The stomach is protected from self-digestion by the mucosal barrier. This barrier has several components. First, the stomach wall is covered by a thick coating of bicarbonate-rich mucus. This mucus forms a physical barrier, and its bicarbonate ions neutralize acid. Finally, stem cells located where gastric glands join the gastric pits quickly replace damaged epithelial mucosal cells, when the epithelial cells are shed. In fact, the surface epithelium of the stomach is completely replaced every 3 to 6 days.
Sometimes, gastric juice eats away at the superficial lining of the stomach mucosa, creating erosions, which mostly heal on their own. Deeper and larger erosions are called ulcers.
Figure Ulcer-causing Bacterium H. Pylori Crossing Mucus Layer of Stomach. Why does the mucosal barrier break down? A number of factors can interfere with its ability to protect the stomach lining.
The majority of all ulcers are caused by either excessive intake of non-steroidal anti-inflammatory drugs NSAIDs , including aspirin, or Helicobacter pylori infection. When caused by H. A potential complication of ulcers is perforation: Perforated ulcers create a hole in the stomach wall, resulting in peritonitis inflammation of the peritoneum. These ulcers must be repaired surgically. The small intestine is a long muscular tube of about 20 feet long and about an inch in diameter.
Chyme moves from the stomach to the small intestine. The small intestine is the organ where the digestion of protein, fats, and carbohydrates is completed and absorb most of the nutrients from what we eat and drink. The coiled tube of the small intestine is subdivided into three regions. From proximal at the stomach to distal, these are the Duodenum, Jejunum, and Ileum.
The three regions of the small intestine are the duodenum, jejunum, and ileum. The wall of the small intestine is composed of the same four layers typically present in the alimentary system. The small intestine has a highly folded inner surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli.
Although their small size makes it difficult to see each microvillus, their combined microscopic appearance suggests a mass of bristles, which is termed the brush border.
These structures are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the blood stream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the blood in hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic substances, including drugs, alcohol, and some pathogens.
Villi are folds on the small intestine lining that increase the surface area to facilitate the absorption of nutrients. The large intestine , illustrated in Figure 13, reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but larger in diameter.
It has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon can be divided into four regions, the ascending colon, the transverse colon, the descending colon and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material.
Most bacteria that enter the alimentary canal are killed by lysozyme, defensins, HCl, or protein-digesting enzymes. However, trillions of bacteria live within the large intestine and are referred to as the bacterial flora. Most of the more than species of these bacteria are nonpathogenic commensal organisms that cause no harm as long as they stay in the gut lumen. In fact, many facilitate chemical digestion and absorption, and some synthesize certain vitamins, mainly biotin, pantothenic acid, and vitamin K.
Fecal transplantation or bacteriotherapy is the transfer of stool from a healthy donor into the gastrointestinal tract for the purpose of treating recurrent C.
Each year, approximately , Americans are diagnosed with colorectal cancer, and another 49, die from it, making it one of the most deadly malignancies.
People with a family history of colorectal cancer are at increased risk. Smoking, excessive alcohol consumption, and a diet high in animal fat and protein also increase the risk. Despite popular opinion to the contrary, studies support the conclusion that dietary fiber and calcium do not reduce the risk of colorectal cancer. Colorectal cancer may be signaled by constipation or diarrhea, cramping, abdominal pain, and rectal bleeding. Bleeding from the rectum may be either obvious or occult hidden in feces.
Since most colon cancers arise from benign mucosal growths called polyps, cancer prevention is focused on identifying these polyps. The colonoscopy is both diagnostic and therapeutic. Colonoscopy not only allows identification of precancerous polyps, the procedure also enables them to be removed before they become malignant. Screening for fecal occult blood tests and colonoscopy is recommended for those over 50 years of age.
The rectum is the terminal end of the large intestine, as shown in Figure The primary role of the rectum is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination.
The anus is an opening at the far-end of the digestive tract and is the exit point for the waste material. The small intestine absorbs about 90 percent of the water you ingest either as liquid or within solid food. Feces is composed of undigested food residues, unabsorbed digested substances, millions of bacteria, old epithelial cells from the GI mucosa, inorganic salts, and enough water to let it pass smoothly out of the body. Feces are eliminated through contractions of the rectal muscles.
Irritable bowel syndrome IBS is characterized by abdominal discomfort associated with altered bowel movements and change in your bowel habits. IBS cause abdominal discomfort in different ways, such as sharp pain, cramping, bloating, distention, fullness or even burning. IBS may be triggered by eating specific foods, following a meal, emotional stress, constipation or diarrhea. Many symptoms are related to hypersensitivity of the nerves found in the wall of the gastrointestinal tract. Each accessory digestive organ aids in the breakdown of food.
Within the mouth, the teeth and tongue begin mechanical digestion. Other accessory organs add secretions enzymes that catabolize food into nutrients. Accessory organs include salivary glands, the liver, the pancreas, and the gallbladder. Detoxifying the blood, producing bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. Cirrhosis is when scar tissue replaces healthy liver tissue. This stops the liver from working normally. Cirrhosis is a long-term chronic liver disease.
The damage to your liver builds up over time. The most common causes of cirrhosis are:. Bile is a mixture secreted by the liver to accomplish the emulsification of lipids in the small intestine. Recall that lipids are hydrophobic, that is, they do not dissolve in water. Food begins its journey through the digestive system in the mouth, also known as the oral cavity.
Inside the mouth are many accessory organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.
The pharynx, or throat, is a funnel-shaped tube connected to the posterior end of the mouth. The pharynx is responsible for the passing of masses of chewed food from the mouth to the esophagus.
