Module 3: Digestion and Taste

Overview

This class will cover the basics of digestion and the digestion system. Through the use of a video, review of material on the website and interactive diagram of the digestive system and class group activities the basic information about the process of digestion will be presented.

Objectives

1. Be able to name the components of the digestive system and understand how the digestive system works.
2. Be able to outline the overall process of digestion and absorption, including the characteristics and roles played by the organs of the digestive system.
3. Become familiar with some of the important enzymes and hormones that are involved in digestion and absorption.

Activities: This is a review activity – it is not graded.

Break into small groups review material on the digestive system by group:

Group A. Mouth, Pharynx and esophagus
Group B. Stomach, small intestine, large intestine
Group C. Accessory organs

Working in your group and using any on-line material, readings and class website prepare a “teaching presentation” for the rest of the class. Your presentation should include :

• A description of the location of the parts of the digestive system assigned to your group. Use descriptions or the diagram.
• An outline of the role that your assigned parts of the digestive system play in digestion and how each part works.
• What enzymes, hormones, or chemicals are used or produced in each part of your assigned piece of the digestive system?
• What gets absorbed?
• Any health problems associated with this part of the digestive system?

digestion review-animation

Notes – Introduction to Nutritional Anthropology

Module 3: Digestive System and Module 5: Primate Digestive System
(See Module 5)

Overview of Digestion and the Digestive System

The digestive system is a group of organs that work to breakdown food that has been ingested through a process called digestion. This process is followed by absorption, through which the nutrients which are freed/ released by digestion pass through the camber walls into the blood system and are carried throughout the body. The nutrients which are absorbed are used to generate energy for the body and build and replace cells and tissues, which are constantly

dying. The digestive system begins in the mouth, continues in the pharynx (throat) and esophagus and into the “gut”region: the stomach, small and large intestines, the rectum and the anus.

Digestion occurs in two phases: 1) mechanical breakdown and 2) chemical breakdown. Mechanical breakdown begins with chewing food which breaks large pieces of food into smaller pieces and particles. The second phase, chemical breakdown, is accomplished by enzymes which break large molecules into their components.

Not all that we eat can be digested, so the waste is disposed of in an efficient way. The unusable material becomes bodily wastes or fecal material that is eliminated from the body.

Digestive System Components

(image of digestive system)

Mouth (including teeth)

The mouth is the entry point to the digestive system. As such it has a number of important features. The function of the mouth and its associated structures is to form a receptacle for food, to begin mechanical digestion through chewing (mastication), to swallow food, and to form words in speech. It can also assist the respiratory system in the passage of air. The lips modify sounds for the production of speech, manipulate incoming food and help control it during the process of chewing. The teeth tear, slice or grind food. These processes increase the surface area of the food thus increasing the area that is exposed to digestive enzymes.

Teeth (diagram of teeth and diagram of dental arch.

Incisors –     Shaped like a blade and set in the front of the dental arcade. The incisors are good for biting off pieces of food.

Canines –     Used for tearing food – especially meat and biting off pieces of food.

Premolars–  Complex surface consisting of cusps and valleys facilitate grinding of tough, fibrous food.

Molars –       Complex surface consisting of cusps and valleys facilitate grinding of tough, fibrous food.

Tongue

There are five primary tastes: sweet, salty, sour,  bitter and umami. The ability to distinguish these tastes enable us to evaluate food and make the critical choice of whether we should swallow it or spit it out. Sweetness, usually associated with sugars, implies a rich source of calories and thus energy. Salty taste appears to be associated with some minerals, like sodium chloride (table salt), and humans seem to have a craving for a certain amount of salt in their diets. Sourness is more problematic. Sour is the taste of unripe ripe fruit. Some researchers suggest we reject unripe fruit because ripe fruit has more nutritional value of more value after it has ripened and thus it is better to leave it to ripe and not eat it unripe. Bitter, which is often characteristic of toxic substances is usually avoided.

Our ability to distinguish the five primary tastes (sweet, salty, sour, bitter, umami) depends on the taste buds which are primarily located on the surface of the tongue, however some can also be found on the inner surface of the cheeks, the roof of the mouth, and in the throat. There seems to be genetic variation in the number of taste buds, but not in the location of them on the tongue. The taste buds serve as the interface between chemicals dissolved in saliva and the sensory neurons that convey the information to the brain. Although each taste bud (and we have about 10,000 of them in our mouth) responds to all of the four basic tastes, they are usually more sensitive to one or two tastes than to the others. Different regions of the tongue seem to have slightly different sensitivity to the different tastes because of the way the taste buds are distributed in the mouth.

Taste Links

Tongue and Taste Information (pdf)

NPR: Sweet, Salty, Bitter and…

Salivary Glands

The mouth contains three pairs of salivary glands. The first, the parotids, is embedded in the cheek in front of the ears. The second, the sublingual glands, are situated beneath the tongue on the floor of the mouth. The third is the submandibular and is found below the angle of the jaw. Together these glands secrete about 1.2 liters of saliva a day. Saliva is important because it lubricates the mouth, mixes with food to moisten it into a mush that is swallowed easily, and dissolves flavor molecules enabling the taste buds to sense them. Saliva also contains antibacterial agents which are important for reducing the bacterial activity in the mouth which can cause decay of the teeth. Although saliva is mostly water it does contain the enzyme salivary amylase which is important in starch and sugar breakdown. Production of saliva is stimulated through the smell, sight and even thought of food.

