Chapter 14

The Digestive System and Body Metabolism

20 Cool Things You Don’t Know

About the Digestive System

Overview Of

Digestion

Pill Cam

Digestive System

Takes in food, breaks it down into nutrient molecules and absorbs them into the bloodstream, then rids the body of indigestible remains

Anatomy of Digestive SystemAlimentary canal –

digests food and absorbs digested fragments through its lining into the blood

GI, tract - continuous hollow tube: mouth, pharynx, esophagus, stomach, small and large intestine

Accessory digestive organs - assist: teeth, tongue, glands

Mouth (Oral Cavity)

Food entersMucus membrane-lined cavitylips, cheeks, hard palate (anterior roof), soft

palate (posterior roof)uvula - fingerlike projection of soft palate

Mouth Tongue - attached to hyoid bone and styloid

processes of skullpapillae containing taste buds on surface

Frenulum - secures tongue and limits its posterior movements

As food enters, it is mixed w/ saliva by tongue and chewed and swallowed

Taste

Salivary Glands - 3 pair

Parotid glands - anterior to ears mumps is inflammation of parotid

glands Submandibular and sublingual glands -

empty secretions into mouth through ducts

SalivaProduct of salivary glands, mixture of

mucus and serous fluidsmucus moistens and binds food

together into a mass (bolus)serous part contains salivary amylase

(enzyme for starch digestion)

Polls Everywhere

Teeth Masticate (chew) Deciduous (baby or milk) teeth - first set; formed

from 6 months to 2 years Permanent teeth - cause baby teeth to fall out b/t 6

and 12 32 permanent teeth 3rd molars (wisdom teeth) form b/t 17 and 25;

sometimes absent or impacted in jawbone and must be surgically removed

Teeth by shape/function

Incisors - chisel-shaped, cutting Canines - fanglike, tearing/piercing Premolars (bicuspids) Molars - broad crowns w/ rounded tips, grinding

Pharynx Nasopharynx (respiratory),

oropharynx (potesterior to oral cavity), and laryngopharynx (continuous w/ esophagus)

Peristalsis: Alternating contraction of muscles propel food into esophagus

Peristalsis

Esophagus

Conducts food from pharynx through diaphragm to stomach

25 cm long

Walls of Alimentary Canal Mucosa - innermost layer; moist membrane Submucosa - blood vessels, nerve endings,

lymph Muscularis externa - inner circular, outer

longitudinal smooth muscle Serosa - outermost layer

Stomach C-shaped, left side, nearly hidden by liver and

diaphragm cardioesophageal sphincter - food enters from

esophagus fundus - expanded part body – midportion 3rd oblique layer in muscularis externa to move,

churn, mix, and pummel foodchemically breakdown proteins

Peristalsis

Stomach Pylorus - funnel-shaped, terminal Pyloric sphincter - goes to small intestine 25 cm long when full, holds 4 liters of food empty - collapses into folds (rugae)

Rugae on Dog Stomach

Stomach Mucosa has gastric pits which lead into gastric

glands that secrete gastric juice chief cells - produce protein-digesting

enzymes (pepsionogen) parietal cells - produce HCl Chyme is produced

Stomach - Food Breakdown Secretion of gastric juice by sight, smell, and

taste of food presence of food and falling pH stimulate

release of hormone gastrin that makes stomach produce enzymes, mucus, & HCl

2-3 liters gastric juice per day

Stomach: Food Propulsion

Peristalsis in lower half, and contractions squirt 3 ml of chyme into small intestine

takes 4 hrs for stomach to empty Irritation (food poisoning) may activate vomiting

Small Intestine Major digestive organ Muscular tube extending from pyloric

sphincter to ileocecal valve average length: 2 m (6 feet) Hangs from coils suspended by mesentery Large intestine encircles and frames it

