The constant value of the total concentration of potassium and sodium in tissue water in animals.

With age, the Musculus longissimus dorsi in pigs showed an increase in the potassium content and a decrease in the sodium content. The total concentration of potassium and sodium in milimoles in one litre of tissue water, here referred to as the "Sum k", is a constant value. The same mean Sum k had been obtained earlier by Blaxter and Rook for various kinds of tissue in cattle of different age. Approximate mean Sums k were calculated from the water, potassium and sodium contents in the muscles of man and pig at different stages of development, as reported in another publication. The constant character of Sum k was shown to be due to the equal concentration of potassium and sodium ions. This allows to calculate the quantity of water in the tissue from the potassium and sodium ions it contains and the percentage of extracellular or cellular water in total water. A consideration of the relationship between Sum k and osmotic pressure in the tissues proved an absence of concentration symmetry between the cellular and extracellular fluids which Sum k had seemed to suggest. Consequently, the total potassium content was multiplied by the asymmetry coefficient, fK, and then the Sum kc = mM (FKK + Na) per litre of water. From Blaxter and Rook's equation it was calculated that fK = 0.856 and Sum kc 148.8 mM.