394
K JELL JOHANSEN
relationship to the relative role of aquatic and air breathing in overall
gas exchange.
The Australian lungfish is an almost exclusive water breather when
at rest in aerated water. It reacts to hypoxic water by promptly increasing its branchial breathing efforts. Water ventilation increased as much
as five times when the 0, tension in the water was reduced to about
80 mm Hg. Figure 14 shows the result of a typical experiment. Ventilation doubled, while the extraction of O2 decreased as did the overall 0,
uptake. The increased ventilation was caused by an increase of the
20
10 20
24
!
a
B R R h 28
32
Bfralh of air
20- -
0- 1
I
I
.
r
- I .
0 10 20 30 40 50 0 10 20 30 40 I
Time (minutes)
20
Fig. 14. Responses to breathing deoxygenated water in Neoceratodus. Ve, ventilation of water; VO,, oxygen uptake from the water; Ext, percent extraction of
oxygen from the water; and BRR, branchial respiratory rate. Arrows mark breaths of
air. Oxygen tensions followed in: Pi, inspired water; PV, pulmonary vein; Pa, pulmonary artery; BA, an afferent branchial artery; and VC, vena cava. From Johansen
et al. ( 1967).