Small Intestine - 3 subdivisions Duodenum - curves around head of

pancreas Jejunum - extends from duodenum to ileum ileum - terminal part that joins large intestine

at ileocecal valve

Small Intestine Chemical digestion begins Small amount of food processed at a time -

controlled by pyloric sphincter Pancreatic enzymes from pancreatic duct

and bile from bile duct enter duodenum

Small Intestine - 3 structures that increase absorption Microvilli - tiny projections that give fuzzy

look (brush border) Villi - fingerlike projections that give velvety

appearance Circular folds - deep folds of both mucosa

and submucosa

Small Intestine: Food Breakdown and Absorption

Takes 3-6 hours By end, digestion is complete and most

absorption has occurred Microvilli have brush border enzymes to break

down sugars and complete protein digestion

Food Breakdown and Absorption

Pancreatic juice digests starch, proteins, fats, and nucleic acids; contains bicarbonate to neutralize chyme

When chyme enters, it stimulates hormones secretin and cholecystokinin to release bile

Food Breakdown and Absorption

Bile is necessary for absorption of fats and fat-soluble vitamins K,D,A

At end, all that remains is water, indigestible food and bacteria which enters large intestine

Food propulsion - peristalsis

Large Intestine Larger in diameter, shorter

in length (1.5 m) Extends from ileocecal

valve to anus Dries out indigestible food

by absorbing water, eliminates residue as feces

Large Intestine Subdivisions Cecum - saclike, first part Appendix - wormlike structure

hanging from cecum; ideal bacteria location - appendicitis

Colon - ascending, transverse, descending, and sigmoid

Rectum Anal canal - ends in anus which

opens to exterior; has external voluntary sphincter and internal involuntary sphincter

Large Intestine

Lots of goblet cells to produce mucus to act as lubricant to ease passage of feces

Large Intestine: Food Breakdown and Absorption Residue is there 12-24 hours Bacteria metabolize nutrients and release

gases (methane, hydrogen sulfide) 500 ml of gas produced each day

Food Breakdown and Absorption

Absorption limited to vitamins, some ions, and most of remaining water

Feces - solid product delivered to rectum; undigested food residue, mucus, bacteria, and some water

Large Intestine: Propulsion and Defecation

Peristalsis and mass movements (long, slow-moving, powerful contractile waves that move over colon 3-4 times daily to push contents toward rectum)occur after eating; fiber increases strength of

contraction

Propulsion and Defecation When feces in rectum, defecation reflex causes

rectum to contract and anal sphincters to relax Diarrhea - food rushes through before water is

absorbed, can result in dehydration and electrolyte imbalance

How fast food Travels thorough.

Propulsion and Defecation

Constipation - food residue remains too long and too much water is absorbed; due to lack of fiber, poor bowel habits, or laxative abuse

A constipated body

Other Accessory Digestive Organs – Pancreas

Soft, pink, triangular gland extending from spleen to duodenum

produces enzymes that break down food and neutralize acidic chyme from stomach, produces hormones insulin, glucagon

Liver Liver - largest gland in body; under

diaphragm on right4 lobesproduces bile which leaves liver

through common hepatic duct

Gallbladder Small, thin-walled green sac in the inferior surface of liver When digestion is not occurring, bile is stored and

concentrated by removal of waterbile stored too long, it crystallizes forming gallstonesYellow-green, watery solution of bile salts, bile pigments

(bilirubin), cholesterol, phospholipids, and electrolytesbile salts emulsify fats to provide more surface area

Disease: Jaundice Bile pigments enter bloodstream Can result from hepatitis (inflammation of

liver from viral infection of contaminated water or blood transfusion) or cirrhosis (severe damage from drinking excess alcoholic beverages)

Nutrition and Metabolism

Most foods used as metabolic fuels (transformed into ATP); some nutrients build cellular molecules

Energy value measured in kilocalories (kcal) or Calories (C)

Nutrition

Nutrient - substance in food used to promote normal growth, maintenance and repair

Carbohydrates, lipids, proteins - bulk of food; vitamins, and minerals in minute amounts

.

Water - 60% of volume of foodMost foods are combination of

nutrients from 5 food groups (grains, fruits, vegetables, meats, and milk products)

Obesity Rates in the US.

