Chapter 005

Williams' Basic Nutrition & Diet Therapy

Chapter 5

Digestion, Absorption, Transport and Metabolism

Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved. 1

14th Edition

Lesson 5.1: Digestion and the Digestive Organs

1. Through a balanced system of mechanical and chemical digestion, food is broken down into smaller substances and the nutrients are released for biological use.

2. Special organ structures and functions accomplish these tasks through the successive parts of the overall system.

2Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights reserved.

Digestion (p. 64)

Basic principles Principle of change

• The body cannot use food as it is eaten.• Food must be changed into simpler substances to be

absorbed and then used by cells to sustain life. Principle of wholeness

• The parts of the digestive process comprise a continuous whole.

• Food components travel through the gastrointestinal (GI) system until they are delivered to cells or excreted.


Digestion (cont’d) (p. 65)


Mechanical and Chemical Digestion (p. 65)

Mechanical and chemical actions make up the digestive process.

Food must undergo these changes to be delivered to cells.

Specific actions occurring during digestion of carbohydrates, proteins, and fats are discussed in other chapters.


Mechanical Digestion (p. 66)

Mechanical digestion: gastrointestinal motility Muscles in GI wall produce:

• Tonic contractions, which produce continuous movement• Periodic muscle contraction and relaxation, which mix

food mass and move it forward


Mechanical Digestion (cont’d)(p. 66)

Mechanical digestion: gastrointestinal motility (cont’d) Nerves from esophagus to anus:

• Control muscle tone in wall• Regulate rate and intensity of contractions• Coordinate various movements


Chemical Digestion (p. 66)

Chemical digestion: gastrointestinal secretions Hydrochloric acid and buffer ions: produce the correct pH

necessary for enzyme activity Enzymes: specific digestive proteins for breaking down

nutrients Mucus: lubricates and protects the GI tract tissues and helps

mix the food mass Water and electrolytes: carry and circulate the products of

digestion through the tract and into the tissues Bile: divides fat into smaller pieces to assist fat enzymes


Case Study

Tim eats a meal consisting of the following foods: Grilled Chicken with lemon seasoning Baked potato with broccoli and cheese Mixed greens with lemon juice and olive oil and

parmesan cheese 1 cup of unsweetened ice tea


Digestion in the Mouth and Esophagus (p. 66)

Mechanical digestion Mastication breaks down food Food is swallowed and passes down esophagus Muscles at tongue base facilitate process Gastroesophageal sphincter at stomach entrance

relaxes, allowing food to enter, then constricts to retain food


Digestion in the Mouth and Esophagus (cont’d) (p. 66)

Chemical digestion Salivary glands secrete material containing

salivary amylase or ptyalin Ebner’s glands at the back of the tongue secrete a

lingual lipase Salivary glands also secrete a mucous material to

lubricate and bind food particles, facilitating the swallowing of the food bolus

Secretions from the mucous glands in the esophagus help move food toward the stomach


Case Study (cont’d)

Explain what is happening in the mouth and esophagus in regard to Tim’s meal


Digestion in the Stomach (p. 67)


Digestion in the Stomach (cont’d) (p. 67)

Mechanical digestion Under sphincter control, the food enters upper

portion of the stomach (fundus) Stomach muscles knead, store, mix, and propel

food mass forward By the time food mass reaches the lower portion

of the stomach (antrum), it is a semiliquid acid/food mix called chyme

Pyloric valve slowly releases chyme into the first section of the small intestine (duodenum)


Digestion in the Stomach (cont’d) (p. 67)

Chemical digestion: three types of acid secretions Hydrochloric acid: parietal cells in the stomach

lining secrete acid to promote gastric enzyme activity

Mucus: secretions protect the stomach lining from the erosive effect of the acid and also bind and mix the food mass and help move it along

Enzymes: pepsinogen is secreted by stomach cells and is activated by acid to become pepsin, a protein-splitting enzyme



Explain what is happening to Tim’s meal in the stomach at this time.


Digestion in the Small Intestine (p. 67)

Mechanical digestion Peristaltic waves push food forward Pendular movements stir chyme Segmentation rings chop food mass into lumps Longitudinal rotation rolls food in spiral motion,

mixing it Surface villi motions stir and mix chyme


Digestion in the Small Intestine (cont’d) (p. 68)

Chemical digestion: pancreatic enzymes Carbohydrate: pancreatic amylase converts starch

to maltose and sucrose Protein: trypsin and chymotrypsin split large

protein molecules into small peptide fragments and eventually into single amino acids; carboxypeptidase removes end amino acids from peptide chains

Fat: pancreatic lipase converts fat to glycerides and fatty acids



Chemical digestion: intestinal enzymes Carbohydrate: Disaccharidases convert

disaccharides into monosaccharides. Protein: Enterokinase activates trypsinogen from

the pancreas to become trypsin; amino peptidase removes end amino acids from polypeptides; dipeptidase splits dipeptides into amino acids.

