Biology || Class-X || Human Digestive System [Part 1]

追加: 2026-05-22

[00:00:03]Hello students. Today we will learn about the human digestive system.

[00:00:07]Uh in the earlier video we have learned about autotrophic nutrition. Today is the second part of the first topic that is nutrition where we are going to learn about the human digestive system.

[00:00:20]Very important system and also very important for your board exam. So this is the overall picture of the human digestive system.

[00:00:29]So it starts from an opening which is called the mouth and it ends at another opening called the anus.

[00:00:39]The organs which you see in between the mouth and the anus, they are concerned with the food that you eat. So what are those organs? Let us see. We have the mouth first, then the esophagus, the liver, then the small intestine, the large intestine. These are the parts that I have underlined with yellow. They make up the alimentary canal of human beings.

[00:01:06]So let us list those parts which makes up the alimentary canal.

[00:01:16]So the alimentary canal starts with the mouth which can also be called the buccal cavity.

[00:01:31]This is followed by the esophagus or the food pipe.

[00:01:37]Then we have the stomach.

[00:01:43]Then we have the small intestine and finally the alimentary canal ends with the large intestine.

[00:01:57]So these are those five parts that make up the alimentary canal. But then, you might tell me, "Ma'am, what about the liver?

[00:02:08]What about the salivary glands? What about the pancreas?"

[00:02:13]Well, these are, if you see the system minutely, they are not a direct part of the tract. They make secretions and they pour it into the alimentary canal. So, let us call these as associated glands.

[00:02:31]The associated glands, what are those associated glands? Number one is the salivary glands.

[00:02:46]Then, we have liver.

[00:02:50]And third, we have pancreas.

[00:02:55]So, you see, on that side I have written down the parts that makes up the alimentary canal. And here we have those glands which are associated with the alimentary canal. Right? So, these together, they make up the entire digestive system of human beings. Okay?

[00:03:13]So, we will start with these parts one by one. We'll be learning about these parts in details one by one. We will also be seeing how complex food is broken down into simple food because that is exactly the job of the digestive system. It is to break down the complex food material which you have ultimately into such simple forms that can be used up by every cell of your body. Let us see how it is done.

[00:03:42]Let us first talk about the buccal cavity.

[00:03:46]The buccal cavity is the inside of your mouth. So, truly speaking, mouth and buccal cavity, there's a small difference between them. Mouth is this opening.

[00:03:59]And the the cavity that this opening leads into, the inner part, that is called the buccal cavity. So, if you see the buccal cavity, which is the first part of the alimentary canal, the buccal cavity will show you teeth, tongue, and the buccal cavity also contains saliva, which comes into the buccal cavity from the salivary glands.

[00:04:35]I told you in my earlier slide that salivary glands are not a part of the alimentary canal. So, we have three pairs of salivary glands, and these three pairs, they are present around the mouth. They make saliva, and they pour it into the mouth or into the buccal cavity. So, we say that in the buccal cavity, we find three such structures or three things, which is the teeth, the tongue, and the saliva. Let us learn about these one by one. So, these are the different teeth that we have. We all know that we have 32 teeth.

[00:05:13]These are the permanent teeth.

[00:05:18]And these are of four types.

[00:05:23]What are those four types? The four types are the teeth in front of your mouth.

[00:05:30]Four over here and four over here. That means four in front of the upper jaw, four in in front of your lower jaw, and those are called incisors.

[00:05:40]The immediate teeth after the incisors on each half of the jaw, that is called the canine.

[00:05:48]Then, the next two teeth after the canine, again on each half of your mouth, are the premolars. And finally, the last three teeth on each side of or on each half of the jaw, the those are called the molars. So, you I see like we have incisors, canine, premolars, and molars on the upper arch or the upper jaw. Similarly, in the lower arch or in the lower jaw, also we have the same arrangement of teeth. Now, what do these teeth help in?

[00:06:24]The first type of teeth that we have learned about are the incisors, and they help us to cut our food. This is the reason why herbivores have very sharp incisors, because you know they have to rip open various parts of a plant, the leaves or the small twigs, and you know, make their food available to them. So, they help us to cut our food. Canines are very important for the carnivores.

