50 15 alo 5 3 °° zio 1.8 ' 1.4 + VERTEBRA 1.4 4 1.24 0 1.0 Blt =|> O.8 o |S -{. 06 tL * . <tj< myn Oo be 0 F FEMUR = << a ° —¢——_—f-— TIBIA O44 0.2 e e “ aeeeeestatinonll cagecs ene eee MID FEMUR eee Q@ecsescneesseseuosensn sresPeeuensenees esennen qusnencanessun nasnes Gueeenasneannnaanas ooo Bes cecccsnccnanssnnecacen Msorcnne ewe R PRU ORE RUE SE EEE PES esmnegperseree” MID TIBIA - ox J | i i wl | al 1 10 30 13 4 ] 3 9 30 Minutes Hours i [ 1090 460 Days Fic. 42.—Thespecific activity of %Sr in different bones of the adult rabbit relative to the total skeleton after a single venous injection. ALL VERTEBRAE Ro@28/kg BONE RATIO( Ra226/kg WHOLE SKELETON ) 1.8 . . ALL VERTEBRAE 0.6 04 L TIBIA RIBS 0.2 0 o L 2 | 4 | 6 ! 8 i oO } «J2 foot fd 4 O16 18 20 ft 22 24 26 28 30 32 34 36 38 40 YEARS CARRIED Fie. 48.—Concentration of 228Re in different human bones relative to the total skeleton RATIO OF THE SPECIFIC ACTIVITY IN THE DIFFUSE COMPONENT TO THAT OF THE WHOLE BODY Loss of Radioisotopes as Seen Microscopically Two theories are currently considered to explain the loss of activity from bone, (1) resorption, i.e., the removal of bone by osteoclasts and (2) the loss of activ- ity by exchange with the extracellular fluids. Bone formation, and by inference boneresorption rates, have been well documented by tetracycline labeling techniques. The rate of loss by exchange process is, how- ever, much moredifficult to quantitate. In Figure 47 I have tried to document what is known abou loss of activity by the exchange process. There ar: main components of uptake of the alkaline eart! seen in autoradiographs of bone, (1) a hotspot ponent usually associated with areas of bone grt and (2) a diffuse component which refers to lab throughout the whole volume of the bone. These components have been studied separately to eva the loss of activity by exchange. The short-term uy} on bone surfaces is ignored in these studies.