Dietary Sources of Major Nutrients - CarbohydratesFrom plants except lactose and

glycogen in meatssugar - fruit, sugar cane, milkstarch - grains, legumes, root

vegetablescellulose - most vegetables

Lipids

Neutral fats: saturated in animal products, unsaturated in seeds, nuts, vegetable oils

Cholesterol - egg yolk, meats, and milk

Phospholipids

Proteins

Animal products, eggs, milkAmino acid polymerslegumes, nuts, and cereals are

low in one or more essential amino acids

Vitamins

Organic nutrients, small amountsNo one food contains all required

vitamins, need balanced dietMost function as coenzymes: act

w/ enzymes for task

Minerals

Requires adequate supplies of 7: Ca, P, K, S, Na, Cl, and Mg; trace amounts of others

Fats/sugars have none, cereals and grains poor sources

In veggies, legumes, milk, meats

MetabolismAll chemical reactions necessary to

maintain lifeCatabolism - substances broken down,

energy released and captured to make ATP

Anabolism - larger molecules built from smaller ones

Metabolism

Carbohydrates (glucose) - broken down to make ATP

Fats - build cell membranes, myelin sheaths, insulate, ATP

Proteins - structural materials hoarded by body cells

Carbohydrate MetabolismCarbs - preferred fuel to produce ATP

from glucose (blood sugar): energy from bonds broken binds phosphate to ADP to make ATP

Carbon atoms leave as CO2 and hydrogen combines w/ oxygen to make water

Carbohydrate Metabolism - Homeostasis of blood glucoseHyperglycemia - high levels;

excess stored as glycogen and converted to fats

Hypoglycemia - low levels; liver breaks down stored glycogen and releases glucose to blood

Fat MetabolismLiver - make ATP, synthesize

lipoproteins, clotting protein and cholesterol for membranes or steroid hormones

Form myelin sheaths and fatty cushions around organs

Most concentrated form of enegy

Fat MetabolismTo be used for ATP synthesis, it must

be broken down into acetic acid; when not enough glucose, acetone accumulates in blood making it acidic (acidosis/ketosis)no carb diets, diabetes, and starvation

Protein MetabolismProteins - bulk of cell structures;

broken down into amino acids for enzymes, membranes

Cells use ATP to actively transport amino acids (8 of the 20 are not made by cells - essential amino acids)

Protein Metabolism

Amino acids make ATP when protein is in excess or no fats or carbsamine groups are removed as ammonia which is toxic so it combines w/ CO2 to form urea

Central Role of LiverManufactures bile, detoxifies drugs

and alcohol, degrades hormones, makes substances vital to body, metabolism

We have more liver tissue than needed, so if damaged, it regenerates rapidly and easily

General Metabolic Functions

Liver maintains blood glucose levels

After high carb meal, glucose is removed from blood and converted to glycogen (glycogenesis) and stored in liver

General Metabolic Functions

As body cells remove glucose from blood, liver breaks down stored glycogen (glycogenolysis)

gluconeogenesis - make glucose from fat and protein

General Metabolic FunctionsHormones insulin, thyroxin, and

glucagon control blood sugarFats are oxidized for energy, broken

down into acetic acid or stored as fat reserves

Makes cholesterol and secretes its breakdown products as bile

General Metabolic FunctionAlbumin - most abundant protein;

holds fluids in bloodstreaminsufficient albumin causes fluid to go from blood to tissues (edema)

Synthesize amino acids and detoxify ammonia

Cholesterol Metabolism and Transport

Cholesterol - structural base for steroid hormones and vitamin D; major building block of plasma membranes

15% from diet, 85% made by liverBroken down and secreted in bile

salts, which leave as feces

Cholesterol MetabolismInsoluble in water, so transported bound

to lipid-protein complexes - lipoproteinsLow-density lipoproteins (LDLs) -

transport cholesterol/lipids to body cells; if too much deposited on arteries - “bad lipoproteins”