[00:06:51]They help you to tear and grasp food.

[00:06:56]So, they help you, say for example, a tiger. When it catches a deer, it will have to hold the deer and then rip open its flesh, right? So, holding as well as tearing flesh, both is done by the sharp pointed canines.

[00:07:12]Herbivores, mind you, they do not have canines because they don't eat flesh.

[00:07:16]So, that part of their jaw where the canine was supposed to have been, that part remains empty. There is no teeth in that part where others have their canines. Next, the premolars. The premolars help us to crush and tear food, the back tooth.

[00:07:37]And finally, the molars, they help us in chewing and grinding food. So, both premolars and molars, they are help us they help us in the process of chewing food and biting them into small and very small pieces. So, what is this process of biting and chewing food called?

[00:07:59]Chewing.

[00:08:02]In scientific terms, it's called mastication.

[00:08:08]So, let me put it simply, teeth helps us in mastication of our food.

[00:08:15]Next, we have the tongue.

[00:08:19]This is the tongue that we have. This is a highly muscular organ present in the buccal cavity and very specifically, there are taste buds located at different regions of the tongue.

[00:08:37]Taste buds help us in tasting food, but there are taste buds dedicated to specific tastes of food. For example, the taste buds concerning the bitter taste are found towards the back portion of the tongue. Those concerned with the taste of sourness is present on the upper sides of the tongue.

[00:09:03]Salty on the lower side of the tongue and finally, the tip of your tongue is concerned with the sense of sweetness.

[00:09:13]So, there are taste buds present and they are localized very specifically in different parts of the tongue so that we are able to taste specific types of food with the help of these taste buds. But, is tasting food the only job of the tongue? No, there are other jobs as well. What are those jobs? There are many functions of the tongue like mastication and deglutition of food. So, when you are chewing the food with your teeth, the tongue works like a natural spoon. It goes on pushing the food towards your teeth. That is why it also helps in mastication of the food. And finally, what is deglutition? The meaning of deglutition is swallowing.

[00:09:59]When you have finished chewing your food, then you swallow the food. And that swallowing is done because the tongue pushes the food into your esophagus or into the food pipe. What else? It helps us in speech.

[00:10:16]It helps you to talk. Without your tongue, you wouldn't be able to talk.

[00:10:19]And it keeps the path of the air open.

[00:10:24]It keeps this passage open so that there is a constant movement of air and we are able to breathe properly. So, all these are the functions of the tongue.

[00:10:35]Let us move on to the next part of the alimentary canal, that is the esoph- uh the esophagus.

[00:10:42]So, this is your esophagus. If you come to the main picture, this is the esophagus. This portion this esophagus is also called food pipe.

[00:10:58]It is also called the food pipe. So, this esophagus or the food pipe, as you can see, it connects the mouth to the stomach. This is your stomach, this J-shaped organ. This organ is the stomach. And this portion is the buccal cavity. So, this pipe which connects the buccal cavity to the stomach is nothing but the food pipe. And what does the food pipe do? Remember, the food pipe, if you see the food pipe closely, then the food pipe helps in carrying food from the mouth to the stomach. Now, what has happened in the mouth? In the mouth, the food has been chewed with the help of teeth. It has been mixed with saliva, and it has become moist. So, this food this moist and chewed food, which is coming from the buccal cavity into the stomach, it is called bolus.

[00:11:52]So, this food is called bolus.

[00:11:59]So, what is bolus? Bolus is the food the chewed food the moistened food, which is coming down from the buccal cavity into the stomach. How does it come? You see this process taking place in the esophagus. You can see that some parts of the esophagus is constricted, whereas the remaining part is wide open.

[00:12:20]Gradually, as the food moves down, the part on the top, which was relaxed so far or which was wide so far, will now contract, and the part immediately above the food will constrict. So, this will again push the food down. This way, the esophagus contracts and relaxes, contracts and relaxes, making a wave-like motion. And due to this wave-like motion, the food moves constantly downwards towards the stomach. So, this wave-like motion this wave-like motion, which is mainly because of the muscle contraction, this is known as peristalsis.

[00:13:01]And remember, peristalsis does not occur only in the esophagus. It occurs across the entire length of the alimentary canal. Okay? Next, let us go into the human digestive system once again. This is a functional picture.