Fat: Intestinal lipase splits fat into glycerides and fatty acids.



Explain how Tim’s meal is progressing through the intestine with each enzyme activity.



Chemical digestion Mucus: protects mucosal lining Bile: emulsifying agent, aids fat digestion and

absorption Hormones: secretin for alkaline environment,

cholecystokinin triggers release of bile



What role does bile play in the digestive process of Tim’s meal?


Lesson 5.2: Absorption and Metabolism

3. Absorption, transport, and metabolism allows for the distribution, use, and storage of these nutrients throughout the body.


Absorption and Transport (p. 69)

At this point in the process: Carbohydrates: reduced to simple sugars

(glucose, fructose, galactose) Fats: changed into fatty acids and glycerides Proteins: changed into single amino acids Vitamins and minerals: liberated from food



Discuss the final products formed from digestion of Tim’s meal that can be readily used by the cells


Absorption and Transport (cont’d) (p. 69)

Amount of nutrients consumed depends on bioavailability: Amount of nutrient present Competition between nutrients for absorption sites Form in which nutrient is present


Small Intestine (p. 71)

Small intestine: three absorbing structures Mucosal folds: surface of small intestine piles into

many folds Villi: small, fingerlike projections Microvilli: cover each villus

Make inner surface 600 times greater than outer surface of intestinal wall


Small Intestine (cont’d) (p. 72)


Absorption Processes (p. 71)

Simple diffusion: particles move outward in all directions toward areas of lesser concentration

Facilitated diffusion: uses a protein channel for carrier-assisted movement of larger particles

Active transport: carrier partner (e.g., sodium) moves particles across a membrane

Pinocytosis: larger materials are engulfed by a cell


Absorption Processes (cont’d)(p. 72)


Absorption in the Large Intestine (p. 73)

Water: main absorptive task of large intestine is to absorb water; small amount remains for feces

Dietary fiber: contributes bulk to help form feces Macronutrients and micronutrients: absorbed to

lymph or blood


Transport (p. 73) Vascular system: veins and arteries

Carry water-soluble nutrients, oxygen, other vital substances

Transport wastes for removal Portal circulation first carries nutrients to liver for

cell enzyme work Lymphatic system

Carries non–water-soluble fatty materials Lymph vessels in villi absorb materials Route to larger lymph vessels Eventually to blood stream through thoracic duct


Metabolism (p. 73)

At this point, individual macronutrients in food: Have been broken down Absorbed into bloodstream or lymphatic system Next, nutrients can be converted into energy or

stored


Energy for Fuel (p. 73)

Mitochondria of cells is where metabolism takes place

Two forms of metabolism Catabolism: breaking down of larger substances

into smaller units Anabolism: building up of larger substances from

smaller units


Energy for Fuel (cont’d) (p. 73)

Metabolic processes ensure that the body has energy in the form of adenosine triphosphate (ATP)

Metabolism of glucose from carbohydrates yields less energy than metabolism of fat, but glucose is the body’s primary source of energy

Protein can be an energy source, but it is relatively inefficient


Stored Energy (p. 74)

Energy beyond that needed at present is stored for future

Glucose is converted to glycogen via glycogenesis, stored in liver or muscles

When glycogen reserves are full, excess is stored as fat via lipogenesis

Excess protein/amino acids converted to glucose via gluconeogenesis


Stored Energy (cont’d) (p. 74)


Errors in Digestion and Metabolism (p. 75)

Genetic defects: cell is missing enzyme controlling metabolism of a specific nutrient Phenylketonuria (PKU)

• Enzyme responsible for metabolizing essential amino acid phenylalanine is missing

• Untreated, causes permanent mental retardation and CNS damage

• With proper treatment, affected children may have normal and healthy lives


Errors in Digestion and Metabolism (cont’d) (p. 75)

Genetic defects (cont’d) Galactosemia

• Enzyme responsible for metabolizing galactose to glucose is missing

• All sources of lactose must be eliminated from diet• Untreated, can cause brain and liver damage• Screening and treatment can enable normal life


Errors in Digestion and Metabolism (cont’d) (p. 77)

Genetic defects (cont’d) Glycogen storage diseases (GSD)

• Group of rare genetic defects• Absence of enzymes required for synthesis or

breakdown of glycogen• Form of disease depends on enzyme missing


Other Intolerances or Allergies (p. 78)

Not genetic inborn errors of metabolism Rather, caused by food intolerances or

allergies Lactose intolerance: deficiency of any of the

disaccharidases in small intestine• Insufficient lactase to break down milk• Abdominal cramping and diarrhea

Produces containing lactose must be avoided


Other Intolerances or Allergies (cont’d) (p. 78)

Allergies Inappropriate immune responses Not necessarily a problem with digestion or

metabolism Covered in Chapter 18


Chapter 005

Business

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