Digestion -- breaking complex (large) nutrients down into simple (small) nutrients

	Mechanical digestion -- large particles into small, without changing molecular structures Examples: chewing, peristalsis, segmentation, mixing/churning
	Chemical digestion -- large molecules into small Examples: enzymatic breakdown of polysaccharides into disaccharides and then into monosaccharides

Absorption -- moving simple nutrients from lumen (external environment) to blood (internal environment)

One-way digestive tract through the body    PP

Section including the stomach and intestines is also called the gastrointestinal (GI) tract     GA

Each section of the canal is considered to be a primary organ

Any digestive organ that is NOT a section of the tract (for example, salivary glands, liver, pancreas) is considered to be a secondary or accessory organ of the digestive system

Hollow area, called the lumen, is functionally an extension of the external environment (contents of the lumen is not really "a part of you")

Tract has four layers (coats) around the lumen

Mucosa (mucous coat) - mucous epithelium

In many sections, this lining of the tract is folded with bumps that have bumps (fractal-like surface) Fractal surfaces have bumps that have bumps that have bumps --seemingly infinitely-- that greatly increase surface area Increases efficiency of secretion of digestive juices and of absorption and at the same time allows each section to expand (like a folded cloth balloon) to accommodate more food

Mucus (water and glycoproteins and other nifty stuff) Protects (from mechanical injury, stomach acid, bacterial infection) Lubricates (keeps food sliding along the tract) Aids in mixing (helps liquify and mix up ingested food)

Submucosa (under-mucous coat) - fibrous connective tissue

Helps stabilize and strengthen mucosa layer

Muscularis (muscle coat) -- smooth muscle

	Two layers: circular (inner) layer and longitudinal (outer) layer Stomach wall has third oblique layer under its circular layer Sphincters Thick groupings of circular muscles that acts as valves to regulate flow through the tract
	Recall that smooth muscle is involuntary muscle
	Often autorhythmic Basal electrical rhythm (BER) = intrinsic, rhythmic fluctuation Pacemaker activity similar to cardiac muscle, allowing the BER to spread to other nearby muscle fibers Migrating motor complex (MMC) during fasting "cleans out" the tract
	Starling's Law of the Gut: smooth muscle contracts when stretched This means that when food enters a section and stretches its wall, the smooth muscle reflexively contracts, either: pushing the food along ahead of the ring of contraction (peristalsis) or     ANIM churning it up in place (segmentation)     ANIM
	Activity of muscles is called motility Defecation is motility that results in movement of the feces out of the body (called "bowel movement" or BM)

Serosa (serous coat)

	Serous membrane (thin epithelial membrane coated with watery, lubricating serous fluid) Same as visceral peritoneum
	Above the diaphragm this layer is instead simply fibrous connective tissue

Among the muscles and other layers is a complex network of nerves

Called by some "the second brain" this network performs complex regulation of secretion and motility of various sections of the tract

Exocrine glands that have ducts leading into the lumen of the digestive tract

Structure     GA

	Boundaries     GA Lips & cheeks Palate (roof) Hard palate (palatine bones and palatine processes of maxillae) is anterior     GA Soft palate (muscle) is posterior Fauces (archway to throat) with uvula (dangling thing) Tongue (and floor)      Root, tip, and body of tongue Covered with papillae     GA Taste buds     GA Frenulum -- fold under tongue; attaches to floor of mouth cavity     GA
	Teeth     GA  GA General tooth structure     GA   GA   GA Crown -- part that is visible covered with hard enamel Neck -- where crown becomes the root Root -- part inside jaw socket     GA Covered with cementum Bound to jawbone by collagenous periodontal membrane (ligament) [peri = "surrounding" odont = "tooth"] Inner shell of tooth (under crown and inside root) is hard dentin tissue with hollow pulp cavity containing nerves and blood vessels 32 permanent teeth 20 deciduous (baby) teeth (not enough room in a small mouth for 32 big teeth)     GA   GA Shape determines function     GA Incisors (2 per quadrant) - cutting teeth Canines (1 per quadrant) - grabbing, tearing teeth Premolars (2 per quadrant) - sawing, tearing teeth Molars (3 per quadrant) - crushing, grinding teeth Human teeth are not very specialized for any of these functions (compare incisors/molars of horse, for example, which are extreme; or compare canines/premolars of a cat, which are extreme) so therefore, we are built to be "Swiss army knives" that can do anything but not all that well
	Salivary glands     GA Parotid glands (largest pair)     GA Parotid duct opens in cheek across from 2nd upper molar Submandibular glands Submandibular duct opens on floor just lateral to frenulum Sublingual (smallest pair) Numerous (8-20) sublingual ducts on floor under tongue