[00:13:23]So, you see a quick recapitulation over here. What happens in the upper portion?

[00:13:28]In the upper portion, mechanical digestion. That means mechanical digestion means here the food is chewed and the food is swallowed. Okay? And chemical digestion of carbohydrates take place. We will see this a little later.

[00:13:45]What? There is something in saliva which also helps in chemical breakdown of the food. We have not covered that now. We will be doing it a little later.

[00:13:55]In the next part, that is in the stomach, what happens? Again, mechanical digestion.

[00:14:00]Mechanical digestion means the food is mixed well. The food is churned in the stomach and that will help in further breakdown of the food. So, mechanical digestion, in addition, chemical digestion means absorption or other chemical digestion means breakdown with the help of enzymes and absorption of lipid-soluble substances. Absorption majorly occurs in the small intestine.

[00:14:26]Small amounts of digested material, uh for example, alcohol or maybe aspirin may be absorbed in through the walls of the stomach also, but we need not pay attention to that. We will not focus on that. We will learn that the primary center for absorption of food is a small intestine.

[00:14:45]Next, coming to the small intestine, here again mechanical digestion, that means mixing of the food with all the digestive juices. Then, chemical digestion means breakdown with the help of enzymes and now absorption of the different products of digestion will take place in this part. Finally, we come to the large intestine where no chemical digestion occurs.

[00:15:13]Mechanical digestion because food constantly keeps moving and some ions and water is reabsorbed back and finally what ends over here, what comes out from here is the undigested food or feces.

[00:15:30]So, the large intestine is mainly concerned with the process of egestion.

[00:15:34]That means removal of the undigested food from the body. We will see these in details now.

[00:15:45]This is again a brief overview of the entire process. See, this is saliva which is which is coming from the mouth.

[00:15:56]So, saliva and the food, this is esophagus. What we have over here is a new term. It is called the acid chyme.

[00:16:05]The food in the stomach is called chyme.

[00:16:08]We will see why because it mixes with hydrochloric acid. We will explain this a little later. Remember that the food in the stomach is called chyme. Now, if you remember, I had told you that the food which enters into the esophagus from the buccal cavity, what is the name of the food if you remember?

[00:16:28]It was called the bolus.

[00:16:31]So, when you swallow, you swallow the bolus. But, when this bolus reaches your stomach, ultimately turns into the acidic chyme. And then, in the stomach, some digestion will take place.

[00:16:46]In the small intestine, a lot of digestion will take place. Why will a lot of digestion take place? Because the pancreas will release a juice into the small intestine. Then, the liver or the gallbladder will release a juice into the small intestine. Not only that, from the walls of the stomach of Oh, sorry.

[00:17:10]The walls of the small intestine also certain juices are going to be released.

[00:17:15]And by the action of these juices, food is completely going to be digested in the small intestine. Okay? Now, we are going to see the exact chemical process what happens during digestion.

[00:17:29]So, moving back to the buckle cavity. I told you in the buckle cavity food is chewed with the help of teeth and the process of chewing is also called mastication.

[00:17:44]The process of chewing is also called mastication. Along with that, the food is moistened with the help of saliva and because of the moistening, it can easily slip down into your esophagus because the food has been lubricated and this food is called the bolus.

[00:18:04]This food is called the bolus.

[00:18:07]Now, suppose this bolus contains a starchy material. Say for example, you've had rice or maybe you've had bread. We know that rice or bread, it contains starch. Then what happens is in the saliva, there is a special enzyme which is called the salivary amylase, which is also called the ptyalin. What does it do? It will act on the starch present in the food and convert it into a simpler form of carbohydrate, which is called maltose. So, you see the process of digestion has begun in the buckle cavity because of the presence of the enzyme salivary amylase or ptyalin. So, if you are asked, "Where does the process of digestion begin?" you have to say it is the buckle cavity. Reason is because the enzyme called salivary amylase, which breaks any starch present in the food and converts it into a simple form into a simpler form, which is called maltose.

[00:19:08]Okay? In the esophagus, I've already told you that there are no digestive juices produced in the esophagus, hence no chemical breakdown or no digestion takes place, but yes, the food moves by the process of peristalsis to the next part of the elementary canal, which is called the stomach.

