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5/14/2010

Digestion
• Digestion

Digestion, Absorption, and
Metabolism

– Process by which food is broken down into smaller
parts, chemically changed and moved through the
gastrointestinal tract

• Two types of action
– Mechanical digestion
– Chemical digestion

Mechanical Digestion
• Food broken into smaller pieces by teeth
• Moved through GI tract through esophagus,
stomach, and intestines
• Process of peristalsis
• Helps prepare food for chemical digestion by
breaking into smaller pieces

Digestion in the Mouth
• Food broken into smaller pieces by teeth and
mixed with saliva
– Saliva contains water, salts and digestive enzyme
salivary amylase
• Acts on complex carbohydrates (starch)

Chemical Digestion
• Changes the composition of CHO, proteins and
fats
• Food broken down to molecules that the
blood can absorb
– Hydrolysis

• Enzymes essential

Digestion in the Esophagus
• Food moved by peristalsis and gravity
• Cardiac sphincter located at the lower end of
the esophagus

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Digestion in the Stomach
• Three sections of stomach:
– Fundus
– Body
– Pylorus
• Transformation of food into chyme
• Gastric juices
– Hydrocholoric acid
– Pepsin
– Mucus

Digestion in the Small Intestine
• Acidity of chyme neutralized
Bile released by gallbladder to emulsify fat
• Chyme triggers pancreas to secrete pancreatic juice
containing enzymes:
– Pancreatic proteases
– Pancreatic amylase
– Pancreatic lipase

• Small intestine produces enzymes
– Lactase
– Maltase
– Sucrase

Absorption
• Passage of nutrients into the blood or
lymphatic system
• To be absorbed nutrients must by in their
simplest forms:
– Carbohydrates into simple sugars
– Proteins to amino acids
– Fats to fatty acids and glycerol

Functions of the Stomach
• Temporary storage of food
• Mixing of food with gastric juices
• Regulation of a slow, controlled emptying of
food into the intestine
• Secretion of the intrinsic factor for vitamin
B12
• Destruction of most bacteria inadvertently
consumed

Digestion in the Large Intestine
• Primary function to absorb water and salts
form undigested food
• Absorption of fatty acids
• Digested food travels out of the body

Absorption in Small Intestine
• Most absorption of nutrients occurs here
• Surface area increased through mucosal folds,
villi, microvilli
– Blood capillaries
– Lacteals

• Villi absorb nutrients from chyme

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Absorption in the Large Intestine
• Absorb water
• Synthesize some B vitamins and vitamin K
• Collect food residue
• Undigested food is excreted as feces by way of
rectum

Metabolism
• After digestion and absorption nutrients
carried by blood to the cells
• Within cells nutrients changed into energy
through the process of metabolism
– Aeorbic metabolism
– Anaerobic metabolism

• In healthy people, 99% of CHO, 95% of fat, and
92% of proteins are absorbed

Metabolism
• As nutrients oxidize energy is released
– Anabolism
– Catabolism

Basal Metabolic Rate
• Energy necessary to carry on all involuntary
vital processes while the body is at rest
– Respiration
– Circulation
– Regulation of body temperature
– Cell activity and maintenance

• Known as BMR
– Also referred to as resting energy expenditure
(REE)

Metabolism and the Thyroid Gland
• Metabolism governed primarily by hormones
secreted by the thyroid gland
– T3
– T4

• Hyperthyroidism
• hypothyroidism

Basal Metabolic Rate
• Affected by
– Lean body mass
– Body size
– Sex
– Age
– Heredity
– Physical condition
– climate

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Lean Body Mass

Estimating BMR

• Muscle as opposed to fat tissue
• Large body frames require more calories
• Men require more energy than women
• Children require more calories per pound
• BMR increase during fever
• BMR decreases during starvation or severely
decreased calorie intake

Energy Balance

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