Transcript DEFENCE AND DISEASE

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Digestion of food
The mouth

The process of food digestion starts in the mouth. When
we think about food or see it and of course when we eat it
we produce saliva from our salivary glands. We have 3
salivary glands. These are the parotid gland, the
submandibular gland and the sublingual gland. These
empty saliva through tiny ducts into the floor of the
mouth. Saliva moistens and lubricates food and together
with the action of the tongue forms the food into a bolus
ready for swallowing. This slide illustrates the relative
size and positions of our three salivary glands.
The parotid gland

Here is a section of the parotid gland
magnified about 500 times. You can see
the saliva producing cell along with the big
duct in the middle that carries the saliva to
the mouth.
The sublingual gland

The sublingual gland sits at the bottom of the
tongue and the saliva produced here contains
mucus which helps the bind the food bolus and
make it sticky. The whitish cells secrete mucus
and you can clearly see the ducts which carry the
saliva to the mouth.
Mastication and Deglutition

Chewing is called mastication. The cheeks
close and hold food between the teeth and
the tongue mixes the food with saliva.
When we swallow (deglutition) several
voluntary and involuntary phases occur
which make sure that the food bolus travels
down our oesophagus. If we talk when we
eat then food may enter the respiratory
path.
Digestion in the mouth

Saliva contains an enzyme called salivary
amylase. This enzyme is especially designed to
start digesting polysaccharides. Most of the
polysaccharides that we eat are starches and these
are polymers of glucose. If the food stays in our
mouth long enough the salivary amylase chops
the starches up into smaller components called
limit dextrins. But if you gulp your food then not
much digestion occurs in your mouth.
Epiglottis

When you swallow the epiglottis retroverts to
deflect a bolus of food from entering the trachea
and making you choke. If you do eat whilst
lounging around there can be a tendency for the
epiglottis not to cover the windpipe adequately
and food can “go down the wrong way”. Can
you see the elastic cartilage? This is very
important as the epiglottis has to bend millions of
times during your life and immediately spring
back into shape. Fortunately for us the elastic
cartilage allows it to do so.
Into the oesophagus

The food bolus then travels through the
oesophagus. This is lined with stratified
squamous cells which are resistant to
abrasion. In this slide you can see these
cells and the mucus producing cells. This
mucus helps lubricate the oesophagus,
making the passage of food easier.
Into the stomach

The oesophagus enters into the stomach through
the lower oesophageal sphincter (LOS). This is a
ring of circular muscle that remains closed and
only opens when you swallow. It remains closed
to stop the acidic contents of your stomach
bubbling up (or reflux) into your oesophagus.
Sometimes babies are born with an incomplete
LOS and the milk that they drink just runs out.
Fortunately, the muscle soon develops correctly.
Look at the diagram of the stomach in this slide.
Stomach

Food gets mixed and churned in the stomach and
the resulting mixture is called chyme. Special
cells in the pyloric region secrete hydrochloric
acid to aid in digestion of proteins. Because the
stomach is so acid (pH about 1.0) any digestion
of carbohydrates that started in the mouth soon
stops as acid destroys salivary amylase. But
fortunately for us the stomach also secretes
mucus that protects its own cells from the acid.
Digestion in the stomach

The sight, smell and taste of food stimulates our
stomach to secrete gastric juice and a hormone
called gastrin which in turn stimulates the special
cells in the pyloric region to secrete more acid,
mucus and a special protein digesting enzyme
called pepsin. It is secreted by cells as
pepsinogen and soon converted to its active form.
Pepsin could not be secreted by cells as it would
break down its own cell and destroy them!
Digestion (continued)


Proteins are long sequences of amino acids and pepsin
attacks proteins at certain points in the protein chain.
These smaller “polypeptides” can be absorbed into the
blood or passed into the duodenum where they are
attacked further by different enzymes.
At the same time the muscle walls of the stomach contract
and churn up the mix. When the chyme is mixed then a
rippling peristalsis begins in the lower part of the stomach
and eventually this reaches the pyloric sphincter, it then
opens a small amount to permit about 3ml of chyme to be
squirted into the duodenum. It takes about 4 hours for all
the stomach contents to be passed into the duodenum or
about 6 hours if you have a fatty meal.
Stomach surface mucus cells

Stomach surface mucus cells survive about 7 days
and are replaced by new cells from stem cells.
Excessive alcohol or aspirin cause damage to the
surface cells but within an hour new cells migrate
upwards. And the acid is neutralised by
bicarbonate ion in mucus.
Duodenum

The function of duodenum is final
digestion of food into smaller units,
absorption of nutrients into mesentery and
production of alkaline mucus to neutralise
the acidic stomach acid contents.
Duodenum histology

The surface area of the duodenum is increased by
massive amounts of villi. We will look at this in
more detail when we consider how nutrients are
absorbed. A brush border of many tiny villi on
the surface of the epithelial cells greatly enhances
absorption and produces special digestive
enzymes.
The bile duct and pancreatic duct

Both the bile duct (from the gall bladder)
and the pancreatic duct from the pancreas
empty their contents into the duodenum.
You can see this illustrated in this slide.
Bile

The liver manufactures bile and passes it to the
gall bladder for concentration and storage. When
fatty foods enter the duodenum this stimulates the
gall bladder to deliver bile. When fatty food
come into the duodenum, they release a hormone
called cholecystokinin into the blood stream and
this causes the contraction of smooth muscle in
the gall bladder forcing the bile out. Fat is not
soluble in water and bile solubilises the fat (just
like a detergent) and makes the fats ready for
digestion.
Pancreatic juice

We’ve just seen how cholecystokinin stimulates the gall
bladder to produce bile. This same hormone also
stimulates the pancreas to release pancreatic juice. This
contains pancreatic amylase (for digesting carbohydrates)
and pancreatic lipase (for digesting fats). Another
hormone called secretin is released by the duodenum
when acid comes into that chamber and this acts on the
pancreatic duct cells to produce an alkaline pancreatic
juice. Obviously, this neutralises the acid contents that
have come from the stomach into the duodenum. This is
essential so that the enzymes can work
Solubilisation of fat

The bile attaches to fat droplets making the
fat droplets soluble in water. This is the
same action as detergent when you wash
fatty dishes.
Fat digestion

Now that the fat is solubilised by the bile
digestion can occur. Pancreatic lipase
breaks down the fats into glycerol and fatty
acids
Carbohydrate digestion

The remaining polysaccharide units from
digestion in the mouth are acted on by
pancreatic amylase within minutes of
entering the duodenum. They are cut up
into much smaller units. Enzymes that are
secreted by the brush border then cut up
these fragments into monosaccharides such
as glucose.
Protein digestion

The remaining protein fragments from
digestion in the stomach are cut into
smaller peptides and eventually into
individual amino acid units by enzymes in
the pancreatic juice. These enzymes
include trypsin, chymotrypsin and
carboxypeptidase.
Large intestine digestion

Most bacteria have been killed but some survive and these
colonise the large intestine. They ferment some of the
undigested polysaccharides such as cellulose and
undigested proteins. In some diets which rich in proteins
not all the protein is digested and these bacteria go to
work. Unfortunately they produce a mixture of gases that
contain sulphur and this does smell somewhat. Beans
have a carbohydrate called raffinose which we cannot
digest but bacteria can and the product is even more gas.
The bacteria also do good things like synthesise Vitamin
K and Vitamin B12
Activity

Write a report outlining the metabolism of
food.