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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.