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Organs of the alimentary canal

The mouth takes up food (ingestion) and is the first place where food is broken down through physical and chemical digestion.

Physical digestion occurs with the use of the teeth and tongue to break up large food.

The salivary glands release saliva (spit) into the mouth to start chemical digestion. Saliva contains the enzyme amylase to break down starch into smaller sugars (mostly maltose).

The oesophagus (or esophagus) allows food to move from the mouth to the stomach. Food in the oesophagus forms a bolus; a mucus-covered food ball.

No digestion takes place during this stage.

The digestive process begins in the mouth
The digestive process begins in the mouth

Peristalsis is a wave-like movement of the walls of the alimentary canal that helps to move food through the canal. This also contributes to physical digestion.

Food is pushed down the oesophagus in only 4-5 seconds!

There are two types of muscle used to produce peristaltic movement:

  • Circular muscles contract above the bolus to prevent backwards movement and push food forwards.
  • Longitudinal muscles relax above the bolus and contract below to widen and shorten the gut.

The oesophagus, stomach and intestines all use peristalsis to move food along the canal.

The stomach is a large hollow organ in the alimentary canal that is important for protein digestion.

The stomach produces gastric juice that is mixed with the bolus inside the stomach. Gastric juice contains:

  • Gastric proteases that digest proteins.

    Meat eaters (carnivores) often have larger stomachs than plant eaters to fully digest their protein-rich diet.

  • Hydrochloric acid that creates an acidic environment to kill bacteria. In addition, gastric proteases need an acidic environment and so the hydrochloric acid supports their activity.

The muscular walls of the stomach contract via peristalsis to break up the bolus. This is a form of physical digestion.

The partially digested food that leaves the stomach is called chyme.

Gastric juice contains a protease called pepsin.
Gastric juice contains a protease called pepsin.

Bile is a liquid that helps with the digestion of fat and neutralises acidic chime in the duodenum.

Bile contains bile salts that break down large fat particles into small soluble fat droplets. This process is called emulsification.

Milk contains water and fat that has been emulsified.

Bile is alkaline (pH 8) and neutralises the acidic chyme that enters the duodenum from the stomach.

Enzymes in the duodenum require a neutral environment for optimal activity.

Bile does not contain any enzymes itself.

Bile is produced by the liver and stored in a small organ called the gallbladder. During digestion, bile is released from the gallbladder into the duodenum.

Small fat droplets have a greater surface area that can be accessed by lipases for chemical digestion.
Small fat droplets have a greater surface area that can be accessed by lipases for chemical digestion.

The small intestine is a long tube involved in chemical digestion and the absorption of nutrients.

The small intestine can be separated into two main sections:

  • The duodenum digests all three food types (proteins, carbohydrates, fats) using pancreatic amylase, trypsin and lipase.

These enzymes come from pancreatic juice from the pancreas, and intestinal juice secreted by the duodenum itself. They require a neutral pH to function optimally.

The products of digestion from the duodenum are ready for absorption.

  • The ileum is the main site of absorption of digested nutrients into the bloodstream.

The walls of the small intestine contain thousands of small folds called villi. Villi increase the surface area of the intestine for absorption.

 A human small intestine is about 7 metres long to enable the maximal absorption of nutrients.
A human small intestine is about 7 metres long to enable the maximal absorption of nutrients.

The intestinal villus (plural villi) is a finger-like fold in the walls of the small intestine that assist in the absorption of nutrients.

Villi are very small (0.5 millimetres long) and increase the area of the intestinal wall 30- to 60-fold. Increasing the surface area of the intestine wall provides a larger space for the absorption of molecules.

The structure of the villus has important links to its function.
The structure of the villus has important links to its function.

The hepatic portal vein is a major blood vessel that delivers blood from the small intestine to the liver.

The blood from the small intestine contains nutrients absorbed from the gut during digestion (glucose, amino acids, fatty acids). These are transported to the liver for assimilation (processing and use).

Despite its name, the hepatic portal vein is not strictly a vein. Unlike veins, it directs blood from one organ to another instead of back to the heart.

The hepatic portal vein acts as a highway between the small intestine and the liver.
The hepatic portal vein acts as a highway between the small intestine and the liver.

The liver is a large organ that processes excess nutrients arriving from the hepatic portal vein.

The processing of nutrients (assimilation) ensures that levels of glucose, fats and amino acids are balanced in the blood. If there is too much of a certain nutrient, the liver converts it into either a storage or waste form.

The liver processes sugars, amino acids and fats differently:

  • Glycogen is a polymer of glucose molecules that temporarily stores sugar.

    $$$\text{excess glucose $\rightarrow$ glycogen}$$$
  • Excess amino acids can be removed as a waste product called urea. This chemical conversion is called deamination

    $$$\text{excess amino acids $\rightarrow$ urea}$$$
  • Glucose can either be used for glucose production or glycogen storage.

    $$$\text{excess fats $\rightarrow$ glucose}$$$

Stained cross-section of the liver under a microscope.
Stained cross-section of the liver under a microscope.

The liver has an important role in the detoxification of blood and the removal of toxins from the body as waste.

Alcohol is a toxin that is broken down by the liver and rapidly removed from the blood stream.

If the liver is exposed to excessive alcohol over a long period of time (e.g. due to alcoholism), the products of alcohol metabolism (processing) can cause liver disease.

Liver disease can be identified by:

  • Build-up of fat in the liver
  • Scarring of liver
  • Loss of other liver functions (such as bile production)

The liver also helps break down old red blood cells that need to be replaced. The products of red blood cell breakdown are found in bile. This gives bile its yellow-green pigment.

Alcohol is a major cause of liver disease.
Alcohol is a major cause of liver disease.

The large intestine is the final section of the alimentary canal. It is responsible for water absorption and the storage of faeces.

The large intestine is composed of three main sections:

  • The colon absorbs water and some minerals from digested food. It is the biggest part of the large intestine.
  • The rectum stores undigested material. This forms solid faeces after sufficient water has been absorbed.
  • The anus is where faeces is removed from the body. This is called egestion.

No further digestion of material occurs in the large intestine.

The large intestine is actually shorter than the small intestine.
The large intestine is actually shorter than the small intestine.

The alimentary canal is arranged so that organs specialised for a particular process are positioned where that process would be most effective.

The ileum (specialised for absorption) is located after the stomach and the duodenum, where the majority of food is digested.