GASTROINTESTINAL TRACT
•
•
Continuous tube, pathway of food
through the body
Four layers of tissue:
1.
2.
3.
4.
Mucosa – innermost, secretes mucus
Submucosa – blood vessels, nerves
Muscularis – two or three muscle layers
Serosa – thin, slippery, connective
DIGESTION
• Mechanical digestion = breaking food into
smaller pieces
• Chemical digestion = break bonds,
produce monomers
• Mouth
– Teeth and tongue begin mechanical digestion
• Teeth slice, tear, or grind
• Crown & root; Enamel, dentin, pulp
• Bacteria + sugar = cavities
– Saliva moistens food, eases swallowing
– Salivary amylase begins starch breakdown
• Pharynx (throat)
– Shared space for breathing, swallowing
– Flaps close off nasal cavity, glottis
• Esophagus
– Connects pharynx & stomach
– Muscular contractions = peristalsis
• Stomach
– Expandable; closed either end by sphicters
– If lower esophageal sphincter weak, heartburn
– Stomach wall: three muscle layers –
muscularis
– Churning action liquefies food  chyme
– Gastric pits secrete pepsinogen and
hydrochloric acid
– Pepsinogen + HCL  pepsin
– Pepsin: enzyme that digests protein to amino
acids
– HCL kills most bacteria, tenderizes tough food
– Why is stomach lining not digested?
• Mucus protects
• Enzymes start out inactive
• Cells divide/replace quickly
• Small Intestine
– Majority of chemical digestion here
– Accessory organs: liver & gallbladder,
pancreas
– Arrival of chyme in duodenum (1st 25 cm of
the small intestine)  hormone release:
• Secretin: causes sodium bicarbonate to be
released by pancreas – neutralizes stomach acid
• Cholecystokinin: causes release of enzymes from
pancreas and bile from liver
• Bile
– Consists of: water, ions, cholesterol,bile
pigments, bile salts
– Bile salts emulsify fats
– Made in the liver and stored in the gallbladder
– Gallstones can form, block ducts
• Digestive Enzymes
– Starch  disaccharides
• Salivary amylase
• Pancreatic amylase – duodenum
– Disaccharides  monosaccharides
• Maltase, sucrase, lactase - in small intestine
– Proteins polypeptides  amino acids
• Pepsin in stomach
• Others in small intestine
– Triglycerides  Glycerol + fatty acids
• Lipase in small intestine
• Absorption of nutrients – wall of small
intestine
– Requires a large surface area
– Circular folds, villi, microvilli
– Each villus has capillaries, lymph vessel
(lacteal)
– Small nutrient molecules diffuse to blood
– Fats are packaged, go to lymph
– Liver adjusts glucose level, removes poisons
• Large Intestine
– Indigestible material + bacteria
– Water absorbed: too much  constipation,
too little  diarrhea
– Bacteria typically harmless – normal flora –
produce vitamins (K). – live on foods we can’t
digest (produce gas)
– Indigestible material (feces) leaves rectum
under control of sphincter muscle
NUTRIENTS
• Energy-providing: fats, carbohydrates,
proteins
• No energy: vitamins, minerals, water
• Fats (triglycerides) – not evil!!!
– Benefits include:
•
•
•
•
•
Cell membrane components
Insulation and cushioning
Carry fat soluble vitamins
Soften skin
Synthesis if hormones
– Saturated and unsaturated
– Risks of high-fat diets include obesity, cancer,
atherosclerosis
• Cholesterol
– More of it found in LDL than HDL
– LDL carries cholesterol from liver to body cells
– excess deposited on artery walls
– HDL brings back to liver; eliminated with bile
• Carbohydrates
– Sugars, starch, fiber
– Starch = “complex” carbohydrates & comes
packaged with other high quality nutrients
– Fiber: soluble and insoluble
• Insoluble fiber: from skins & tough parts of fruits
and vegetables
– Adds bulk to material in colon, which speeds up
peristalsis
– Contact with carcinogens reduced
– Less risk of constipation
• Soluble fiber: from oat bran, apples, beans, etc.
• Combines with bile salts and cholesterol in small
intestine and prevents them from being absorbed;
similar effects to insoluble fiber
• Protein
– Can be energy source, but typically used as
amino acid building blocks
– Amino acids are not stored: need 8 essential
ones at every meal
– Animal proteins complete, but plant protein
not
– Vegetarians need to think about
complementing
• Recommended Diet
– Carbohydrates: 55-60% of calories
– Fats: 30% of calories (no more), unsaturated
better
– Protein: 10-15% of calories
– Calories = energy measure (Kcal)
– Fats: 9 cal per gram; carbs & protein 4 cal per
gram
• Nutrition Labels
– Check serving size
– Total fat = saturated + unsaturated
– Total carbohydrate = sugars + fiber + starch
– Calories per gram: Fat 9, Carbohydrate and
Protein 4
• Vitamins & Minerals
– Needed in tiny amounts (metabolic purposes);
if lacking, deficiency diseases
– Vitamins organic (carbon containing)
– Minerals inorganic
– Often part of enzymes (cofactors)
– Vitamins grouped as water-soluble & fat
soluble
• Vitamin C
– Deficiency is scurvy
• Bleeding gums, poor wound heealing, loose teeth,
painful joints
• British sailors in 1700’s; citrus fruits (“limey’s”)
• Now know required for collagen synthesis
• Vitamin A
– Example of fat-soluble
– Night-blindness, dry skin, bones, hair, etc.
– Component of rhodopsin
– Can be synthesized in the body from carotene
• Examples of Minerals
– Calcium, phosphate: bones and teeth
– Potassium, sodium: body water balance,
nerve message
– Iron: hemoglobin
– Zinc: cofactor in digestive enzymes
OBESITY
• When calorie intake exceeds energy
usage, you gain weight
• Eat less OR burn more calories
• Rapid weight loss is self-defeating
because valuable water and lean muscle
lost
• Body fights calorie reduction; appetite
increases; metabolic rate slows
EATING DISORDERS
• Obsession with weight loss, distorted body
image – anorexia
• Life-threatening; treatment involves
psychological & medical help, involvement
of family & friends