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  • (Narrator) This is Armando. This video was created as part of the Hungry Microbiome Project,

  • 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 were 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.