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The Digestive System
Transcript of The Digestive System
Chemical vs. Mechanical Digestion
Chemical Digestion of Nutrients
The digestive system's purpose is to break down the food that is ingested by the organism, so nutrients can be absorbed and converted into usable energy.
The Digestive System
Many organs in the digestive system secrete different substances that break down food by chemical means. During chemical digestion, enzymes may break down large food molecules (macromolecules such as carbohydrates and proteins) by hydrolysis. This is a process where a water (H20) molecule is added where a bond is broken.
The liver is the largest internal organ in the human body. It produces bile, which is a bitter-tasting alkaline fluid that aids in digestion by helping break down fats. Everyday, the liver produces about 1 liter of bile.
The gallbladder is the small organ beneath the liver where excess bile from the liver is stored until it is needed by the small intestine.
The end portion of the digestive tract is known as the large intestine. It's about 1.5 meters long and includes the colon, the rectum, and a small sac-like appendage (something that is added or attatched to something larger or more important) called the appendix.
The true purpose of the appendix is unknown. However, it may need to be surgically removed if it becomes inflamed or swollen. This condition is known as appendicitis.
The primary function of the colon is to absorb water from the chyme (semi-fluid mass of partly digested food). This makes the indigestable material more solid and is called feces. Perstalsis moves the feces throughout the colon by a series of muscle squeezes and contractions.
The rectum is the second to last part of the digestive system. It provides temporary storage for feces before they are excreted. When peristalsis causes feces to reach the end of the colon to the rectum, the walls of the rectum stretch. This induces a reflex that makes the final sphincter muscle relax, so the feces can be eliminated from the body through the anus.
A swallowing reflex is stimulated when the tongue pushes chewed food to the back of the mouth.
This forces the food into the upper portion of the esophagus, which is
a muscular tube that connects the pharynx -also known as the throat- to the stomach. The smooth muscles that line the esophagus
use the process of peristalsis to move the food down to the stomach.
This is a process where muscles contract consecutively to squeeze food throughout the digestive system.
Food leaves the esophagus and passes through a sphincter, or a circular muscle. This sphincter is called the cardiac sphincter. After, it moves into the stomach. Three overlapping layers of smooth muscle form the stomach walls. They take part in mechanical digestion, using contractions to break down the food and mix it with the secretions of the gastric glands that line the inner wall of the stomach. The acidic enviornment is beneficial to the action of pepsin, an enzyme that is involved in the chemical digestion process of proteins. Cells in the stomach lining secrete a mucus that prevents damage from the pepsin and the high acidity of the environment.
When a person swallows food, a small plate of cartilage called the epiglottis covers the trachea, where air that is breathed in and out passes through. The trachea leads to the lungs, and if the epiglottis is not closed when a person swallows, food will be able to enter the trachea and cause a person to choke. The body induces a coughing relfex to expel the food from the trachea and prevent food from entering the lungs.
The small intestine is the longest part of the digestive tract, at 7 meters (about 21 feet). It is called the "small" intestine because it has a diameter of only 2.5 centimeters, while the "large" intestine has a diameter of 6.5 centimeters. Here, mechanical digestion is continued by the smooth muscles of the small intestine walls, and peristalsis continues to move the food along. However, the completion of chemical digestion in the small intestine requires the participation of 3 other accessory organs- the pancreas, the liver, and the gallbladder.
Acid reflux is a disorder where a person experiences a severe and chronic form of heart burn, where the sphincter between the end of the esophagus and the entrance to the stomach is faulty or inadequate and leads to reguritations of stomach acids.
The main functions of the pancreas are to produce enzymes that digest carbohydrates, proteins and fats, and to produce hormones, which are substances that act on certain target cells and tissues to produce a specific response.
In the mouth, the process of carbohydrate digestion begins with actions of the saliva. Amylase, an enzyme in saliva, starts chemically digesting starch, converting it from a polysaccharide to a disaccharide called maltose. This is further broken down to glucose in the small intestine so the body can absorb and utilize it.
Millions of gastric glands line the inside of the stomach walls. They secrete 400-800 mL of gastric fluids every meal. Three kinds of cells are found in the gastric glands; parietal cells secrete hydrochloric acid, "chief" cells secrete pepsinogen, and mucus-secreting cells secrete, of course, mucus. The substance pepsinogen is a very important constiuent of gastric fluids. The acidity of the stomach's contents is increased by the secreted hydrochloric acid to a pH of 2 or lower. This pH causes the pepsinogen to be converted into the enzyme, pepsin, which begins the chemical digestion of proteins.
Side note: Once amylase enzymes reach the stomach they go inactive because they function at a pH of 6 or 7 and the stomach's pH is at about 2 due to the hydrochloric acid. Therefore, starches cannot continue to be chemically digested after leaving the mouth where the amylase enzymes fucntion properly.
