Secretin is a peptide hormone that plays a significant part in the digestive process, acting as a chemical messenger released into the bloodstream to coordinate organ function. This hormonal signal is integral to regulating the chemical environment within the upper small intestine, primarily by influencing pH levels.
Where Secretin is Produced and Released
The source of secretin is a group of specialized cells known as S cells, located in the lining of the small intestine, specifically within the duodenum and the upper jejunum. These S cells act as chemical sensors, constantly monitoring the acidity of the contents entering the intestine. The primary trigger for secretin release is the arrival of highly acidic, partially digested food, called chyme, from the stomach.
When the pH in the duodenum drops below approximately 4.5, S cells become activated and promptly secrete secretin directly into the bloodstream. This mechanism ensures the small intestine has a rapid response to protect itself from the corrosive nature of the stomach’s hydrochloric acid. The speed and volume of this hormonal signal are proportional to the amount of acid that has entered the small intestine.
The Essential Function: Stimulating Bicarbonate Secretion
The primary function of secretin is to stimulate the pancreas to release fluid rich in bicarbonate. Bicarbonate is a powerful base that acts as a natural antacid, neutralizing the incoming acidic chyme. This neutralization raises the pH of the intestinal contents to a neutral range, typically between 6 and 8.
This action is crucial because the delicate lining of the duodenum is vulnerable to acid damage, which can lead to ulceration. Furthermore, pancreatic digestive enzymes, such as amylase, lipase, and proteases, function optimally only within this neutral pH range, allowing for the continued breakdown of nutrients.
Secretin achieves this by binding to specific receptors on the ductal cells of the pancreas. This binding increases the secretion of bicarbonate and water into the pancreatic duct. A similar action occurs in the liver, where secretin stimulates bile duct cells (cholangiocytes) to secrete bicarbonate into the bile. This combined effort from the pancreas and liver quickly buffers the acid, thereby enabling efficient and safe nutrient digestion.
Secondary Digestive System Regulation
Beyond its primary role in acid neutralization, secretin has several other regulatory effects throughout the digestive tract. It acts as a brake on the stomach, slowing the rate at which acidic chyme is released into the small intestine. This is achieved by inhibiting gastric acid secretion from the parietal cells and reducing gastric motility (muscular contractions that propel food).
By slowing gastric emptying, secretin provides the duodenum with sufficient time to process and neutralize the acid, preventing an overwhelming surge. Secretin also promotes the normal growth and upkeep of the exocrine pancreas, exhibiting trophic effects.
Another element is that secretin influences bile flow from the liver, increasing the volume of bile produced, a process known as choleresis. This increase supports fat digestion and absorption in the small intestine, working in concert with other hormones like cholecystokinin. These secondary actions demonstrate secretin’s comprehensive role in coordinating the entire upper digestive process.
Uses in Clinical Diagnostics
The unique physiological action of secretin has made it a valuable tool in medical diagnostics, most notably through the Secretin Stimulation Test. This test is primarily used to diagnose Zollinger-Ellison syndrome (ZES), a rare condition caused by gastrin-producing tumors (gastrinomas).
In a healthy person, an injection of secretin inhibits the release of the hormone gastrin. However, in individuals with ZES, the tumor cells paradoxically release a massive surge of gastrin into the bloodstream. A rise in serum gastrin levels above 200 pg/mL is considered diagnostic for ZES.
The test is also used to assess the function of the exocrine pancreas, helping to diagnose conditions like chronic pancreatitis. In this application, synthetic secretin is administered, and a diminished bicarbonate response indicates pancreatic insufficiency.