The esophagus is a portal, a passageway, a transit corridor for the food you eat and the liquids you drink. It's a muscular tube that conveys these solids and liquids from your mouth to your stomach.

Esophageal muscles are partly under conscious control and partly involuntary. As we swallow, muscles in the esophagus involuntarily contract and relax in a wave that propels anything in it to the stomach. This wave of relaxation and contraction is called peristalsis, and it's the method used throughout the digestive tract to send food along its way.

A slippery mucous membrane covers the surface of the esophagus, helping ease the way for food. The muscle lies underneath. Slight constrictions at each end of the esophagus help close off each end as needed to prevent contents from flowing the wrong way. The one near the stomach, the lower esophageal sphincter, increasingly fails to function correctly as we age. This causes gastroesophageal disease, or GERD.

When the lower sphincter doesn't close, stomach contents can travel backward into the esophagus. Because the contents are mixed with stomach acid, this condition is called acid reflux. The esophagus is damaged when this happens because it is not protected against stomach acid. It causes the pain misleadingly named as heartburn.

However, people who have coronary artery disease may mistake the chest pains for heartburn, so it's important to see a doctor if you are unsure. There are ways of telling the difference: An antacid will stop heartburn, but has no effect on heart-related pain. If the pain comes after meals, it is likely to be heartburn.

While heartburn is painful, it can also lead to more serious problems, including cancer. The damaged esophageal tissue changes, becoming red and more resistant to stomach acid. However, the changed tissue, called Barrett's esophagus, is at a higher risk of becoming cancerous.

Heartburn in a pregnant woman can mean something quite different: that the baby will be born with a full head of hair. A recent study from Johns Hopkins University found that this bit of folklore was true. The study was published in December in the journal Birth. The researchers think that the two are linked by hormones such as estrogen, which are produced in higher amounts in pregnancy. These hormones not only influence hair growth in the fetus, they relax the lower esophageal sphincter.

In some fetuses, the esophagus does not completely develop. In many of these cases, the upper end of the esophagus connects to the windpipe and does not join up with the lower portion of the esophagus. This condition, called esophageal atresia, must be corrected by surgery. Among the dangers, the infant's saliva, lacking a proper outlet, may flow into the lungs and cause pneumonia. In other cases, the stomach may be connected to the windpipe, with similarly bad results if not treated.

In esophageal atresia, the gap between upper and lower esophagus may be too great to correct immediately. In such cases, the upper esophagus can be stretched over a few weeks with the pressure from repeatedly inserting a tube and pressing down. Doctors have also used intestinal grafts to achieve the same end.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.