumptions that circulating cells pick up CO and that complete 

 mixing occurs rapidly, the total blood volume can be calculated from concen- 

 tration of carboxyhemoglobin. Dyes are also used: Evans blue (T-1824) 

 combines with plasma albumin in vertebrates and leaves the blood very 

 slowly; a known amount of dye is injected and the dilution of dye in the 