[00:19:29]In the stomach, what happens? In the stomach, two types of digestion happens.

[00:19:34]One is called the mechanical breakdown of food, the other one is called the chemical breakdown of food. So, in mechanical breakdown of food, there's a word used which is called churning.

[00:19:45]Churning means as soon as food enters into the stomach, the stomach is a highly muscular bag. Its walls are highly muscular, so these walls, they start contracting and relaxing in different ways so that the food inside now is properly broken down. Right? It is properly broken down. Not only is it broken down, it is also mixed up with hydrochloric acid. Now, where is this hydrochloric acid coming from?

[00:20:15]Let us go back to the structure once again. If you see the stomach, if you see the stomach, along the wall of the stomach, along the wall of the stomach, you have these glands which are called the gastric glands.

[00:20:35]The gastric glands.

[00:20:38]What do these gastric glands do? The gastric glands produces a juice which is called the gastric juice.

[00:20:48]What are the components of the gastric juice? The components of the gastric juice is hydrochloric acid and it contains a protein breaking enzyme called pepsin and it also contains mucus.

[00:21:05]Very important, the gastric juice, which is produced by the gastric glands present in the walls of the stomach, contains hydrochloric acid. It contains an enzyme called pepsin, and it also contains mucus. So, the food which is coming from the esophagus, this bolus, now mixes with hydrochloric acid, and it gives rise to acidic food, which is called chyme. So, bolus, when it comes down into the stomach, it is not only mechanically churned, it is also mixed with hydrochloric acid, and it is converted into chyme. What is the reason for this?

[00:21:49]There are two reasons for this. First is hydrochloric acid, it has two functions.

[00:21:55]It kills the germs coming in along with the food.

[00:22:00]It's a very, very powerful acid. Very strong acid. So, whatever germs are coming in along with the food, hydrochloric acid can kill those germs.

[00:22:10]Second is, it makes the medium acidic.

[00:22:14]It makes the medium acidic.

[00:22:19]It makes the medium acidic. Why does it have to make the medium acidic? Because if it did not make the medium acidic, the enzyme pepsin wouldn't have been able to act on the food. So, it makes the enzyme acidic for pepsin to act on the food. What type of food is pepsin going to act on? Pepsin is going to act on the proteins present in the food, and convert them into simpler peptones and proteoses.

[00:22:46]Peptones and proteoses are not yet the end products. They have to be still broken down, but the first breakdown of proteins takes place in the stomach with the help of pepsin, where the proteins are converted into peptones and proteoses. Okay, that was all about the stomach. Let us see now what happens in the small intestine.

[00:23:08]I told you in the small intestine when this food with the food which is present in the stomach stays in the stomach for 3 to 4 hours during which the food is converted into chyme and pepsin acts on the food and finally now after 3 to 4 hours the food is now ready and the food moves down to the next part of the alimentary canal which is the small intestine. So now from here the acidic chyme the chyme which is acidic now it moves into the small intestine. And if you remember I told you in the small intestine there are three juices which acts on the food. So the food which is coming from the stomach is called chyme and on the chyme three types of juices are going to act. One is bile coming from the liver.

[00:24:03]Then we have pancreatic juice coming from the pancreas.

[00:24:11]And we have the intestinal juice which comes from the walls of the small intestine.

[00:24:21]So chyme is acted upon by these three juices. Let us see them one by one. So when food or a chyme enters into the small intestine the same thing mechanical breakdown or churning of the food happens in the small intestine so that it can mix well with these juices.

[00:24:41]What is the first juice which is poured over the food? The first juice is bile.

[00:24:47]Where does it come from? It is produced in the liver.

[00:24:52]In the liver the bile juice is produced.

[00:24:56]Liver produces bile juice, but liver does not store bile. Liver conveys this bile the bile is taken from the liver and stored in the gallbladder.

[00:25:09]So, gallbladder is not an organ which produces bile. It only stores bile that has been produced by the liver. So, when when this bile produced by the liver, when it first falls on the chyme, what does it do? Its first job is to provide an alkaline medium. Remember when I was telling you about the stomach, I said that hydrochloric acid is important because it makes the food acidic, otherwise pepsin will not act on the food. But, now this food which is coming from the stomach is acidic.

