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Small Intestine

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Introduction

The bulk of absorption occurs in the small intestine (SI). This is made difficult by the need to maintain a barrier against toxins and pathogens. The breakdown of nutrients into small constituents facilitates this.

The duodenum

The jejeunum

The Ileum

The terminal ileum is the site of vitamin B12/Intrinsic Factor absorption and also the site of bile salt recycling.

Digestion

Digestion is the breakdown of macromolecules to facilitate their absorption and use. Macronutrients must be digested to be absorbed.

Levels of pancreatic and gall bladder secretion must be closely matched to digestive requirements, necessitating sophisticated communication.

Chemical digestion occurs largely in the small intestine.

Pancreas is important for digesting carbs, protein, and I think some lipids.

Liver is very important for fat.

Incretins come from the small intestine

Absorption

The top two-thirds of intestinal mucosa is responsible for digestion and absorption, while the bottom is primarily involved with secretion.

absorption

illustration by Kathyrn Dorman

Carbohydrates, proteins, and lipids are primarily absorbed in the duodeum, but also along the rest of the small intestine.

Calcium, iron, and folate are also primarily absorbed in the duodenum, but calcium is also taken up along the rest of the small intestine.

Bile acids are primarily absorbed in the terminal ileum.

Vitamin B12, in association with intrinsic factor, is absorbed in the terminal ileum.

Protein Digestion

Carbohydrate Digestion

Lipid Digestion

Lipids are emulsified by bile salts and digested by pancreatic lipase in the small intestine.

Intrisinc factor is made by parietal cells in the stomach. It binds to Vitamin B12, which is cotransported in the terminal ileum. Destruction of parietal cells causes lack of intrisic factor and malabsorption of B12, leading to pernicious anemia.