Before diving, pinnipeds typically exhale to empty their lungs of half the air and then close their nostrils and throat cartilages to protect the trachea. Their unique lungs have airways that are highly reinforced with cartilaginous rings and smooth muscle, and alveoli that completely deflate during deeper dives. While terrestrial mammals are generally unable to empty their lungs, pinnipeds can reinflate their lungs even after complete respiratory collapse. The middle ear contains sinuses that probably fill with blood during dives, preventing middle ear squeeze. The heart of a seal is moderately flattened to allow the lungs to deflate. The trachea is flexible enough to collapse under pressure. During deep dives, any remaining air in their bodies is stored in the bronchioles and trachea, which prevents them from experiencing 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.
The main adaptations of the pinniped circulatory system for diving are the enlargement and increased complexity of veins to increase their capacity.Retia mirabilia form blocks of tissue on the inner wall of the thoracic cavity and the body periphery. These tissue masses, which contain extensive contorted spirals of arteries and thin-walled veins, act as blood reservoirs that increase oxygen stores for use during diving. As with other diving mammals, pinnipeds have high amounts of hemoglobin and myoglobin stored in their blood and muscles. This allows them to stay submerged for long periods of time while still having enough oxygen. Deep-diving species such as elephant seals have blood volumes that make up to 20% of their body weight. When diving, they reduce their heart rate and maintain blood flow only to the heart, brain and lungs. To keep their blood pressure stable, phocids have an elastic aorta that dissipates some energy of each heartbeat.