The openings of the esophagus and trachea are connected in the pharynx. The biggest risk of spitting up is that milk flows back from the esophagus into the pharynx and then accidentally enters the trachea during inhalation; this is what's known as choking on milk. A large amount of milk entering the trachea can cause a blockage, preventing breathing and endangering life; whereas a smaller amount can directly reach deep into the lungs, causing aspiration pneumonia.