| Summary: | 1. The pH of the blood of the third instar Gastrophilus larva is 6.64 at 38° C. with a pH-temperature coefficient of -0.007 Per 1° C. rise in temperature. 2. The total CO 2 content of the blood varies from 40.6 to 131.4 vol. % with an average of 72.4 vol. %. The CO 2 content of the tissues minus the cuticle is very close to, and follows the variations in, the CO 2 content of the blood. 3. The CO 2 tension in the blood is from 300 to 500 mm. Hg. From 30 to 50% of the CO 2 is in solution, the rest in the form of bicarbonate. Carbamate formation does not occur in the blood. 4. The apparent dissociation constant for carbonic acid, (pK' 1 ), has a value of 6.08 (S.D. ±0.06) at 38° C. and 6.19 (s.d. ±0.13) at 16° C. 5. CO 2 dissociation curves have been drawn for 38 and 16° C. The slope of the curves indicates that the whole of the CO 2 is given off at zero CO 2 tension, and that the blood is adapted for functioning at high CO 2 tensions.
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