Lesson Video: The Digestive Process Biology

In this video, we will learn how to describe the processes of buccal, gastric, and intestinal digestion in the human body.


Video Transcript

In this video, we will learn how to describe the digestive process in the human body and why digestion is so important. We will investigate the different types of digestion that occur as food moves through the digestive system from the mouth, which is the site of buccal digestion, through the stomach, where gastric digestion occurs, and into the small intestine, where intestinal digestion occurs. We will also learn about the important roles of various enzymes that are involved in chemical digestion and the role of muscles and other body structures in carrying out mechanical digestion.

The human digestive system works tirelessly, whether we are standing upright or balancing upside down, when we’re awake, and even when we’re asleep, by using chemicals, muscles, and other structures to break down the food we eat into smaller pieces. Digestion is a process by which large molecules are broken down into smaller molecules. This is essential, as we need to break down the large nutrients in the food we ingest into a form small enough to be absorbed into the bloodstream.

But how does this actually happen? These large molecules are broken down into their smaller constituent subunits by chemicals called enzymes, which are specific to the type of substrate that they break down. The enzymes in the digestive system mostly function to break bonds within a substrate molecule to form products, which are smaller. Most of these products are now small enough to move into capillaries, which surround certain parts of the digestive system. The blood within these capillaries can then transport the products away around the body to all the cells that might require them.

This diagram shows an outline of some of the major organs in the human digestive system. Keep in mind that they have been color-coded here, but in reality they mostly all appear a similar fleshy pink or brown color. We can use this diagram throughout the video to pinpoint the different organs that food travels through, which is known as the alimentary canal or the gastrointestinal tract. We can also use it to identify the different organs which food does not directly travel through so are not part of the alimentary canal but are still useful for the digestive process. These organs, which are known as accessory organs, have been color-coded here in blue, and we’ll look at them in more detail as we go through each stage of the digestive process.

So let’s get started. Say that you’ve just taken a bite of a tasty chicken sandwich. The first place the food enters is the mouth, where the food is turned into a ball that is now referred to as a bolus. The mouth is the site of buccal digestion. The word buccal refers to the mouth itself. And buccal digestion includes many different structures and chemicals within the mouth and in the areas surrounding it.

Let’s take a closer look at the mouth and these surrounding areas, so we can see how buccal digestion occurs in more detail. Human teeth are excellently adapted to chew different types of food, as we are biologically adapted to be omnivores, eating both animal and plant products. Our incisors in the front of our jaw cut our food, while the adjacent fairly sharp canines grip and tear the food into smaller pieces. At the back of the jaw, the molars and premolars grind down food to give it a larger surface area. We mostly think of our tongue as a tool to taste food, but it’s actually also a muscle. And it helps to play an important role in moving food and helping the teeth to grind it down.

These processes are all examples of mechanical digestion, using muscles in the jaw to grind down food into smaller chunks. Mechanical digestion results in the same volume of food being broken down to have a larger surface area. This makes it easier for enzymes, which are represented here by green arrows, to digest food, as it increases the surface area that these enzymes can act on to break down the large nutrients and the food even further.

Generally, mechanical digestion includes hard structures like teeth or muscles like the tongue, and it occurs in other regions in the digestive system, too. But let’s finish off buccal digestion first.

Aside from mechanical digestion, chemical digestion also occurs in the mouth, as this term is generally used to refer to the role of enzymes in digestion. This diagram shows a close-up side view of the mouth and some surrounding areas. We’ve already looked at the functions of the tongue and the teeth in mechanical digestion, but let’s see how some of these other structures play a role in chemical buccal digestion.

You may recall that we refer to the blue structures in the diagram as accessory organs. In this close-up of the mouth, we can see three out of the three paired salivary glands, which are our first example of accessory organs. The role of the salivary glands is to secrete saliva into the mouth. Saliva contains mucus, which softens food by making it more fluid. Saliva also contains enzymes called amylases.

Let’s see how amylases work. Amylase starts off the breakdown of carbohydrates, such as large starch molecules in food, into smaller sugars, such as the disaccharide maltose. This occurs through a process called hydrolysis, which literally means breaking a molecule down using water. Maltose will eventually be broken down itself into smaller monosaccharides, called glucose, by another enzyme called maltase. But this doesn’t happen till later on in the digestive tract.

Amylase works best in slightly alkaline conditions, and its optimum pH is around 7.4. It’s important to note that an optimum pH is very specific to each particular enzyme, so it will differ depending on which enzyme we’re talking about. Amylase is just one example of an enzyme involved in chemical digestion in the digestive process, more of which will come later.

