The Human Body: Digestive System (2nd ed)

[music] >> Getting the nutrients out of food for the trillions of cells that make up your body is the work of your digestive system. >> [music] >> Substances your body must have are in the food you eat. >> [music] >> From your food come the proteins, starches and sugars, carbohydrates and fats, vitamins and minerals that help you grow, that keep you well, that keep you alive. >> [music] >> But food can't be used by your body in the form it has when you eat it. It has to be broken down into parts that are smaller [music] physically and simpler chemically. And this is the work of your digestive system. Your food spends most of its time moving through three organs during the process of digestion. Your stomach, your small [music] intestine, and your large intestine. Let's follow this path of digestion now from the time food enters your mouth until the nutrients get to your cells and your body disposes of the rest. You're eating. You take a bite of something. And now you're chewing. Your teeth cut and chop the food, grinding it while your tongue moves it around, mixing [music] it with your saliva. The digestion process is beginning because two things are happening. The food is being broken up into [music] smaller pieces. That's the chewing part. And the food starts being broken down chemically. That's the work of the saliva. In saliva, there are certain chemicals called enzymes that begin the job of converting [music] starches in your food into simple sugars. Saliva is secreted [music] from glands in your mouth. And because it's mainly water, saliva also softens the food, putting it into a form that will make it easy to swallow. When [music] you're ready to swallow a mouthful of food, the tongue pushes it to the back of your mouth. Swallowing starts [music] as a conscious act. You swallow when you want to. Voluntary muscles are at work. But once swallowing begins, the process becomes [music] automatic. Muscles begin working that you have no control over, involuntary muscles. >> [music] >> A flap, the epiglottis, closes the trachea, the tube that leads to [music] your lungs. This prevents food from going down that tube and instead sends it into the proper tube, the esophagus, [music] that leads to your stomach. Two layers of muscles surround the esophagus. These muscles begin working automatically when you swallow, contracting and relaxing rhythmically, pusing the food through it down toward your stomach. This steady relaxation and contraction is called peristalsis and it produces a wave-like effect. It is this peristaltic wave that moves the food along with the help of gravity. And now the food reaches your stomach. So far, very little of the digestive process has taken place. Here, much more will happen. The important work begins. Your stomach is in layers of muscles, just as your esophagus is. But, there are three layers >> [music] >> instead of two. These three layers of muscles produce the same sort of peristaltic movement that moved food down your esophagus. Peristalsis moves the food around, mixing it with digestive liquids that your stomach [music] produces. These are the gastric juices that are secreted by the microscopic glands lining the inner wall of your stomach. The glands secrete two substances, the inactive enzyme >> [music] >> pepsinogen and hydrochloric acid. The acid converts the pepsinogen [music] into the active enzyme pepsin, which has an important digestive function. Pepsin [music] can break down complex protein molecules into simpler chemical parts. Important steps in the breakdown of protein begin here in your stomach. But, not much happens to carbohydrates or fats. Their breakdown [music] takes place further on during the digestive process in your small intestine. The substances that glands in your stomach secrete to digest food would also digest the lining of your stomach if it weren't for other glands that secrete mucus. This mucus coats the lining of your stomach and keeps the gastric juices from acting on it. In your stomach, the churning and mixing continue, carrying on the work of breaking the food down physically [music] and chemically. Eventually, the food particles will be small enough to pass into your bloodstream. This happens in your stomach to some of the water, salt, simple sugars, and alcohol. These substances pass through the stomach walls into the blood vessels that surround them. But, most of the food that enters your stomach moves on. At regular intervals, a ring of muscle opens and allows the digesting food to pass from your stomach into your small intestine. This organ is called small, not because of its length, but because of its diameter, about 3 cm, slightly more than an inch. The small intestine is actually very long, about 7 m, >> [music] >> nearly 25 ft, and coiled into a twisted pile. Like your stomach and esophagus, your small intestine's walls are made up of muscles, two layers of them. And the same peristaltic movement that pushed the food down your esophagus and through your stomach also moves the digesting food through your small intestine. The beginning of the small intestine is called the duodenum. The arrival of digesting food in this part of the intestine stimulates the production of certain hormones by the duodenum. These hormones, in turn, activate two glands, the liver and the pancreas. These glands begin producing digestive chemicals, the enzymes that are sent to the duodenum. Most of the chemicals that digest your food enter your small intestine here. The pancreas produces various [music] kinds of enzymes. Some help break down proteins into amino acids. Other enzymes [music] from the pancreas break starch into simple sugars. And some of them break fats into fatty acids and glycerol. >> [music] >> The breakdown of fats is aided by a substance called bile. Bile may come [music] into the duodenum either directly from the liver, where it is made, or from the gallbladder, where bile is stored. The bile breaks the fat into tiny droplets, the way detergent breaks up grease. When the fat is in this droplet form, the enzymes in the pancreatic juice can break it down more effectively. Now, not only do the liver and pancreas glands deliver chemicals from outside the duodenum, but there are also glands in the wall of the duodenum that secrete enzymes of their own. These enzymes also aid in the breakdown of