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Overview of Digestion & GI Secretions

Overview of Digestion & GI Secretions

Here we will learn an overview of digestion and gastrointestinal secretions.

Digestion is the process of breaking down food particles into their chemical building blocks so that they can be absorbed through the gastrointestinal tract into the blood.

In this tutorial, we'll learn about the enzymes and acids that are secreted to aid in digestion; we'll also highlight segments where mucus secretion is significant.

Digestive System Overview

Food enters the GI tract via the oral cavity (the mouth), which receives saliva from the nearby salivary glands.

Chewing (aka, masticating) uses the teeth and tongue to mechanically break up the food and mix it with saliva.

Upon swallowing, food is pushed into the pharynx (the throat) and then through the esophagus.

From the esophagus, food enters the stomach, where churning converts food to liquid chyme.

From the stomach, this chyme passes through the small and large intestines; most nutrient absorption occurs in the small intestine.

The large intestine concludes with the rectum and anus.

Featured Accessory Organs

The liver creates bile.
The gallbladder stores bile.
The pancreas creates pancreatic enzymes.

Digestion & GI Secretions

Now, let's track the breakdown of a food bolus (for instance, a bite of turkey sandwich) into the four macromolecules: carbohydrates, proteins, nucleic acids, and fats

Carbohydrate digestion begins in the oral cavity; the salivary glands secrete saliva, which itself comprises amylase, mucus, and lysozyme.

When we chew a food bolus, salivary amylase breaks down large polysaccharides into smaller polysaccharides and maltose (a disaccharide).

The esophagus secretes mucus, which lubricates the passage of food to the stomach; no digestion occurs in the esophagus.

In the stomach, gastric juice is active; it comprises hydrochloric acid, pepsin, and mucus.

Pepsin breaks down proteins into smaller polypeptides. Review gastric mixing and secretions.

The small intestine is a major site of digestion and absorption.

Major secretions and products: bile, pancreatic enzymes, and brush border enzymes.

Pancreatic enzymes include: Amylase, maltase, trypsin, chymotrypsin, carboxypeptidase, lipase and nuclease, all of which digest substrates within the duodenum (the first segment of the small intestine).

Pancreatic amylase breaks down polysaccharides into maltose and disaccharides (similar to salivary amylase).

Trypsin and chymotrypsin break polypeptides into smaller polypeptides.

Then, these smaller polypeptides plus those received directly from the stomach are broken down by carboxypeptidase.

Brush border enzymes are created within the small intestine, and include: Disaccharidases and aminopeptidases.

Specifically, brush border enzymes include lactase, sucrase, isomaltase, and glucoamylase.

Brush border disaccharidases breakdown disaccharides into monosaccharide.

Brush border aminopeptidases digest polypeptides.

Nucleases break down DNA and RNA into their component nucleotides; they are further digested into nitrogenous bases, sugars, and phosphates.

Fat globules are digested by bile salts and lipase to form glycerol and fatty acids; we will describe the emulsification and digestion of fats elsewhere.

The large intestine secretes mucus. Although minimal digestion occurs in this segment, it is important to note that enteric bacteria do digest some food residues, which creates gas, and they are also instrumental in producing Vitamins K and B.

Overview of Digestion & GI Secretions

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