1. The systemic blood pressure of Rana pipiens and R. temporaria is slightly higher than the pulmocutaneous pressure at systole and much higher at diastole. The pulses differ in shape and a conus component can be seen in the systemic wave.

2. Submersion of the animal causes a fall in systolic pressure in both arches, the diastolic pressure remaining relatively constant. The shape of the pulse wave changes, the conus component being accentuated and visible in recordings from both arches.

3. Heart rate and stroke volume fall during submersion so that after 30 min. under water the minute volume may be 20-50% of the value at the surface. The heart becomes increasingly full of blood.

4. The differences in systemic and pulmocutaneous pressures are explained in terms of resistance, compliance and flow in lung and body circuits. The same general relationships persist during submersion but selective increases in peripheral resistance must occur to maintain the central blood pressure in face of falling heart output.

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