Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Digestive System and...

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings

The Digestive System and Body Metabolism Digestion

Breakdown of ingested food

Absorption of nutrients into the blood

Metabolism

Production of cellular energy (ATP)

Constructive and degradative cellular activities


Processes of the Digestive System

Mechanical propulsion

Secretion

Digestion - mechanical and chemical breakdown

Absorption

Elimination


molars (12)

premolars (8)

canines (4)

incisors (8)

lower jaw upper jaw

Mechanical Processing

• Teeth break down food


Stomach Functions Acts as a storage tank for food

Mechanical, chemical breakdown of protein begins

Delivers chyme to the small intestine


Propulsion in the Stomach Food must first be well mixed

Rippling peristalsis occurs in the lower stomach

Figure 14.15


Chemical Digestion in the Small Intestine

Figure 14.6

Pancreatic enzymes digest starch, proteins, nucleic acids, fats; reset pH.Gall bladder -


Propulsion in the Small Intestine Peristalsis is the major

means of moving food

Segmental movements

Mix chyme with digestive juices

Aid in propelling food


Absorption in the Small Intestine

Site of nutrient absorption into the blood

Villi = Fingerlike structures on the mucosa surface

Microvilli on cells

All increase surface area

Figure 14.7a


Absorption of Proteins and Carbohydrates

Figure 14.13

Amino acids, sugars move by active transport into blood vessels

Next stop: liver, where glucose glycogenamino acids protein

Excess moleculesare converted to fats


Absorption of Fats

Figure 14.14

Lipids are absorbed into lymph system, which drains into heart

Emulsified with bile saltsDigested into fatty acids, glycerolWith bile salts, diffuse into cellsReassembled into fats, exocytosis


Food Breakdown and Absorption in the Large Intestine No digestive enzymes are produced

Resident bacteria digest remaining nutrients

Produce some vitamin K and B

Release gases

Water and vitamins K and B are absorbed

Undigested fiber keeps materials moving and is eliminated via feces


Control of Digestive Activity

Regulation dependent on volume and content of food

Nervous system: sight, smell of food, stretch receptors in stomach

Hormones:

Gastrin: stimulates release of gastric juice

Secretin: stimulates pancreas to secrete water and bicarbonate

Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes


Nutrition Carbohydrates: major energy source, simple

or complex

Lipids: cell components and energy sources, saturated or unsaturated

Proteins: 20 amino acids

Vitamins: fat soluble and water soluble

Minerals: recommended daily allowance

Fiber


Food Guide Pyramid

Figure 14.16


Body Energy Balance Energy intake = total energy output (heat +

work + energy storage)

Energy intake from food oxidation

Energy output

Heat is usually about 60%

Storage energy is in the form of fat or glycogen


Regulation of Food Intake

Mechanisms that may regulate food intake

Levels of nutrients in the blood

Hormones

Body temperature

Psychological factors


Basal Metabolic Rate

BMR– amount of heat produced by the body per unit of time at rest

Factors that influence BMR

Surface area

Gender

Age – children and adolescents have a higher BMR

thyroxine from thyroid gland


Total Metabolic Rate (TMR) Total amount of kilocalories the body must

consume to fuel ongoing activities

TMR increases with an increase in body activity

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings The Digestive System and...

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