importance of digestion

The Importance of Digestion

The human digestive system is made of a tube from the mouth to the anus, this is called the Gastrointestinal Tract (GI Tract).

Digestive systems is needed to breakdown large materials into smaller ones that can be used by the tissues of the body.

Digestion and Nutrition 211

6.3

Figure 1The human digestive system and accessoryorgans

stomach

Figure 2Taste buds are located along the tongue.

papillae

mouth

pharynx

esophagus

small intestine

large intestine

rectum

anus

salivary glands

liver

gall bladder

pancreas

The tiny projections on your tongue that giveit a velvety appearance are called papillae.It is within the papillae that the majority oftaste buds on the tongue are found. Themost numerous papillae (which are downthe middle of your tongue) do not have tastebuds. These papillae are involved in tactilesensation.

DID YOU KNOW ?

equipped with receptors (Figure 2) that have a specific geometry that permit theidentification of sweet tastes from carbohydrates. Nerve cells for taste are stimu-lated when receptor sites are filled by chemical compounds with a complemen-tary shape. You can find out the significance of dissolving foods by drying yourtongue and then placing a few grains of sugar or salt on it. You will not detect anyflavour until the crystals dissolve.

TeethThe teeth are important structures for physical digestion. Eight chisel-shapedteeth at the front of your mouth, called incisors, are specialized for cutting. Theincisors are bordered by sharp, dagger-shaped canine teeth that are specializedfor tearing. Next to the canine teeth are the premolars. These broad, flattenedteeth are specialized for grinding. The molars are found next to the premolars.These teeth tend to be even broader and have cusps that are even more flattened.They are designed for crushing food. The last set of molars are the wisdom teeth,so called because they usually do not emerge until about 16 to 20 years of age.Often these molars are troublesome and must be removed because there is notenough space for them to grow in.

Each tooth has two divisions: the root and the crown. An enamel crowncovers the tooth with calcium compounds and forms the hardest substance in thebody. Immediately inside the enamel is dentin, a bonelike substance, which is

tastebuds

taste poremicrovilli

connectivetissue

sensorynerve fibre

epitheliumof tongue

taste cell

The tongue and teeth contribute to the physical digestion occurring in the mouth.

The Mouth

The Tongue

The tongue is a muscle attached to the lower jaw. It freely moves in all directions and is sensitive to touch, taste and temperature.

physical digestion begins in the mouth where food is chewed and formed into bolus.

Bolus ~ in latin means “ball of form” and contains food and saliva.

Function:

1. Pushes food around to be chewed

2. Helps form the bolus

3. Aids in swallowing. When swallowing the tongue moves up and pushes the food down the throat.

Salivary Glands:

These are glands which secrete saliva (which is mostly water) into the mouth.

1. Submaxillary: base of the jaw

2. Sublingual: under the tongue

3. Paratoid: below the ear lobe

1 & 2 helps food stick together

3 helps with

digestion

Saliva contains amylase enzymes, which breaks down starches. Function:

1. Moisten food to make swallowing easier

2. Helps food dissolve into a solution

3. contains amylase which starts to break down carbohydrates

4. Has a neutral pH (6.5 - 6.8)

Saliva

•prevents food from blocking the pharynx when swallowing • initiates the gag reflex

• hangs from the middle of back edge of soft palate

Uvula

Types of Teeth

1. Incisor ~ cut

2. Pre Molars ~ grind

3. Canines ~ tear

4. Molar ~ grind

Structures of the Tooth Include

1. Enamel ~ a hard outer coating

2. Dentin ~ a bone like inner layer

3. Pulp Cavity ~ contains soft tissue with blood vessels and nerves

Moving Food Past The Mouth:

The bolus of food moves down the esophagus by rhythmic contractions known as peristalsis.

When the food reaches the stomach, it enter through the muscular valve called the cardiac sphincter.

When food is swallowed the epiglottis shuts off the trachea (for breathing) letting the bolus go down esophagus (for food).

The entrance of the stomach is guarded by the cardiac sphincter which controls the flow of materials into the stomach.

