RESPIRATION
The Narwhal, like most marine mammals, have a blowhole that they utilize for air intake and to shut out water while diving. They have two nasal passages connected by the premaxilla bag that merges with the larynx. The epiglottis creates a link to the nasal cavity and the trachea. When the animal is relaxed, the peri laryngeal sphincter controls the amount of air that is inhaled by the lungs. Leading to the lungs, the trachea is made up of cartilaginous rings. Similar to humans, air travels around the various bronchioles and bronchiole tubes, leading to the alveoli. When narwhals have to undergo long dives, they are capable of collapsing their lungs to prevent damage from the increasing pressure. To further protect their collapsing lung, they have a jointed rib cage that allows their thoracic cavity to collapse with the lung. They limit the gas exchange from lungs to blood to prevent excess nitrogen bubble accumulation, which could result in decompression sickness. Narwhals have to use their limited supply of oxygen strategically. They slow their heart rate down in order to reduce the flow of oxygen. Several organs undergo anaerobic metabolism resulting in a redirection of available oxygen to organs, such as the heart and the brain. But because anaerobic metabolism results in the production of lactate, the narwhal has to spend more time at the surface before going on another dive in order to get back to the pre-dive lactic acid levels. ​​
According to "Escaping narwhals can freeze and flee at the same time," narwhals have adapted to hold their breath for a long time so they can dive deep down into the ocean away from the predators. To conserve oxygen while foraging for food, they can slow their body functions. The normal heart rate for a narwhal is about 60 beats per minute. But during a long underwater feeding session, a narwhal can drop its heart rate to just 10 or 20 beats per minute.
According to "Heart monitors on wild narwhals reveal alarming responses to stress" by Tim Stephens, although narwhals dive to depths exceeding 1000 m (maximum recorded depth was 1900 m; Laidre et al., 2003), they are not considered fast swimmers. Based on contraction times, the dominance of slow-twitch muscle fibers, and exceptionally high myoglobin concentrations, narwhals have been characterized as slow, aerobic swimmers (Williams et al., 2010). From monitored escape dives, narwhals required 97 percent of the animal's oxygen supply and often exceeded its aerobic dive limit (meaning depletion of oxygen stores in the muscles, lungs, and blood, followed by anaerobic metabolism). Normal dives of similar duration and depth used only about 52 percent of a narwhal's oxygen store, the study found (Williams et al., 2017).
Between dives, the narwhal will lay still at the surface, occasionally rising to inhale and then lowering beneath the surface with its blowhole closed. After some time, the stationary narwhal will exhale explosively and resurface to inhale again. This process is repeated several times, rising higher out of the water each time. A complex network of blood vessels and the ability to hold large amounts of oxygen in blood vessels and muscle tissue sustain the narwhal on their deep dives. "After ATP is used the wasted CO2 is carried by blood vessels to the lungs, where it is quickly replaced with oxygen through diffusion as soon as the narwhal inhales." Deep diving air-breathing species by necessity must balance submergence time and level of exercise during breath-holding: a low activity level preserves oxygen stores and allows longer duration submergence whereas high activity levels consume oxygen quickly and shorten submergence time (Tervo et al., 2021).
Figure 2. Risk factors for gas bubble formation and Gas Emboli Pathology (GEP), and 3 hypothetical regions [Shallow (0–~30 m), Intermediate (~30–~200 m), and Deep (beyond ~200 m)].
https://www.frontiersin.org/articles/10.3389/fmars.2021.598633/full.