1. Human Digestive System

2. 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

3. 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.

4. 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

5. Digestion
Mechanical / 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 maltose
amylase

6. Starch (carbohydrate) digestion: in mouth and small intestine.
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

7. 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

9. Swallowing
(1)The initial stages of eating and swallowing are under voluntary control.
(2)Once food enters the mouth the teeth break it down into smaller and smaller pieces. This has the dual function of making the food easier to swallow and increasing the surface area of food on which the saliva can act.
The tongue, lips and cheeks assist the teeth in the process by allowing the food to be "rolled" around the oral cavity.
The mechanical action described above produces a softened bolus of food which is now ready to be swallowed. The correct biological term for swallowing is deglutition.
(3) shows the voluntary stage of deglutition. Here the bolus is pushed into the upper part of the pharynx (known as the oropharynx) by the action of the tongue. 
The pharyngeal stage of deglutiton is stimulated when the bolus enters the oropharynx. This stage of swallowing is mainly due to a reflex response. Various nerve receptors send messages to the deglutition centre of the brain stem. (see medulla and pons in your notes on the central nervous system).
(4)This sets off muscular contractions in the pharynx. The soft palate closes off the nasopharynx. The vocal cords in the larynx are moved up and towards the front of the throat thus closing it off to the passage of food. This is extremely important in preventing food from entering the airway.I am sure we have all experienced the unpleasant feeling of food or drink going the "wrong way"!!
Another effect of the process is to widen the opening of the oesophagus thus making the passage of the bolus along the alimentary canal easier.
(5) As the bolus pushes it's way into the oesophagus it automatically pushes the epiglottis downwards further closing off the airway.
(6) The bolus then enters the oesophagus and the final stage of swallowing begins.
This final stage is known as the oesophageal stage of deglutiton.

10. What Happens During Breathing and Swallowing?
Normally, air passes into the trachea (windpipe) while food passes into the oesophagus.
pharynx
During breathing, the larynx is lowered and the glottis is open.
air
larynx (voice-box)
Figure 6.4 page 92
oesophagus
glottis
trachea (windpipe)

11. What Happens During Breathing and Swallowing?
During swallowing, the larynx is raised and the glottis is covered by the epiglottis. This prevents food particles from entering the trachea.
pharynx
food particles
Figure 6.4 page 92
epiglottis
oesophagus
glottis
larynx (voice-box)
trachea (windpipe)

12. What Happens During Breathing and Swallowing?
Occasionally, small particles of food or water may get into the larynx or trachea.
food particles
Figure 6.4 page 92
larynx (voice-box)
trachea (windpipe)

13. What Happens During Breathing and Swallowing?
This automatically induces violent coughing to force the food particles or water out and to prevent choking.
food particles
Figure 6.4 page 92
larynx (voice-box)
trachea (windpipe)

14. 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.

15. Peristalsis
Part of the gut wall
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.
circular muscles
Figure 6.6 page 93
longitudinal muscles

16. Direction of movement of food Circular muscles relax
Wall here dilates
Direction of movement of food
Circular muscles relax
Wall here constricts. Circular muslces contract; longitudinal muscles relax
Longitudinal muscles
contract

17. 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.

18. 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

19. HCl converts inactive pepsinogen and prorennin
to their active forms

20. The stomach is “guarded” at the entrance and exit points by sphincter muscles which control the amount of food entering and leaving the stomach.
Compare the stomach wall n oesophagus wall? Which is thinker? Why? ( pro

21. Small Intestine Subdivided into duodenum, jejunum and ileum
In the small intestine, chyme stimulates
Pancreas to secrete pancreatic juice
Gall bladder to secrete bile
Intestinal glands to secrete intestinal juice
All three juices secreted are alkaline,
pH 8.5

22. Pancreatic and intestinal juice contain many digestive enzymes.2
bile 1
bile duct
pancreatic juice
Pancreatic and intestinal juice contain many digestive enzymes.
Bile does not contain
enzymes. Bile emulsifies fats, increasing the
surface area for lipase
action
pancreatic duct 3
intestinal juice

23. Duodenum Ileum Note that the small intestine is the main site of
pancreatic amylase
Starch maltose
Protein polypeptides
Fats fatty acids + glycerol
Ileum
Maltose glucose
Polypeptides amino acids
Lactose glucose + galactose
Sucrose glucose + fructose
proteases
lipase
maltase
protease
lipase
lactase
sucrase
Note that the small intestine is the main site of
digestion of food and absorption of nutrients.

26. 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)

27. 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 faeces
Anus – egests (= removal of undigested matter) faeces

28. Organs associated with the alimentary canal
These organs do not digest food but aid in digestion
Gall bladder
Pancreas
Liver

29. 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

30. 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

31. 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

33. One cell thick epithelium – for efficient absorption of food particles
Blood capillaries –
transport sugars and amino acids away from the small
intestine
Lacteal – fatty acids
and glycerol recombine
in the epithelium to
form fat which then
enters the lacteal as
fine fat droplets
This continual transport of digested food substances
maintains the concentration gradient for the absorption
of digested food substances.

34. The concentration of simple nutrients (e. g
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.

35. What happens to amino acids and glucose after absorption?
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
Products released from liver into general blood circulation

37. 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.

38. 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

39. 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.

40. Villi – absorption by diffusion
To liver
From intestine To

41. 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

42. Functions of the Liver Regulation of blood glucose concentration
70-90mg of glucose / 100cm3 of blood (normal conditions)
Production of bile
Liver produces bile which is stored in the gall bladder

43. Functions of the liver 3. Iron storage 4. Protein synthesis
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

44. Functions of the liver 5. Deamination of amino acids
Excess amino acids are transported to the liver, where their amino groups are removed and converted to urea.

45. Functions of the liver 6. Detoxification 7. Heat production
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