Chapter 14 Digestive System

Biology 112

Tri-County Technical College

Pendleton, SC

Functions of Digestive System• Digestion is mechanical and chemical

breakdown of foods into nutrients that cell membranes can absorb

• System consists of alimentary canal (GI tract)– Extends from mouth to anus– Includes several accessory organs that release

secretions into the canal

Functions, cont.

• Alimentary canal digests food and absorbs fragments through its lining into the blood

• Accessory organs (teeth, tongue, and several large digestive glands) assist the digestive breakdown in several ways

• Digestive system makes nutrition possible

• One could accurately state that a human is a digestive tube with a body built around it

Organ Sequence

• Bolus defined as rounded mass of food prepared by the mouth for swallowing

• Mouthpharynxesophagusstomach small intestinelarge intestineanal canal

• Accessory organs include the salivary glands, liver, gallbladder, and pancreas

Digestive System Visual

Layers of the Tract• Wall of tract consists of 4 distinct layers (tunics) • Innermost mucosa (mucous membrane) layer

composed of surface epithelium, underlying connective tissue, and small amt. of smooth muscle– Develops folds/tiny projections in some regions that

extend into passageway (LUMEN)– Increase mucosa’s absorptive surface area– May contain glands that secrete mucus/digestive

enzymes

• Mucosa secretes/absorbs & protects tissues beneath it (really exterior to it)

Layers, cont.• SUBMUCOSA—loose connective tissue,

glands, blood vessels, lymphatic vessels, & nerves (organized into a plexus)– Vessels nourish surrounding tissue & carry

away absorbed material

• MUSCULAR layer moves tube and has 2 coats of smooth muscle tissue and some nerves (plexus)– Fibers of inner coat encircle tube (circular)– when these contract, tube diameter gets smaller

Layers, cont.

• Fibers of outer coat run lengthwise (longitudinal)– fibers contract, tube length gets shorter

• SEROUS layer comprised of visceral peritoneum– Outermost layer

– Cells protect underlying tissues & secrete serous fluid-moistens and lubricates outer surface

– Organs within abdominal cavity slide freely against one another

Layers Visual

Function(s) of the Stomach

• Located under diaphragm in upper left portion of abdominal cavity

• Thick folds (rugae) of mucosal and submucosal layers mark inner lining– Disappear when stomach distends

• Stomach receives food from esophagus, mixes it with gastric juice, initiates protein digestion, does limited absorption, and moves food into small intestine

Regions of the Stomach

• Stomach divided into cardiac, fundic, body, and pyloric regions

• Cardiac-small area near esophageal opening• Fundic-balloons above cardiac portion & is

temporary storage area• Body-main part of stomach that lies between

fundic and pyloric portions• Pyloric-narrows and becomes pyloric canal as

approaches small intestine

Stomach Functions, cont.

• At end of pyloric canal, wall thickens to form pyloric sphincter– muscular valve controlling stomach emptying

• After meal, mixing movements of stomach help produce semifluid paste called chyme

• Peristalic waves push chyme toward pyloric region

• Rate stomach empties depends on chyme’s fluidity and type of food present

• Liquidscarbsproteinsfatty foods

Stomach Digestive Secretions

• Stomach mucosa studded with gastric pits– gastric pits are ends of tubular gastric glands

• Gastric glands contain 3 types of secretory cells• Mucous (goblet cells) occur in necks near

openings of gastric pits• Chief cell occur in deeper parts of pits and secrete

digestive enzymes• Parietal cells occur in deeper parts and secrete

hydrochloric acid

Gastric Pits Visual

Stomach Functions, cont.

• Products of mucous, chief, and parietal cells form gastric juice

• PEPSIN most important enzyme in juice• Made in inactive form (pepsinogen) which is

converted to active form (pepsin) by contact with HCl—begins protein digestion

• Intrinsic factor (parietal cells) important for vitamin B12 absorption (small intestine)

• Mucous membrane produces mucus (viscous and alkaline secretion to coat stomach walls

Digestive Secretions into Duodenum• Duodenum is first part of small intestine• As chyme enters, pancreas, liver, and

gallbladder add digestive secretions• Pancreas secretes pancreatic juice• Liver secretes bile which is stored in

gallbladder– Bile salts important in lipid digestion

• Chyme moving into duodenum contains array of enzymes/substances

Digestive Secretions, cont.

