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Chapter 3
The Human Body:
From Food to Fuel
Taste and Smell:
The Beginnings of Our Food Experience
• Sight, smell, thought, taste, and sound
– Trigger a set of responses that prepare the
digestive tract to receive food
The Gastrointestinal Tract
• Variety of functions, including:
– Ingestion of food
– Transport of ingested food
– Secretion of digestive enzymes, acid, mucus,
and bile
– Absorption of end products of digestion
– Movement of undigested material
– Elimination of digestive waste products
The Gastrointestinal Tract
• Several layers to the GI tract, including
– Mucosa
– Circular muscle
– Longitudinal muscle
– Sphincters
Overview of Digestion
• Physical movement
– Peristalsis
• Wavelike, muscular
contractions
• Transport food and nutrients
along the GI tract
– Segmentation
• Series of muscular
contractions in the small
intestines
• Divides and mixes the chyme
Overview of Digestion
• Chemical breakdown
– Enzymes
• Proteins that catalyze chemical reactions
– Other secretions
• Acid
• Base
• Bile
• Mucus
Overview of Absorption
• The road to nutrition absorption
– Passive diffusion
– Facilitated diffusion
– Active transport
Assisting Organs
• Salivary glands
– Moisten food
– Supply enzymes
Assisting Organs
• Liver
– Produces bile
– “Chemical factory”
– “Dynamic warehouse”
Assisting Organs
• Gallbladder
– Stores and secretes bile
• Pancreas
– Secretes bicarbonate
– Secretes digestive enzymes
Putting it All Together:
Digestion and Absorption
• Mouth
– Enzymes
• Salivary amylase
acts on starch
• Lingual lipase acts
on fat
– Saliva
• Moistens food for
swallowing
Putting it All Together:
Digestion and Absorption
• Esophagus
– Transports food to
stomach
– Esophageal sphincter
Putting it All Together:
Digestion and Absorption
• Stomach
– Hydrochloric acid prepares protein for
digestion and activates enzymes
– Pepsin begins protein digestion
– Gastric lipase has some fat digestion
– Gastrin (hormone) stimulates gastric
secretion and movement
– Intrinsic factor is needed for absorption of
vitamin B12
Putting it All Together:
Digestion and Absorption
• Small intestine
– Sections of small intestine
• Duodenum, jejunum, and ileum
– Nutrient digestion
• Bicarbonate neutralizes stomach acid
• Pancreatic and intestinal enzymes
–Carbohydrates
–Fat
–Protein
Putting it All Together:
Digestion and Absorption
• Small intestine
– Absorption
• Folds, villi, and microvilli expand absorptive
surface
• Most nutrients absorbed here
• Fat-soluble nutrients go into lymph
• Other nutrients go into blood
Putting it All Together:
Digestion and Absorption
• Large intestine
– Sections
• Cecum, colon, rectum, anal canal
– Digestion
• Peristaltic movement is slow, taking 18–24
hours for material to travel
• Some digestion of fiber by bacteria
Putting it All Together:
Digestion and Absorption
• Large Intestine
– Absorption
• Water
• Sodium, potassium,
and chloride
• Vitamin K
(produced by
bacteria)
– Elimination
Circulation of Nutrients
• Vascular system
– Veins and arteries
– Carries oxygen and nutrients to tissues
– Removes wastes
• Lymphatic system
– Vessels that drain lymph
– Empties into the bloodstream near the neck
Circulation of Nutrients
• Excretion and elimination
– Lungs
• Excrete water and carbon dioxide
– Kidneys filter blood
• Excrete waste; maintain water and ion
balance
Signaling Systems:
Command, Control, and Defense
• Nervous system
– Regulates GI activity
• Enteric nervous system
• Autonomic nervous system
• Hormonal system
– Increases or decreases GI motility and
secretions
– Influence your appetite
Signaling Systems:
Command, Control, and Defense
• Immune system
– Protects us from foreign invaders
– Role of GI tract
• Barrier
• Immune response
–Natural killer cells
–Macrophages
• Location of lymphoid tissues
–Lymphocytes
»Antibodies
Influences on Digestion and Absorption
• Psychological influences
– Taste, smell, and
presentation of food
• Chemical influences
– Type of protein you eat and
the way it is prepared
• Bacterial influences
– Hydrochloric acid kills most
bacteria
Nutrition and GI Disorders
• Constipation
– Hard, dry, infrequent stools
– Reduced by high fiber, fluid intake, exercise
• Diarrhea
– Loose, watery, frequent stools
– Symptom of diseases/infections
– Can cause dehydration
– Broth, tea, toast, and other low-fiber foods can
help reduce
Nutrition and GI Disorders
• Diverticulosis
– Pouches along colon
– High-fiber diet reduces formation
• Heartburn and GERD
– Reduced by smaller meals, less fat
– Smoking weakens the esophageal sphincter
– Being overweight often worsens symptoms
Nutrition and GI Disorders
• Irritable bowel syndrome
– Causes abdominal pain, diarrhea or
constipation, and cramps
– Stress and certain foods aggravate the
symptoms
– Can usually be controlled by diet and lifestyle
modifications
• Stress management
Nutrition and GI Disorders
• Colorectal cancer
– Fiber-rich diet may reduce risk
• Gas
– Most foods that contain carbohydrates can cause
• Ulcers
– Pain in the upper abdomen
– Can cause nausea, vomiting, loss of appetite, and
weight loss
– Bacterial cause (H. pylori)
Nutrition and GI Disorders
• Functional dyspepsia
– Chronic pain in the upper abdomen
– Treat with medicine and stress reduction