Compendium II Unit II

I. Digestion and Nutrition

A. Overview

B. Take Me to the Factory: Digestion, Part I

C. Stomach and Small Intestine

D. Pancreas, Liver, Gallbladder and Their Juices

E. Out With It!

F. Nutrition and Weight Control



I. Digestion

A. Overview
All the organs of the digestive system are found in the gastrointestinal tract (GI): a very long tube running through the body. Digestion is the way the body breaks down macromolecules in food into basic molecules for absorption and use by cells. The five main processes of digestion are: ingestion(intake of food through the mouth), digestion (the breakdown of food from, mechanical to chemical, for nutrients), movement (transport from organ to organ to expulsion.), absorption (broken down unit molecules pass through the GI tract then into the bloodstream.) and elimination (removal of undigested waste.)
The GI tract has four layers. The inside space is called lumen. The first layer is the mucosa. It produces mucus that protect the wall from enzymes. It can secrete enzymes or recieve them from other enzyme-secreting organs. The second layer is the submucosa. This layer is made of loose tissue containing blood and lymphatic vessels and nerves. It also contains lymph nodules that protect from infection. The third layer is the muscularis. It contains two layers of muscle. The first layer circles the tract and the second lies the same way as the tract. These muscles contract to move food through the GI. The fourth layer is the serosa. The serosa is also part of the lining of the abdominal cavity.

The Digestive system. A: mouth, B: esophagus, C: stomach
D: small intestine, E: large intestine.

B. Take me to the factory: digestion, part I
The mouth, pharynx and esophagus are the first section of digestion in the first part of the GI tract. The mouth receives food. It begins mechanical and chemical digestion. The roof of the mouth separates the nasal and oral cavities. The front part is the hard palate. The back is the soft palate. There are three pairs of salivary glands that send saliva to the mouth. The openings of the salivary ducts are on either side of the mouth by the molars, underneath the tongue and on the floor of the oral cavity. Saliva contains mucus, water, bicarbonate and the enzyme, salivary amylase.
Teeth are used in mechanical digestion to break down food. there are 32 teeth in the adult mouth. The crown of the tooth covers the dentin and pulp in a very hard bone-like material called enamel. The tongue is covered with a mucous membrane. In this are the taste buds, for sensing taste. The tongue moves food around for the teeth to chew and lumps food into a mass then pushes it toward the pharynx.
The pharynx is a cavity that contains both food and air passages. The esophagus is found here. Swallowing is voluntary from the mouth to the pharynx. However, from the pharynx down the process is reflexive. The soft palate closes of the nasal cavity. The The larynx rises which causes the glottis to block of the epiglottis (so food does not go down the air passage.) Peristalsis is a contracting that pushes food along the esophagus and through the digestive tract. Sphincters are muscles around digestive tubes that act as valves that keep food and acid down.

C. Stomach and small intestine

Continuing through the GI we come to the stomach. It is located beneath the diaphragm on the left side of the body. Thick-walled organ stores food, begins the digestion of protein and controls the flow of the now semi-fluid chyme into the small intestine. The stomach does not absorb food. The muscularis around the stomach have three layers of muscle. A layer of oblique muscle is added to the wall at the stomach which makes it able to stretch and mechanically digest even further. Openings in the stomach, called gastric pits release gastric juice from the gastric glands. An enzyme called pepsin is contained in the gastric juice. Also found is hydrochloric acid that kills most bacteria entering the stomach. It also activates pepsin in protein-rich foods. Due to peristaltic waves, the contents of the stomach are released into the lower intestine a bit at a time.
The small intestine is on average about 18 feet long. It contains every type of enzyme for digesting all kinds of foods. The enzymes come from the pancreas. Bile also enters, which breaks up fat into droplets and is then hydrolyzed by lipase. Amylase and intestinal enzymes break down carbohydrates into glucose. Pancreatic trypsin and intestinal enzymes break down proteins into amino acids. The basic content of the small intestine neutralizes the acidity of chyme. The mucosa of the small intestine is modified for nutrient absorption. Projections called villi contain microscopic, microvilli (projections.) All in all, this greatly increases the surface area of the small intestine for greater absorption. Capillaries run through the villi as well as lymphatic vessels called lacteal. The lacteal absorb chylomicrons (villus-packaged combo of fatty acids and lipoproteins.) Some people lack the brush border enzyme, lactase which breaks down the sugar, lactose (the main sugar of milk and other dairy.) The resulting disorder is lactose intolerance. It causes gas, bloating and even diarrhea. Cheese and yogurt have already had the lactose broken down and is safe for the lactose intolerant. Intake of too much sugar and fat causes obesity which can lead to diabetes two and/or cardiovascular disease. Healthy diet and exercise can combat this.