• Amylase from salivary glands

• Pepsin from gastric glands

• Amylase, lipase, proteolytic enzymes (trypsin, chymotrypsin, carboxypeptidase), and nucleases from the pancreas

• Peptidase, sucrase, maltase, lactase, lipase, and enterokinase from the duodenum

Small Intestine Surface Area• Three structures > surface area • Microvilli (brush border)-tiny projections of

plasma membrane• Inside each villus is rich capillary bed and

modified lymphatic capillary called a lacteal• Circular Folds (plicae circulares) are deep folds

of both mucosa & submucosa layer• Peyer’s Patches-lymphatic tissue found in

patches near end of small intestine; help prevent bacteria from entering bloodstream

Surface Area Visual

The Appendix

• Is a subdivision of large intestine

• Hangs from saclike first part of large intestine

• Is wormlike and potential trouble spot

• Usually twisted and ideal location for bacteria to accumulate & multiply

• Inflammation of appendix = appendicitis

• “Remnant” from “grass-digestion” days????

The Pancreas• Pancreatic enzymes secreted into duodenum

in fluid rich in bicarbonate (ph 9) which helps neutralize acidic chyme

• Secretes pancreatic amylase (starch), trypsin, chymotrypsin, carboxylpeptidase, & others (proteins), lipases (fat), and nucleases (nucleic acids)

• Also has endocrine functions—produces the hormones insulin and glucagon

Hold the liver..pass the onions• Liver largest gland in body with 4 lobes• Suspended from diaphragm and abdominal

wall by mesentery cord (falciform ligament)• Liver’s digestive function is to produce bile• Bile contains bile salts, bile pigments

(bilirubin from hemoglobin), cholesterol, phospholipids, and variety of electrolytes

• Leaves liver via common hepatic duct and enters duodenum through bile duct

Liver, cont.

• ONLY bile salts and phospholipids aid digestive process

• Bile salts emulsify fats by breaking down large fat globules into smaller ones; >ing surface area for fat digesting enzymes

• Gallbladder is small green sac in shallow fossa in inferior surface of liver

• Food digesting not occurring, bile backs up cystic duct

Enough liver…already

• Bile enters gallbladder via cystic duct• While being stored; bile [ ]ed by removal of water• Later, when fatty foods enter duodenum, hormonal

stimulus prompts gallbladder to release bile into duodenum

• Bile stored too long or too [ ]ed = gallstones• Jaundice occurs if blockage of common hepatic

bile ducts prevent bile from entering small intestine (also from hepatitis/cirrhosis)

The Salivary Glands

• Three pairs of salivary glands

• Parotids are large/lie anterior to the ears– Mumps is an inflammation of the parotids

• Submandibulars and small sublinguals empty secretions into floor of mouth

• All produce saliva (mixture of mucus and serous fluids

• Mucus moistens/binds food into bolus

Salivary Glands, cont.

• Clear serous portion contains salivary amylase (enzyme) which begins process of starch digestion in the mouth

• Saliva also contains lysozyme and IgA antibodies that inhibit bacteria

• Also serves to dissolve food chemicals so they can be “tasted”

Human Dentition• Are 32 permanent teeth in full set• Same number and arrangement of teeth

exist in both upper and lower jaw• 2 incisors; 1 canine; 2 premolars

(precuspids); 3 molars (3rd molar = wisdom tooth)

• Incisors for cutting; canines for tearing/piercing; premolar and molars for grinding

Human Dentition Visual

Anatomy of a Tooth

• Tooth consists of 2 major regions: crown and root• Enamel-covered crown is exposed part of tooth

above the gingiva (gum)• Portion of tooth embedded in jawbone=root• Root and crown connected by “neck”• Outer surface of root covered by cementum

which attaches tooth to periodontal membrane (ligament)– this ligament holds tooth in place in bony jaw

Tooth Anatomy, cont.

• Dentin is bonelike material underlying enamel & forms bulk of tooth– Dentin surround “pulp cavity” containing

connective tissue, blood vessels, and nerve fibers (collectively called “pulp”

• Where pulp extends into root, it becomes the “root canal” which provides route for blood vessels, nerves, and other pulp structures to enter pulp cavity of tooth

Tooth Anatomy Visual

Digesting Digestion• Mechanical digestion prepares food for

degradation by enzymes– biting, chewing, mixing of food in mouth by tongue,

churning of food in stomach, and segmentation in small intestine

• Chemical digestion = large food molecules broken down to their building blocks by enzymes (hydrolytic reactions)

• Ingestionpropulsionmechanical digestion chemical digestionabsorptiondefecation

To Digest or to Absorb…??

