0610 · 7.2Digestive system

Alimentary canal and associated organs

0:00Think about the sandwich you ate for lunch. Right now, pieces of it are travelling through a tube inside you that is several metres long. Some of it will end up inside your muscle cells. Some of it will end up in your brain. And some of it will never really enter your body at all. How can food be inside you, but not really in your body? By the end of this video, you will know the answer.

0:19By the end of this video you will be able to identify the main organs of the digestive system on a diagram, describe what each organ does, and use five key words correctly: ingestion, digestion, absorption, assimilation and egestion. You need two ideas from earlier videos. First, an organ system is a group of organs working together to carry out a job. Second, large food molecules, such as starch and protein, are too big to pass into the blood.

0:47They must be broken down into small, soluble molecules first. Enzymes, which you met in the enzymes video, help to do this. Here is the big idea. The digestive system is built around one long tube that runs from your mouth to your anus. This tube is called the alimentary canal. Food passes along it, in one direction, from start to finish. The inside of this tube is actually open to the outside world at both ends.

1:11So food in your stomach or intestines is inside the tube, but it has not yet crossed into your body tissues. Food only truly enters your body when it is absorbed into the blood. That is the answer to our opening question. There are also some organs that are connected to the alimentary canal, but food never passes through them. These are called the associated organs. They are the salivary glands, the pancreas, the liver and the gall bladder.

1:35Their job is to make or store juices that are released into the canal to help digest the food.

Mouth, oesophagus and stomach

1:40Let us build the whole system on a diagram. One quick warning first. Diagrams of the body are drawn as if the person is facing you. So the person's right side is on the left of the picture. Keep that in mind when you look at where the liver and the stomach are. The journey starts at the mouth. This is where ingestion happens. The syllabus defines ingestion as the taking of substances, for example food and drink, into the body.

2:04In the mouth, the teeth also break the food into smaller pieces. You will see how teeth do this in the video on physical digestion. Next to the mouth are the salivary glands. They make saliva, which flows into the mouth. Saliva moistens the food so it is easier to swallow, and it contains an enzyme called amylase, which starts to digest starch. When you swallow, the food goes down the oesophagus.

2:27This is a muscular tube that carries food from the mouth to the stomach. It runs down behind the windpipe. Muscles in its wall squeeze the food along. This squeezing is called peristalsis, and it moves food along the whole of the alimentary canal. The oesophagus leads into the stomach. The stomach is a muscular bag that holds food for a few hours. Its walls churn the food and mix it with stomach juices.

2:50The stomach starts the digestion of protein, using a protease enzyme. It also makes hydrochloric acid, which kills harmful microorganisms that were taken in with the food. The details of the stomach's enzymes and acid come in the video on chemical digestion. This is a good moment to define digestion. The syllabus defines digestion simply as the breakdown of food. Large pieces of food are broken into smaller pieces, and large molecules are broken into small, soluble molecules.

3:17Digestion starts in the mouth, continues in the stomach, and is completed in the small intestine.

Small intestine and nutrient uptake

3:22After the stomach, food enters the small intestine. The small intestine has two parts. The first, short part is called the duodenum. Three associated organs work with the duodenum. The pancreas makes pancreatic juice, which contains digestive enzymes. It releases this juice through a tube into the duodenum. Notice that it goes into the duodenum, not into the stomach. The liver makes bile. Bile is stored in the gall bladder, a small sac under the liver.

3:50When food arrives, the gall bladder releases bile through a tube into the duodenum. Bile contains no enzymes, but it helps fats to be digested. So the duodenum is where pancreatic juice and bile are mixed with the food, and digestion continues. Exactly how bile helps is covered in the videos on physical and chemical digestion. The second, much longer part of the small intestine is called the ileum. Here, digestion is finished, and the small, soluble molecules are absorbed into the blood.

4:19This is where most absorption happens. The syllabus defines absorption as the movement of nutrients from the intestines into the blood. So absorption is the step where food really enters your body. The blood then carries the nutrients all around the body, and many of them go to the liver first. How the ileum is adapted for absorption is the topic of the absorption video. Once nutrients are in the blood, they are delivered to cells.

4:44This leads to the fourth key word, assimilation. The syllabus defines assimilation as the uptake and use of nutrients by cells. For example, a muscle cell takes up glucose and uses it in respiration to release energy. A cell takes up amino acids and uses them to build new proteins for growth and repair. The liver also helps here. It takes up spare glucose from the blood and stores it as glycogen.

