The Digestive System Dr. Timmel

What is the digestive system? The digestive system is a one-way tube the begins at the mouth and ends at the anus. It is also known as the alimentary canal. The purpose of the digestive system is to break down large organic molecules (Carbohydrates, Proteins, and Lipids) into smaller ones that can diffuse into the circulatory system. The digestive system also absorbs vitamins, minerals and water (inorganic molecules). These do not need to be broken down into smaller molecules.

Interactive diagram of human digestive system

Lets eat! To understand what happens in the digestive system we will investigate the path a roast beef sandwich with mayo and lettuce takes when we eat it. Roast beef= Protein; Bread= Carbs; Mayo = Lipid; Lettuce = roughage. (Vitamins, minerals and water are also present in the sandwich)

Enter the mouth As you bite into the sandwich, your teeth help to shred and mash up the food. This is a type of physical or mechanical digestion. As you chew, your salivary glands secrete saliva which helps moisten your food. Saliva also contains Salivary Amylase, an enzyme which begins to break down carbohydrates. This is a type of chemical digestion. Your tongue helps to position the food between your teeth for chewing, and then helps move the food to the back of your mouth for swallowing. Once your food leaves your mouth and enters your throat it is known as bolus.

Down the gullet Once the bolus leaves the mouth it goes through your throat and into your esophagus. No digestion occurs here, the esophagus is just a tube which connects your mouth to your stomach. Prior to entering the stomach it passes through the gastro-esophogeal spincter. This muscular ring helps prevent the contents of your stomach from moving back into the esophagus. Sometimes the acidic contents of your stomach does get into the esophagus. This is what causes the painful condition known as “heart burn”.

Going down the “wrong pipe.” As you swallow, a flap of tissue called the epiglottis covers the top of the trachea to prevent food from entering the lungs. Sometimes food or drinks can get past the trachea which causes a fit of coughing to help get it out of there and back into the esophagus. It is impossible to breathe and swallow at the same time.

Get in my stomach! The environment of the stomach is very acidic. This is because the enzymes present which begin the digestion of proteins are only active at a pH of about 2. Enzymes which break down proteins are called proteases. The acidic environment also helps to kill the bacteria which are present in/on all food.

Stomach continued. The churning of your stomach is a type of physical digestion. All the food you eat becomes completely liquefied in the stomach. This semi-digested, acidic liquid is now referred to as chyme. Chyme is then squirted through the pyloric spincther into the small intestine.

The small intestine. It is here in the small intestine where most of the business of digestion takes place. The first 10 centimeters of the small intestine is called the duodenum. The chyme is very acidic when it first enters the S.I., so it needs to be neutralized.

The Pancreas and the Liver These are two very important organs in the digestive system. Though food never enters into either the pancreas or the liver, their secretions are vital to digestion. First we will look at the pancreas

The pancreas The pancreas neutralizes stomach acid by secreting bicarbonate into the duodenum. The pancreas also secretes three types of digestive enzymes: Proteases, Amylases and Lipases. These secretions travel from the pancreas to the duodenum through the common bile duct.

The liver The liver secretes a substance called bile. Bile helps in the physical digestion of lipids by emulsifying them. This means it breaks a large glob of fat into smaller ones for easier digestion. Bile is stored in the gall bladder and secreted into the duodenum through the common bile duct.

Duodenum summary. In this first section of the small intestine: The chyme is neutralized. Carbohydrates complete their chemical digestion. Proteins complete their chemical digestion Lipids begin and complete their chemical digestion.

Absorption in the S.I. As your nutrients pass out of the duodenum into the rest of the S.I., they begin to be absorbed into the blood stream. Absorption is carried out by special finger-like extensions in the walls of the S.I. called villi. The villi’s shape greatly increases the surface area of the S.I., allowing it to absorb nutrients at a much faster rate. By the time your food reaches the end of the S.I., all that remains are roughage, water and bacteria.

Drawing of villi

The Appendix Near the point where the small intestine and large intestine meet, there is a small fingerlike projection called the appendix. In modern humans, the appendix serves no purpose. It is believed that in our evolutionary past the appendix did aid in digestion. Organs, such as the appendix are called vestigial organs.

The Large intestine Here in the large intestine most everything useful except water and some vitamins has been absorbed. The main job of the large intestine is to reabsorb water. When the Large intestine removes too much water, constipation is the result. If not enough water is reabsorbed, diarrhea is the result. Both conditions can actually kill!

Bacteria and the large intestine. The large intestine is also home to E.coli bacteria. These bacteria are actually beneficial to use here as they synthesize vitamin K.

The rectum After water has been removed, your food, now called feces, is temporarily stored in the rectum. At this point all that is left is roughage, bacteria and a small amount of water. Feces are then eliminated from the body through the anus.

Recap. Here is the pathway your food takes through your body. MouthEsophagusStomach Small IntestineLarge Intestine  Rectum Anus.

Chemical digestion Chemical digestion for carbohydrates begins in the mouth and ends in the Small Intestine. Chemical digestion of proteins begins in the stomach and ends in the small intestine. Chemical digestion of lipids begins and ends in the Small intestine.

Physical (mechanical) digestion. In the mouth, all food in physically digested by the action of your teeth, tongue and jaws. In the stomach, all food is physically digested by churning. In the duodenum lipids are physically digested by bile. (Emulsification).