which I made at CSIRO.

The digestive system is responsible for the breakdown,

the digestion and absorption of food.

The digestive tract, also known as the alimentary canal,

is the pathway where food travels through after being ingested.

The digestive tract consists of the mouth,

oesophagus,

stomach,

small intestine,

large intestine also known as the colon,

and the rectum.

Therefore we can refer to the digestive tract

as a long tube that runs from the mouth to the rectum.

Food enters the mouth and leaves through the rectum.

But there are also other organs called accessory organs

that play fundamental roles in digestion.

These organs include the salivary glands,

tongue, teeth,

the liver, gall bladder and the pancreas.

Without the proper function of these accessory organs

food will not be digested properly, and consequently

will not be absorbed, resulting in

gastrointestinal diseases, such as malnutrition.

So looking at a general overview of the digestive system,

food can be grouped into three main categories

carbohydrates, such as bread; protein, such as meat;

and lipids, such as oil.

These foods are ingested by the human.

The food will be digested by the digestive system

and then absorbed into the blood stream,

where it will be then delivered to body tissues as energy,

or for storage.

Finally, waste is excreted.

Another fundamental point to take in

is that the digestive tract

all share three similar anatomical properties.

Here I am drawing a diagrammatical cross section

representation of the digestive tract.

The digestive tract have three main layers.

The lumen is the inside space of the digestive tract.

The first layer is

mucus, and we find it around the lumen.

Mucus lubricates the food

and also protects the lining of the digestive tract.

Then you have the epithelial cells,

the lining of the digestive tract,

that forms the tract itself.

Some of these cells are what produce and secrete mucus.

The third layer is the smooth muscle layer, the outer layer,

which is important in contraction.

Through contraction

the smooth muscle allows the food

to move through the digestive tract after being ingested.

Now that we have a better feel for the digestive system,

let us look at each of the organs of the digestive system

and what they do in relation to foods being consumed.

So food enters the oral cavity,

the process called mastication occurs, which is essentially chewing.

Food will be broken down mechanically by the mouth,

teeth playing a key role.

The tongue plays a role in tasting the food,

as well as mixing the food around,

while the salivary glands within the oral cavity secrete saliva,

which lubricates the food.

There are three salivary glands,

the sublingual, meaning below the tongue,

parotid, and submandibular, which means below the mandible.

The salivary glands also secrete an enzyme called amylase,

which will initiate carbohydrate digestion.

Once the food leaves the oral cavity

in a partially digested form it is swallowed,

a term called deglutination.

The food is actually now referred to as a bolus.

The bolus, which means ball in Latin,

is a mass of food that has been chewed up.

The bolus will travel through the oesophagus

thanks to peristalsis.

Now peristalsis is the involuntary contraction of the smooth muscles

that line the digestive tract.

Peristalsis allows the movement of food

through the digestive tract essentially.

And so the bolus will eventually enter the stomach.

The stomach will temporarily store and churn the bolus.

The stomach is able to churn the bolus

because it has three layers of muscle.

The stomach cells also secrete chemicals and enzymes,

such as hydrochloric acid, that helps break down the food,

kill bacteria, and stimulate enzyme secretions.

The stomach cells also secrete mucus

that helps protect the lining of the stomach,

as well as pepsin that begins protein digestion.

The vigorous contraction of stomach muscles

and the stomach secretions,

result in the liquefaction of the food,

which is then slowly released into the small intestine.

The pylorus sphincter

is the barrier between the stomach and the small intestine.

During digestion the pylorus sphincter opens in phases,

allowing the liquefied food,

now referred to as chyme,

to enter the small intestine.

Now chyme is the term used to describe a semi-fluid mass

of partially digested food.

So essentially before the stomach the food was referred to as bolus,

after the stomach it is chyme.

The small intestine runs from the pylorus

to the ileocecal value,

which... where it joins to the large intestine.

The small intestine is divided into three segments,

the duodenum, the jejunum, and ileum.

The ileum connects to the start of the large intestine here.

The small intestine is very important

because it is where most of the digestion and absorption

of food takes place.

However, it cannot do this without the help of the accessory organs,

the liver, the gall bladder and the pancreas.

