12 80 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Return 



chamber}; 



chamber 



I P 1 



5cm 



fig. 3. Sectional view of the Dawes' flowmeter. The cham- 

 bers are made of Perspex, the washers of Portex sheet, the con- 

 necting tubes of rubber, and the electrodes of silver wire. The 

 bag is molded of rubber solution on a form (see text). [From 

 Dawes et al. (22).] 



to return to the arterial stream driven by a pressure 

 difference between the two limbs. This pressure dif- 

 ference is supplied by the density difference between 

 blood and silicone oil. Therefore the authors have 

 called their apparatus "density flowmeter." The tech- 

 nical details are more involved than the description 

 of the principle indicates. As can be seen from figure 3, 

 in the actual device blood is not allowed to enter the 

 electrode chamber. A rubber bag filled with saline 

 placed in the lower part of the proximal limb is com- 

 pressed by the inflowing blood, emptying its contents 

 into the part of the limb containing the electrode. 



The apparatus of the dimensions given in figure 3 

 can measure blood flow at rates as high as 45 ml per 

 min with an absolute accuracy of ±4 per cent. The 

 pressure drop does not exceed 3 to 4 mm Hg at maxi- 

 mum rates. The dead space to be filled with blood 

 amounts to about 4 to 5 ml. The range of flow may be 

 extended by using larger measuring chambers. 



For measurement of time intervals any kind of 

 ordinate writer (29) can be used. Gaddum's drop- 

 timer (1938) was used by the authors (44). 



A --till simpler sell-recording flowmeter making use 

 of a single electromagnetic tap in the bypass was first 

 described by Dawes et al. and has been constructed 

 recently by Wretlind (88). When the bypass is closed 

 the blood enters the U-tube, consequently bulging 

 a membrane in proportion to the volume flow. The 

 displacement of the membrane is recorded by a lever 

 on a smoked drum. The tap is automatically opened 

 every 2 sec so that blood in the U-tube returns to the 

 artery, allowing the membrane to return to its original 

 position. Then the cycle begins again with the closing 

 of the tap. Since the lever indicates blood flow per 2 

 sec, the record gives direct readings of flow rate. 



BUBBLE FLOWMETER 



The bubble flowmeter developed by Soskin et al. 

 (75) consists of a glass tube of known caliber and 



Amwiii 



fig. 4. Schematic drawing of a bubble flowmeter :B, bubble 

 reservoir; E, entrance of the bubble into the flowmeter; Mi Aft, 

 measuring points (platinum electrodes) for timing the passage 

 of the bubble; S, rubber tubing; V, magnetic tap used as auto- 

 matic bubble injector. [From Rockemann (72).] 



