SurgiSense

What is the small intestine

The small intestine is a long, folded tube where most digestion and nutrient absorption take place. In an adult, it is about 6 meters long on average, though this can vary from person to person.

What does it do?

It receives food that the stomach has partially digested, breaks it down further and, with the help of enzymes, absorbs vitamins, minerals and other nutrients, sending most into the bloodstream; while some fats are absorbed into the lymphatic system. This process provides the energy and materials the body needs to function and grow.

Afterward, the small intestine passes any remaining material to the large intestine for further processing.

Where is it located?

The small intestine sits between the stomach and the large intestine, coiled in the middle of the abdomen.

It is divided into three sections, each with a slightly different role in digestion:

• Duodenum
  The first part, about 25–30 cm long, receives partially digested food from the stomach and mixes it with digestive juices from the pancreas and bile from the liver and gallbladder, which help break down fats, proteins, and carbohydrates (sugars).
• Jejunum
  The middle section, roughly 2.5 meters long, is where most nutrient absorption takes place, especially sugars, amino acids and fatty acids.
• Ileum
  The final and longest part, about 3.5 meters long, absorbs vitamin B12, bile salts and any remaining nutrients before passing the contents to the large intestine.

What is it made of?

The small intestine, like the rest of the gastrointestinal (GI) tract, is made up of four main layers, each with distinct roles:

1. Mucosa (innermost layer)
   Lines the inside of the small intestine and is covered with tiny finger-like projections called villi, which increase the surface area for absorption hundreds of times!
   
   Produces mucus to lubricate the intestine and protect it from digestive juices and irritation.
   
   Produces digestive enzymes.
   
   Absorbs nutrients.
   
   Contains its own immune cells, known as gut-associated lymphoid tissue (GALT), which help defend the gut against harmful microbes and other irritants and support a healthy balance of gut bacteria (gut microbiome).

2. Submucosa
   Supportive layer of connective tissue.
   
   Contains blood vessels, lymphatic vessels and nerves.
   
   The first parts of the small bowel (mainly the duodenum) contains Brunner’s glands, that produce alkaline mucus (rich in bicarbonate) to neutralise the gastric acid.
   
3. Muscularis
   Made up of two layers of smooth muscle (longitudinal and circular). These muscles are involuntary, meaning you cannot consciously control them
   
   They contract in waves (peristalsis), not only to move food along the intestine, but also to mix it with digestive juices and gently churn it, helping nutrients be absorbed more effectively.
   
4. Serosa (outermost layer)
   Reduces friction with other organs and provides structural support.

What is its role in digestion?

The small intestine digests and absorbs nutrients from food. To do this, the small intestine mucosa produces several digestive enzymes that help break down food so nutrients can be absorbed.

• Peptidases
  Break small protein fragments into amino acids.
• Disaccharidases (maltase, lactase and sucrase)
  Convert complex sugars into simple sugars.
• Enterokinase or enteropeptidase
  Activates pancreatic enzymes to help digest proteins.
• Nucleosides and phosphatases
  Break down the DNA in the food we eat – like plants, fish and meat – into basic building blocks. Our body then absorbs these and uses them to make its own DNA and other essential molecules needed for energy, growth and overall body function.

These enzymes, together with pancreatic secretions, ensure that the proteins, fats, sugar and nucleic acids in our food are broken down into smaller molecules for easier absorption.

• Proteins => amino acids
• Fats => fatty acids and glycerol
  Long-chain fats, such as those found in meat, fish, nuts and seeds, mostly enter the lymphatic system first; while short and medium-chain fats (mainly in dairy and coconut oil) enter the bloodstream.
• Carbohydrates => simple sugars (monosaccharides)
• Nucleic acids (DNA and RNA) => nucleotides*

* A nucleotide is a basic building block of DNA and RNA, made of a sugar, a phosphate group and a nitrogen-containing base. These units link together like beads on a string to form DNA and RNA, the molecules that carry genetic information.