The pyloric sphincter, between the antrum of the stomach and the duodenum of the small intestine, regulates the passage of chyme from the stomach to the small intestine. The antrum has rhythmic contractions that force chyme against the pyloric sphincter, allowing approximately five milliliters to flow through every contraction; thus, the pyloric sphincter allows for the gradual but continual digestion of gastric contents.

The upper and lower esophageal sphincters are involved in the process of swallowing, while the ileocolic sphincter joins the small intestine and large intestine.

The small intestine is composed of the duodenum, jejunum, and ileum. The duodenum receives chyme from the stomach and continues to breakdown food particles with additional enzymes from the pancreas. The jejunum is responsible for absorbing most micro- and macronutrients, and the ileum is responsible for absorption of bile salts, vitamin B12/IF, and ascorbic acid. The colon is a part of the large intestine and is primarily involved in water reabsorption.

The correct answer is:

This question tests the basic principles of what happens to the acidity of food throughout the digestive tract. In this example, the food began with a pH of 7.0 when the subject ate it. The question then tests your knowledge of what happens to the pH of food after digestion in the stomach and small intestine.

The stomach acidifies food, as parietal cells produce H+ that enters in the stomach. Therefore, we would expect the pH of the food to be lower than when it was first consumed. In this case, 3.0 is the most appropriate listed pH for post-stomach, pre-small intestine digestion.

When food enters the small intestine from the stomach, the small intestine alkalinizes the acidic stomach contents, causing the pH of the food to rise above what it was in the stomach. In this case, if it is leaving the stomach with a pH of 3.0, a pH of 6.0 after small intestinal digestion is reasonable.

None of the other answer choices accurately reflect that food is acidified in the stomach and alkalinized in the small intestine.

The ileum is primarily responsible for absorbing ascorbic acid, vitamin B12 complexed to intrinsic factor (IF), and bile salts complexed with fats ingested as part of a meal. Iron is primarily absorbed in the jejunum of the small intestine.

A low pH in the stomach is necessary in order to activate pepsinogen, the zymogen form of pepsin. Pepsin is necessary for breaking down proteins into smaller polypeptides. In a stomach with a higher pH (less acidic), pepsin production will be decreased, and proteins will not be digested as well.

D cells in the antrum of the stomach are neuroendocrine cells that secrete somatostatin, a neuropeptide that increases stomach pH by decreasing gastric acid secretion from fundal parietal cells. In a negative feedback mechanism, when the stomach pH falls too low, hydrogen ions (H+) will stimulate D cells to secrete somatostatin. This somatostatin blocks histamine release from enterochromaffin-like (ECL) cells in the stomach, preventing the stimulation of parietal cells to release hydrochloric acid.

Histamine is released by ECL cells, stimulating acid secretion from parietal cells. Gastrin is released by G cells, also stimulating acid secretion from parietal cells. Ghrelin is a neurohormone that stimulates hunger.

A layer of mucous forms between the epithelial cells of the stomach and the acid within the stomach. This mucous is secreted by mucous cells lining the stomach. When the mucous layer is broken down, certain complications can take place (e.g. stomach ulcers).

The patient should be most concerned with nutrient absorption. Microvilli provide additional surface area to the small intestine, allowing for nutrient absorption. If the lining of the intestine were completely smooth, nutrients would pass too quickly through the digestive system. Enzyme secretion is primarily provided by the liver and pancreas.

Microvilli increase the surface area in the small intestine, and thus allow a greater amount of absorption of nutrients once digestion occurs. They do not actually participate in digestion, nor are they linked to intestinal cancer.

The small intestine is the location of where the majority of enzymatic digestion occurs because enzymes from the pancreas are incorporated into the digestive process at this point. It is important to begin the enzymatic breakdown of food as early as possible to maximize nutrient absorption through the intestinal system.

The large intestine is responsible for water absorption and waste removal. The mouth begins enzymatic digestion with salivary amylase, and the stomach introduces pepsin, but these do not constitute the majority of enzymatic activity in the digestive tract.