[00:25:45]But, the remaining enzymes, that means the enzymes present in the pancreatic juice or the intestinal juice, they will not be able to act on the food till the acidity uh the chyme is neutralized. And that neutralization of the acidity of chyme is done by bile. So, bile provides alkaline medium. In other words, it neutralizes the acidity of the chyme so that the enzymes of the pancreatic juice and intestinal juice can carry forward the job of digestion. And it causes emulsification of fats. Very important.

[00:26:19]Emulsification means what?

[00:26:21]Emulsification means fats normally are very big chunks of substances. Right?

[00:26:28]They are uh high molecular weight substances. So, they have to be broken down. They have to be actually broken down into smaller pieces. They are not broken down chemically, remember.

[00:26:40]Emulsification means the fats, the big fat chunks are broken down into smaller droplets. And so, when they are broken down into smaller droplets, the surface area of the fat is increased and that helps in better uh uh action of enzymes. So, the job of emulsification of fat is to increase surface area for the action of the fat digesting enzymes.

[00:27:23]So, this is the job of bile.

[00:27:28]So, first job is to provide an alkaline medium. Second job is to job is to cause emulsification so that the fat digesting enzymes can act on the fats. Next, let's learn about the pancreatic juice and the intestinal juice. Pancreatic juice is produced by the pancreas and put into the small intestine. What does pancreatic juice contain? Pancreatic juice contains pancreatic enzymes. What are these enzymes? Trypsin, pancreatic amylase, pancreatic lipase.

[00:28:07]If you remember, we had learned about amylase somewhere else also. Where did we learn about amylase?

[00:28:13]We learned about amylase here.

[00:28:18]We learned about salivary amylase present in the saliva, which is a starch digesting enzyme. Similarly, another starch digesting enzyme called the pancreatic amylase is present in the pancreatic juice. And let us see what the job of these enzymes are. So, trypsin acts on the proteins and peptides that were broken down earlier in the stomach.

[00:28:43]So, proteins and peptides are broken down with the help of the enzyme trypsin into smaller peptides and amino acids.

[00:28:51]Right?

[00:28:52]Amylase, the pancreatic amylase, any starch that was left over, any starch which might have been left over by the salivary amylase in the buccal cavity, those leftover starch is digested by amylase into maltose and the emulsified fats, emulsified by bile, the emulsified fats are broken down by the pancreatic lipase into fatty acids and glycerol.

[00:29:21]So, we are gradually going from complex food to simpler food. That means we are still into the process of digestion.

[00:29:28]Now, these will be further broken down.

[00:29:31]Further broken down by what? By the enzymes present in the intestinal juice.

[00:29:37]Let us see what those enzymes are.

[00:29:39]The enzymes of the intestinal juice are peptidase, maltase, sucrase, lactase, and intestinal lipase. We will see their jobs one by one.

[00:29:55]So, peptidase works on peptides. If you see this chart, these smaller peptides, these smaller peptides are finally converted into amino acids by the peptidase present in the intestinal juice. Maltose is a sugar, breakdown product of starch. So, maltose is converted by maltase into glucose.

[00:30:24]Sucrose is sugar, the normal sugar that we have.

[00:30:28]Sucrose is converted by sucrase into glucose and fructose. Lactose is the milk sugar converted by lactase into glucose and galactose. And the emulsified fats, if any emulsified fats are still left behind, then they are converted by lipase into fatty acids and glycerol.

[00:30:50]Okay? So, by and large, over here, the process of digestion is completed.

[00:30:57]Now, what is the next step? The next step is absorption. Food is now going to be absorbed. What is going to be absorbed? All the smaller substances which we have received.

[00:31:10]Carbohydrates have been broken down into glucose, proteins have been broken down into amino acids, fats have been broken down into fatty acids and glycerol. So, these are the end products of digestion of all the complex nutrients present in food. Now, the next process is absorption after digestion.

[00:31:33]Now, we have the process of absorption.

[00:31:37]So, this shows the inner surface of the small intestine. Which part of the small intestine?