Function

	Ingestion = putting food into digestive tract (usually implies swallowing also)
	Mastication = chewing
	Deglutition = swallowing (through pharynx and esophagus to stomach)
	Gustation = tasting (chemical analysis of food)
	Digestion Mostly mechanical (see above) Perhaps some chemical (salivary amylase digests polysaccharides, but not much time for this)
	Nondigestive functions (for example: smiling, kissing, spitting, talking, singing, whistling, raspberries, licking, and so on)

Structure

	Three divisions Nasopharynx Behind nasal cavity Oropharynx Behind oral cavity Laryngopharynx Behind larynx (voicebox)
	Extends to the esophagus
	Bolus (ball or chunk) of food from oral cavity enters middle of pharynx and moves into esophagus

Function

Deglutition (swallowing)     ANIM Three phases: oral, pharyngeal, esophageal

Structure

	Collapsible, muscular tube specialized for peristalsis (approx. 25 cm)
	Posterior to trachea and heart     GA
	Passes through hiatus in diaphragm
	Lower esophageal sphincter (LES) is a ring of smooth muscle at opening of stomach to prevent backflow (esophageal reflux) Reflux can cause "heartburn" and lead to more serious complications Also sometimes called the cardial or cardiac sphincter
	There is also an upper esophageal sphincter (UES) that guards the opening of the esophagous from the pharynx opens during deglutition

Function

	Deglutition
	Vomiting = emptying of stomach and beginning of small intestine via esophagus/throat/mouth

Structure

	Just inferior to diaphragm     GA
	Regions of stomach     GA Cardial region Fundus Body Pylorus
	Curves of stomach Greater curvature (inferior) Lesser curvature (superior)
	Size varies greatly     GA  GA
	Folds (rugae) help mix/liquefy bolus to become chyme     GA
	Very muscular (has 3, not 2, layers of muscle)
	Has gastric pits that include gastric glands
	Two sphincter valves control the openings Lower esophageal (cardial) sphincter controls superior opening Pyloric sphincter controls inferior opening

Function

	Storage of food until small intestine is ready This is the MAIN function of the stomach, other functions occur because you might as well do something with the food while it's sitting there so you start what digestion you can ahead of time
	Mechanical digestion Liquifaction of bolus into chyme Propulsion = forward movement of chyme Retropulsion = backward movement of chyme Goblet cells produce mucus (part of "gastric juice") that protects stomach lining and aids in mixing & lubrication
	Chemical digestion Hydrochloric acid (HCl) Produced by parietal cells of gastric mucosa CO2 + H2O --> H2CO3 --> HCO3- + H+ H-K pump (proton pump) moves H+ ions into gastric juice Parietal cells can quickly increase their surface area by 100x, thus rapidly increasing their output Parietal cells also make intrinsic factor (protects vitamin B12 and allows its absorption) Denatures (unfolds) proteins so enzymes have a shot at peptide bonds between amino acids Pepsin Produced by chief cells of gastric mucosa Secreted as pepsinogen (an inactive proenzyme that is later converted to active pepsin) All digestive enzymes do this, or else they'd eat the cell that made them Hydrolysis of long polypeptide chains into shorter peptide chains Hydrolysis, used by all digestive enzymes, uses water (H2O or HOH) to break apart subunits of macromolecules
	Absorption Almost NO absorption of water & nutrients in the stomach Chyme must move to small intestine for absorption If nutrients are already in their simplest form, they are almost immediately absorbed by the small intestine Rate of gastric emptying determines how fast a substance will be absorbed This is important for rapid rehydration, for example Gastric emptying can be hastened if The ingested material is dilute, if it is room temp or cooler, and if it is low in protein or fat

Structure

	Small diameter (2-3 cm) and long length (6 m)
	Three sections forming many loops in the abdominopelvic cavity Duodenum Short, C-shaped section under stomach     GA Has opening of common bile duct     GA Entry of secretions of liver and pancreas Jejunum Ileum
	Lined with circular folds (plicae circulares) Each fold is covered with fingerlike villi (sing. villus)     GA  GA Each villus covered with cells that have microscopic microvilli This surface is called the "brush border" and is the boundary between the external and internal environment Massive surface area for efficient absorption Inside each villus is a network of blood capillaries and a single lymph capillary (lacteal)     GA Intestinal glands produce mucus (have goblet cells)     GA Intestinal crypts have stem cells that divide and thus replenish cells of villus that slough off    fig

Function

	Receives chyme from the stomach in small spurts
	Continues mechanical digestion
	Most chemical digestion occurs here
	Absorption of most nutrients occurs here Most nutrients are absorbed through the brush border into the blood capillaries Lipids are absorbed instead into the lacteal so that the blood flow is not "gummed up" with fats and oils

