The Human Body:
From Food to Fuel
BIOL 103, Chapter 4
Today’s Topics
•
•
•
•
•
•
Taste and Smell
GI Tract
Digestion and Absorption
Assisting Organs
Circulation of Nutrients
Signaling Systems: Command, Control, and Defense
– The Hormonal System
– The Immune System
• Influences on Digestion and Absorption
– Psychological, Chemical, Bacterial Influences
• Nutrition and GI Disorders
Taste and Smell: The Beginnings
of our Food Experience
• Sight, smell, thought, taste, and sound
– Can trigger a set of responses that prepare the
digestive tract to receive food
The Gastrointestinal (GI) Tract
• Organization
– Parts (mouth  anus)
• Mouth, esophagus,
stomach, small intestine,
large intestine, rectum
– Accessory organs
• Salivary glands, liver,
gallbladder, and pancreas
Organization of the GI Tract
• Functions:
1. Ingestion (the receipt and softening of food)
2. Transport of ingested food
3. Secretion of digestive enzymes, acid, mucus, and
bile
4. Absorption of end products of digestion
5. Movement of undigested material
6. Elimination of waste material
Organization of the GI Tract
• Structural organization of
the GI tract
– Mucosa (innermost layer)
• Glands and absorptive cells
– Circular muscle and
longitudinal muscle
• Mix and move the food
– Sphincter: valve that
controls the movement of
food material so that it
travels through the GI tract
in only one direction.
Overview of Digestion
• Mouth
– Chewing  break food into smaller pieces
– Saliva  lubricates the food  ____________
• Down the GI tract:
– Physical movement
• Peristalsis
• Segmentation
Overview of Digestion
Overview of Digestion
• Chemical breakdown:
– Bolus  __________________
– Enzymes: are proteins that catalyze (____________)
reactions but are not altered in the process.
• Examples: amylase, lipase, etc.
– Other secretions:
–
–
–
–
Stomach Acid
Base
Bile
Mucus
Overview of Absorption
• 3 Main processes allow nutrients to be absorbed
from GI tract  blood/circulation
– Passive Diffusion: movement of molecules through
cell membrane from high to low concentration
gradient without _____________ use
– Facilitated Diffusion: movement of molecules through
cell membrane from high to low concentration
gradient with a help of a transport _____________
– Active Transport: movement of molecules through
cell membrane requiring both _____________ and
_______________.
Overview of Absorption
Assisting/Accessory Organs
1. Salivary glands
– Moisten food
– Supply enzymes
2. Liver
1.Produce bile
2.“Detox center”
3.“Chemical factory”: >500
chemicals
4.“Dynamic Warehouse”
Assisting/Accessory Organs
3. Gallbladder
– Stores and excretes bile
– Bile’s Enterohepatic circulation:
• Liver  Gallbladder  Intestine 
Liver
4. Pancreas
– Secretes bicarbonate, digestive
enzymes
– Secretes hormones (insulin and
glucagon)
Putting It All Together:
Digestion and Absorption
• Mouth
– Enzymes
• Salivary amylase
• Lingual lipase
– Saliva
• Moistens food for swallowing  bolus
• Esophagus
– Transports food to stomach
– Esophageal sphincter (“cardiac sphincter”)
• “Heartburn”
Putting It All Together:
Digestion and Absorption
• Stomach Enzymes:
– Hydrochloric acid/gastric acid
1. kills bacteria
2. prepares protein for digestion
1.
2.
breaks down 3D structure of protein
Pepsinogen (inactive)  Pepsin (active)
– Pepsin: breaks down ___________ into smaller pieces
– Gastric lipase: some ____________ digestion
– Gastrin (hormone) stimulates gastric secretion and
movement
– Intrinsic factor is needed for vitamin ____ absorption.
Putting It All Together:
Digestion and Absorption
• Small intestine (~10ft)
– Pyloric Sphincter
– Sections of small
intestine:
• Duodenum
• Jejunum
• Ileum
– Nutrient digestion
• ____________________
neutralizes stomach acid
• Pancreatic and intestinal
enzymes to digest
carbohydrates, lipids, and
proteins.
Putting It All Together:
Digestion and Absorption
• Small intestine
– Completes absorption:
• Folds, villi, microvilli expand
_________________________ surface
– 600x fold increase/tennis court!
• Most nutrients absorbed here
• Fat-soluble nutrients  lymphatic
system (lymphatic vessel in the
intestinal villus)
• Water-soluble nutrients  bloodstream.
Putting It All Together:
Digestion and Absorption
• Large Intestine
– Ileocecal valve
– Sections
• Cecum, colon, rectum,
and anal canal
– Digestion
• ____________ movement
is slow, taking 18-24 hours
for material to travel
• Some bacterial activity
(e.g. fiber digestion)
Beans, Beans, Beans!
• Beans are made up of oligosaccharides (e.g.
raffinose and stachyose), a component of
fiber.
• They are ignored until they are met by 700+
species of bacteria in your large intestine.
