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CSIRO_The Hungry Microbiome Project_The Digestive System-SD
[Image shows a person’s hand writing on a whiteboard and text appears: The Hungry Microbiome. A diagram is drawn
beneath the title]
Narrator: This is (indistinct word – 0:01). This video was created as part of the Hungry Microbiome Project, which I
made at CSIRO.
[Image changes to show a person’s hand writing on a whiteboard and text appears: Digestive System]
The digestive system is responsible for the breakdown, the digestion and absorption of food.
[Image changes to show a person’s hand drawing a diagram of a man and text appears: Oral cavity, oesophagus,
stomach, small intestine, large intestine]
The digestive tract, also known as the elementary 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.
[Image changes to show a person’s hand drawing on the diagram of a man and text appears: Teeth, tongue, salivary
glands, liver, gall bladder, pancreas]
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.
[Image shows a person’s hand writing on a whiteboard and text appears: Digestive System]
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.
[Image changes to show a person’s hand writing on a whiteboard and text appears: Digestive System, carbohydrates,
proteins, lipids]
[Image changes to show a person’s hand drawing a diagram and text appears: Ingestion, Digestion, Absorption]
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.
[Image changes to show a person’s hand drawing a diagram and text appears: Excretion]
Finally, waste is excreted.
[Image changes back to the drawing of the man]
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.
[Image changes to show a person’s hand drawing a diagram of the digestive tract]
The digestive tract have three main layers. The lumen is the inside space of the digestive tract.
Transcribed by: www.transcriberonline.com
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[14m:57s]
[Image changes to show a person’s hand drawing on the diagram and text appears: Lumen, mucus lubricates food,
protects lining of 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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Epithelial cells]
Some of these cells are what produce and secrete mucus.
[Image changes to show a person’s hand drawing on the diagram and text appears: Smooth muscle, contraction]
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.
[Image changes back to the diagram of the man, and the person’s hand indicates the path of the digestive tract]
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.
[Image changes to show a person’s hand drawing a diagram and text appears: Mastication, “chewing”]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Teeth, tongue, salivary glands]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Sublingual, parotid,
submandibular]
There are three salivary glands, the sublingual, meaning below the tongue, parotid, and submandibular, which means
below the mandible.
[Image changes to show a person’s hand drawing on the diagram and text appears: Secrete amylase]
The salivary glands also secrete an enzyme called amylase, which will initiate carbohydrate digestion.
[Image changes to show a person’s hand drawing on the diagram and text appears: Deglutination, “Swallowing”]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Bolus]
The bolus will travel through the oesophagus thanks... thanks to peristalsis.
[Image changes to show a person’s hand drawing on the diagram and text appears: Peristalsis, involuntary
contraction of smooth muscles]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Stomach, mixes food content]
Transcribed by: www.transcriberonline.com
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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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Secrete HCL, mucus and pepsin]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Pylorus Sphincter]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Chyme]
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.
[Image changes to show a person’s hand drawing on the diagram]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Duodenum, Jujenum, Ileum]
The ileum connects to the start of the large intestine here.
[Image changes to show a person’s hand drawing on the diagram]
The small intestine is very important because it is where most of the digestion and absorption of food takes place.
[Image changes to show a person’s hand indicating to the diagram of the small intestine]
However, it cannot do this without the help of the accessory organs, the liver, the gall bladder and the pancreas.
[Image changes back to the diagram of the man]
So let us learn a bit more about these accessory organs and learn about their role in digestion.
[Image changes to show a person’s hand drawing on the diagram and text appears: Liver, Gall Bladder]
So here we’re zooming into the liver and the gall bladder. The liver produces bile, which has a critical role in lipid
digestion.
[Image changes to show a person’s hand drawing on the diagram and text appears: Liver produce bile]
The gall bladder stores the bile, and when needed the gall bladder will contract and release the bile into the small
intestine.
[Image changes to show a person’s hand drawing on the diagram and text appears: Gall Bladder store bile. Bile duct.
Bile]
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.
Transcribed by: www.transcriberonline.com
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[Image changes to show a person’s hand drawing on the diagram and text appears: Chyme]
So the other important accessory organ that has to be mentioned here is the pancreas.
[Image changes to show a person’s hand drawing on the diagram and text appears: Pancreas]
The pancreas is an important endocrine and exocrine gland. During digestion the pancreas secretes many enzymes.
[Image changes to show a person’s hand drawing on the diagram and text appears: Secretes many digestive enzymes]
These pancreatic digestive enzymes will be secreted into the small intestine as well through the pancreatic duct.
[Image changes to show a person’s hand drawing on the diagram and text appears: 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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Lipase, Amylase, Trypsin]
So the chyme will encounter all these enzymes and will be digested further.
[Image changes to show a person’s hand drawing on the diagram and text appears: Digestion]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Maltase, Sucrase, 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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Monosaccharides, amino acids,
fatty acids and glycerol]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Absorption, Blood stream,
monosaccharides, amino acids]
The fatty acids are absorbed by the small intestine into the lymphatics, with the help of bile.
[Image changes to show a person’s hand drawing on the diagram and text appears: Lymphatics]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Small intestine]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Villi, Crypts]
Transcribed by: www.transcriberonline.com
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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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Thin mucus layer]
The small intestine also have... has a thin layer of mucus.
[Image changes back to the diagram of the large intestine]
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.
[Image changes to show a person’s hand indicating to the diagram and text appears: Non-digestible “foods”]
And so we will soon see what will happen to these non-digestible foods.
[Image changes back to the diagram of the man]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Caecum, Ascending, Transverse,
Descending, Sigmoid, Rectum]
[Image changes back to the diagram of the large intestine]
This pointy bit here is known as the appendix.
[Image changes to show a person’s hand drawing on the diagram and text appears: Caecum, Appendix, Immune
System]
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.
[Image changes to show a person’s hand drawing a diagram]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Colon cells, mucus, lumen,
bacteria]
Residing within the lumen are many types of bacteria that can digest these foods that have escaped digestion in the
small intestine.
[Image changes to show a person’s hand drawing on the diagram and text appears: Foods that are not digested and
absorbed in the small intestine undergo fermentation by bacteria in the colon]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Large intestine]
Transcribed by: www.transcriberonline.com
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[Image changes to show a person’s hand drawing a line on the diagram to text below: Crypts]
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.
[Image changes to show a person’s hand drawing on the diagram and text appears: Thick mucus layer]
[Image changes to show a person’s hand drawing a line on the diagram to text above: Thin mucus layer]
The difference in mucus thickness is thought to be because of the denser population of bacteria that reside in the
large intestine.
[Image changes to show all the diagrams drawn]
And of course substances, foods and contents that are not digested and absorbed, will be excreted by the body as
faeces.
[Image changes to show a person’s hand indicating to the diagram]
And that is where I finish. Thank you for watching.
[Music plays and CSIRO logo appears with text: Big ideas start here www.csiro.au]
Transcribed by: www.transcriberonline.com
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