A DEEP BREATH

A col­lapsed lung helps cetaceans dive to great depths and avoid the bends

Scuba Diving - - ASCEND - BY MELISSA GASKILL

Beaked whales can spend two hours be­neath the sur­face. Dol­phins de­scend down to 1,000 feet and rou­tinely make as many as 20 dives in a row to 300 feet.

Good luck find­ing that type of pro­file on a dive ta­ble.

So, how do these mam­mals avoid get­ting hit with de­com­pres­sion sick­ness?

Spe­cial lung ar­chi­tec­ture helps pro­tect them from the bends, ac­cord­ing to a study by re­searchers at Woods Hole Oceano­graphic In­sti­tu­tion and Spain’s Fun­da­cion Oceanografic.

When air-breath­ing an­i­mals dive un­der­wa­ter, in­creas­ing pres­sure causes ni­tro­gen bub­bles to col­lect in the blood­stream and tis­sue. As­cend­ing slowly al­lows ni­tro­gen to re­turn to the lungs and be ex­haled. As­cend too fast, and ni­tro­gen bub­bles don’t have time to dif­fuse back into the lungs. In­stead, they be­gin to ex­pand in blood and tis­sues, caus­ing pain and dam­age — DCS, or the bends.

Un­der deep-sea pres­sure, the lungs of cetaceans — whales, dol­phins and por­poises — cre­ate two dif­fer­ent re­gions, one filled with air and one col­lapsed. This cre­ates a gra­di­ent in the amount of blood flow and gas ex­change, tak­ing ad­van­tage of dif­fer­ences in sol­u­bil­ity of oxy­gen, car­bon diox­ide and ni­tro­gen.

“These an­i­mals have the abil­ity to change that rate,” says bi­ol­o­gist Michael Moore, se­nior sci­en­tist at WHOI and a study co-author. “They can ma­nip­u­late the gra­di­ent to fa­vor con­di­tions that trans­fer oxy­gen and car­bon diox­ide but not ni­tro­gen, so as not to in­crease the risk of DCS. Blood flow­ing mainly through the com­pressed re­gion al­lows ab­sorp­tion of some oxy­gen while min­i­miz­ing or pre­vent­ing the ex­change of ni­tro­gen.”

The sci­en­tists ob­served this phe­nom­e­non by in­flat­ing the lungs from dif­fer­ent an­i­mals and put­ting them in a hy­per­baric wa­ter cham­ber to sim­u­late dives to dif­fer­ent depths. “We com­pared dol­phin, seal and pig lungs, and found dra­matic dif­fer­ences,” says Moore. “Ter­res­trial mam­mals just don’t have the anatom­i­cal and func­tional adap­ta­tions that marine mam­mals do.”

Marine mam­mals are not com­pletely im­mune to DCS, how­ever. Sci­en­tists have de­tected de­com­pres­sion gas bub­bles in seals and dol­phins that drowned at depth in gill nets. Four­teen dead whales in a 2002 strand­ing event linked to U.S. Navy sonar ex­er­cises had gas bub­bles in their tis­sues — a sign of DCS.

“We know that loud noises are stress­ful for marine an­i­mals,” says Moore. “It can cause a fight-or-flight re­sponse, in­creas­ing heart rate and vas­cu­lar di­la­tion. That messes with this pro­tec­tive mech­a­nism — with the way the an­i­mal has pro­grammed its dive — and in­creases ab­sorp­tion of ni­tro­gen in blood.”

The re­search doesn’t ex­plain why DCS might cause a cetacean to beach, says co-author An­dreas Fahlman.

“But just know­ing that stress can cause fail­ure of this adap­ta­tion means we might find ways to mit­i­gate it,” he says. “The so­lu­tion could be as sim­ple as start­ing sonar at low lev­els so the an­i­mals don’t freak out, then in­creas­ing lev­els grad­u­ally to give them a chance to move away.”

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