Chapter 24

7 – The Small Intestine

The Small Intestine

• The small intestine is a tube of the GI tract that begins at the pyloric sphincter of the stomach, and ends at the large intestine.

The Small Intestine

• The small intestine is divided into three regions:

1. The duodenum

2. The jejunum

3. The ileum

The Small Intestine

1. The duodenum is the first part of the small intestine between the stomach and the jejunum.

The Small Intestine

2. The jejunum is the middle part of the small intestine between duodenum and the ileum.

The Small Intestine

3. The ileum is the terminal part of the small intestine between the jejunum and the large intestine.

The Small Intestine

• **Recall – the four layers of the GI tract are: the mucosa, the submucosa, the muscularis, and the serosa.

The Small Intestine

• **Special structural features of the small intestinal wall facilitate the process of digestion and absorption.

The Small Intestine

• The special structural features of the small intestinal wall include:

1. Circular folds

2. Villi

3. Microvilli

The Small Intestine

1. Circular folds are folds of the mucosa and the submucosa that enhance absorption by increasing surface area and causing the chyme to spiral.

The Small Intestine

2. Villi are fingerlike projections of the mucosa that increase the surface area available for absorption and digestion.

The Small Intestine

3. Microvilli are microscopic fingerlike projections of the mucosa that increase digestive surface area and contain several “brush-border” enzymes that have digestive functions.

The Small Intestine

• **Digestion in the small intestine is both mechanical and chemical.

The Small Intestine

• The two types of mechanical movements of the small intestine are:

1. Segmentation

2. Migrating motility complex

The Small Intestine

1. Segmentation is the localized mixing and contracting of the small intestine that mixes chyme with digestive juices and brings the particles of food in contact with the mucosa for absorption.

The Small Intestine

– Sequences of segmentations cause chyme to slosh back and forth, they do not push the intestinal contents along the GI tract.

The Small Intestine

2. The migrating motility complex is a type of peristalsis that occurs in the small intestine that pushes chyme forward.

The Small Intestine

• **Recall – in the mouth, salivary amylase begins the breakdown of starch; in the stomach pepsin begins the break down of proteins, & lingual and gastric lipases begin the breakdown of triglycerides.

The Small Intestine

– Thus, chyme entering the small intestine contains partially digested carbohydrates, proteins and lipids.

The Small Intestine

• **Chemical digestion in the small intestine completes the digestion of carbohydrates, proteins, and lipids and is a collective effort of pancreatic juice, bile and intestinal juice in the small intestine.

The Small Intestine

• **Recall – ‘salivary amylase’ is an enzyme secreted by the salivary glands that initiates the breakdown of starch in the mouth.

The Small Intestine

• Pancreatic amylase secreted from pancreatic acinar cells continues the digestion of carbohydrates into disaccharides.

The Small Intestine

• **Carbohydrate digestion is completed in the small intestine by three brush-border enzymes secreted by microvilli:

1. Sucrase

2. Lactase

3. Maltase

The Small Intestine

1. Sucrase breaks down sucrose

2. Lactase breaks down lactose

3. Maltase breaks down maltose

The Small Intestine

• **Recall – ‘pepsin’ is an enzyme secreted by the chief cells of the stomach that starts fragmenting proteins into peptides in the stomach.

The Small Intestine

• Trypsin and other enzymes secreted from pancreatic acinar cells continue the digestion of proteins into peptides in the small intestine.

The Small Intestine

• **Protein digestion is completed in the small intestine by two brush-border pepidases secreted by micorvilli:

1. Aminopeptidase

2. Dipeptidase

The Small Intestine

1. Aminopeptidase is an enzyme secreted by microvilli that cleaves off the amino acids at the amino end of a peptide.

The Small Intestine

2. Dipeptidase is an enzyme secreted by microvilli that splits dipetides into single amino acids.

The Small Intestine

• **Recall – ‘lingual lipase & gastric lipase’ are enzymes secreted by the tongue and the chief cells of the stomach that start the digestion of triglycerides in the stomach.

The Small Intestine

• Pancreatic lipase secreted from pancreatic acinar cells completes the digestion of triglycerides in the small intestine.

The Small Intestine

• **Recall – ‘bile’ is a secretion of the liver that emulsifies lipids in the small intestine prior to their digestion.

The Small Intestine

• **Emulsification must happen before triglycerides are broken down by pancreatic lipase into fatty acids and monoglycerides.

The Small Intestine

• **Recall – ‘pancreatic enzymes’ also include “nucleases” nucleic acid digesting enzymes.

The Small Intestine

• The two nucleases contained in pancreatic juice are:

1. Ribonuclease

2. Deoxyribonuclease

The Small Intestine

1. Ribonuclease is an enzyme secreted from pancreatic acinar cells that digests RNA in the small intestine.

The Small Intestine

2. Deoxyribonuclease is an enzyme secreted from pancreatic acinar that digests DNA in the small intestine.

The Small Intestine

• **All the chemical and mechanical phases of digestion from the mouth through the small intestine are directed at changing food into forms that can pass through the absorptive cells lining the mucosa.

The Small Intestine

• About 90% of all absorption of nutrients occurs in the small intestine; the other 10% occurs in the stomach and large intestine; any unabsorbed materials left in the small intestine passes on to the large intestine.

Homework

• Finish handout:

– “The Small Intestine”

• Study for mini-quiz