Oxygen equilibrium curves and the relationships between the partial pressure of CO2 and pH were determined for the haemolymph of the arachnids Eurypelma californicum, Pandinus imperator and Cupiennius salei. A new type of experimental apparatus was constructed, tested and used to make these measurements on small undiluted cell-free haemolymph samples. Most of its components were made in our workshop and were inexpensive. The apparatus proved to be very reliable as demonstrated by control (oxygen concentration) experiments. In previous reports on O2 equilibrium curves of E. californicum haemocyanin, Tris­HCl buffer was used to set the pH, whereas in our experiments, pH was adjusted by altering the partial pressure of CO2. The O2 concentration measurements demonstrate an increase in oxygen affinity of E. californicum haemocyanin when using Tris­HCl buffer. The position and shape of the O2 equilibrium curves of E. californicum and P. imperator haemocyanin show a distinct dependency on pH. Oxygen affinity is lower in E. californicum and cooperativity changes with pH in E. californicum, but not in P. imperator. Oxygen transport in the haemolymph of E. californicum during rest, activity and recovery was calculated on the basis of the O2 equilibrium curves of undiluted haemolymph. Apart from oxygen transport, there are indications that haemocyanin is also involved in other physiological processes. For example, it may function as a storage protein.

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