The Small Intestine
Millions of microscopic finger-like projections called villi cover the walls of the small intestine. In the small intestine, nutrients in the food are absorbed into the body through the villi. Intestinal enzymes produced by the cells in the villi complete the digestion of peptides and sugars. The first part of the small intestine after the stomach is called the duodenum. This location is the most active in digestion, using secretions from the liver and pancreas. Epithelial cells of the duodenum secrete a watery mucus to protect the lining from any harm from pepsin or hydrochloric acid. The pancreas secretes digestive enzymes and sodium bicarbonate (Tums), a stomach acid-neutralizer. The sodium bicarbonate makes the acidic chyme neutral to allow the enzymes in the small intestine to function. This neutralization also protects the intestinal wall.
The process where food is physically broken down by movements of muscles is called mechanical digestion.
the teeth, tongue and palate work together to grind and mash up food into a soft mush that can be swallowed.
the smooth muscles in the the stomach churn food to help mechanically break it down, and mix it with the stomach secretions.
peristalsis in the small intestine further breaks down the food by squeezes and contractions.
In the mouth, carbohydrates are chemically digested by amylase enzymes in your saliva. They break down the complex carbs into simple sugars, called monosaccharrides. The digestion of carbohydrates ends once the food leaves the mouth and travels down to the stomach. This is because after the roughly 10 seconds the food spends in the esophagus, it passes into the stomach where the pH is too acidic for amylase to properly function and break down the carbohydrates anymore. (The enzyme's active sites are denatured and the substrates are no longer able to fit to them and fulfill their job.)
In the stomach, the chemical digestion of proteins begins. Because proteins are too complex for the body to properly absorb and utilize in this state, they must be broken down into simpler molecules that are subunits of proteins. These are called amino acids and they provide building blocks for the essential compounds in your body, such as cell proteins, enzymes, hormones and genetic material. Stomach cells create acids and an enzyme called pepsin, and these cooperate to break the proteins down into smaller proteins or amino acids. These broken down particles pass into the small intestine where they meet a different set of enzymes that continue the work of the pepsin, deconstructing proteins into amino acids. These simpler molecules are transported into the intestinal walls and travel through the bloodstream and throughout the body.
At the back of your tongue and in your stomach there are enzymes called lipases that break the bonds between the small fatty acids. Once free, they travel down the digestive tract to the small intestine where cells in the intestinal walls absorb them into the bloodstream. To break the links between the long fatty acids, bile salts from the gallbladder and enzymes from the pancreas are secreted into the small intestine, where they work with other enzymes and hormones. The fatty acids in their simplified form can be used to build cell membranes, making certain hormones, and absrobing fat-soluble vitamins. These acids can be broken down by the body for energy.
The small intestine is where the majority of absorption occurrs. This is the final site where hydrolysis (the process of adding an H2O molecule where a bond is broken in a macromolecule) takes place. Digested molecules of food, as well as water and minerals from the diet are absorbed in the upper cavity of the small intestine. These materials pass through the mucus membrane and into the bloodstream and are carried off to other parts of the body for storage, further chemical reactions or utilization. This part of the process varies from nutrient to nutrient.
A sphincter is a surrounding circle of muscle that serves to guard or close off an opening or tube such as the entrance to the stomach. Sphincters are important to the digestive system because, like the entrance to the stomach, they are needed to block the hydrochloric acid from traveling up the esophagus.
Digestive Systems of Organisms in other Phyla
A stovepipe sea sponge
There is no digestive system present in the sea sponge organisms. They rely on a constant water flow to obtain food and oxygen and to remove waste products.
(disorder of the GIT)
The Role of Villi in Nutrient Absorption
When chemical digestion is completed, most of the nutrients from the food are absorbed into the bloodstream by finger-like structures in the small intestine called villi (singular is villus). It increases the surface area of the small intestine walls to almost the size of a tennis court. This is beneficial because the greater the surface area, the more nutrients can be taken into the body.
Food begins its journey along the GIT of the goldfish when it enters the mouth. To create suction, goldfish open their mouths, drawing food items along with water, into their mouth. Food items are kept in the mouth while the water is expelled through the gills. Then, the food items then enter the pharynx, located at the very back of the mouth. There are taste buds as well as teeth called pharyngeal teeth in the pharynx. This creates a process of mechanical digestion to break the food items down into small pieces that can pass through the narrow intestines. After the food items have been chewed by the pharyngeal teeth, they go through a very short esophagus and enter the intestinal bulb. Goldfish intestines can be divided into two basic parts; the intestinal bulb for temporary storage of food items and to absorb lipids, and the caudal intestine's function is to absorb protein from the food items as they pass through.
Digestive System Fun Facts
We eat about 500kg of food per year. 1.7 liters of saliva is produced each day.
The esophagus is approximately 25cm long. Muscles contract in waves to move the food down the esophagus. This means that food would get to a person's stomach, even if they were standing on their head.
An adults stomach can hold approximately 1.5 liters of material.
Every day 11.5 liters of digested food, liquids and digestive juices flow through the digestive system, but only 100mLs is lost in faces.
In the mouth, food is either cooled or warmed to a more suitable temperature.
Hundreds of different kinds of enzymes are needed to properly digest food. Cooking destroys food enzymes, forcing the body to make its own. Over time the body may tire of this extra work, leaving room for possible indigestion.