1 Human Human Digestive Digestive System System
Nov 18, 2014
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Human Human
Digestive Digestive SystemSystem
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What you will be learning...(a) identify the main regions of the alimentary canal and the associated
organs: mouth, salivary glands, oesophagus, stomach, duodenum, pancreas, gall-bladder, liver, ileum, colon, rectum, anus
(b) *describe the functions of these parts in relation to ingestion, digestion, absorption, assimilation and egestion of food, as appropriate
(c) explain why most foods must be digested(d) describe:
(i) digestion in the alimentary canal(ii) the functions of a typical amylase, protease and lipase, listing the
substrate and end-products(e) describe the structure of a villus (including role of capillaries and lacteals) in
absorption(f) State the function of the hepatic portal vein as the route taken by most of
the most absorbed from the small intestine(g) state the role of the liver in:
(i) carbohydrate and fat metabolism(ii) breakdown of red blood cells(iii) metabolism of amino acids and the formation of urea(iv) breakdown of alcohol, including the effects of excessive alcohol
consumption
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Recall. . . What are the main organs of the alimentary canal?
Mouth Oesophagus Stomach Small intestine Large intestine Anus
Although not part of the alimentary canal, the liver, gall bladder and pancreas are closely associated with it. They play an important role in digestion by secreting digestive enzymes.
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5 Digestion Processes (IDAAE)• Ingestion: taking in of food into the body.
• Digestion: breaking down of food into simpler substances
• Absorption: diffusion of food from small intestine into the blood
• Assimilation: using digested nutrients to make new material
• Egestion: removal of undigested waste material
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DigestionMechanical / physical digestion physically
breaks down the food in the mouth (chewing). Smaller pieces of food increase surface area for digestion. It also takes place in the stomach (churning of food by the muscular stomach walls)
Chemical digestion uses enzymes to chemically break down complex food substances into their simplest form. e.g.
Starch maltoseamylase
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Chemical digestion:• Starch (carbohydrate) digestion: in mouth and
small intestine.• Protein digestion: in stomach and small
intestine• Fat digestion: only in small intestine
Why must food be digested???Large molecules of food are unable to pass through cell membranes, thus must be broken down into small molecules so that they can diffuse through cell membranes into the blood stream
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Mouth (Ahhh....)• Mouth ingests food • Teeth masticates food into small pieces to
increase surface area for digestion• Saliva (pH 7) moisten and soften food
• Starch maltose• Tongue mixes food with saliva and rolls
food into a bolus before swallowing• Saliva - water, mucus, salivary amylase
Salivary amylase
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Swallowing
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trachea (windpipe)
glottis
During breathing, the larynx is lowered and the glottis is open.
pharynx
oesophagus
larynx (voice-box)
air
What Happens During Breathing and Swallowing?
Normally, air passes into the trachea (windpipe) while food passes into the oesophagus.
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During swallowing, the larynx is raised and the glottis is covered by the epiglottis. This prevents food particles from entering the trachea.
pharynx
trachea (windpipe)
oesophagus
glottis
epiglottis
food particles
larynx (voice-box)
What Happens During Breathing and Swallowing?
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Occasionally, small particles of food or water may get into the larynx or trachea.
trachea (windpipe)
larynx (voice-box)
food particles
What Happens During Breathing and Swallowing?
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What Happens During Breathing and Swallowing?
This automatically induces violent coughing to force the food particles or water out and to prevent choking.
trachea (windpipe)
larynx (voice-box)
food particles
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Oesophagus• Minimal digestion
• Carries food from mouth to stomach by peristalsis
• Oesophagus has circular and longitudinal muscles which are antagonistic.
• When circular muscles contract, longitudinal muscles relax and vice-versa.
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The two layers of muscles cause rhythmic, wave-like contractions of the gut walls. Such movements are known as peristalsis.
Peristalsis:
• enables food to be mixed with the digestive juices; and
• moves the food along the gut.
Part of the gut wall
Peristalsis
circular muscles
longitudinal muscles
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Wall here constricts. Circular muslces
contract; longitudinal muscles relax
Wall here dilates
Direction of movement of food
Circular muscles relax
Longitudinal muscles contract
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Peristalsis – Move the food down!
• When circular muscles contract, longitudinal muscles relax. Gut wall constricts i.e. gut becomes narrower and longer. Food is squeezed or pushed forward.
• Gravity and slippery mucous lining helps push food down too.
• http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html
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Stomach• Stores food temporarily
• Stomach muscles churns and mixes food (also by peristalsis) with gastric juice to form chyme.
• Gastric juice contains hydrochloric acid (HCl) and enzymes like rennin and pepsin
– HCl is very acidic (pH2), thus it kills bacteria and other microorganisms, as well as stopping the action of salivary amylase
– Provides acidic medium for gastric enzymes to work
• Only protein digestion here
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HCl converts inactive pepsinogen and prorennin to their active forms
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• The stomach is “guarded” at the entrance and exit points by sphincter muscles which control the amount of food entering and leaving the stomach.
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Small Intestine
• Subdivided into duodenum, jejunum and ileum
• In the small intestine, chyme stimulates1. Pancreas to secrete pancreatic juice
2. Gall bladder to secrete bile
3. Intestinal glands to secrete intestinal juice
• All three juices secreted are alkaline,
pH 8.5
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bile
intestinal juice
pancreatic duct
pancreatic juicebile duct1
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2•Pancreatic and intestinal juice contain many digestive enzymes.