[00:31:45]All this process of digestion All this process of digestion that you've seen That means the breakdown in the by the pancreatic juice, by the intestinal juice, and the action of All this digestion part is completed here.

[00:32:02]The first part of the small intestine, which is called the duodenum, here the job of digestion is completed. Now, this digested food, as it moves down the lower part As it moves down the lower part of the small intestine, it has to be absorbed.

[00:32:21]It has to be taken into blood so that blood can distribute these small nutrients to all the cells of the body.

[00:32:29]And that absorption is aided by the presence of these structures.

[00:32:36]What are these structures? These are the villi. Each of these finger-like projection, each one is called a villus.

[00:32:45]Plural is villi.

[00:32:47]It is Say, for example, if I show you my palm like this, and this is the surface area for absorption. This much is the surface area for absorption. Okay? Now, if I separate my fingers, so what happens to the surface area of absorption? This entire finger, then this finger, then this finger, this finger, and this finger. That means the surface area increases. So, the small intestine, the lower part of the small intestine, if you see the inner side, it is not plain.

[00:33:21]It is not smooth, but it is thrown into such finger-like projections. The The result is it increases the surface area for absorption of food. It's a very important uh part, and many questions come from this. So, it increases What is the job of villi? It increases the surface area for absorption of digested food. Uh very common mistake with children make is when we are asked to write about the function of villi, what is written is it increases the absorption of digested food. This doesn't increase the absorption. It increases the surface area through which absorption is going to take place. And one more thing, of digested food. Don't forget the word digested, because what is absorbed is digested food. Fine? So, the function of villi is to increase the surface area for the absorption of digested food.

[00:34:33]Let us see a villi. So, this is how a villi looks like. You observe a This is how See, these are the finger-like projections.

[00:34:42]These are the finger-like projections.

[00:34:44]Digested food is present over here.

[00:34:47]Digested food is present over here, and there are blood capillaries in the villi through which digested food is absorbed. If you see a closer section of the villi, you will find that each of the cells which are present here on the walls of the villi, they have these hair-like projections on them, which further increases the surface of area. So, surface area for absorption, these are called microvilli.

[00:35:20]These are called microvilli. So, microvilli present on the villi, so microvilli and villi both increase the surface area for absorption of the digested food. And finally, food has been digested, but a lot of the food has also been left undigested or maybe very slightly digested, and that will have to be thrown out of the body. Who's going to do that for us? That job is going to be done by the last part of the of the alimentary canal, which is the large intestine. I'm talking about this part.

[00:35:55]I'm talking about the orange part.

[00:35:59]Right? So, this part is the large intestine. Truly speaking, the small intestine actually is much longer. It's around 7 to 8 m in human beings, whereas the large intestine is only 2 m. In spite of that, it is called large intestine because it is wider than the small intestine. So, the process of egestion Egestion can also be called defecation.

[00:36:27]Defecation means removal of the undigested food in the form of feces.

[00:36:33]So, when the undigested food reaches into the large intestine from the small intestine, let me show you the picture once again.

[00:36:50]So, from the small intestine, this is the small intestine, the pinkish part.

[00:36:55]This is the small intestine. So, from the small intestine, when the food reaches the large intestine, what is going to happen in the large intestine?

[00:37:04]In the large intestine, a lot of water is going to be taken back. Lot of water is going to be absorbed. So, in the large intestine, this is the picture of the large intestine.

[00:37:23]So, what happens in the large intestine?

[00:37:26]Water is reabsorbed.

[00:37:32]Because if reabsorption of water uh does not take place, then the person is going to have diarrhea.

[00:37:43]And next is after reabsorption of water, finally, feces is formed.

[00:37:51]Feces feces or fecal pellets.

[00:37:56]Dry fecal pellets are formed, and finally, these are removed from the system in the form of feces. So, that is elimination of feces.

[00:38:14]Or defecation takes place. So, the main job of large intestine is to reabsorb water from the feces and throw the feces out of the body in the form of feces or fecal pellets, which is called egestion or defecation. Okay? So, this broadly is the entire process of the human digestive system, how each of the organs of the system works I've told you. Very importantly, you have to remember the role of the enzymes and remember the small points, the small tips that I've given you. Thank you. I hope you've enjoyed the session. Have a nice day.