5:07So absorption and assimilation are two different steps. Absorption is nutrients moving from the intestine into the blood. Assimilation is cells taking those nutrients out of the blood and using them.

Large intestine and key mistakes

5:20Whatever cannot be digested or absorbed passes from the ileum into the large intestine. The large intestine has three parts: the colon, the rectum and the anus. The first and longest part is the colon. The colon absorbs water from the undigested food, so the waste becomes more solid. In fact, most water is absorbed in the small intestine, but the colon absorbs much of what is left. The semi-solid waste is now called faeces.

5:46Faeces are stored in the rectum, the last short section of the large intestine. Finally, faeces leave the body through the anus. This final step is egestion. The syllabus defines egestion as the removal of undigested food from the body as faeces. Egestion happens at the anus. And here are the five processes in order. Ingestion, taking food and drink into the body, at the mouth. Digestion, the breakdown of food, in the mouth, stomach and small intestine.

6:16Absorption, nutrients moving from the intestines into the blood, mainly in the ileum. Assimilation, cells taking up and using the nutrients. And egestion, removing undigested food as faeces, through the anus. Now for the most important common mistake. Many students say that faeces are excreted. That is wrong. Excretion means removing the waste products of metabolism, such as carbon dioxide and urea, and substances in excess of requirements. Metabolism is the chemical reactions inside cells.

6:45Undigested food was never absorbed and never used by cells. It just passed straight through the tube. So removing it is egestion, not excretion. If a question asks about faeces, the word you need is egestion. Here are four more mistakes to avoid. The first is mixing up the oesophagus and the windpipe. The oesophagus carries food to the stomach. The windpipe, which sits in front of it, carries air to the lungs.

7:11The second is saying that the liver stores bile. The liver makes bile. The gall bladder stores it. And both the bile and the pancreatic juice go into the duodenum, not the stomach. The third is thinking that food passes through the liver, the pancreas or the gall bladder. No, it does not. These are associated organs. Food only passes along the alimentary canal. Digested nutrients do reach the liver, but they get there in the blood, after absorption.

7:36The fourth is confusing the parts of the intestines. The duodenum and the ileum are parts of the small intestine. The colon and the rectum are parts of the large intestine. Most absorption of nutrients happens in the ileum, while the colon absorbs water. Also, remember that absorption means moving into the blood, not just moving into the intestine.

Digestive system exam question

7:57Let us try an exam-style question. The diagram shows the human digestive system. First, name the organs labelled A, B, C and D. Then, state one function of organ C, and state one function of organ D. Start by finding your landmarks. Organ A is a large organ on the left of the picture, which is the person's right side, high up under the ribs. That is the liver. Organ B is the small sac tucked underneath the liver.

8:23That is the gall bladder. Organ C is the flat organ just below the stomach. That is the pancreas. Organ D is the wide tube around the outside of the coiled small intestine. That is the colon, part of the large intestine. Now the functions. Organ C, the pancreas, secretes digestive enzymes into the duodenum. Organ D, the colon, absorbs water from undigested food. A model answer would read: Organ A, liver.

8:49Organ B, gall bladder. Organ C, pancreas. Organ D, colon, or large intestine. The pancreas secretes enzymes into the duodenum. The colon absorbs water. Notice two things. For the pancreas, saying where the enzymes go, into the duodenum, makes the answer precise. And for the colon, the key word is water. Saying that the colon digests food, or just that it absorbs nutrients, would not gain the mark. The answer they want is water.

9:18Let us recap. The alimentary canal is one long tube: the mouth, oesophagus, stomach, small intestine, made of the duodenum and ileum, and large intestine, made of the colon, rectum and anus. The salivary glands, pancreas, liver and gall bladder are associated organs. Food does not pass through them, but they release juices into the canal. The salivary glands make saliva, which starts the digestion of starch. The oesophagus carries food to the stomach.

9:45The stomach digests protein and kills microorganisms with acid. The pancreas releases enzymes into the duodenum. The liver makes bile, and the gall bladder stores it. In the duodenum, digestion continues with pancreatic juice and bile. The ileum completes digestion and absorbs nutrients into the blood. The colon absorbs water, the rectum stores faeces, and the anus is where egestion happens. And the five processes are ingestion, digestion, absorption, assimilation and egestion.

10:15Remember, removing faeces is egestion, not excretion. In the next video, we will look more closely at physical digestion, and how your teeth and stomach break food into smaller pieces.