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Function of the Small Intestine (Digestive System)

Here is a brief description of what happens in the human small intestine.

The small intestine is the part of the gastrointestinal tract (also called the 'digestive tract' and the alimentary canal) located after the stomach and before the large intestine. It is the part of the digestive tract where approx 90% of the digestion and absorption of food occurs, the other 10% taking place in the stomach and large intestine.

The main function of the small intestine is absorption of nutrients and minerals.

That is, absorption of the nutrients and minerals in the food ingested, usually via the mouth, at an earlier stage in the digestive process.

 

Digestion in the Small Intestine

Quick re-cap re. digestion (from "Basic Stages of Digestion"):

Digestion is the process by which ingested (food) material is broken down into a form that can then be absorbed, then assimilated into the tissues of the body.
It is one of the main stages in the digestive process and takes two forms:

  • Mechanical digestion (e.g. chewing, grinding, churning, mixing), and
  • Chemical digestion (e.g. action of digestive enzymes, bile, acids, etc.).

Chemical digestion occurs in the small intestine (and, to a lesser extent, also in some other part of the gastrointestinal tract - incl. the action of saliva on food in the mouth and the actions of some chemicals secreted by cells located in the lining of the stomach).

The three main categories of nutrients that undergo digestion within the small intestine are proteins, lipids (fats) and carbohydrates.

  • Proteins
    Proteins and peptides amino acids.
    Proteolytic enzymes e.g. including trypsin and chymotrypsin, secreted by the pancreas, break proteins into smaller peptides. (Chemical breakdown begins in the stomach and continues in the large intestine.)

  • Lipids (Fats)
    Lipids (fats) fatty acids and glycerol.
    Pancreatic lipase breaks triglycerides into free fatty acids and monoglycerides.
    It is helped by bile salts secreted by the liver and the gall bladder. They attach to triglycerides, which aids access to the triglycerides by the pancreatic lipase. This is because lipase is water-soluble but the fatty triglycerides are hydrophobic so position themselves towards each other and away from the watery intestinal surroundings. The bile salts hold the triglycerides in the watery environment until the lipase can break them into the smaller parts that can enter the villi for absorption - see below.

  • Carbohydrates
    Some carbohydrates simple sugars, or monosaccharides (e.g., glucose).
    Pancreatic amylase breaks down some carbohydrates, e.g. starch into oligosaccharides.

    Other carbohydrates pass undigested into the large intestine where they may, depending on their type, be broken-down by intestinal bacteria.

 

Absorption in the Small Intestine

In order for digested material to be absorbed into the bloodstream it must first be broken-down into particles that are small enough to pass, or "be transported", across the epithelial cells of the gastrointestinal tract. (This is the result of the processes of digestion that must already have occurred before absorption is possible.)

Quick re-cap re. absorption / assimilation (from "Basic Stages of Digestion"):

Absorption is the uptake of fluids or other substances by the tissues of the body.

Digested material is absorbed into the bodily fluids (blood and lymph). Most of the absorption part of the digestive process occurs in the jejunum and the ileum of the small intestine, though alcohol is readily absorbed through the stomach.

Assimilation is the process by which broken-down parts, e.g. the chemical components of, or derived from, ingested food and beverages are taken into the cells of the body.

Digested material may be transported into blood vessels in the wall of the small intestine by the processes of simple/passive diffusion, facilitated diffusion, primary active transport, or secondary active transport (see below).

The structure of the small intestine is suited to these processes of absorption due to its very large surface area. That is best explained using a diagram of the anatomy of the small intestine. Briefly, the inside surfaces of small intestine have many folds called plicae circulares, from which project many tiny finger-like structures of tissue called villi. The individual epithelial cells also have finger-like projections, which are called known as microvilli.

The function of the 3 structures, (1) the plicae circulares, (2) the villi and (3) the microvilli is to:

increase the surface area available for the absorption of nutrients.



The following three sections are inter-related.

1. What are the molecule transport mechanisms by which different types of molecules / nutrients are absorbed from the lumen of the small intestine into the epithelial cells of the mucosa (inner-layer) of the small intestine, then from those epithelial cells into the underlying blood and lymphatic vessels in the submucosa of the small intestine ?

Note: Knowledge of the types of diffusion, osmosis, and active transport is not required for all first-level courses in human anatomy.
If included these topics may be taught in another module, i.e. not about digestion specifically but with other aspects of general science, biology or molecules and cells. Only the names of the mechanisms are stated here, see elsewhere for descriptions and further explanation.

Type of Transport

Examples of nutrients / molecules transported by this mechanism:

Passive / Simple Diffusion

  • Lipids
  • Short-chain fatty acids

Facilitated Diffusion

  • Fructose

Primary Active Transport

  • Amino acids (primary or secondary active transport with Na+, followed by diffusion through the epithelial cells of the villus)

Secondary Active Transport

  • Glucose (secondary active transport with Na+, followed by facilitated diffusion through the epithelial cells of the villus)

Thorough understanding of this part of the topic of digestion involves knowledge of the specific transport mechanisms for different types of molecules - which may move by different mechanisms within the lumen of the small intestine vs. through the epithelial tissue formed by the cells lining the villus.


2. Absorption in the small intestine of specific nutrients

Nutrients / Molecules

Absorption from the Small Intestine, then into the Blood:

Monosaccharides

Transport into the epithelial cells (of the villi): Glucose and galactose are transported by active transport. Fructose is transported by facilitated diffusion.

Transport from epithelial cells into the bloodstream is by facilitated diffusion.

Amino Acids, Dipeptides,
Tripeptides

Transport into the epithelial cells (of the villi) is generally by active transport processes - mainly in the duodenum and jejunum.

Transport from epithelial cells into the bloodstream is by passive diffusion.

Lipids (Fats)

Dietary lipids are absorbed by diffusion.

Water

Most of the water in ingested food and beverages is absorbed by osmosis.
Approx 80% is absorbed by the small intestine, 10% by the large intestine and the remaining 10% excreted in the faeces.

Electrolytes

Some electrolytes are from gastrointestinal secretions and others from ingested foodstuffs.

Vitamins

Fat soluble vitamins (A, D, E and K) are absorbed together with dietary triglycerides.
Most water-soluble vitamins (C and the B vitamins) are absorbed by diffusion.
Vitamin B12 combined with intrinsic factor (from the stomach) is absorbed by active transport.


3. Which parts of the small intestine absorb which nutrients ?
Most nutrients are absorbed in the jejunum.

Part of the Small Intestine

Nutrients absorbed - with notes

from the stomach

 

 

Pyloric sphincter

 

Duodenum

Iron (Fe) is absorbed in the duodenum.

Jejunum

Most nutrients are absorbed in the jejunum

Ileum

Vitamin B12 and bile salts are absorbed in the terminal (later part of) the ileum.

 

to the large intestine

 

 

Particles that have been absorbed into the blood vessels as described above are then transported from the small intestine via the hepatic portal vein to the liver, then on via the body's overall network of blood vessels, to organs and tissues of the body where they are used according to the functions of the organs they supply.

Material within the small intestine that has not been digested and absorbed passes into the large intestine.

See also the diagram of the structure of the small intestine.

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