User:WyattMillion/sandbox

There are many components that make up sea lion physiology and these processes control aspects of their behavior. Physiology dictates thermoregulation, osmoregulation, reproduction, metabolic rate, and many other aspects on sea lion ecology including but not limited to their ability to dive to great depths. The sea lions' bodies control heart rate, gas exchange, digestion rate, and blood flow to allow individuals to dive for a long period of time and prevent side-effects of high pressure at depth.

The high pressures associate with deep dives cause gases such as nitrogen to build up in tissues which are then released upon surfacing, possibly causing death. One of the ways sea lions deal with the extreme pressures is by limiting the amount of gas exchange that occurs when diving. The sea lion allows the alveoli to be compressed by the increasing water pressure thus forcing the surface air into cartilage lined airway just before the gas exchange surface[2]. This process prevents any further oxygen exchange to the blood for muscles, requiring all muscles to be loaded with enough oxygen to last the duration of the dive. However, this shunt reduces the amount of compressed gases from entering tissues therefore reducing the risk of decompression sickness[2]. The collapse of alveoli does not allow for any oxygen storage in the lungs however, this means that sea lions must mitigate oxygen use in order to extend their dives. Oxygen availability is prolonged by the physiological control of heart rate in the sea lions. By reducing heart rate to well below surface rates, oxygen is saved by reducing gas exchange as well as reducing the energy required for a high heart rate [3]. Bradycardia is a control mechanism to allow a switch from pulmonary oxygen to oxygen stored in the muscles which is need when the sea lions are diving to depth [3]. Another way sea lions mitigate the oxygen obtained at the surface in dives is to reduce digestion rate. Digestion requires metabolic activity and therefore energy and oxygen are consumed during this process, however sea lions can limit digestion rate and decrease it by at least 54% [4]. This reduction in digestion results in a proportional reduction in oxygen use in the stomach and therefore a correlated oxygen supply for diving. Digestion rate in these sea lions increase back to normal rates immediately upon resurfacing [4]. Oxygen depletion limits dive duration, but carbon dioxide (CO2) build up also plays a role in the dive capabilities of many marine mammals. After a sea lion returns from a long dive, CO2 is not expired as fast as oxygen is replenished in the blood, due to the unloading complications with CO2. However, having more than normal levels of CO2 in the blood does not seem to adversely affect dive behavior [1]. Compared to terrestrial mammals, sea lions have a higher tolerance to storing CO2 which is what normally tells mammals that they need to breath [1]. This ability to ignore a response to CO2 is likely brought on by increase carotid bodies which are sensor for oxygen levels which let the animal know its available oxygen supply [1]. Yet, the sea lions cannot avoid the effects of gradual CO2 build up which eventually causes the sea lions to spend more time at the surface after multiple repeated dives to allow for enough built up CO2 to be expired [1].

References:

Gerlinsky, C. D., Rosen, D. A., & Trites, A. W. (2014). Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus). Journal of Comparative Physiology B, 184(4), 535-544.

Kooyman, G.L.; Sinnett, E.E. 1982. Pulmonary shunts in harbor seals and sea lions during simulated dives to depth. Physiological Zoology. 55: 105-111.

McDonald, B. I. and Ponganis, P. J. 2014. Deep-diving sea lions exhibit bradycardia in long-duration dives. The Journal of Experimental Biology. 217: 1525-1534.

Rosen, D. A., Gerlinsky, C. D., & Trites, A. W. (2015). Evidence of partial deferment of digestion during diving in Steller sea lions (Eumetopias jubatus). Journal of Experimental Marine Biology and Ecology, 469, 93-97.