Food digestion in the human body is a fascinating subject, as it operates almost completely on its own. The only part we have conscious control over is what we eat, chewing, swallowing, and controlling when we go to the bathroom. Even with our conscious control, we get signals from our bodies when it is time to put things into our bodies and when to take them out. Here is a step-by-step explanation of the process our food goes through from beginning to end.
Most foods we eat are insoluble, meaning they can't be broken down by water. Because of this, our food needs to be digested so we can use it for energy for our bodies. One common place people don't think about digestion occurring is in the mouth. This is the first of many important steps in digestion.
We have salivary glands in the back of our mouths and under the tongue that release saliva for breaking down food. Even before we put food in our mouths, the thought or smell of food makes us start salivating. That way there is plenty of saliva to aid in digestion. Saliva is mostly made of water, but it has other things in it like mucus and enzymes. These enzymes do most of the food breakdown in saliva. Amylase is one of those enzymes. It breaks down starches like rice and crackers into sugar.
Chewing food also helps break our food into smaller pieces that are easier to swallow. As saliva surrounds the food we eat, it makes it much easier to swallow the food and bring it to the next stage of digestion.
The next important place the food goes to after the mouth is down your pharynx to the stomach. The stomach can expand to hold up to two liters of food. Inside the stomach there are various gastric glands that make gastric juice. In the gastric juice is hydrochloric acid, or HCL. With a pH of 2, this acid in your stomach is strong enough to burn holes in your clothes as well as dissolve your food and kill bacteria. HCl makes pepsin, which can break down proteins into smaller peptides so the body can absorb them. The stomach can be thought of as a giant muscle that mixes your food with gastric juice. Once gastric juice is mixed with food, it is then known as chyme.
The first section of the small intestine is called the duodenum. Chyme enters through a sphincter in little squirts as the stomach forms a squeezing motion to push the chyme through. In the small intestine is where most digestion occurs. Before this point, only carbs and proteins have been partially digested, and lipids or fats have not been digested at all.
From here the pancreas releases pancreatic juice into the small intestine, which has pancreatic amylase that breaks starch into maltose or sugar, much like the amylase found in saliva. Pancreatic juice also has bicarbonate to neutralize the acid in chyme from the stomach. Also in pancreatic juice is chymotrypsin and trypsin, which digest proteins into peptides, as well as lipase, which digests fats into glycerol and fatty acids.
The gallbladder contains bile, which is produced by the liver. The gallbladder releases bile into the duodenum, so it can emulsify fats, or break them into smaller drops. Digested fats are changed into glycerol and fatty acids.
The small intestine has many ridges called microvilli that increase its surface area. Here glucose and amino acids are moved into the intestinal cells by active transport, where blood capillaries can pick them up, and move them into cells. Glycerol and fatty acids, however, enter villi by diffusion and reassembling them into fat or triglycerides. Here the triglycerides combine with proteins and move into the lacteals so they can be transported into the lymphatic system.
After the small intestine, the remaining products get sent to the large intestine. Here water, sodium, and vitamins are absorbed. In fact, about 10 liters of water from food and drinks pass through the large intestine each day. However, 95% of it gets reabsorbed. It is important that so much water is reabsorbed, or else diarrhea can result, as well as dehydration from the loss of water. The last 20 centimeters or so of the colon is called the rectum, where feces exit. Feces are about 75% water, and 25% solid waste. Most of the solid waste is bacteria, with the rest being undigested materials such as fiber.
Food digestion in the human body is a slow but effective process. It helps us turn food into energy we can use to keep us moving and our bodies healthy and strong. To keep a healthy digestive system, it is important to drink plenty of water, and eat a healthy diet with plenty of fibrous foods like fruits and vegetables.
Digestive System, from Clinton Community College of New York: https://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/digestive%20system/digestive%20system.htm
Lakin S and Patefield J. Essential Science for GCSE from Google docs: https://books.google.com/books?id=8CYzLwdXBtIC&pg=PA16&dq=food+digestion&hl=en&ei=tD-OTe3OD4iasAOVztCVCQ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CFYQ6AEwBg#v=onepage&q=food%20digestion&f=fal