The pharynx also plays an important role in the respiratory system, as air from the nasal cavity passes through the pharynx on its way to the larynx and eventually the lungs.
Because the pharynx serves two different functions, it contains a flap of tissue known as the epiglottis that acts as a switch to route food to the esophagus and air to the larynx. The esophagus is a muscular tube connecting the pharynx to the stomach that is part of the upper gastrointestinal tract.
It carries swallowed masses of chewed food along its length. At the inferior end of the esophagus is a muscular ring called the lower esophageal sphincter or cardiac sphincter.
The function of this sphincter is to close of the end of the esophagus and trap food in the stomach. The stomach is a muscular sac that is located on the left side of the abdominal cavity, just inferior to the diaphragm. In an average person, the stomach is about the size of their two fists placed next to each other.
This major organ acts as a storage tank for food so that the body has time to digest large meals properly. The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth. The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower gastrointestinal tract. It is located just inferior to the stomach and takes up most of the space in the abdominal cavity.
The entire small intestine is coiled like a hose and the inside surface is full of many ridges and folds. These folds are used to maximize the digestion of food and absorption of nutrients. The liver is a roughly triangular accessory organ of the digestive system located to the right of the stomach, just inferior to the diaphragm and superior to the small intestine. The liver weighs about 3 pounds and is the second largest organ in the body. The liver has many different functions in the body, but the main function of the liver in digestion is the production of bile and its secretion into the small intestine.
The gallbladder is a small, pear-shaped organ located just posterior to the liver. The gallbladder is used to store and recycle excess bile from the small intestine so that it can be reused for the digestion of subsequent meals. The excess sugars in the body are converted into glycogen and stored for later use in the liver and muscle tissue. Glycogen stores are used to fuel prolonged exertions, such as long-distance running, and to provide energy during food shortage.
Fats are stored under the skin of mammals for insulation and energy reserves. Proteins in food are broken down during digestion and the resulting amino acids are absorbed. All of the proteins in the body must be formed from these amino-acid constituents; no proteins are obtained directly from food.
Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy, and fatty acids are required for the construction of lipid membranes. Fats are also required in the diet to aid the absorption of fat-soluble vitamins and the production of fat-soluble hormones. While the animal body can synthesize many of the molecules required for function from precursors, there are some nutrients that must be obtained from food.
These nutrients are termed essential nutrients, meaning they must be eaten, because the body cannot produce them. The fatty acids omega-3 alpha-linolenic acid and omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids.
Vitamins are another class of essential organic molecules that are required in small quantities. Many of these assist enzymes in their function and, for this reason, are called coenzymes. Absence or low levels of vitamins can have a dramatic effect on health. Minerals are another set of inorganic essential nutrients that must be obtained from food. Minerals perform many functions, from muscle and nerve function, to acting as enzyme cofactors.
Certain amino acids also must be procured from food and cannot be synthesized by the body. The human body can synthesize only 11 of the 20 required amino acids; the rest must be obtained from food. With obesity at high rates in the United States, there is a public health focus on reducing obesity and associated health risks, which include diabetes, colon and breast cancer, and cardiovascular disease.
How does the food consumed contribute to obesity? Fatty foods are calorie-dense, meaning that they have more calories per unit mass than carbohydrates or proteins. One gram of carbohydrates has four calories, one gram of protein has four calories, and one gram of fat has nine calories.
Animals tend to seek lipid-rich food for their higher energy content. Excess carbohydrate is used by the liver to synthesize glycogen. When glycogen stores are full, additional glucose is converted into fatty acids. These fatty acids are stored in adipose tissue cells—the fat cells in the mammalian body whose primary role is to store fat for later use.
The rate of obesity among children is rapidly rising in the United States. The goal of this campaign is to educate parents and caregivers on providing healthy nutrition and encouraging active lifestyles in future generations.
This program aims to involve the entire community, including parents, teachers, and healthcare providers to ensure that children have access to healthy foods—more fruits, vegetables, and whole grains—and consume fewer calories from processed foods. Another goal is to ensure that children get physical activity. With the increase in television viewing and stationary pursuits such as video games, sedentary lifestyles have become the norm.
Visit www. There are many organs that work together to digest food and absorb nutrients. The mouth is the point of ingestion and the location where both mechanical and chemical breakdown of food begins.
Saliva contains an enzyme called amylase that breaks down carbohydrates. The food bolus travels through the esophagus by peristaltic movements to the stomach. The stomach has an extremely acidic environment. The enzyme pepsin digests protein in the stomach. Further digestion and absorption take place in the small intestine. The large intestine reabsorbs water from the undigested food and stores waste until elimination.
Carbohydrates, proteins, and fats are the primary components of food. Some essential nutrients are required for cellular function but cannot be produced by the animal body. These include vitamins, minerals, some fatty acids, and some amino acids. Food intake in more than necessary amounts is stored as glycogen in the liver and muscle cells, and in adipose tissue. Excess adipose storage can lead to obesity and serious health problems.
Learning Objectives By the end of this section, you will be able to: Explain the processes of digestion and absorption Explain the specialized functions of the organs involved in processing food in the body Describe the ways in which organs work together to digest food and absorb nutrients Describe the essential nutrients required for cellular function that cannot be synthesized by the animal body Describe how excess carbohydrates and energy are stored in the body.
Figure Concept in Action Explore this interactive United States Department of Agriculture website to learn more about each food group and the recommended daily amounts. Obesity With obesity at high rates in the United States, there is a public health focus on reducing obesity and associated health risks, which include diabetes, colon and breast cancer, and cardiovascular disease.
Glossary amylase: an enzyme found in saliva and secreted by the pancreas that converts carbohydrates to maltose.
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