Digestion Continued...

• Mechanical breakdown of food continues, this includes the churning of food across the rugae (lines in the wall (muscles) of the stomach)

• Gastric juices are released (HCl, mucus, pepsinogen)

• HCl chemically breaks down food and kills foreign substances (eg. bacteria), HCl converts pepsinogen into active form pepsin.

• pepsin breaks down proteins into shorter chains

• the food becomes a semi-liquid called chyme

• mucus protects stomach from being digested by HCl

The Pyloric SphincterControls the movement of food into the small intestine. Moves like a drawstring on a bag.

At this point the food is semi liquid and 5% digested

The undigested matter is known as chyme.

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

Small IntestineAbout 2.5 cm wide, 7 m long

Coiled Loop

There is Gastric Juice here as well to help with chemical digestion of chyme.

Three Parts of the Small Intestine:

1. Duodenum: first 25 cm where the majority of digestion occurs

2. Jejunum: 3m long

3. Ileum: 4 m long

Small Intestine

Small Intestine - Duodenum

•Chemical digestion of food • has tubes (ducts) leading from liver and pancreas • bile and digestive enzymes enter from the ducts and are mixed with the chyme • disaccharidases (sucrase, maltase, lactase) are released to further break down carbs

Small Intestine -Jejunum & Ileum

• responsible for nutrient absorption • molecules of fat, protein and carbohydrates get absorbed into the bloodstream and then are transported to liver • pushes undigested material into large intestine

Absorption in Small Intestine

The interior of the small intestine is covered in finger-like tube extensions called villi (singular = villus) villi greatly increase the surface area of the small intestine thus increased absorption

Each villus has hair like extensions called on them called microvilli (singular = microvillus)

these will also increase the surface area of the interior small intestine and thus increase absorption

Digestion and Nutrition 225

6.7

villus

artery

lymph vessel

vein

Figure 1Villi greatly increase the surface area forabsorption of nutrients. Each villus has bloodcapillaries and lymph vessels.

villi: small fingerlike projections that extendinto the small intestine which increase surfacearea for absorption

microvilli: microscopic fingerlike outwardprojections of the cell membrane

The large intestine houses bacteria, such as E. coli, which are essential to life,and use waste materials to synthesize vitamins B and K. Cellulose, the long-chain carbohydrate characteristic of plant cell walls, reaches the large intestineundigested. Although cellulose cannot be broken down by humans, it serves animportant function: Cellulose provides bulk. As wastes build up in the largeintestine, receptors in the wall of the intestine provide information to the cen-tral nervous system which, in turn, prompts a bowel movement. The bowelmovement ensures the removal of potentially toxic wastes from the body.Individuals who do not eat sufficient amounts of cellulose (roughage or fibre),have fewer bowel movements. This means that wastes and toxins remain in theirbodies for longer periods of time. Scientists have determined that cancer of thecolon can be related to diet. Individuals who eat mostly processed, highlyrefined foods are more likely to develop cancer of the colon.

Medical reports that state fibre may also reduce cholesterol levels have beenembraced by many food manufacturers. Although the actual benefits of bran, aform of roughage, in reducing cholesterol can be debated, its value in providinga balanced diet is undeniable.

Although some water and vitamins are absorbed in the large intestine andthe stomach, most absorption takes place within the small intestine. Long fin-gerlike tubes called villi (singular: villus) greatly increase the surface area of thesmall intestine (Figure 1). One estimate suggests that villi account for a tenfoldincrease in surface area for absorption. The cells that make up the lining of eachvillus have microvilli, which are fine, threadlike extensions of the membrane thatfurther increase the surface for absorption (Figure 2).

Figure 2

Villi in the mammalian intestine

Microvilli on the surface of an epithelial cell

Digestion and Nutrition 225

6.7

villus

artery

lymph vessel

vein

Figure 1Villi greatly increase the surface area forabsorption of nutrients. Each villus has bloodcapillaries and lymph vessels.

villi: small fingerlike projections that extendinto the small intestine which increase surfacearea for absorption

microvilli: microscopic fingerlike outwardprojections of the cell membrane

The large intestine houses bacteria, such as E. coli, which are essential to life,and use waste materials to synthesize vitamins B and K. Cellulose, the long-chain carbohydrate characteristic of plant cell walls, reaches the large intestineundigested. Although cellulose cannot be broken down by humans, it serves animportant function: Cellulose provides bulk. As wastes build up in the largeintestine, receptors in the wall of the intestine provide information to the cen-tral nervous system which, in turn, prompts a bowel movement. The bowelmovement ensures the removal of potentially toxic wastes from the body.Individuals who do not eat sufficient amounts of cellulose (roughage or fibre),have fewer bowel movements. This means that wastes and toxins remain in theirbodies for longer periods of time. Scientists have determined that cancer of thecolon can be related to diet. Individuals who eat mostly processed, highlyrefined foods are more likely to develop cancer of the colon.

Medical reports that state fibre may also reduce cholesterol levels have beenembraced by many food manufacturers. Although the actual benefits of bran, aform of roughage, in reducing cholesterol can be debated, its value in providinga balanced diet is undeniable.

Although some water and vitamins are absorbed in the large intestine andthe stomach, most absorption takes place within the small intestine. Long fin-gerlike tubes called villi (singular: villus) greatly increase the surface area of thesmall intestine (Figure 1). One estimate suggests that villi account for a tenfoldincrease in surface area for absorption. The cells that make up the lining of eachvillus have microvilli, which are fine, threadlike extensions of the membrane thatfurther increase the surface for absorption (Figure 2).

Figure 2

Villi in the mammalian intestine

Microvilli on the surface of an epithelial cell

Large Intestine

Parts of the Large Intestine

Anus

Appendix

Ascending Colon

Transverse Colon

Descending Colon

Rectum

Large Intestine -Colon•water and dissolved minerals are absorbed from undigested food

• bacteria break food down further and produce vitamins (B12, K) and amino acids

• forms and expels feces

Large Intestine -Rectum

• acts as temporary storage area for feces

Large Intestine -Anus

• egestion of wastes by peristalsis • sphincters control the release of feces

Turn to Pg. 418, Table 10.6

→appendix

VESTIGIAL