Cholesterol MetabolismHigh-density lipoproteins (HDLs) -

transport cholesterol from tissue cells to liver for disposal in bile; high levels are good

Both types necessary; ratio determines whether cholesterol will go to arterial walls

Body Energy Balance

When fuel is burned, it consumes O2 and generates heat

Energy intake = total energy output (heat + work + energy storage)

Body Energy Balance

Energy intake - liberated during food oxidation

Energy output - lost as heat (60%) + used to do work (driven by ATP) + stored as fat or glycogen

Regulation of Food IntakeWhen energy intake and energy outflow

are balanced, body weight is stableFood intake controlled by rising or

falling blood levels of nutrients, hormones, body temp. and psychological factors

Basal Metabolic RateCarbs & proteins yield 4 kcal/g, fats

yield 9 kcal/gBasal metabolic rate (BMR) - amount of

heat produced by body per unit time at rest; energy supply for breathing, heartbeat, and kidney function

Basal Metabolic RateAvg. adult has BMR of 60-72 kcal/hr;

influenced by surface area, gender, age, and thyroxin production (more thyroxin produced by thyroid gland, higher O2 consumption and ATP use and metabolic rate)

Basal Metabolic RateHyperthyroidism - excessive rate,

lose weight despite increased hunger and food intake, bones and muscles weaken

Hypothyroidism - slow rate, obesity, diminished thought process

Total Metabolic Rate

When active, more glucose must be oxidized to provide more energy for activities

Total Metabolic Rate (TMR) - total amount of kcal body must consume to fuel all activities

Total Metabolic RateWhen total calories = TMR, weight is

constantIf eat more, excess calories appear as

fat depositsIf active w/o enough food, break down

fat reserves and even tissues to satisfy TMR

Body Temp RegulationHeat warms tissues and blood

keeping them at homeostatic temps

Reflects balance b/t heat production and heat loss, controlled by hypothalamus, regulated b/t 96-100ºF

Heat-Promoting MechanismsWhen cold, heat is conserved by

vasoconstriction of blood vessels and shivering makes blood route to vital body organs causing temp of skin to dropif extended, skin cells w/o O2 die leading to frostbite

Heat-Promoting Mechanisms

Hypothermia - extremely low body temp from prolonged exposure to cold; vital signs decrease, person becomes drowsy and can progress to coma and death as metabolic rate stops

Heat Loss MechanismsMost loss occurs through skin by

radiation (when body temp increases, blood vessels dilate and heat radiates off surface) or evaporation (too hot - perspiration off skin’s surface; effective unless humid)

Heat Loss Mechanisms

Hyperthermia (elevated body temp) depresses hypothalamus and positive-feedback cycle occurs: soaring body temp increases metabolic rate, which increases heat production

Heat Loss MechanismsHeat stroke - skin hot and dry - fatal unless

immersed in cool water and given fluidsHeat exhaustion - collapse during vigorous

activity due to excessive loss of fluids (dehydration), causing low blood pressure, fast heart rate and cool, clammy skin

Heat Loss MechanismsFever - controlled hyperthermia - results

from infection, cancer, or allergies; pyrogens are released to hypothalamus that set thermostat at higher levelchills - reset lower - sweattoo high - protein denatures

Developmental Aspects5th week - alimentary canal formscleft palate/lip - child unable to suck

properlytracheoesophageal fistula - connection

b/t esophagus and trachea - causes drool, cyanosis during feedings

Development AspectCystic fibrosis - blockage of pancreatic

ducts so that fats and fat-soluble vitamins are not digested or absorbed

PKU - inability of tissue cells to use phenylalanine (amino acid) causes brain damage

Developmental AspectsNewborn: rooting & sucking reflexAppetite decreases in elementary

age and increases in adolescenceGastroenteritis - inflammation of GI

tract due to contaminated foodAppendicitis - common in teens

Developmental AspectsMiddle age - metabolic rate

decreases 5-8% every 10 yrsulcers & gallbladder problems

Old age - activity of GI tract declines, taste and smell decreasecancer of stomach and colon