2. Humans have a a complete digestive tract, which begins with
the mouth and ends with the anus.
The major structures of the human digestive tract are the :
5. Why is digestion
To convert large, often insoluble
molecules of food into smaller
soluble molecules capable of
being absorbed into the blood or
lymph capillaries associated with
the digestive tract and used for
various metabolic processes.
8. Ingestion: Food is placed into
- Incisors cut or bite the food.
- Canines used to tear meat.
- Molars and pre-molars grind the food
10. Secreted by the salivary
glands (parotid gland,
sublingual gland and sub-
Mix with the food to form a
Saliva contains the enzyme
amylase, that breaks down
cooked starch into maltose.
11. Mixes food with saliva and pushes food
Makes swallowing easier.
12. The bolus is forced down the esophagus when
the muscular pharynx contract – swallow
Peristalsis (contraction and relaxation of
antagonistic circular and longitudinal muscle)
of the esophagus pushes the food downwards
into the stomach, through the cardiac valve.
No absorption takes place
The epiglottis that covers the
trachea prevents food from going
into the trachea when you swallow.
16. Food enters the stomach (fundus region) through
the cardiac valve.
Remains a half an hour still before the stomach
muscles (circular-, longitudinal- and oblique
muscles) starts contracting and relaxing
The food move with circular movements through
the stomach (corpus and pyloric regions) and
mixes with all the gastric juices.
17. Gastric juices are secreted after
the hormone, gastrin stimulates
the parietal cells in the fundus
region of the stomach.
Gastric juices consist of HCl
(Acidify stomach and neutralizes
the bolus, antiseptic
fats), digestive enzymes
(pepsin, rennin and
lipase), mucus (help protect the
inner lining of the stomach
against enzyme activity) and
18. These gastric juices help with chemical
digestion of food and the bolus is now called
Some absorption occurs in the stomach.
Some water, glucose, salt and certain drugs
and alcohol pass into the blood capillaries of
the stomach wall.
20. As the chym enters the duodenum (first part
of the small intestine) through the pyloric
valve it mixes with bile (excreted from the
liver or gall bladder) and pancreatic juices.
21. Secretin is the hormone that stimulates the
pancreas to secrete pancreatic juice into the
The pancreatic juice contain sodium
bicarbonate (neutralizes the chym, antiseptic)
and digestive enzymes (Trypsin, amylase and
Bile is produced in the liver and stored by the
When fatty rich food enters the small
intestines, bile is secreted by the gall bladder
and transported to the duodenum.
23. Neutralizes acid from stomach
Emulsifies fats (increase surface
area of fats for better digestion)
Aid in the absorption of fats.
Reduce the fluidity of the chym
Antiseptic – prevent decomposition
Assist in the absorption of the fat-
24. The chym then moves through the ileum
(second part of small intestines)
It mixes with intestinal juice (succus
entericus) that contains the digestive
enzymes for the final digestion of food –
Peptidase, lipase, lactase, maltase, sucrase
Succus entericus is secreted by 2 glands:
Crypt of Lieberkuhn and the glands
of Brunner (both situated in the lining of the
25. The 2 muscle layer of the small intestine helps
with the process of peristalsis.
Cross section through the small intestine
26. The liver produces and secretes bile.
It stores glucose in the form of glycogen and is controlled
by the hormone insulin. When the body requires glucose,
glycogen is changed back into glucose.The liver also
converts glycerol, into glucose.
It can convert excess carbohydrates into fatty acids which
combine with glycerol to form fats.
The liver stores blood temporarily, blood is also formed in
the liver of embryos.
The liver manufactures plasma proteins e.g. albumen and
The liver forms heparin which prevents the clotting of
blood inside the blood vessels.
The liver detoxifies the body.
27. •The liver breaks down surplus amino
acids through the process of
• Change amino acid into ammonia.
•Two ammonia molecules combine
with one molecule of carbon dioxide to
form urea and water.
• Deaminated amino acids are
converted into glucose (glucose is
converted to glycogen and stored in
28. The hepatic portal vein (transports digested
food from the small intestine to the liver) and
the hepatic artery (transports oxygen and
nutrients to the liver) enter the liver.
Inside the liver the blood of these 2 blood
vessels mix and the products transported is
exchanged between the blood and the liver
The waste moves out of the liver cells and is
transported away from the liver by means of
the hepatic vein which joins up with the
inferior vena cava.
30. Food is mainly absorbed from the
small intestine, where all final
digestive processes take place and
the end-products of digestion is
31. The final products of digestion (glucose, amino
acids, fatty acids and glycerol) are formed in the
The absorptive surface area is increased by:
The great length of the small intestine (7m)
The circular folds of the mucosa lining of the
The millions of villi lining the folds.
The chym is pushed along very slowly through the
small intestine, allowing time for absorption to
33. A villus consist of several capillary arteries,
which supply the villus with OXYGEN and
capillary veins which carry food away from the
Lacteals (lymph vessels) are found in the centre
of each villus used for the absorption of fats.
The vessels are surrounded by connective tissue
and a layer of columnar epithelial cells in which
goblet cells occur
35. The columnar epithelial cells play an
active part in the absorption and are
able to allow substances to enter the
villi against the concentration gradient.
Amino acids, glucose, mineral salts,
water and vitamins are absorbed
directly into the blood.
36. Fatty acids (insoluble) combine with bile salts
To form fatty acid-bile salt complex which is soluble in water.
This complex plus the glycerol component of fat is absorbed by
the columnar epithelial cells of the villi.
Inside the villus the fatty acids are freed from the bile salts and
recombine with glycerol to form tiny fat globules.
Some of the fat globules are absorbed directly into the
Most fat globules are absorbed by the lacteals.
The lacteals unite to form small lymph vessels which enter
the main lymphatic system.
This fat in the lacteal is now known as chyle, and it reaches
the bloodstream in the end.
37. No digestion takes place in the colon.
Undigested food remains from the small intestine enter
the caecum through the ileocaecal valve.
In the colon, water is absorbed so that the chym becomes
Symbiotic bacteria present in the colon act upon the food
remains, decomposing them and turning them into faeces.
The bacteria synthesize vitamins of the B group and
vitamin K which is essential for the blood clotting process.
Peristalsis in the colon is facilitated by mucus produced by
numerous mucous glands.
Mucus assists the passage of faeces and protects the wall
of the colon.
39. By the time the faeces reach
the rectum, they consist of
approximately 70% water.
Bacteria account for about 30%
of the dry mass of faeces
The remainder is made up of
food residue, mainly cellulose.
41. Defication is brought about by
contraction of the circular muscles of
the rectum, and relaxation of the
muscles which make up the anal