MIXING RATE OF PHOSPHATE BETWEEN PLASMA AND INTERSTITIAL BODY FLUID OF COWS* BY MAX KLEIBER, ARTHUR H. SMITH,$ ~a~D

Isotopes as tracers offer the possibifity of studying the kinetics of phosphate metabolism in the normal intact animal. Our aim in such a study on dairy cows is to describe the cow's phosphorus exchange by a system of quantitative terms, especially capacities and rates of exchange. According to the varying mobility, the total amount of phosphorus in the animal may be partitioned among a series of pools. Each of these pools may be characterized by its exchange rates with other pools and its capacity. Biochemically, it is of interest to establish the correlation between the chemical nature of phosphorus compounds in the body, the metabolic processes involved, and the exchange rates. Physiologically, it is important to study the distribution of the various pools among the organs and possibly to correlate physiological function with pool capacity and exchange rate. This paper deals mainly with the analysis of the decrease of radioactivity with time in plasma from cows for the first 20 minutes after intravenous injection with radioactive phosphate. The data are interpreted as the result of mixing of plasma phosphate with the phosphate of the interstitial body fluid. In order to avoid confusion between postulates and conclusions., we shall number, instead of name, the phosphate pools and the exchange processes dealt with in this paper. The tracer, P~, is injected as sodium phosphate into the first pool whose capacity is the phosphate content of the plasma. The second pool is a postulated phosphate reservoir which is in most rapid exchange with the first pool. The capacity of pool 2 is derived as an empirical constant from the time relation of tracer activity in the first pool. Correspondingly we distinguish 3 processes of different rapidity: Process 1 is the mixing of the tracer within the first pool (plasma). This process presumably is practically completed at the starting time of our calculations (5 minutes after injection). 1 Process 2 is the mixing of phosphate between the first and second pool. This process takes place mainly between 5 and 20 minutes after injection. Process 3 is characterized by the loss of activity from the first (or second) pool after the mixing between these