Brunner’s gland in the duodenum secretes:
**Core Concept**
Brunner's glands in the duodenum are specialized mucous glands that play a crucial role in protecting the duodenal mucosa from the acidic chyme entering from the stomach. These glands secrete a bicarbonate-rich fluid to neutralize the acidity and create a favorable environment for pancreatic enzyme activity.
**Why the Correct Answer is Right**
The correct answer is a bicarbonate-rich fluid. Brunner's glands contain mucous cells that secrete a bicarbonate-rich mucus, which is essential for neutralizing the acidic chyme entering from the stomach. This bicarbonate-rich fluid helps to maintain an optimal pH environment in the duodenum, allowing for proper pancreatic enzyme activity. The bicarbonate ions (HCO3-) are produced by the enzyme carbonic anhydrase, which catalyzes the reaction between carbon dioxide and water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
**Why Each Wrong Option is Incorrect**
**Option A:** Mucus. While Brunner's glands do secrete mucus, it is not the primary component of their secretion. The mucus secreted by Brunner's glands is rich in bicarbonate ions, which is essential for neutralizing acidity.
**Option B:** Pancreatic juice. Pancreatic juice is secreted by the pancreas and contains digestive enzymes, but it is not secreted by Brunner's glands.
**Option C:** Gastric juice. Gastric juice is secreted by the stomach and contains digestive enzymes and mucus, but it is not secreted by Brunner's glands.
**Clinical Pearl / High-Yield Fact**
Brunner's glands are unique in that they are the only mucous glands in the gastrointestinal tract that secrete a bicarbonate-rich fluid. This adaptation is essential for protecting the duodenal mucosa from the acidic chyme entering from the stomach.
**Correct Answer: D. Bicarbonate-rich fluid.**