At the back of the mouth, there are two tubes, one of which is the trachea that leads down to the lungs, and the esophagus which leads to the stomach. Usually a flap of tissue called the epiglottis is held open, which allows gases like oxygen to move down the trachea and into the lungs. To prevent food from entering the trachea, when the food bolus reaches the pharynx at the back of the mouth, the trachea and the larynx, which is otherwise known as the voice box, rise upwards, which causes the epiglottis to close over the entrance to the trachea. So no food and drink enters the trachea, only the esophagus. If the epiglottis did not close, food or drink might enter the lungs and cause choking.

Once food is in the esophagus, it needs to move down to the stomach. So let’s see how the esophagus manages this. The esophagus is a long tube with glandular cells in its lining that secrete mucus onto the bolus. The esophagus lining also contains circular muscles. These circular muscles rhythmically contract and relax to move the bolus through the esophagus to the stomach. This process is called peristalsis, which is the rhythmic contraction and relaxation of these circular muscles, which can move food and sometimes can mix up the bolus with digestive juices in certain organs of the body, though this does not occur in the esophagus. Peristalsis also occurs in the stomach, the small intestine, and the large intestine. As muscles are involved, this shows us a nice example of mechanical digestion.

Let’s look at gastric digestion next, which refers to processes occurring in the stomach. We will need to see a closer view to understand this clearly. So let’s see a larger image of the stomach where it meets the bottom of the esophagus. When the food bolus approaches the end of the esophagus, it reaches a thick muscular ring called the cardiac sphincter. The cardiac sphincter is usually closed but opens when the bolus reaches it, allowing peristaltic movements to push the bolus out of the esophagus and into the stomach.

The stomach is a muscular sac which is able to move food via peristalsis. And like the mouth, mechanical digestion also occurs in the stomach. Once the bolus reaches the stomach, it mixes with gastric juice and becomes more watery. At this point it is no longer referred to as the bolus, and instead it is known as chyme.

Gastric juice, which is sometimes known as stomach acid, contains strong hydrochloric acid. This is helpful for two main reasons. Firstly, if pathogens, which are disease-causing biological agents such as bacteria, where to enter the digestive system as a result of being ingested as part of food, the hydrochloric acid in the stomach would kill most of them. This prevents them from entering the rest of the digestive system, or worse the bloodstream. Hydrochloric acid also provides the acidic conditions which are optimal for the enzymes that function in the stomach.

Gastric juice also contains a substance called pepsinogen, which is secreted by the cells lining the stomach. Pepsinogen is an inactive form of an enzyme. The acidic pH provided by the hydrochloric acid allows pepsinogen to be activated and converted into an enzyme called pepsin.

Let’s take a closer view as to how pepsin functions as an enzyme in the stomach. Pepsin is an example of a protease enzyme. The stem “prote” shows us that protease enzymes break down proteins. Pepsin specifically catalyzes the hydrolysis of proteins into smaller polypeptides. These polypeptides will eventually be broken down by other protease enzymes in the small intestine into amino acids. The optimum pH of pepsin is between 1.5 and 2.5, so it functions very effectively in the acidic stomach environment.

The cells in the wall of the stomach itself are protected from being digested by pepsin by mucus, which lines the inside of the stomach. Digestion of proteins into polypeptides is another example of chemical digestion.

At the base of the stomach is another ring of muscle called the pyloric sphincter. The pyloric sphincter is also usually closed, but a peristaltic contraction causes it to open briefly, allowing some chyme to pass from the stomach into the first section of the small intestine, which is called the duodenum.

The final part of the digestive process is intestinal digestion, which primarily occurs in the small intestine. The small intestine is where the majority of chemical digestion occurs in the digestive tract as proteins, carbohydrates, and lipids will all be digested by enzymes there. Most digestive enzymes are secreted into the small intestine by an accessory organ called the pancreas. By adjusting the location of the other organs slightly, we can see how the pancreas attaches to the duodenum of the small intestine more clearly.

This green structure running through the pancreas represents the pancreatic duct, which is responsible for secreting pancreatic juice, which contains all these enzymes into the duodenum, where pancreatic juice and intestinal juice will mix.

Let’s take a closer look at the enzymes that pancreatic juice contains that will be acting in the small intestine. Pancreatic juice contains many carbohydrases that catalyze the hydrolysis of carbohydrates, which you might remember began in the mouth with amylase. In the small intestine, for example, carbohydrases like maltase can break down maltose into molecules of glucose. Pancreatic juice will also contain lipases, which catalyze the hydrolysis of lipids in food into fatty acids and glycerol.

