Final answer:
Heartburn is caused by the reflux of acidic digestive juices from the stomach into the esophagus due to the failure of the lower esophageal sphincter to remain closed.
Step-by-step explanation:
The physiological cause of heartburn is the reflux of acidic digestive juices from the stomach into the esophagus. This occurs when the lower esophageal sphincter, a muscular valve at the end of the esophagus, fails to remain completely closed. When the sphincter opens, it allows the gastric juices to escape into the esophagus, causing a burning sensation known as heartburn.