Gastrointestinal pH

pH ranges from 0 (acidic) to 7 (neutral) to 14 (basic or alkaline). pH is important in discussing reflux because it is the primary measure of acid, whether normal or pathologic, aiding in the diagnosis of GERD.

Esophagus (pH 5-7)
The esophagus is divided anatomically, functionally, and in terms of pH into proximal/upper and distal/lower (Figure 1).

Proximal:
The pH of the upper or proximal esophagus should be more than 5 and is often 5 to 7. The amount of time that the esophageal pH measures less than 5 is a standard for diagnosing reflux problems when looking at the proximal esophagus. A pH greater than 5 must be maintained to allow the healing of damage from reflux in the proximal esophagus.

Distal:
The pH of the lower or distal esophagus should be more than 4 and is often 5 to 7. The amount of time that the esophageal pH measures less than 4 is a standard for diagnosing reflux problems, when looking at the distal esophagus. A pH greater than 4 must be maintained to allow the healing of damage from reflux in the distal esophagus.

Stomach (pH 2-5)
The pH of the stomach varies throughout the day, depending on the amount of food it holds (Figure 1). Overall, it is rather acidic because of the parietal cells pumping out stomach acid (hydrochloric acid, HCl), which has a pH as low as 0.8. On an empty stomach, the pH can be as low as 1, rising to close to 5 after a full meal.

Gastric pH drops especially while we sleep, without food to help neutralize the acid. This event is termed nocturnal acidity and it can lead to nighttime gastric acid breakthrough (NAB). The combination of high acidity and lying flat while sleeping gives acid reflux the perfect opportunity to cause damage to the esophagus and airways. Additionally, symptoms such as night time awakening, coughing fits, and choking may occur due to the acid.

Small intestine (pH 6.8)
The duodenum (first 10-12 inches of the small intestine) is significantly less acidic than the stomach (Figure 1).
This difference is relevant to the way that enteric-coated drugs are made to work. The coating dissolves at the more neutral pH, releasing the drug, allowing it to be absorbed into the bloodstream through the intestinal wall.

 

The esophagus, stomach, and small intestine each have their own mechanisms for protection against the dangerously low pH of stomach acid, whether keeping acid away from sensitive tissues or neutralizing acid already present.

Esophagus

	Peristalsis clears the esophagus by pushing stomach contents back into the stomach; gravity also aids this process.			Layers of cells called stratified squamous epithelium line the esophagus and keep acid from seeping in deep enough to do serious damage.
	Saliva naturally contains bicarbonate, a chemical that helps to neutralize acid in the digestive tract.			Mucous glands in the wall of the esophagus secrete a protective lubricant.

Stomach

	Mucus-secreting cells coat the entire surface of the stomach with a thick layer of mucus that has buffering, lubricating and antibacterial properties.			Tight junctions (arrow) between the epithelial cells act as a seal to prevent acid from seeping between the cells and into the stomach lining.
	A high turnover rate to replace damaged cells allows surface mucous cells to renew themselves every 3-5 days.			The parietal cell’s membrane is highly impermeable to acid, protecting the cell from acid degradation.

Small intestine

Pancreatic juices containing bicarbonate are secreted into the small intestine. This neutralizes much of the acid leaving the stomach.

Glands in the wall of the duodenum (first segment of the small intestine) also secrete bicarbonate.

 

For more on stomach acid, go to

• Acid Production
• Complications Associated with Acid Reflux
• Disorders: What Are GERD & EER?