carbohydrates, proteins, and fats. Of course, the purpose of all this chemical activity is to turn the food you eat into something small enough to pass through the walls of the intestine. The walls of the small intestine are wrinkled and folded. The folds are made up of microscopic finger-like parts called villi. Each tiny villus contains lymph vessels that carry water-like lymph and capillaries that carry blood. It is through the villi that food passes out of your digestive system and into your circulatory system. When they are small enough physically and simple enough chemically, nutrient molecules pass through the thin walls of the villi. Amino acids, sugars, vitamins and minerals are picked up by the blood flowing through the capillaries. Some fats are absorbed into the lymph vessels and pass into the lymphatic system. The rest of the fats pass through the capillaries into the blood. Capillaries of the villi then pass blood into the veins of the small intestine and eventually into one large vein, the portal vein. The portal vein carries the nutrient-filled blood to the liver for processing. In the liver, the amino acids that resulted from the digestion [music] of food proteins are now reassembled into different kinds of proteins in a form your body [music] can use for growth and repair. Or the amino acids may be broken down even further to generate energy. Some sugar and fat is stored in the liver. Nutrients that are not held in the liver for storage are returned to your circulating blood and distributed to your body cells. And so, the main function of digestion has been accomplished. Food has been simplified chemically and physically to a point where it can be used by your body. Food that has not been digested in the small intestine passes on into the large intestine for removal from your body. The large intestine is about two and a half times as big as the small intestine in diameter, but it's only about 1/4 as long. As the undigested food moves through your large intestine, water is reabsorbed back into your body. This reabsorption and the elimination of the undigested [music] food are the main functions of your large intestine. While your digestive system is carrying food that hasn't been digested out of your body, your circulatory system is carrying the digested food to all your trillions of cells. In each cell, two opposite kinds of chemical processes take place as the nutrients are used. Both of these processes together make up what is called cell metabolism. With the help of enzymes, [music] one process breaks down the nutrients and produces both energy and smaller chemical units. As the atoms of carbon, hydrogen, and oxygen break [music] the chemical bonds that form them into carbohydrate molecules, energy is [music] released. Energy is released as molecules of fat break apart in a similar way. At the same time [music] that nutrients are being broken down, another process is going on in which nutrients are being combined into new material that is more complicated chemically. Again, with the help of enzymes, simple amino acids are [music] built into more complex proteins. One after another, the amino acids [music] link to form chains of proteins. There are 20 different kinds of amino [music] acids. These can be put together in thousands of different ways to form [music] thousands of different proteins. Proteins are the basic components of all your bones, muscles, and nerves, blood, and membranes. Some of the amino acids can be manufactured in your body from carbohydrates and nitrogen. But most have to be brought in as parts of the proteins in the food you eat. Human life depends on the taking apart and reassembly of animal and plant proteins. Also essential to human life and health are certain components of enzyme molecules that keep the process of metabolism moving along efficiently. These are the substances that we lump together under the name vitamins. The cells of your body need a variety of them. Many of these vitamins can't be stored because they're constantly being excreted by your body. So, you have to keep bringing in fresh supplies. Eating a well-balanced diet does that. A good diet also brings in adequate supplies of minerals that are basic to the functioning of your cells. You need zinc, manganese, magnesium, copper, and cobalt for the construction and functioning of your enzymes. You need iron for the hemoglobin that binds oxygen to red blood cells. Calcium and phosphorus are needed for the structural rigidity of your bones and teeth. Sodium and potassium chlorides are important components of your tissue fluids. You need iodine to make the hormone released by your thyroid gland. And so, your digestive system makes available to your whole body substances that are vital to it, to its health and growth and proper functioning. Because the proper amounts of nutrients are so important to the body's existence, parts of your body constantly keep track of how well it's being supplied with food. Areas in your brain stem sense and monitor the level of sugar dissolved in your blood and whether your stomach is empty or full. But your emotions also play a large role in what, when, >> [music] >> and how much you eat. Your mind and your culture affect your eating habits, [music] too. And often you eat simply because it's time for a meal. For your digestive system to completely process a meal, it takes about 24 hours. After spending just a few seconds in your mouth and [music] esophagus, the food stays nearly 4 hours in your stomach. It spends another 4 to 6 hours in your small intestine. At this point, most of the nutrients have been absorbed. [music] Then what's left of your meal moves from the small intestine into the large intestine. Various colonies of bacteria live here. They thrive on foods like plant cellulose that your enzymes can't digest. Products of their metabolic processes are vitamins K and B complex, which you absorb through the walls of your large intestine. Finally, some 24 hours after you brought it in, the remains of your meal, along with the residue of used-up digestive enzymes and worn-out cell fragments, are ready to be disposed of. In the meantime, [music] a number of other meals have started working their way down in the continuing process [music] of digestion. In the continuing process of drawing nutrients [music] from food for the life of your cells. >> [music] [music] [music] [music] [music]