The Small Intestine

By Alex & Bella

Where is the Small Intestine Located?

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Figure 1: Location of the Small Intestine, General Surgery, 2014
The Small Intestine is located below the stomach and the liver and is surrounded by the large intestine (colon).

Function of the Small Intestine

90% of digestion and absorption takes place in the small intestine. The main function is to absorb the nutrients from our food and drink into the blood stream to be transported to the cells to be used for energy (...). The small intestine is divided into three parts, the duodenum, jejunum and the ileum (Wikimedia Foundation, 2014).

The Macromolecules

  • All four macromolecules carbohydrates, lipids, proteins and nucleic acid are digested in the small intestine.
  • These nutrients are then absorbed into the blood stream to be transported around the body to be used for various functions.
The breakdown of carbohydrates into glucose molecules begins immediately when the food enters the mouth due to the digestive enzyme salivary (Boundless, unknown). These food molecules continue to breakdown when they reach the stomach where acidic gastric juices are secreted. They continue through the digestive system to the small intestine where they are digested by the enzyme pancreatic amylase and absorbed into the blood steam to be used for energy or stored as glucose in the muscles (Unknown, 2001).
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Figure 2: A simple diagram of carbohydrate digestion and where glucose molecules end up after absorption from the small intestine, Competition Affairs, 2014
The pancreas and small intestine secrete enzymes that break down proteins, the enzymes peptidase are secreated by the stomach and trypsin and chymotrypsin, are secreted by the pancreas (Boundless, unknown).
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This figure shows where the pancreas is compared to the small intestine. It is joint, therefore making the digestive enzymes easy to transport into the small intestine from the pancreas.

Figure 3: Wake Gastroenterology, 2014

The digestion of lipids in the small intestine is different from the other macromolecules. Bile, which is secreted from the gall bladder, covers the droplets of fat. This separates them from each other creating a larger surface area, allowing the enzymes to break down the fats quicker into fatty acids.
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Figure 4: Absorption of Lipids by Cells in the Small Intestine, Unknown, n.d.
Nucleic acid is digested in the small intestine by pancreatic enzymes in the duodenum. These digested molecules are then absorbed into the blood stream in the ileum (Biology Mad, 2004).

Parts of the Small Intestine


This is the first part of the small intestine and it is 25 cm long and 5 cm wide. The function of this is to absorb iron. It is also where the majority of chemical digestion takes place.


The lining of the jejunum is specialised for the absorption of small nutrient particles which have previously been digested by enzymes in the duodenum.


The function of this is to absorb vitamin B 12 and bile salt, as well as any products of digestion that were not previously absorbed. This is where the villi are located.

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Figure 5: The three regions of the small intestine are the duodenum, jejunum, and ileum, OpenStax CNX, n.d.

Mechanical & Chemical Digestion

Mechanical Digestion

Mechanical digestion involves the physical digestion of food. This process occurs before the nutrients in the food reach the small intestine and therefore does not occur in this organ.

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Figure 6: Mechanical Digestion, Human Anatomy and Physiology, 2014

Chemical Digestion

Chemical digestion is the chemical breakdown of food into smaller parts that are able to be more easily absorbed into the blood stream, this digestion has been discussed in the macromolecule section of the flyer but here is some more information:

  • The digestive enzymes enter the small intestine in response to the cholecystokinin, which is a hormone released by the body due to nutrients entering the small intestine.

  • This cholecytokinin hormone not only causes the enzymes to enter the small intestine but also causes bicarbonate to be released, which works to neutralise any harmful acid that may be coming from the stomach.

Digestive Enzymes Present in the Small Intestine

  1. Nuclease: Any of a group of enzymes that split nucleic acids into nucleotides and other products.
  2. Protease: Any of various enzymes, including the proteinases and peptidases, that catalyze the hydrolytic breakdown of proteins.
  3. Collagenase: Any of various enzymes that catalyze the hydrolysis of collagen and gelatin.
  4. Lipase: Any of a group of lipolytic enzymes that cleave a fatty acid residue from the glycerol residue in a neutral fat or a phospholipid.
  5. Amylase: Any of a group of enzymes that catalyze the hydrolysis of starch to sugar to produce carbohydrate derivatives.
  6. Elastase: An enzyme capable of catalyzing the digestion of elastic tissue.
  7. Trypsin: A proteolytic digestive enzyme produced by the exocrine pancreas that catalyzes in the small intestine the breakdown of dietary proteins to peptones, peptides, and amino acids.
  8. Chymotrypsin: A proteolytic enzyme produced by the pancreas that catalyzes the hydrolysis of casein and gelatin.

Source (Online Medical Dictionary)

These enzymes allow the particles of food and nutrients to be broken down and consequently absorbed in to the blood stream.

What are the Villi? How do they Help with Digestion?

Villi are finger shaped and are covered in tiny hairs called microvilli.

Thousands of villi are attached to the muscular wall of the small intestine, which greatly increase the surface area to volume ratio that aids absorption. The microvilli located on the villi aid in increasing the surface area. Inside the villi there are many blood capillaries that absorb the food.


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