Pancreatic juice also contains proteases, which, you might recall, break down proteins. Trypsinogen is secreted as a part of pancreatic juice and, like pepsinogen, is an inactive form of a protease enzyme. Trypsinogen is activated by another enzyme, which is present in intestinal juice, called enterokinase. When trypsinogen enters the small intestine and comes into contact with enterokinase, it’s converted into its active form, trypsin. Trypsin is a protease enzyme that catalyzes the hydrolysis of polypeptides and peptides into smaller units and eventually into the monomer subunit amino acids.

Some gastric juice from the stomach will enter the small intestine. But pancreatic juice contains another substance called sodium bicarbonate, which neutralizes the acidic gastric juice entering the duodenum from the stomach. This means that the pH at the start of the duodenum is around six, but the pH increases as the small intestine continues to provide an optimal pH for many of the enzymes that will be acting there.

The liver is another accessory organ that is involved in the digestive system. By magnifying our summary diagram and adjusting the organs slightly, we can see where this interaction occurs more clearly. The duodenum, the first part of the small intestine shown here in pink, is connected to the pancreas, which is one of the accessory organs that we already explored. And it’s also connected to the liver. The liver is described as an accessory organ of the digestive system, as it produces a substance called bile.

Bile is stored after its production by the liver in another accessory organ called the gall bladder before being transported along a duct called the bile duct, which joins up with the pancreatic duct so that both pancreatic juice and bile are both secreted into the duodenum.

Now that we know how it gets there, let’s see how bile can be helpful in digestion in the small intestine. Bile emulsifies lipids, which means that it disperses them into smaller droplets. These smaller globules are sometimes called emulsion droplets. Lipids are insoluble in water, but the enzymes that break them down, lipases, are soluble in water. So the two don’t naturally mix well. Emulsification by bile, however, means that these droplets of lipids are more interspersed throughout water, which means that the lipases have a larger surface area upon which they can act, as shown by these green arrows. This increases the efficiency of lipid hydrolysis in the small intestine.

Once the nutrients have been broken down sufficiently by all of these different enzymes, the simple sugars, amino acids, fatty acids, and glycerol are absorbed across the wall of the small intestine. The diagram on the right shows us how the small intestine wall is highly folded. This gives the small intestine a really large surface area for nutrient absorption. Amino acids and simple sugars are small enough that they can move directly across the cells that line the wall of the small intestine into the blood capillaries, which are shown here as these dense, red networks of blood vessels. Once they’re in the capillaries, these essential nutrients can be transported via the blood to the body cells that require them.

As fatty acids and glycerol are larger and are not water-soluble, they cannot pass directly into the capillaries. Instead, they move into structures called lacteal vessels, which sit in the same area as the capillaries just outside the cells lining the wall of the small intestine. The lacteal vessels transport these fatty acids and glycerol and any undigested lipids into the lymphatic system, where they can enter the bloodstream at a larger junction.

Let’s summarize the processes that we’ve covered so far. First, food is placed in the mouth, where it is mixed with saliva and mashed into a ball called a bolus. This is called buccal digestion, and it involves amylase enzymes breaking down starch in food into smaller sugars, such as the disaccharide maltose. From the mouth, the food passes down the esophagus to the stomach. This is where gastric digestion occurs and proteins in food are broken down into polypeptides.

Once the bolus reaches the stomach, it mixes with digestive juices and becomes more watery. The solid pieces break down, and they have a lower surface area. And the bolus is now known as chyme. Chyme can then move into the small intestine, where intestinal digestion will occur. This includes the majority of chemical digestion, where proteins, polypeptides, and peptides will be broken down into amino acids by protease enzymes. Larger carbohydrates like maltose can be broken down into comparatively smaller simple sugars like glucose by carbohydrase enzymes. And lipids are converted into fatty acids and glycerol by lipase enzymes.

The chyme then passes into the large intestine, which reabsorbs water, any remaining vitamins, and salts to form solid feces. Feces is then stored in the rectum before it is passed out of the body by egestion via the anus.

Let’s have a look at the key points that we’ve covered in this video. Digestion is the process by which large food molecules are broken down into smaller and generally more soluble molecules that can be absorbed into transport systems like the blood to be transported to the body cells that require them. The digestive process begins with buccal digestion in the mouth, which involves the teeth, the tongue, and the salivary glands.

Gastric digestion occurs next in the stomach and involves gastric juice containing hydrochloric acid and protease enzymes. Finally, intestinal digestion in the small intestine uses pancreatic juice, bile, and additional enzymes to carry out the majority of chemical digestion. And it’s also the site where the smaller nutrients will be absorbed into the bloodstream.

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