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Digestive System:- Ingestion (1st Stage), Food movement
(2nd Stage), Absorption (3rd Stage), Assimilation (4th
stage), O-levels Biology
Written by:- Maria Batool

Table of Contents
What are the 4 stages of digestion? (Ingestion, Food movement, Assimilation and
Absorption)
What is nutrition?
Ingestion:
The Human Digestive System:
Mouth and the Buccal Cavity:
The Pharynx:
The Oesophagus:
Stomach:
The Small Intestine:
The Large Intestine:
Organs associated with the gut:
The liver:
The Gallbladder:
The Pancreas:
What is absorption?
Adaptations of Small Intestine:
Factor 1:
Factor 2:
Factor 3:
How does absorption take place in the intestines (ileum)?
What happens to the undigested matter?
What is assimilation?
Absorbed substances that go to the liver are:
1
...
Amino Acids:
3
...


What is nutrition?
Nutrition is the process by which organisms obtain food and get energy for growth,
repair and maintenance of the body
...

Now, how does the food move through the digestive system?

The Human Digestive System:
The human digestive system consists of the gut or the alimentary canal and the
organs associated with it
...


Mouth and the Buccal Cavity:
Food enters our body through the mouth which leads to the buccal cavity
...
The chewing action of teeth makes it easier for
the abdominal cavity to digest the large pieces of food
...
Saliva flows into the
buccal cavity via the salivary duct
...

● Tongue: it helps to mix the food with the saliva and rolls down the food into
the oesophagus through the pharynx
...
Both food and air first pass through the pharynx and then
the food goes into the oesophagus and air passes into the trachea
...
During swallowing, the larynx moves upwards and epiglottis
moves downwards, hence covering the trachea (windpipe)
...
The wall of the oesophagus contains two
layers of muscles
...
The two layers of muscles are:
● Longitudinal Muscles: These are on the outer side of the muscles
...

Both muscles are antagonist muscles, that is when one muscle contracts the other
relaxes
...

Peristalsis are the rhythmic, wave-like muscular contractions in the wall
of the alimentary canal
...
When the circular muscles constrict and the longitudinal muscles relax, the wall
of the gut becomes narrower and longer (constricts)
...

When the longitudinal muscles constrict, the wall of the gut becomes wider and
shorter (dilates) for the food to pass through
...
The stomach has
numerous pits which lead to gastric glands that secrete gastric juice
...
There is a ring of muscle called the pyloric sphincter
located where the stomach joins the small intestine
...


The Small Intestine:
The small intestine consists of a u-shaped duodenum, jejunum and the much coiled
ileum
...
The walls of the small intestine
contain glands which secrete digestive enzymes
...


The Large Intestine:
The large intestine is 1
...
It is much broader than the small intestine
...
At the junction between the
colon and small intestine are the appendix (no specific function) and caecum
(sac-like structure)
...
Faeces are stored in the rectum, when the rectum contracts the faeces are
expelled from the anus
...
No digestion occurs in the large
intestine
...
The
liver cells produce and secrete bile
...
Bile pigments give bile its colour and are
removed from the body through the faeces
...
Bile flows into the
duodenum via the bile duct
...
The pancreas
produces the pancreatic juice which contains digestive enzymes
...


What is absorption?
Absorption is the process whereby digested food substances are absorbed into the
body cells
...
The
absorbed nutrients pass through the small intestine into the bloodstream
...
Most of the water,
about 90%, is absorbed in the colon
...

● The membrane that separates the food substances from the blood capillaries
--- a thinner membrane will allow the food molecules to pass through it in a
shorter time
...

Factor 1:
The inner surface of the small intestine is folded
...
The epithelial cells of these villi, have in turn
numerous microvilli to increase the surface area
...

Factor 3:
The small intestine is long enough (6 metres) to provide sufficient time for
absorption
...
The continual transport of
digested food substances maintain the concentration gradient for the absorption of
digested food substances
...
However active
transport is also required for the absorption
...
Glycerol and fatty acids diffuse into the epithelium
...


What happens to the undigested matter?
The undigested matter (fibre, bile salts, and other substances) are stored temporarily
in the rectum, before it is discharged as faces through the anus
...


What is assimilation?
Assimilation is the process of converting the absorbed food particles into new
protoplasm or using it to provide energy to the body for different vital activities
...
The blood vessels carrying
the nutrients unite to form a bigger vein, hepatic portal vein, which transports
nutrients from the small intestine to the liver
...
Glucose:
Glucose is used by all the cells as a source of energy
...
Blood
containing glucose passes through the liver, where excess glucose is stored as
glycogen, by a hormone insulin
...
Then the glucose is transported
around the body
...
Amino Acids:
Amino acids which enter the cells are used to make new protoplasm for growth and
repair of worn out parts
...
Deamination is the process of
removing amino groups (NH2) from amino acids and are converted to urea
...
The deaminated amino acids are brought to
the liver where they are converted to glucose and stored as glycogen
...
Fats:
Fats are a good source of energy and are carried by lymphatic capillaries
...
Blood carries fats to all parts of the body, especially the liver
...
Fats is not required in a large
amount by the body in the presence of glucose, however when there is less amount of

glucose, like during fasting, fats are transported to all body cells to provide energy for
vital activities
...
Fats are stored in special
tissues called adipose tissues
...
They protect the organs and act as shock absorbers

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