Pharynx and Esophagus

When food is swallowed, it is pushed into the pharynx or throat. It is first manipulated in the mouth until it is shaped into a ball (bolus), and then the tongue pushes it into the throat. The presence of food in the pharynx triggers a reflex reaction called peristalsis. Peristalsis is a series of involuntary contractions of the muscles in the wall of the esophagus. These contractions move the food down the esophagus into the stomach and then on to the rest of the digestive tract.

Stomach

The stomach is an expandable pouch-like organ connected at one end to the esophagus and to the duodenum (the first part of the small intestine) at the other end. The stomach can hold about 4 cups of food for several hours, which enables us to eat only two or three meals a day. If this weren’t possible, we would have to eat about every twenty minutes. The first major stage of chemical digestion occurs in the stomach. The wall of the stomach is unique in the digestive system because it is composed of three layers of smooth muscle. The contractions of the stomach wall muscles knead the contents, throughly mixing food with the stomach’s secretions. The mixture is called chyme. The secretions produced by the stomach include hydrochloric acid, intrinsic factor as well as enzymes. The hydrochloric acid partially serves as a germicide which helps kill the “bad” bacteria in food. Intrinsic factor is important in the absorption of Vitamin B12. In addition the stomach produces pepsinogen which reacts with hydrochoric acid to produce pepsin, an enzyme that breaks down proteins. The stomach is stimulated to begin producing secretions by the sight, smell or thought of food.

In summary, the stomach has several functions:(1) storage, holding a meal in the upper portion and then releasing slowly into the lower portion where it is processed; (2) food mixing; (3) sterilization; (4) digestion, digestive fluid is produced which splits and cracks the chemicals in food. There is very little absorption of nutrients in the stomach, except for some alcohol. The pyloric sphincter, a muscular valve, is located at the end of the stomach and controls the movement of the chyme into the small intestine.

Small intestine

The small intestine, which is actually about 10 feet long, is considered small because of its narrow diameter ( about i inch / 2,5 cm).It is bounded on the “north” by the stomach and on the “south” by the large intestine. The small intestine is looped back and forth upon itself, this enables it to fit into the abdominal space it occupies. It is held in place by tissues which are attached to the abdominal wall Most of the digestion and absorption of food occurs in the small intestine. The small intestine has three segments:

1.   Duodenum (10 in/ 25 cm)- this section receives the highly acid chyme from the stomach. It is the part of the small intestine that is most susceptible to ulcers because it comes in contact with the chyme. Secretions from the liver, pancreas, and gall bladder are released into the duodenum. The bile salts act as emulsifiers they break fat into small drops which then are broken down by enzymes. The secretions from the pancreas are important because they are alkaline and neutralize the acidic chyme. They also include enzymes that break down starch, protein, fats and nucleic acids.

2.   Jejunum (8 ft./2.5 m) – most of the nutrients are absorbed into the blood from the jejunum.

3.   Ileum ( 12 ft/ 3.6 m) – the site where the remaining nutrients are absorbed before moving into the large intestine.

The surface area of the small intestine is increased by the presence of numerous folds. Within these folds there are many finger like projections called villi. The villi trap food and enhance absorption. When undigested food reaches the end of the small intestine it passes through the ileocecal sphincter to the large intestine.

Large intestine

The large intestine, which is larger in diameter than the small intestine, does not have villi or digestive enzymes, consequently very little absorption occurs in the large intestine. However, water some vitamins, some fatty acids and minerals(sodium and potassium)are able to be absorbed in the large intestine. The large intestine does contain mucus producing cells. The mucus is important because it helps hold the feces together. The large intestine also contains a large number of bacteria some of which breakdown starches such as lactose that may have not been broken down in the small intestine. Feces produced by the large intestine are comprised of water, protein, fat, undigested food, roughage, dried digestive juices, cells shed by the intestine, and dead bacteria.

The large intestine has four main portions:

1.   ascending

2.   transverse

3.   descending

4.   sigmoid

The colon depends on roughage or bulk to function properly. Most of the bulk is composed of carbohydrates that can not be digested such as cellulose. If there is not enough fiber in the large intestine, it will take longer for the contents to move through it and be expelled. This can lead to prolonged exposure to toxins which have been implicated in cancer. A lack of fiber is also associated with constipation. A shortage of fiber can also lead to the development of weak spots and “balloons.” These can become infected and inflamed, a condition known as diverticulosis.

Rectum and Anus

The feces or stool remains in the last portion of the large intestine called the rectum. Muscular contractions push the stool into the anus and then it is eliminated from the body. The rectum contains two sphincters (one internal and the other external). Individuals learn to control the external sphincter when they are “toilet trained” as children.

Accessory Organs

The liver, gallbladder and pancreas are considered to be accessory organs of the digestive system because while they do not participate directly in digestion, they provide fluids that aid the digestive process.

1. Liver and Gallbladder

The liver produces bile which is stored in the gallbladder. Bile is essential for the absorption of fat. The liver is also important for its detoxifying ability. It is capable of detoxifying harmful substances and destroy harmful microorganisms.. After removing them from the blood the liver disposes this material through the duodenum and then the feces. The liver also controls numerous aspects of carbohydrate, lipid, protein and alcohol metabolism.

2. Pancreas

The pancreas produces glucagon and insulin. These hormones are important for regulating blood glucose levels. Insulin affects blood glucose in a number of ways. First, it promotes glycogen synthesis. Glycogen is the storage form of glucose, synthesized and stored in the liver and muscles. Secondly, insulin affects the uptake of glucose by cells . Both of these activities lower blood glucose levels and help return it to normal levels after eating.

Assignment (See assignments page)