– Bacteria digests these sugars  gases accumulate
 flatulence
Putting It All Together:
Digestion and Absorption
• Large Intestine:
– Absorption
• Water
• Na, K, Cl
• Vitamin K (produced by
bacteria)
– Elimination at anal
sphincter
• Feces: 60% solid (bacteria,
dietary fiber, digestive
secretions), 40% water
Circulation of Nutrients
Vascular System
• Veins and arteries
• Water soluble nutrients are
absorbed into the capillaries
of the intestines.
• Blood carries nutrients
_____________ portal vein
to the liver before
dispensing them through
the body.
Lymphatic system
• Vessels that drain lymph
(clear fluid formed in the
spaces between cells)
• Fat soluble-vitamins are
absorbed into lymph
vessels in the intestine.
• ______________ liver and
delivers nutrients to veins
in the neck  enter blood
Circulation of Nutrients
Circulation of Nutrients
• Excretion and Elimination
– Lungs
• Excrete water and carbon dioxide (metabolic wastes)
– Kidneys filter blood
• Excrete digestive wastes
– Urine: urea + salts + water
• Maintain water and ion balance
Summary of Digestion and
Absorption of Nutrients in
the Human Body
Review!!!
Signaling Systems:
Command, Control, Defense
• Nervous system
– Nerves carry info back and forth between tissues
and the brain using chemical signals known as
neurotransmitters.
– The Central Nervous System (CNS) regulates GI
activity in 2 ways:
1. Enteric nervous system: nerves located in the GI wall.
2. Autonomic nervous system: part of CNS that controls
organ function.
– Responses to sight, smell, thought of food by enhancing GI
movement and secretion
Signaling Systems:
Command, Control, Defense
Signaling Systems:
Command, Control, Defense
• Hormonal system
– Hormones: chemical messengers that travel in the
bloodstream
1. Increases or decreases GI motility and secretions
2. Influence your appetite by sending messages to
your CNS.
• Thus, your CNS and hormones work together
to coordinate movement and secretions of the
GI tract.
Signaling Systems:
Command, Control, Defense
• Immune System
– Purpose: protects us from foreign invaders
– Role of GI tract
• Immune Response
– Macrophages: “big eaters”  engulf and kill pathogens and oldcells
– Natural killer cells: attack virus-infected cells or cancerous cells
• Location of lymphoid tissues
– Lymphocytes: white blood cells present in blood and lymph
» Some types produce antibodies – protein molecules that tag,
neutralize, and help destroy bacteria/viruses or toxins.
Influences on Digestion and
Absorption
• Psychological influences
– Positive: Taste, smell, presentation of food
– Negative: Stress, fear, depression
• Example: Stress  activate autonomic NS  reduced gastric
secretion, reduced blood flow to stomach, inhibition of peristalsis
 upset stomach
• Chemical influences
– Cooking  protein changes + produce chemicals that may
influence digestion  easier food digestion/absorption
processes
• Bacterial influences
– HCl acid kills most bacteria
• If lives  food borne illnesses, gastritis, ulcer, etc.
Nutrition and GI Disorders
• Constipation (< 3x/week)
– Hard, dry, infrequent stools
– Reduced by high fiber, fluid intake, and exercise
• Diarrhea (> 3x/day)
– Loose, watery, frequent stools
– Symptom of diseases/infections  peristalsis
– Can cause dehydration
– Broth, tea, toast, and other low-fiber foods can
help reduce diarrhea
Nutrition and GI Disorders
• Diverticulosis
– Pouches along colon
develop as people age
• 10-25% people 
pouches become infected
– High-fiber diet reduces
formation:
• Low-fiber diet  harder
stools  more pressure
on colon  weak spots
bulge outwards and form
pockets
Nutrition and GI Disorders
• Heartburn and GERD
– Chronic heartburn 
GERD
– Reduced by smaller
meals, less fat
– Smoking weakens the
esophageal sphincter
– Being overweight often
worsens symptoms
Nutrition and GI Disorders
• Irritable Bowel Movement (IBS)
– Causes abdominal pain, diarrhea, or constipation
and cramps
– Stress and certain foods aggravate the symptoms
• Examples: milk, beans, chocolate, alcohol
– Associated with reproductive hormones
– Can usually be controlled by diet and lifestyle
modifications
• Stress management
Nutrition and GI Disorders
• Colorectal cancer: cancer in colon or rectum
– _______ leading cause of cancer-related deaths in
the US
– Eating a lot of red meat and low fiber food can
increase risks
– ______________________ may reduce risk:
• Hypothesis: more fiber  goes down the GI tract faster,
dilution of carcinogens in a bulkier stool, etc…
• BUT, human/animal studies do not support those
theories.
Nutrition and GI Disorders
• Ulcers
– Pain in the upper abdomen
• Sore is in duodenum: duodenal
ulcer
• Sore is in stomach: gastric ulcer
– Can cause nausea, vomiting,
loss of appetite, and weight
loss
– Cause: Bacteria (H. pylori) and
over-usage of NSAIDS (aspirin,
ibuprofen)
Summary