structural:summarization, quality:correction, structural:expansion
Physiology

Given the large body mass and long neck of sauropods like Apatosaurus, physiologists have encountered problems determining how these animals breathed. Beginning with the assumption that, like crocodilians, Apatosaurus did not have a diaphragm, the dead-space volume (the amount of unused air remaining in the mouth, trachea, and air tubes after each breath) has been estimated at about 0.184 m (184 L) for a 30 t (30 long tons; 33 short tons) specimen. Paladino calculates its tidal volume (the amount of air moved in or out during a single breath) at 0.904 m (904 L) with an avian respiratory system, 0.225 m (225 L) if mammalian, and 0.019 m (19 L) if reptilian.

On this basis, its respiratory system would likely have been parabronchi, with multiple pulmonary air sacs as in avian lungs, and a flow-through lung. An avian respiratory system would need a lung volume of about 0.60 m (600 L) compared with a mammalian requirement of 2.95 m (2,950 L), which would exceed the space available. The overall thoracic volume of Apatosaurus has been estimated at 1.7 m (1,700 L), allowing for a 0.50 m (500 L), four-chambered heart and a 0.90 m (900 L) lung capacity. That would allow about 0.30 m (300 L) for the necessary tissue. Evidence for the avian system in Apatosaurus and other sauropods is also present in the pneumaticity of the vertebrae. Though this plays a role in reducing the weight of the animal, Wedel (2003) states they are also likely connected to air sacs, as in birds.

James Spotila et al. (1991) concludes that the large body size of sauropods would have made them unable to maintain high metabolic rates because they would not have been able to release enough heat. They assumed sauropods had a reptilian respiratory system. Wedel says that an avian system would have allowed it to dump more heat. Some scientists state that the heart would have had trouble sustaining sufficient blood pressure to oxygenate the brain. Others suggest that the near-horizontal posture of the head and neck would have eliminated the problem of supplying blood to the brain because it would not have been elevated.

James Farlow (1987) calculates that an Apatosaurus -sized dinosaur about 35 t (34 long tons; 39 short tons) would have possessed 5.7 t (5.6 long tons; 6.3 short tons) of fermentation contents, though he cautions that the regression equation being used is based on living mammals which are much smaller and physiologically different. Assuming Apatosaurus had an avian respiratory system and a reptilian resting-metabolism, Frank Paladino et al. (1997) estimate the animal would have needed to consume only about 262 litres (58 imp gal; 69 US gal) of water per day.