GASTROINTESTINAL TRACT

• Continuous tube, pathway of food through the body

• Four layers of tissue:1. Mucosa – innermost, secretes mucus

2. Submucosa – blood vessels, nerves

3. Muscularis – two or three muscle layers

4. 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