Digestive systems
#### Digestive systems of multicellular animals:
Multicellular organisms have increasing levels of organisation that enable them to digest a wider range of foods and to digest them more efficiently. They have digestive systems that carry out extracellular digestion, but within the organism. The products of digestion are then absorbed, and undigested food egested.
Hydra:
This organism is related to jellyfish, and has a simple, sac-like gut with only one opening to the external environment.
- Digestion is a combination of extracellular and intracellular digestion. This large scale digestion takes place in the digestive sac (called the gastrovascular cavity) to break down food into fragments that can be absorbed into the cells lining the gut.
- Digestion in the gut is a combination of mechanical digestion, due to the contraction of the body, and chemical digestion, through the action of extracellular hydrolytic enzymes.
Fragments of food are absorbed into the cells lining the gut cavity by phagocytosis, and large molecules (such as proteins) are absorbed by pinocytosis. Intracellular digestion completes the breakdown of food with vacuoles.
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Flatworms:
Flatworms are larger than Hydra and have a more complex level of organisation than hydra, but still have a single gut cavity for the digestion of food.
- The gastrovascular cavity is highly branched - as a result, food can be digested, and the products of digestion can b absorbed throughout the organism. Therefore, there is no need for a designated transport system to deliver raw materials to the tissues.
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### Digestive systems - higher order animals:
More complex animals have developed a tube gut which has an opening for ingestion and a separate opening for egestion. Different regions of the gut are adapted for different functions:
- Mechanical digestion by teeth, muscular action.
- Chemical digestion by acids
- Chemical digestion by enzymes with different optimum ph.
This type of gut increases the efficiency of digestion and also enables an organism to eat and digest a wider range of foods.
- The human gut is an example of a generalised digestive system which is adapted to eating both plant and animal material.
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## Human digestive system:
Buccal cavity:
In the buccal cavity, or mouth, a mixture of mechanical and chemical digestion takes place:
- Lips tongue and teeth work together to - Capture and receive food, move food about the mouth, cut, grind and chew food, mix the food with saliva to lubricate food, forms food into a bolus to make swallowing easier.
- Salivary glands secrete about 1500 cm^3 of saliva a day - saliva is slightly acidic, containing the enzyme salivary amylase (breaks down starch into maltose).
Oesophagus:
The oesophagus is a straight, narrow tube with a muscular wall (which carried food to the stomach by peristalsis). This is due to waves of contraction of muscles within the gut wall, which push the food forwards through the gut.
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- When you begin to swallow, the epiglottis closes to prevent food from entering the trachea and lungs.
Stomach:
The stomach carries out chemical and mechanical digestion
- Chemical - through the action of enzymes and hydrochloric acid
- Mechanical - through the action of muscles in the stomach which contract and relax to mix food with gastric juice and further breakdown large particles of food into smaller particles with a larger surface area for chemical action.
The end result of digestion in the stomach is a semi-liquid called chyme.
- The stomach wall contains a range of gastric glands/pits which produce a mixture of chemicals, called gastric juice. The chemicals are produced by the following cells:
Zymogenic/Chief cells:
- Secrete pepsinogen (precursor of pepsin)
- Digestion of protein inti polypeptides; digestion of milk protein
Oxyntic cells:
- Secrete hydrochloric acid
- Provides optimum pH for enzymes; denatures proteins and softens connective tissues in food; activates pepsin
Goblet/neck cells:
- Secrete mucus
- Forms a barrier between the stomach lining and the gastric juice; protects the stomach wall and glands from self-digestion by pepsin and hydrochloric acid.
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Small intestine:
- Comprised of the duodenum and ileum (as well as the jejunum which is a part of the duodenum between the duodenum and ileum).
Duodenum
- The stomach opens in the first part of the intestine called the duodenum. This is about 30cm and receives secretions from accessory organs:
- Bile from the liver
- Pancreatic juice from the pancreas
The wall of the small intestine is folded to increase surface area for the final stages of digestion and absorption. These folds are called villi.
- At the base of the villi are also intestinal glands called the crypts of Lieberkühn.
- The duodenum also has Brunner's glands that secrete alkaline fluid (containing sodium hydrogen carbonate) and mucus to neutralise the acid chyme from the stomach
Ileum:
- The ileum is nearly 5cm long and is where digestion is completed and is the main site of absorption of the products of digestion
- Due to its length, food takes a relatively long time to pass through the ileum, increasing the time available for digestion/absorption.
- The villi also increase the surface area and have a rich blood supply to remove the products of digestion. Most of the water in our food is absorbed in the ileum.
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Large intestine:
- The ileum opens into the first part of the large intestine called the caecum This has no role in the human digestive system. However, in some animals this is the site of cellulose digestion. The appendix is a blind-ended sac which open from the caecum, but has no known role in digestion in humans.
- The undigested food and remaining water and minerals first pass into the colon.
Colon:
- The main part of the large intestine; this is where the rest of the water and mineral salts are absorbed.
- Vitamins produced by microorganisms in the colon are also absorbed into the blood
- This leaves a semi-solid mass of undigested food, dead intestinal cells and bacteria - forms faeces.
Rectum:
- This is the last part of the large intestine; it is a muscular tube that temporality stores the faeces before they are eliminated from the body.
Anus:
- This is made from sphincter muscles that control when defaecation occurs
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### Accessory organs:
The liver produces bile by breaking down haemoglobin. Bile is stored in the gall bladder and is secreted into the duodenum through the bile duct.
The liver receives almost all the products of digestion from the small intestine, carried there by the hepatic portal vein.
- Excess glucose is stored as glycogen
- Amino groups are removed from amino acids (de-amination) and converted into urea
- Many vitamins are also stored in the liver
The pancreas has two main secretory functions:
- endocrine - it secretes the hormones insulin and glycogen for control of blood glucose levels. Blood carried secretions to site of action
- Exocrine - Secretes pancreatic juice, a mixture of: enzymes, enzyme precursors and sodium hydrogen carbonate.