A fitting (1) for a blood circulating circuit (4) of a dialysis machine has a first ans a second mouth (8, 9) for connecting the fitting (1) to a first ans a second portion (2, 3) of the blood circulating circuit (4) respectively; the fitting (1) defining a third portion (10) of the blood circulating circuit (4) by means of a tuby (6) by which to determine the absorption of electromagnetic waved by the blood, and by means of a chamber (7) having a wall (15) movable as a function of the pressure of the blood.