Stomach and Small Intestine - MCAT Biological and Biochemical Foundations of Living Systems

Digestion in the mouth involves the mechanical chewing of food and the action of salivary amylase, which breaks down starch. Proteins do not start getting digested until they are in the stomach, where pepsin breaks them down. Fats and carbohydrates are broken down in the small intestne by lipases and other enzymes like lactase.

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.

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.

The lower esophageal sphincter separates the esophagus from the stomach. When stomach acid and contents leak through the lower esophageal sphincter, the acid irritates the esophagus and can lead to pain around the area of the heart. The heart is not actually affected.

The duodenum is the first part of the small intestine. This is where the acidic chyme from the stomach mixes with pancreatic secretions and bile. The pancreatic secretions and bile contain bicarbonate, which helps to neutralize the acidic chyme.

The small intestine is before the large intestine. It consists of the duodenum, followed by the jejunum, then ileum. The large intestine comes directly after the small intestine and constists of the cecum, followed by the colon, then rectum.

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 pyloric sphincter is found between the stomach and the duodenum of the small intestine. The other 3 sphincters are found in different parts of the body.

Microvilli surround villa which line the small intestine. These structures greatly increase the surface area (and therefore nutrient absorption capability) 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.

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.

Food enters the stomach from the esophagus through the cardiac sphincter.

The pyloric sphincter connects the stomach to the duodenum. The ileocecal sphincter connects the ileum to the large intestine, while the internal (anal) sphincter controls the exit of fecal matter from the body. There is also an external anal sphincter, which is under voluntary control, while the internal is involuntary.

The majority of digestion in the small intestine (and the body in general) occurs in the duodenum, the first section of the small intestine, after passing through the stomach. The digestive enzymes secreted by the pancreas enter the digestive tract in the duodenum.

The next two sites, the jejunum and ileum, are sites of absorption. The cecum is part of the large intestine, not the small intestine.

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.

Iron, usually ingested in the Fe3+ form (ferric iron), must be reduced to Fe2+ (ferrous form) in order to be absorbed in the duodenum of the small intestine. The iron undergoes an oxidation-reduction reaction in the stomach, facilitated by the low pH of the stomach lumen.

Calcium is absorbed in the jejunum of the small intestine. Iron is absorbed in the duodenum, calcium in the jejunum, and vitamin B12 (complexed with intrinsic factor) is absorbed in the ileum.

The villi, characteristic of the small intestine, serve to increase the surface area over which nutrients can be absorbed and allow for the appropriate uptake of micro- and macronutrients. While other parts of the gastrointestinal system contains pits and folds that allow for the secretion of digestive enzymes, only the high surface area of the small intestine allows for the necessary uptake of nutrients.

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.