The stomach is a muscular bag where both chemical and physical digestion occur.

The exit of material from the stomach is guarded by the pyloric sphincter which controls flow into the small intestine.

The Stomach

214 Chapter 6

6.4 The Stomach and DigestionThe stomach is the site of food storage and initial protein digestion. The move-ment of food to and from the stomach is regulated by circular muscles calledsphincters. Sphincters act like the drawstrings on a bag. Contraction of the car-diac sphincter closes the opening to the stomach located nearer the heart, while itsrelaxation allows food to enter. A second sphincter, the pyloric sphincter, regulatesthe movement of foods and stomach acids to the small intestine (Figure 1).

The J-shaped stomach has numerous ridges that allow it to expand so that itcan store about 1.5 L of food. Millions of cells line the inner wall of the stomach.These cells secrete the various stomach fluids, called gastric fluids or gastric juice,that aid digestion. Approximately 500 mL of these fluids are produced followinga large meal. Gastric fluid includes mucus, hydrochloric acid (HCl), pepsinogens,and other substances. Mucus provides a protective coating. Hydrochloric acid killsmany harmful substances that are ingested with food. It also converts pepsinogeninto its active form, pepsin, which is a protein-digesting enzyme. Pepsin breaksthe long amino acid chains in proteins into shorter chains, called polypeptides.

The pH inside the stomach normally ranges between 2.0 and 3.0, but mayapproach pH 1.0. Acids with a pH of 2.0 can dissolve fibres in a rug! It is the highacidity (low pH) of hydrochloric acid that makes it effective at killing pathogensand allows pepsin to do its work. How does the stomach safely store these strongchemicals, both of which dissolve the proteins that make up cells? A layer of alka-line mucus protects the stomach lining from being digested. Pepsinogen movesthrough the cell membrane and mucous lining, is activated by HCl, and becomespepsin. The pepsin breaks down the proteins in the food, but not the proteins ofthe stomach’s cells because these proteins are protected by the mucous layer.

UlcersWhen the protective mucous lining of the stomach breaks down, the cell mem-brane is exposed to the HCl and pepsin. The destruction of the cell membraneleads to a peptic ulcer. Beneath the thin layer of cells is a rich capillary network.

sphincters: constrictor muscles thatsurround a tubelike structure

mucus: a protein produced by a layer ofepithelial cells known as a mucous membrane

pepsin: a protein-digesting enzyme producedby the stomach

ulcer: a lesion along the surface of an organ

capillary: a blood vessel that connectsarteries and veins. Capillaries are the sitesof fluid and gas exchange.

Figure 1The stomach is the initial area of proteindigestion. The cardiac sphincter controls themovement of food from the esophagus to thestomach. The pyloric sphincter controls themovement of food from the stomach to thesmall intestine.

esophaguscardiac

sphincter

J-shapedstomach

pyloricsphincter

Rennin is another enzyme found in thestomach. Rennin slows the movementof milk through the gastrointestinal tract,thereby permitting more time for the break-down and absorption of nutrients. Renninhas little impact in the stomachs of adults;however, it causes milk to curdle in thestomachs of infants, allowing more nutrientsto be absorbed. Curds and whey, mentionedin the nursery rhyme “Little Miss Muffet,”are made by adding a type of rennin to milk.The proteins are the milk curds; the wateryportion is the whey. Rennin is used to makecheese.

DID YOU KNOW ?Understanding Concepts

1. How are the digestive system and other organ systems interde-pendent?

2. What advantage is gained from digesting food along a digestivetract rather than in a single gastrovascular cavity?

3. How do toothless animals such as birds break down food parti-cles? Suggest advantages of being able to swallow food withoutneeding to chew.

4. Differentiate between physical and chemical digestion. Provideexamples of each.

5. Is the movement of food through your digestive system volun-tary or involuntary? What mechanisms are responsible formoving food along the alimentary tract?

6. The type of teeth that a mammal has is matched to diet. Keepingin mind the function of different types of teeth, name an animalthat would have well-developed (a) canines, and (b) molars andpremolars.

Sections 6.1–6.3 Questions