• Food consumed generally a polymer• Must be taken apart into monomers small

enough to cross PMs of cells lining small intestine

• Materials NOT broken apart cannot be absorbed (cellulose)

• Carbohydrates (glucose, fructose, galactose)– Sucrose, lactose, maltose, and starch

• Proteins and Fats

Digestion in the Mouth• Mouth begins mechanical/chemical process• Saliva contains salivary amylase that begins

starch digestion—maltose• Essentially NO food absorption in mouth

– nitroglycerin and alcohol are exceptions

• Pharynx and esophagus have NO digestive functions– Simply provide passageways to carry food to

stomach

Digestion in the Stomach

• Food must be swallowed (deglutition)

• Involves coordinated activity of tongue, soft palate, pharynx, and esophagus

• BUCCAL phase occurs in mouth when food has been chewed & mixed with saliva– BOLUS is forced into pharynx by tongue– Enters pharynx passes from conscious control– Now in realm of reflex activity

Stomach Digestion, cont.• PHARYNGEAL-ESOPHAGEAL phase

transport bolus through– Parasympathetic controls events (Vagus)– Promotes motility of digestive organs

• Bolus reaches end of esophagus, presses against cardioesophageal sphincter– Open and allows food to enter stomach

• Secretion of gastric juice regulated by neural and hormonal factors

Stomach Digestion, III• Presence of food in stomach and falling pH

stomach cells release gastrin– causes release of protein-digesting enzymes

(pepsinogens), mucus, and HCl

• If CE sphincter fails to close tightly gastric juices back up into esophagus heartburn (hiatal hernia)

• Acid environment activates pepsinogen into PEPSIN– Rennin also produced in stomach of young

Stomach Digestion, IV

• Stomach will compress, pummel, break apart, and mix food with enzymes into chyme (semisolid fluid)

• Protein digestion begins in stomach but very little chemical digestion occurs

• Alcohol and aspirin ONLY absorbed through stomach walls

• Pyloruspyloric valveduodenum

Digestion in Small Intestine

• Carbs and protein digestion begun by time food reaches small intestine but virtually NO fat digestion

• Digestion completed and nearly all food absorption by time reaches end of SI (3-6 hrs)

• Microvilli of SI cells bear few important enzymes called brush border enzymes

SI Digestion, cont.

• Break down disaccs and complete protein digestion

• Dextrinase, glucoamylase, lactase, maltase, sucrase

• Aminopeptidase, carboxypeptidase, dipeptidase

• Foods entering SI deluged with enzyme-rick pancreatic juice

SI digestion, cont.• Mucosa cells produce secretin and CCK

(cholecystokinin) which influence release of pancreatic juice and bile

• Absorption of water and end products of digestion occurs all along length of SI

• Most absorbed by active transport• Enter capillary bed in villus to be

transported in blood to liver via hepatic portal vein

Digestion, Visual

SI digestion, cont.• Lipids absorbed by diffusion

• Enter capillary bed and lacteal in villus for transport via blood and lymphatic fluids

• At end of ileum, all that remains is water, indigestible food materials, and large amounts of bacteria

• Debris enters large intestine through the ILEOCECAL VALVE

The Large Intestine• Colon produces NO digestive enzymes• Spends 12-24 hours in LI• Normal flora metabolize some remaining

nutrientsmethane/hydrogen sulfide gas– Vitamin K and some B vitamin synthesis

• Absorption in LI limited to these vitamins, some ions, and water

• Feces delivered to rectum for elimation• Undigested food, mucus, bacteria, and enough

water for “smooth” passage

Important Terms• Metabolism, anabolism, and catabolism• Glycogenesis = combining glucose to make

glycogen (occurs in liver) for storage in liver and muscle cells

• Glycogenolysis = glycogen splitting; blood glucose levels fall; liver breaks apart glycogen

• Gluconeogenesis = formation of new sugar– Liver can make glucose from fats/proteins

Ole Gly…and more

Metabolism and more…

• Carbohydrate ( fig. 14.9a on p. 461)• Fat (fig. 14.9b)• Protein (fig. 14.9c)• ATP formation (14.9d)• Hypoglycemia = low blood glucose level• Hyperglycemia = excessive high level of

blood glucoseglycogenstill too much = fat

You ate yet, Brutus?

Basal Metabolic Rate• BMR amount of heat produced by body per unit

time when under basal (rest) conditions• Reflects energy needed to perform essential life

activities (breathing, heartbeat, & kidney function)• 70-80 kg adult = 60 to 72 kcal/hour• Amount of thyroxine most important factor in

determining BMR– Dubbed “metabolic hormone”– More thyroxine produced, > use of oxygen and more

ATP produced

Body Temperature

• 60 percent of energy of food = heat

• Warms tissues and bloodhomeostatic conditionsefficient metabolism

• Body temp balance between production and loss

• Body’s thermostat is hypothalamus

• Heat promoting=vasoconstriction/shivering– Frostbite and hypothermia

Body Temperature, cont.

• Heat loss=radiation/evaporation– heat exhaustion, heat stroke, fever

• Fever is controlled hyperthermia

• Pyrogens produced by macrophages, WBCs, and injured tissue reset thermosat in hypothalamus

• Time for an Estesism…Yeah!!!

To rise…or not to rise???