48

key concepts

48.1

Respiratory Gas Exchange Is Governed by Physical Factors

48.2

Enhancing Diffusion Maximizes Respiratory Gas Exchange

48.3

Humans Have Tidal Respiration

48.4

Respiratory Gases Are Transported by the Blood

48.5

Breathing Is Homeostatically Regulated

The Breath of Life

Elephant seals (Mirounga angustirostris) on a beach are impressive. Typical adult males are 5 meters long and weigh 2,500 kilograms. They fight viciously to defend areas of beach where the females come out of the water to give birth and rear their young. Thus the beach masters are really defending their opportunities to father the next generation.

Physiologically, these animals are even more impressive in the water. Female elephant seals, half the size of the males and therefore easier and safer to manage for experiments, have been fitted with instruments to record the depth and duration of their dives after they leave the beach where they gave birth. They spend up to 7 months at sea, and during that entire time they dive continually—day and night—to feed on eels, skates, rays, squid, and other prey. Most dives are 300–600 meters deep and last 20 minutes, but dives lasting over an hour and exceeding 2,000 meters have been recorded. The seals spend only 4–5 minutes at the surface between dives. Thus for the 80 percent of their lives that they spend at sea, they are breathing only about 20 percent of the time—12 minutes per hour!

The best human divers are perhaps the pearl divers of Japan and Korea. They typically dive repeatedly during their workday to about 20 meters, and their dives last about a minute. Record breath-hold dives for humans resting facedown in a pool are about 10 minutes, but that is a one-time event that cannot be repeated for many hours. The elephant seals are not resting—they are descending and ascending great distances and pursuing prey.

How can these behemoths spend so much time not breathing? How can they survive such crushing depths? How can they not get the bends, which is a danger for human divers? How much oxygen do these animals need to carry out their normal behavior? How much oxygen can they store in their bodies during the short times they spend at the surface? And what adaptations do they have to make those oxygen stores last as long as they do? To answer these questions, elephant seals and other diving animals have been studied in unique laboratory facilities and in the field.

What adaptations make seals champion breath-hold divers?