102 M. Brookes 



Materials and methods 



Female albino rats, mean weight 140 gm were used. All investigations were 

 carried out under ether anaesthesia. Results are given on the middle third of the 

 femoral cortex, the marrow core inside it, the superior and inferior metaphyses and 

 the inferior epiphysis. 



Blood Volume. A red cell dilution technique was employed. Rat red cells 

 were labelled with •^iCr, washed in saline and centrifuged. The labelled cells 

 were prepared and used on the same day. Each rat was given 0.3 ml i.v. packed 

 labelled cells. After 15 minutes to allow complete mixing of the label, the tail of the 

 animal was swiftly amputated and a drop of blood caught on a glass slide. The 

 anaesthetised rat was then dropped into acetone at — 50°C, and the circulation 

 thereby suddenly stopped. The femora were fixed in ice-cold formalin. All extra- 

 osseous soft tissue including cartilage was removed from the bone. It was then broken 

 down into the various samples to be investigated. The weight of each sample was 

 taken and its radioactivity measured as also that of a 0.005 ml sample of tail blood 

 collected in a micropipette. 



From these readings, the blood volume/ 100 gm selected tissue was calculated In 

 terms of ml tail blood. The red cell volume was also arrived at knowing the haemato- 

 crit of rat tail blood (43.4''/o). 



Rate of flow 



The above procedure was modified as follows. Each rat was weighed and a 

 0.005 ml sample was taken of the measured volume of labelled blood given to the rat. 

 The rats were killed by freezing at known intervals varying from 5 — 45 sec. after in- 

 jection i.v. of the label. The radioactive concentration of the tissue was calculated and 

 appropriate corrections made for variation in rat weight, dose volume of label, 

 radioactive decay, and radioactive concentration of the dose given to each rat. 

 Graphs were then constructed relating measured radioactive concentration to time in 

 seconds after injection. From the graphs, it was possible to estimate the rate of flow 

 through individual tissues in ml packed red cells/100 gm/min, or In terms of peri- 

 pheral whole blood applying an arbitrary haemotocrit of 43.4o/o. 



Circulation time and velocity 



The former is given by Vol/Flow. A comparative estimate of the velocity of the 

 cells through the tissue is given by the reciprocal of the circulation time. 



Results 



Blood volume 

 Tables 1 and 2 summarize the findings on 21 rats. 



Flow, circulation time, and velocity 

 These are derived from flow curves (Figs. 1, 2) constructed from the mean read- 

 ings of 61 rats. The area A under the initial wave divided by T, the transit time, 

 gives a measure of the bolus of label traversing 1 gm tissue in time T. The radio- 

 activity of 0.005 ml label is known. Fience the flow rate F can be calculated. V/F 

 then yields the circulation time C.T. Velocity is given by I/C.T. (Tables 3 and 4). 



