Humans

The Digestive System 

 

The digestive system plays an important role in sustaining human life. It takes the food we eat, breaking it down and converting it into usable energy and is also the process in which human bodies absorb and store nutrients. Energy can also be stored and converted into work the rate at which this happens is measured by metabolism. The breaking down of large materials to small materials such as in digestion is called catabolism and the building up of smaller materials allowing humans to grow is known as anabolism, both of which are extremely important for human life. The human is a complex animal, it has an alimentary canal and separate opening for ingestion and digestion called the gastrointestinal tract.

 

The gastrointestinal tract starts at the mouth with ingestion. When we eat a portion of food, it enters our mouth and right away physical digestion occurs. As we chew the food it begins to break down into smaller pieces. Humans have teeth such as incisors to cut food, canines to tear it and premolars to grid it. The tongue also helps mash food and the soft and hard palates keep the food contained in the mouth and keep it out of the nasal cavity. Chemical digestion also occurs as saliva is produced in the mouth by three salivary glands (the parotid, the sublingual and the submandibular glands) also aiding in the break down of the food. Saliva contains amylase, which breaks down starch into disaccharides. It also contains mucus making swallowing food much easier. The broken down food now called a bolus is pushed into the pharynx by the tongue and the soft palate rises to keep it from entering the nasal passage. The epiglottis also covers the larynx to stop the food from going into the trachea and entering the respiratory system. The bolus travels down a tube called the esophagus powered by gravity and with the help of peristalsis which are wavelike contractions caused by smooth stomach muscles is the esophagus. The bolus then enters the stomach. The stomach is made up of four different layers. The Mucosa is made up of epithelial tissues and is where mucus and gastric juices are secreted. The submucosa is what holds the tissues together and contains the nerves and blood vessels. The muscularis is smooth muscles tissue that does most of the physical digestion by contracting to mix and break down foods. Finally the serosa holds the stomach in place and prevents friction from damaging the organ. In the stomach hormone gastrin activates gastric cells and the break down of the food into a liquid form. This includes triggering the enzyme pepsin from its inactive form pepsinogen to break down proteins. It contains hydrochloric acid, which kills microorganisms and breaks down food. Hydrochloric acid is also what triggers the release of pepsin into the stomach. In the stomach much of the physical digestion happens along with some chemical digestion that breaks down the food turning it into a liquid form known as chyme, which is acidic. From the stomach the chyme moves into the first section of the small intestine called the duodenum. This is where most of the chemical digestion takes place. When chyme enters the duodenum it activates prosecretin into its active form called secretin. Secretin then activates bile production in the liver and stimulates the pancreas. From this the gall bladder releases bile into the duodenum through the bile ducts aiding in the physical digestion of fats breaking them down into micelles. Along with producing bile the liver also cleans the blood of toxins and stores vitamins and nutrients. The secretin in the stomach causes the hormone CCK to stimulate the pancreas. The pancreas then releases bicarbonate ions to make the acidic chyme become more neutral also inactivating pepsin. Enterokinase then activates the enzymes trypsin, which breaks down larger protein chains and erepsin, which breaks down the protein chains into amino acids. The pancreas also produces amylase and lipase to break down fats along with glucagon and insulin to manage blood sugar. The next section of the small intestine is the jejunum and the ilium where most of the nutrient absorption happens. These sections are very long and contain many villi and microvilli to maximize surface area. Absorption happens in two main ways, active transport which requires the use of energy or passive transport, which does not. When active transport occurs it can happen in the form of endocytosis, which is when a cell membrane wraps around a substance and pulls it into a cell (the purpose is to move larger substances quickly across cells/packing or getting rid of a substance). It can also happen in the form of ion pumps, which uses electricity to move ions across a cell or co-transport where the energy of a molecule is used by another molecule to move. When absorption happens by using a form of passive transport it occurs as simple diffusion where particles spread from high to low concentration. It can also happen as facilitative diffusion where particles too big to fit through the membrane, needs help to move substance across a cell (like opening a door). Or through osmosis, which is the diffusion of water across a membrane, water always follows salute concentration (water moves to higher salt/sugar concentrations). From here the contents moves into the cecum, a small pouch attached to a sphincter at the end of the small intestine where most water absorption happens. E coli is a bacterium that produces vitamin K and D, which is also absorbed in the cecum along with some minerals. The large intestine is split into four sections, the ascending, descending, transverse and sigmoid colon. From this point the waste materials collected (called feces) are stored in the rectum and are then expelled from the body with the help of and internal and external sphincter. And the process begins again (Biology 11, 2011).