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Structure and function of the alimentary canal
The mouth and oesophagus
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Teeth: Chew the food into smaller pieces
Saliva: Contains water, mucus and amylase. Water helps dissolve substances in food
(important for taste), mucus (produced in goblet cells) helps bind chewed food together and
lubricates the oesophagus, amylase begins turning starch and animal glycogen into maltose
Tongue: Mixes food with salive and forms it into a bolus
Swallowing: Subconscious contractions automatically swallow a bolus
The stomach
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Oesophagus carries food to the stomach
Sphincter muscles at the entrance and exit of stomach control when food gets in and out.
The stomach can store food for up to 1-2 hours
The stomach has strong muscular walls that contract and relax to churn food and mix it with
the stomach enzymes, stomach acid and mucus. This mixture is called chyme.
The mucus is produced by goblet cells
The stomach enzyme pepsin (a protease) starts the protein digestion by breaking down
proteins into polypeptides. It is produced in specialized cells
The stomach enzyme rennin is only produced in the stomach of young mammals and causes
milk protein to clot (making it easier to digest). It is produced in specialized cells
The stomach acid (hydrochloric acid) kills potentially harmful bacteria and helps the stomach
enzymes work. It is produced by place cells.
The three important cells types (enzyme cells, place cells and goblet cells) are all found in
gastric pits, pits in the stomach wall.
The chyme released out of the stomach goes to the duodenum (first part of the small
intestine)
The pancreas
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The pancreas is a cream coloured organ that exits into the duodenum and produces
pancreatic juice
Pancreatic juice contains enzymes amylase (turns starch into maltose), trypsin (a protease,
turns proteins into polypeptides) and lipase (turns fats into fatty acids and glycerol)
Because the pancreatic enzymes don’t work well in acidic environments, the pancreatic juice
also contains sodium bicarbonate, a substance
neutralizes the hydrochloric acid in the chime
Apart from being an exocrine gland (secreting pancreatic juice into the alimentary canal) the
pancreas is an endocrine gland as well (secreting the hormones insulin and glucagon into the
blood stream). The hormones are produced in the islets of langerhans. Insulin lowers the
blood sugar level, glucagon increases the blood sugar level
The small intestine
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Digestion is completed in the small intestine
About 5 meters long
Can be split up into two parts: the duodenum and the ileum
All along the duodenum water (5-10 liters a day), salts and vitamins are absorbed into the
blood stream
To help with this absorption the small intestine has a specialized structure
o The surface is folded, covered with protruding villi which again carry cells on their
surface that carry microvilli (protruding parts of the cell surface).
The surface area for absorption is increased
o The surface covering the villi is only one cell thick and contains blood vessels
Direct and easy transport of nutrients into the blood stream
o The villi also contain lacteals, which are part of the lymphatic system
Fatty acids can be absorbed into the lymphatic system
In the duodenum:
o The pancreatic juice as well as the bile (produced in the liver and stored in the gall
bladder) are secreted into the entrance of the duodenum
o The sodium bicarbonate in the pancreatic juice neutralizes the hydrochloric acid in
the chyme released out of the stomach.
o The bile salts emulsify the fats from the chyme, making them accessible to lipase
o The pancreatic enzyme do the first step of nutrient digestion (starch->maltose,
proteins-> polypeptides, fats -> fatty acids and glycerol)
In the ileum:
o The cells covering the villi produce intestinal enzymes (maltase, sucrose, lactase,
peptidases and lipases) that finish digesting nutrients into molecules small enough to
be absorbed
o Maltase: maltose -> glucose
o Sucrase: sucrose (regular sugar) -> glucose and fructose
o Lactase: lactose -> glucose and galactose
o Peptidase: polypeptides -> amino acids
o Lipase: Fats –> fatty acids and glycerol
o The produced enzymes stay around the surface of the ileum and digest the nutrients
just before they are aborbed
The liver
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Apart from producing bile the liver is where the blood from the small intestine (carrying all
the absorbed nutrients) is transported through the hepatic portal vein
The liver converts excess nutrients so that they can be stored (e.g. glucose into glycogen,
amino acids into fats)
It also removes nitrogen from the nutrients we take in (mainly amino acids) and breaks down
toxins into less dangerous substances
The liver also breaks down old red blood cells, leaving the yellowish bile pigments which are
also part of the bile and excreted with the faeces.
Large intestine: can be split up into colon and rectum
Colon
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Main function is the absorption of water and salts out of the non-digestible parts of food
The colon absorbs much less water than the small intestine (about 0.3 to 0.5 liters per day)
Rectum
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By the time the food comes to the rectum most usable substances have been absorbed into
the blood stream
Leftovers are the faeces (fiber, other indigestible roughage, bacteria and dead cells). These
are egested out of the body through the anus.
Faeces can be temporarily stored in the rectum, during which more water and salts are
absorbed.