A cutaway of the villi, of the small intestine.

D. Pancreas, gallbladder, liver and their juices
These particular organs are called accessory organs because they are accessed for digestion without having food pass directly through. them. The pancreas produces pancreatic juice. It contains sodium bicarbonate and the enzymes: pancreatic amylase, trypsin and lipase. It also secretes insulin into blood. Insulin regulates glucose levels. The liver is the largest accessory organ. The liver receives blood to filter out wastes and toxins. The liver also stores iron and vitamins:A, E, K, D and B12 that have come from the blood. It stores insulin-present glucose as glycogen and releases it steadily between meals. After converting amino acids into glucose the combine the unused acids with CO2. to make urea for expulsion. In addition the liver produces bile which regulates the amount of cholesterol in the blood. It is stored in the gallbladder, just below the liver, until released into the small intestine.
Hepatitis and cirrhosis are two disorders that affect the liver. They are serious and life-threatening. . Jaundice can occur, a symptom in which bile leaks into the blood giving the skin and eyes a yellowish tint. Hepatitis, in all its types, is a virus which can be spread through sewage-contaminated drinking water (Hepatitis A), sexual contact or blood transfusion (Hepatitis B, C.) Cirrhosis causes the liver to become overly fatty. Then scar tissue replaces liver tissue. This occurs from over activity from fatty foods and/or alcohol. Artificial livers have been tested and liver transplants are the only option for liver failure, otherwise: death.

Jaundice has caused the eyes of this person, with
Hepatitis A, to turn yellow.

E. Out with it!
After the small intestine, comes the large intestine, which includes the cecum, colon, rectum and anal canal. The cecum has a the appendix projecting from it, which may help fight infection. The colon makes up the majority of the large intestine. It has three parts: ascending, transverse and descending. Then comes the rectum which opens at the anus. The large intestine does not absorb nutrients. It does, however, absorb water for hydration and vitamins produced by bacteria. Feces are formed in the large intestine. 25% of the feces is made up of dietary fiber, bacteria and indigestible material. The other 75% is fluid. Bacterial breakdown causes the odor and gas associated with feces. Defecation is the final phase of digestion. When peristalsis occurs at the rectum the feces are forced into the rectum. Nerve endings send a signal to relax the anus and contract the rectum muscles. Exit occurs.
Many disorders can occur in the large intestine. Diarrhea is usually caused by infection in the lower intestinal tract. Peristalsis increases with irritation. Prolonged diarrhea can causes dehydration and heart contraction disturbance. Constipation is caused frequently from the resistance of the urge to defecate. Hemorrhoids may develop from chronic constipation. These are inflamed blood vessels at the anus. Diverticulosis is caused from patches of mucosa pushing out through the muscularis. Irritable Bowel Syndrome occurs when the muscularis contracts strongly and irregularly. Inflammatory Bowel Disease is the title given to a collection of disorders which cause ulcers. Dietary fat may cause the promotion of colon cancer while fiber appears to inhibit the development of polyps and colon cancer.

The bulge shown is colonic polyp.

