Digestive System Review

Intestinal gas is primarily produced by bacterial fermentation of undigested carbohydrates in the large intestine.

Chylomicrons are large lipoprotein particles formed in enterocytes that are too large to enter blood capillaries, so they are absorbed into lacteals (lymphatic vessels).

Micelles are small aggregates of bile salts, fatty acids, and monoglycerides that transport these lipids to the intestinal surface for absorption.

H2 blockers inhibit the H2 receptors on parietal cells, which reduces the secretion of hydrochloric acid (HCl). Since HCl is required to convert pepsinogen into active pepsin, reducing acid levels leads to a decrease in this conversion.

The stomach produces proteases (pepsin) and gastric lipases, but it does not produce amylase. Salivary amylase is inactivated by the low pH of the stomach, and pancreatic amylase is produced by the pancreas, not the stomach.

The internal anal sphincter is composed of smooth muscle and is under involuntary control. The external anal sphincter is composed of skeletal muscle and is under voluntary control.

After fats are broken down and processed by enterocytes, they are reassembled into triglycerides and packaged with proteins and phospholipids to form chylomicrons. These chylomicrons are too large to enter blood capillaries and are instead absorbed into the lacteals.

Due to their large size, chylomicrons cannot pass through the basement membrane of blood capillaries. They enter the lymphatic capillaries, known as lacteals, which have larger pores.

Brush border enzymes are located on the microvilli of the small intestine. They perform the final stage of chemical digestion, breaking down small peptides and disaccharides into absorbable monomers.

Pancreatic lipase is the primary enzyme responsible for the chemical digestion of triglycerides into free fatty acids and monoglycerides. Bile salts emulsify fats, but they do not chemically digest them.

Cholecystokinin (CCK) is released by the duodenum in response to fats and proteins. It stimulates the pancreas to release digestive enzymes.

CCK causes the gallbladder to contract and the sphincter of Oddi to relax, which results in the release of bile into the duodenum to aid in fat digestion.

The intestinal phase occurs when chyme enters the duodenum. This triggers the enterogastric reflex, which inhibits gastric activity to allow the small intestine time to process the incoming chyme.

The epithelial lining of the stomach is replaced very rapidly, typically every 3 to 6 days, due to the harsh acidic environment. Three weeks is far too long.

A zymogen is an inactive precursor of an enzyme. Chymotrypsinogen is the inactive form that is converted into the active enzyme chymotrypsin.

Parietal cells secrete intrinsic factor, which is essential for the absorption of vitamin B12 in the ileum. A lack of intrinsic factor leads to pernicious anemia.

Pepsinogen is an inactive zymogen. When it encounters the highly acidic environment created by hydrochloric acid (HCl) in the stomach, it is cleaved into its active form, pepsin.

Chief cells are responsible for the secretion of pepsinogen and gastric lipase into the stomach lumen.

Parietal cells are specialized cells in the gastric glands that secrete hydrochloric acid (HCl) and intrinsic factor.

The stomach is divided into the cardia, fundus, body, and pylorus. The pyloric region is the terminal part of the stomach that connects to the duodenum of the small intestine.

Peristalsis consists of waves of smooth muscle contraction that move the bolus or chyme forward through the digestive tract.

Deglutition (swallowing) has three phases: the buccal phase (voluntary), the pharyngeal phase (involuntary), and the esophageal phase (involuntary).

Salivary amylase is denatured by the low pH of the stomach. Therefore, it ceases to function shortly after the food enters the stomach.

Salivary amylase specifically targets the glycosidic bonds in starch, which is a carbohydrate polymer.

Saliva is generally neutral, with a pH typically ranging between 6.2 and 7.6, making 7 the best approximation.

Saliva contains lingual lipase for fat digestion, lysozyme for antibacterial defense, and IgA antibodies for immune protection.

The alimentary canal (primary organs) includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine (including the colon). The pancreas, gallbladder, and salivary glands are accessory organs.

The myenteric plexus (Auerbach's plexus) is located between the circular and longitudinal muscle layers and primarily controls gastrointestinal motility, including peristalsis and segmentation.

The mucosa is the innermost layer of the gastrointestinal tract that lines the lumen. It consists of an epithelial lining, a lamina propria, and a thin layer of smooth muscle.

Absorption is the process where digested nutrients move from the lumen of the GI tract into the blood or lymph. This is the point at which nutrients officially enter the body's internal environment.