So let us learn a bit more about these accessory organs

and learn about their role in digestion.

So here we’re zooming into the liver and the gall bladder.

The liver produces bile,

which has a critical role in lipid digestion.

The gall bladder stores the bile,

and when needed the gall bladder will contract

and release the bile into the small intestine.

So bile will enter the small intestine

through the bile duct.

So to see where the food is,

the chyme, the chyme is actually coming from the stomach

and is here within the small intestine now.

So the other important accessory organ that has to be mentioned here

is the pancreas.

The pancreas is an important endocrine and exocrine gland.

During digestion the pancreas secretes many enzymes.

These pancreatic digestive enzymes

will be secreted into the small intestine as well

through the pancreatic duct.

The pancreatic duct actually connects with the bile duct

and... and consequently to the small intestine.

The main enzyme secreted by the pancreas

for digestion are lipases for lipid digestion,

pancreatic amylase

for carbohydrate digestion,

and protease that helps in protein digestion.

So the chyme will encounter all these enzymes

and will be digested further.

The small intestine itself have some enzymes,

called brush border enzymes,

that are actually found on the cell membranes.

These brush border enzymes include maltase,

lactase, sucrase and peptidase.

The brush border enzymes are sort of the final step of food digestion.

The chyme will encounter all these enzymes and chemicals

which will further digest

it into smaller molecules.

These smaller molecules are the monosaccharides, amino acids,

fatty acids and glycerol,

which are the building blocks of carbohydrates, proteins and fats.

And it is only in this form

that the body can absorb them.

So the monosacchardies and amino acids

are able to be absorbed by the small intestine

into the blood stream,

where it will then travel to specific areas in the body.

The fatty acids are absorbed by the small intestine

into the lymphatics, with the help of bile.

So from all this we can see how the small intestine is the major place

for the digestion and absorption of food,

therefore it is important that we understand

more about the histology of this organ.

If we zoom into the small intestine

we can find the lining of the intestine,

with its rich blood supply.

The small intestine

is composed of finger like projections

called villi and crypts.

Now below the crypts

are stem cells that keep dividing,

renewing the cells above.

The villi is important in food digestion and absorption

because it increases surface area.

The small intestine also have... has a thin layer of mucus.

Now we move onto the large intestine, also referred to as the colon.

The main function of the large intestine

is to absorb water and ions,

as well as storing and transporting waste

that will be expelled by the body.

But there are other things that happen within the large intestine,

because what happens is that foods

that are not digested and absorbed in the small intestine,

these foods will reach the colon.

And so we will soon see what will happen

to these non-digestible foods.

But first we need to learn a bit more

about the anatomy of the large intestine.

The large intestine is divided into a few sections,

the caecum, the ascending colon,

transverse colon, the descending colon,

the sigmoid colon and the rectum.

There is also the anus region.

This pointy bit here is known as the appendix.

It has a role in the immune system.

What people... what most people may not know about the large intestine,

in particular the proximal area of the large intestine,

so I’m talking about the caecum,

is that a lot of these non-digested foods

undergo fermentation by the trillions of bacteria that live here.

So let’s have a look at what happens.

So here we are zooming into this area.

Here we have the colon cells

and we can find mucus above the colon cells, and so here is the lumen.

Residing within the lumen are many types of bacteria

that can digest these foods

that have escaped digestion in the small intestine.

So foods that are not digested and absorbed in the small intestine

can undergo fermentation by bacteria in the colon.

Through fermentation the bacteria can produce

many substances that the human body can use,

such as short chain fatty acids.

Finally it is important to compare the histology

between the small intestine and the large intestine.

So let’s take a section of the large intestine.

It also has a rich blood supply to the lining.

The large intestine has crypts where stem cells keep dividing,

renewing the cells above.

However the large intestine do not have villi.

And to add further to this, the large intestine has a very

thick mucus layer compared to the small intestine,

with an additional thinner mucus layer on top.

The difference in mucus thickness

is thought to be because of the denser population of bacteria

that reside in the large intestine.

And of course substances, foods

and contents that are not digested and absorbed,

will be excreted by the body as faeces.

And that is where I finish. Thank you for watching.

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