F. Nutrition and weight control
Obesity is on the rise worldwide and especially in the United States. This is of great concern because excess fat is linked to diabetes type 2, hypertension, heart disease and premature death. The Body Mass Index (BMI) shows healthy weights as well as warns of overweight and obesity. This can be determined by dividing one's weight in kilograms by the square of the individual's height. Healthy BMIs are between 19.1 and 26.4. Overweight is 26.5 to 31.1. Obesed is 32.3 to 39.9 and 40 and over is morbid obesity. Although, this is only a general rule. The BMI does not take into account factors such as gender, bone structure, muscle content and fitness.
Nutrients are the part of food that is used by the body. Carbohydrates, as studied in chapter two, are consumed and turned into, if not already available as, glucose. It is a major energy source and vital to brain function. Foods with complex carbohydrates can be very good for you, especially since many, naturally, contain vitamins, minerals and fiber. However refined carbohydrates lack other nutrients. The overconsumption of them is probably responsible for obesity in the US. Proteins are digested into amino acids. Some are absolutely integral to the diet. Eggs, milk, meat, poultry and most other animal-derived foods contain all the essential amino acids. No plant source has in itself all of the essential amino acids, except soy products. Amino acids are not stored so a daily supply is necessary. It does not take much, though. Many people take in too much protein daily. Excess amino acids can be ingested by those who think it will increase muscle mass. They are important to muscle building, but too much is just excreted by the body in urine, which can cause dehydration. Lipids are any of the oils, fats and cholesterols. Saturated fats mostly come from animals, save for palm and coconut oil. Polyunsaturated fats, in oils, are important to the diet. Canola oil and safflower oil contain linoleic and linolenic acids, which the body doesn't produce. Olive oil and canola oil contain good amounts omega-3 fatty acids which are great for fighting heart disease. Saturated fats and cholesterol are the major contributors to cardiovascular disease and must be consumed moderately. Minerals are important to the body and are used in different ways. Calcium contributes to strong bones and teeth and to muscle contraction. Phosphorus is essential to bone and soft tissue growth and ATP. Potassium is used in nerve conduction and muscle contraction. Sulfur. Stabilizes protein shape and neutralizes toxins. Sodium is important to pH and water balance. Magnesium is contained in many enzymes for nerve contraction and muscle contraction and is integral to protein synthesis. Zinc is needed for protein synthesis and contributes to healing and fetal development. Iron is very important to hemoglobin. Copper is as well. Iodine is essential to thyroid hormone production. Selenium is found the antioxidant enzyme and manganese is an essential ingredient to enzymes.
Vitamins, organic compounds, are used for metabolic purposes. The body does not produce an adequate amount of vitamins. Some vitamins are parts of coenzymes. Vitamins have various functions. Vitamins C, E and A are antioxidants which fight cancer-causing free radicals. Vitamin D is essential to proper calcium absorption.
A nutritious diet is important to healthy living. It is important to: eat food from all food groups. A lot of fruits, vegetables, whole grains and low-fat milk should be eaten. Eat less food high in saturated and trans fat, added sugar, cholesterol, salt and alcohol. Eat lean meat and dry beans and peas for protein. Lastly, it is very important to remain physically active.
Some people suffer from different eating disorders. Anorexia nervosa is a disorder in which a person has an unreasonable fear of being fat. People with anorexia usually starve themselves and have marked binges of eating. Bulimia Nervosa is marked by binge-eating and then purging to keep from gaining weight. Purging is done through self-induced vomiting and overuse of laxatives. Binge-eating disorder is marked by periods of overeating. Muscle dysmorphia is a disorder where a person believes he or she is not muscular enough. These people obsess over diet and weight training.

Tofu, a soy product, is an excellent source of
essential amino acids.

Sources:

Digestive system http://www.i-can-drink-milk-again.com/images/Human-Digestive-System-Picture.jpg

Jaundice http://www.in.gov/isdh/programs/hivstd/hepatitis/figure2.gif

Polyp http://www.bowelcheck.co.nz/images/polyp-colon.jpg

Tofu Robot http://zedomax.com/blog/wp-content/uploads/2007/04/tofu-robot-1.gif