There was also the question of breathing that needed to be settled; a neck that long produces some rather difficult engineering problems. The trachea of an Apatosaurus is a long anatomical structure. The pressure in the lungs and that outside of the body needs to be great enough that the volume of oxygen required for breathing can be taken into the lungs and carbon dioxide expelled or the animal, our Apatosaurus, will effectively suffocate. How does a 20 to 22 ton animal create the pressure required for breathing? How much pressure might that be exactly? Studies have shown that an estimated 30 ton Apatosaurus may have had up to 184 liters of dead-space volume in the thoracic cavity. Dead-space volume is air between breaths in oral cavity, trachea, and lungs. The assumption is that Apatosaurus possessed a crocodilian like breathing apparatus that lacks a diaphragm. Some estimates of the ability to replace this air have been done comparing Apatosaurus and ideal conditions of taxa specific respiratory systems. Reptilian systems are vastly inadequate to replace this volume and would have suffocated Apatosaurus; mammalian and avian systems are better equipped to replace the volumes of air proposed in Apatosaurus systems. Mammalian systems are adequate, but under powered as well, meaning that avian systems remain as the best equipped systems for allowing Apatosaurus to breathe efficiently; another possibility put forth is an unknown and undescribed system that has been lost in the modern world.
It is assumed that a four chambered 500 liter heart and 900 liter lungs filled a 1700 liter thoracic cavity (300 liters of tissue) in Apatosaurus. These assumptions in conjunction with reptilian resting metabolic rates and avian respiration, it has been hypothesized that an Apatosaurus required 69 gallons of water a day to live in addition to massive amounts of vegetation that it must have consumed.