•Bile does not containenzymes. Bile emulsifies fats, increasing thesurface area for lipase action
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Duodenum• Starch maltose• Protein polypeptides • Fats fatty acids + glycerol
Ileum• Maltose glucose• Polypeptides amino acids • Fats fatty acids + glycerol • Lactose glucose + galactose• Sucrose glucose + fructose
pancreatic amylase
proteases
lipase
maltase
protease
lipase
lactase
sucrase
Note that the small intestine is the main site of digestion of food and absorption of nutrients.
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Large Intestine (colon)
• Large inverted U shaped tube.
• No digestion takes place here
• Absorbs water and minerals salts
• Stores the faeces (dead cells, mucus, germs, undigested food)
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Is the colon the main region for water absorption?
No! About 94% of the total amount of water passing through the alimentary canal is absorbed by the small intestine! The large intestine absorbs most of the remaining 6% of water.
Rectum – temporarily stores faecesAnus – egests (= removal of undigested
matter) faeces
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Organs associated with the alimentary canal
These organs do not digest food but aid in digestion
• Gall bladder
• Pancreas
• Liver
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Gall bladder• Temporarily stores bile (smelly green
substance) secreted by liver. • Secretes bile in the presence of chyme.• Bile breaks up large fat droplets into very
small fat droplets to increase surface area for lipase action (Emulsification) Bile emulsifies fats
• *Bile is not an enzyme, so it is not affected by temperature
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Pancreas• Connects to small intestine by pancreatic
duct• Produces pancreatic juice• Secretes hormones like insulin (controls
blood glucose concentration) and glucagon (controls carbohydrate metabolism)
Liver• Produces bile, which is stored in the gall
bladder
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Absorption Adaptations of the small intestine
• Small intestine is very long (~5 m)
• Internal surface of the small intestine has many folds.
• On these folds, there are many finger-like projections called villi
• These 3 adaptations increase surface area for absorption
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Lacteal – fatty acids and glycerol recombine
in the epithelium toform fat which then enters the lacteal as
fine fat droplets
Blood capillaries – transport sugars and amino acids away from the
small intestine
One cell thick epithelium – for efficient absorption of
food particles
This continual transport of digested food substances maintains the concentration gradient for the absorption
of digested food substances.
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• The concentration of simple nutrients (e.g. glucose, amino acids, fatty acids and glycerol) is higher in the lumen of the small intestine then in the blood capillaries that pass through the villi.
• Thus, nutrients diffuse across a region of high concentration (lumen of the small intestine) to the bloodstream, which has a lower concentration. Note that absorption by active transport is also possible.
• The blood capillaries in the small intestine unite to form larger blood vessels, which unite to form the hepatic portal vein, which transports the nutrients to the liver.
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What happens to amino acids and glucose after absorption?
Products released from liver into general blood circulation
Molecules pass into the epithelial cells
Through walls of capillaries in the villus and into bloodstream
The capillaries join up to form veins
Veins unite to form 1 large vein: Hepatic Portal Vein
Hepatic portal vein carries blood to liver
Liver stores or alters products of digestion
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Glucose Amino Acids
• Glucose is used by all cells as a source of energy.
• Excess glucose returned to liver and stored as glycogen. Insulin stimulates liver to convert glucose into glycogen. When the body needs energy, glycogen is converted back to glucose.
• Amino acids which enter the cells are converted into new protoplasm that is used for growth and repair.
• Amino acids used to form enzymes and hormones.
• Excess amino acids deaminated by liver.
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What happens to fatty acids and glycerol after absorption?
Molecules pass into the epithelial cells
Recombine into fats again in the epithelial cells
Fats enter the lacteals
Lymph (fluid in lacteals) + fat = chyle
Lymphatic vessels discharge chyle into bloodstream
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Fats• Blood carries fats to all parts of the body,
especially to the liver.• When there is enough glucose, fats are not
broken down but are used to build protoplam.
• When there is insufficient glucose, fats are broken down to provide energy.
• Excess fats stored in adipose tissues.
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Villi – absorption by diffusion
DiffusionDiffusion
From intestine
To
To liver
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Assimilation• After travelling through the blood stream to
the rest of the body, cells can now make use of– glucose as source of energy– amino acids to build new cytoplasm and tissue
cells– fatty acids to build new cell membranes
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Functions of the Liver1. Regulation of blood glucose concentration
– 70-90mg of glucose / 100cm3 of blood (normal conditions)
2. Production of bile– Liver produces bile which is stored in the gall bladder
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Functions of the liver3. Iron storage
– Red blood cells are destroyed in the spleen and their haemoglobin is sent to the liver to be broken down. The iron released is then stored in the liver. Bile pigments are also formed from the breakdown of haemoglobin.
4. Protein synthesis– Liver synthesizes proteins found in blood plasma,
e.g. albumins, globulins, fibrinogen
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Functions of the liver5. Deamination of amino acids
– Excess amino acids are transported to the liver, where their amino groups are removed and converted to urea.
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Functions of the liver6. Detoxification
– Liver cells contain alcohol dehydrogenase to break down alcohol.
– Excessive alcohol is harmful. Alcohol stimulates acid secretion in the stomach and increases risk of gastric ulcers.
– Prolonged alcohol abuse may lead to liver cirrhosis (destruction of liver cells), which can lead to liver failure and death.
7. Heat production