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Introduction to the digestive tract

The digestive tract includes the mouth, esophagus, stomach and the small and large intestines. Many of us are familiar with the role of the mouth and stomach, but did you know that the intestines are highly specialized to help us absorb nutrients, and even play a role in keeping us healthy?

The small intestine is the longest part of the digestive tract, being about 10 ft (3 m) long.1 The main role of the small intestine is to absorb nutrients from your food. To help with this, on the surface of cells of the small intestine there are hair-like projections called villi and micro-villi. These increase the surface area of the small intestine to approximately 320 sq. ft (30 m2), which means that there is a large area for absorption to take place.1

Despite its name, the large intestine, or colon, is shorter than the small intestine at approximately 5 ft (1.5 m), but it is wider in diameter.1 The colon is responsible for water retention and is also where the bacteria that live in your gut ferment any nutrients that are still undigested by the time they reach this portion of the digestive tract.

The whole of the intestine is lined by a single layer of cells called the intestinal epithelium. This forms a barrier between the contents of the intestines and the rest of the body. This helps to keep harmful molecules or intestinal bacteria confined to the gut. Intestinal epithelial cells are much more than just a barrier, though, they also play a role in the absorption of nutrients.

On top of the epithelial cells, next to the inside of the intestine, is a constantly renewed layer of mucus that keeps intestinal bacteria away from the epithelium, but at the same time allows nutrient to be absorbed.2,3

Holding the epithelial cells in place is a layer of connective tissue, called the lamina propria. The lamina propria contains a large number of immune cells.4 In fact, as much as 70% of all immune cells in the body are located in the lamina propria.5

Given how specialized the sections of the digestive tract are, it is perhaps not surprising that different probiotics work in different ways in different parts of the gastrointestinal tract.

The article is provided for informational purposes regarding probiotics and is not meant to suggest that any substance referenced in the article is intended to diagnose, cure, mitigate, treat, or prevent any disease.


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Microbiome dysbiosis

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Surviving the stomach

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Mucus Layer

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Immune system

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Intestinal barrier

Reference list

  1. Helander HF, Fändriks L.Surface area of the digestive tract - revisited. Scand J Gastroenterol. 2014;49(6):681–9. (Pubmed)
  2. Johansson MEV.Fast renewal of the distal colonic mucus layers by the surface goblet cells as measured by in vivo labeling of mucin glycoproteins. PLoS One. 2012;7(7): e41009. (Pubmed)
  3. Wells JM, et al.Homeostasis of the gut barrier and potential biomarkers. Am J Physiol Gastrointest Liver Physiol. 2017;312: G171–G193. (Pubmed)
  4. Shi N, et al.Interaction between the gut microbiome and mucosal immune system. Mil Med Res. 2017;4(1):1–7.(Pubmed)
  5. Vighi G, et al.Allergy and the gastrointestinal system. Clin Exp Immunol. 2008;153(SUPPL. 1):3–6. (Pubmed)