Which gastrointestinal organs are involved in digestion?

The gastrointestinal (GI) tract is a tube that extends from mouth to anus and is approximately 9 m in length. It consists of the mouth, oesophagus, stomach, small intestine and large intestine. In addition, there are a number of accessory organs of digestion:

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  The teeth and tongue are involved in the initial mixing of food with saliva.

  
  
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  The salivary glands, liver, gallbladder and pancreas secrete substances involved in the chemical and enzymatic breakdown of food.

How is the small intestine anatomically and histologically arranged?

The small intestine is divided into three parts:

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  Duodenum;

  
  
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  Jejunum;

  
  
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  Ileum.

Most dietary nutrients are absorbed in the jejunum, but some are absorbed at other sites:

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  Vitamin B₁₂ and bile salts are absorbed in the terminal ileum.

  
  
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  Iron is absorbed in the duodenum.

  
  
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  Dietary fat and water are absorbed throughout the small intestine.

The small intestine is made up of four layers:

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  The adventitia, the outermost layer, is composed of loose connective tissue.

  
  
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  The muscularis externa consists of an outer layer of longitudinal smooth muscle and an inner layer of circular smooth muscle. Peristalsis results from coordinated contraction of the smooth muscle. The myenteric plexus, part of the enteric nervous system, lies between the muscle layers, where it coordinates smooth muscle contraction.

  
  
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  The submucosa contains nerve cells making up Meissner’s plexus (a secondary enteric nervous system plexus), blood vessels, lymphatic vessels and elastic connective tissue.

  
  
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  The mucosa, the innermost layer, is divided into (from outermost to innermost):

  
  
  
  
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     – Muscularis mucosae, a layer of smooth muscle that provides continuous agitation of the mucosa, increasing contact with the luminal contents and preventing their adherence.

     
     
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     – Lamina propria, which contains blood vessels and collections of immune cells. In the ileum, the immune cells are organised into lymphoid nodules called Peyer’s patches.

     
     
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     – Epithelium: the absorptive cells of the intestine are called enterocytes.

  
  

Although the length of the small intestine is only 7 m, the absorptive surface area of the small intestine is enormous: over 250 m². The absorptive surface area is increased as a result of:

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  Valvulae conniventes, mucosal folds that project into the lumen of the small intestine.

  
  
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  Villi, tiny finger-like projections of the intestinal wall. In between the intestinal villi are goblet cells, which secrete mucus, and intestinal crypts, which secrete the brush border enzymes and contain stem cells.

  
  
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  Microvilli, microscopic projections on top of the villi. The fuzzy microscopic appearance of the microvilli superimposed on the intestinal villi gives rise to the name brush border.

Each villus has three vessels:

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  A single arteriole. This gives rise to a capillary network at the tip of the villus.

  
  
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  A single venule. The capillary network drains into a single venule, which returns blood to the liver through the portal vein.

  
  
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  A single lacteal. Each villus also has a single lymphatic capillary called a lacteal, which transports absorbed dietary fats as chylomicrons to the thoracic duct.

What are the three main classes of dietary nutrients?

The three main dietary nutrients are carbohydrates, amino acids and fats. Each is broken down and absorbed very differently.

How are carbohydrates digested and absorbed?

Dietary carbohydrate polymers (e.g. starch) must be broken down into their constituent monosaccharides before they can be absorbed:

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  In the mouth. Salivary amylase breaks down complex carbohydrates into smaller carbohydrate polymers and monosaccharides.

  
  
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  In the duodenum. Pancreatic amylase continues the breakdown of complex carbohydrates.

  
  
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  At the brush border. Specific brush border enzymes (e.g. sucrase, maltase and lactase) hydrolyse the smaller carbohydrate polymers into their constituent monosaccharides. For example, sucrase hydrolyses the disaccharide sucrose into glucose and fructose. The brush border enzymes are integral membrane proteins attached to the villi. Individuals with brush border enzyme deficiencies cannot digest certain carbohydrates. For example, lactose intolerance is caused by brush border lactase enzyme deficiency. Some carbohydrates, such as cellulose, pass through the GI tract unchanged, as humans do not have the brush border enzymes necessary for hydrolysis.

  
  
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  At the enterocyte, monosaccharides are absorbed:

  
  
  
  
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     – Glucose and galactose can only be absorbed by secondary active transport through an Na⁺ co-transporter. This co-transporter (called the sodium-glucose linked transporter, SGLT1) requires a low enterocyte intracellular Na⁺ concentration, generated as a result of basolateral Na⁺/K⁺-ATPase pump activity.

     
     
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     – Fructose is absorbed by facilitated diffusion, not by Na⁺ co-transport.

     
     
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     – Pentose sugars are absorbed by simple diffusion.

  
  
  
  
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  Within the enterocyte. Once absorbed into the enterocyte, glucose and galactose travel down their concentration gradients. They pass through the basolateral membrane and into the capillary by facilitated diffusion (via the transmembrane glucose transporter, GLUT-2). As monosaccharides are osmotically active, their absorption across the enterocyte also results in the absorption of water.

How are proteins digested and absorbed?

Ingested proteins must be broken down into single amino acids, dipeptides and tripeptides before they can be absorbed:

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  In the stomach, protein digestion begins. The proenzyme pepsinogen is released by chief cells in the stomach. The low-pH environment then converts pepsinogen into pepsin. Pepsin cleaves the peptide bonds of dietary protein, resulting in shorter polypeptides.

  
  
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  In the duodenum, protein digestion continues. The two most important peptidases are trypsin and chymotrypsin, both of pancreatic origin. Polypeptides are progressively cleaved by these powerful peptidases, resulting in dipeptides and tripeptides (but not single amino acids).

  
  
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  At the brush border, peptidases cleave dipeptides and tripeptides into single amino acids. Again, these brush border enzymes are integral membrane proteins attached to the villi.

  
  
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  At the enterocyte, single amino acids are absorbed in a similar way to glucose, through Na⁺-linked co-transport. There are different co-transporters for neutral, basic and acidic amino acids. Short peptides (two or three amino acids in length) are also absorbed by secondary active transport using an H⁺-linked co-transport system.

  
  
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  Inside the enterocyte, these short peptides are broken down into single amino acids, which then exit the enterocyte by facilitated diffusion across the basolateral membrane. Amino acids are osmotically active: as they are transported from the gut to the capillary, water molecules are also absorbed.