Editing Pinniped (section)

===Diving adaptations===
{{see also|Physiology of underwater diving#Pinnipeds}}
[[File:Weddell seal swims underwater in McMurdo Sound (Image 3).jpg|right|thumb|[[Weddell seal]] underwater]]
To dive, a pinniped must first exhale much of the air out of its lungs and shut its nostrils and throat cartilages to protect the [[vertebrate trachea|trachea]].{{sfn|Riedman|1990|p=25}}{{sfn|Berta|2012|p=69}} The airways are supported by [[cartilage|cartilaginous rings]] and [[smooth muscle]], and the chest muscles and [[Pulmonary alveolus|alveoli]] can completely deflate during deeper dives.{{sfn|Berta|Sumich|Kovacs|2006|p=245}}<ref name="miller2006b"/> While land mammals generally cannot empty their lungs, pinnipeds can reinflate their lungs even after alveolar collapse.<ref name="miller2006b">{{cite journal |last1=Miller|first1=N. J. |last2=Postle|first2=A. D. |last3=Orgeig|first3=S. |last4=Koster|first4=G. |last5=Daniels|first5=C. B. |year=2006b |title=The composition of pulmonary surfactant from diving mammals |journal=Respiratory Physiology & Neurobiology |volume=152 |issue=2 |pages=152–68 |doi=10.1016/j.resp.2005.08.001|pmid=16140043 |s2cid=23633245 }}</ref> The middle ear contains [[sinus (anatomy)|sinus]]es that probably fill with blood during dives, preventing [[Ear clearing|middle ear squeeze]].<ref name="Costa"/> The heart of a seal is moderately flattened to allow the lungs to deflate. The trachea is flexible enough to collapse under pressure.{{sfn|Riedman|1990|p=25}} During deep dives, any remaining air in their lungs is shifted to the [[bronchiole]]s and trachea, which stops gas exchange with the blood, and thereby prevents them from developing [[decompression sickness]], [[oxygen toxicity]] and [[nitrogen narcosis]]. In addition, seals can tolerate large amounts of [[lactic acid]], which reduces skeletal muscle fatigue during intense physical activity.<ref name="Costa">{{cite book |author=Costa, D. P. |year=2007 |chapter=Diving physiology of marine vertebrates |title=Encyclopedia of Life Sciences |doi=10.1002/9780470015902.a0004230 |url=http://www.ucmp.berkeley.edu/about/shortcourses/costa_divingphysiology2007.pdf |isbn=978-0-470-01617-6}}</ref>

The [[circulatory system]] of pinnipeds is large and elaborate; [[rete mirabile|retia mirabilia]] line the inside of the trunk and limbs, allowing for greater oxygen storage during diving.{{sfn|Berta|Sumich|Kovacs|2006|p=241}} As with other diving mammals, pinnipeds have large amounts of [[hemoglobin]] and [[myoglobin]] stored in their blood and muscles respectively. This provides enough oxygen storage for them to stay submerged for long periods. Deep-diving species such as elephant seals have [[blood volume]]s that represent up to 20% of their body weight. When diving, they reduce their heart rate, and blood flow is mostly restricted to the heart, brain and lungs.<ref name="Costa"/> Pinnipeds have bulb-shaped [[ascending aorta]]s which are largest in deeper and longer diving species, allowing them to better maintain their [[blood pressure]].<ref>{{cite journal|last1=Storlund|first1=R. L.|last2=Rosen|first2=D. A. S.|last3=Trites|first3=A. W.|year=2024|title=Pinnipeds with proportionally wider aortic bulbs make longer dives|journal=Marine Mammal Science|volume=40|issue=4|page=e13145|doi=10.1111/mms.13145|doi-access=free}}</ref>