Structure

	Wider than small intestine (5-6 cm) but shorter (1.5-2 m)
	Muscles in wall are "bunched up" causing the large intestine to look puckered Pouched sections are called haustra Tapelike taeniae coli are longitudinal strips of muscle
	Several sections: Cecum     GA  GA Ileocecal sphincter controls flow of chyme from ileum of small intestine into cecum of large intestine Vermiform  appendix (literally "worm-shaped addition" ) Acts as an incubator for intestinal flora (microbes) Colon is inhabited by interacting communities of microbes and other organisms  fig Appendix "regrows" the desirable mix of organisms after it has been damaged by antibiotics or other events Ascending colon Transverse colon Descending colon Sigmoid ( meaning "S-shaped") colon Rectum     GA Anus is opening to outside     GA Internal anal sphincters are involuntary     GA External anal sphincters are voluntary (thank goodness) Hemorrhoids are varicose veins of the anus     GA

Function

	Continued digestion and absorption of nutrients
	Formation of feces
	Defecation (elimination) Feces tells a lot about a person's digestive health     PP Timing issues can warn of diarrhea (literally, "flow through") or constipation    ANIM Color, consistency can warn of health issues Gray feces indicates lack of bile Black feces indicates bleeding in early part of tract Red feces indicates bleeding in lower part of track Presence of bacteria, parasites, swallowed objects

Serous membrane     GA  GA  GA

	Visceral layer (against organs)
	Parietal layer (lines abdominopelvic cavity)     GA
	Produces lubricating peritoneal fluid

Mesenteries

	Inward folds of membrane
	Support and protect digestive glands
	Greater and lesser omentum are apronlike folds

Peritonitis

	Inflammation of peritoneum
	If infectious, can quickly spread to many organs & into blood stream

Structure

	About 1.5 kg (largest gland in the body)
	Located in right upper portion of abdominopelvic cavity  -across from stomach     GA
	Left and right lobes     GA Right lobe is made up of three smaller lobes Lobules are microscopic subdivisions of lobes Each lobule processes blood and produces bile Sinusoids (for blood flow)     GA     FIG Bile ducts collect bile as it forms Hepatic ducts drain bile from liver Common hepatic duct formed by joining of left & right hepatic duct Common bile duct formed by joining of common hepatic duct with cystic duct, then later the pancreatic duct Enters duodenum at major duodenal papilla Minor duodenal papilla is where (optional) accessory duct of pancreas joins duodenum
	Hepatic portal system Blood coming from digestive tract would normally go directly back to the heart, but instead it goes through the liver first Thus, blood containing nutrients stops off at the liver before flowing to all other tissues of the body

Function

	Lobules detoxify many poisons (toxins) coming in from the digestive tract Waste products are secreted into bile ducts
	Lobules skim off excess glucose and other nutrients from blood before it gets to rest of body and stores them for later use
	Lobules break down old RBCs, releasing pigments from the breakdown of hemoglobin into the bile ducts
	Produces most plasma proteins
	Bile contains water, bile pigments, bile salts (fat emulsifiers), detox products, other stuff

Structure

	Located below liver, attached to the bile duct system     GA
	Hollow, muscular bag ("bladder" = "bag")     GA
	Cystic duct from gall bladder joins with common hepatic duct from liver to form common bile duct

Function

	Acts as an overflow reservoir for bile coming down from liver
	Stores and concentrates bile
	Contracts to release concentrated bile when there is chyme passing by

Structure

	12-15 cm (length) organ cradled in the C of the duodenum, below stomach     GA
	Most is exocrine tissue that secretes pancreatic juice into pancreatic duct Acinar cells are exocrine cells that produce "pancreatic juice"
	2-3% of tissue is endocrine tissue, secreting insulin, glucagon, other hormones into blood stream Pancreatic islets are endocrine cells
	Pancreatic duct joins common bile duct just before it enters the duodenum     GA

Function

Pancreatic juice contains many substances Water Sodium bicarbonate (NaHCO3)     PP CO2 + H2O <--> H2CO3 <--> HCO3- + H+ HCO3- neutralizes stomach acid (while its H+ counterpart enters the blood to neutralize the HCO3- formed when gastric HCl is secreted) Trypsin & chymotrypsin -- protein-digesting enzymes Lipases -- lipid-digesting enzymes Amylase -- carbohydrate-digesting enzyme

Very complex, and still not completely understood, interaction of many different nervous and endocrine reflexes

The nervous system of the gut has been called the "second brain" because of its ability to make decisions and coordinate activity somewhat independently of the CNS

Example: phases of gastric secretion

Cephalic phase (psychic phase) Anticipation of food and initial ingestion Parasympathetic stimulation  Increased HCl secretion  Increased pepsin secretion  Increased secretion of gastrin (hormone released by stomach mucosa) Amplifies stimulation of HCl and pepsin secretion

Gastric phase Begins when food enters stomach Stimuli: stretch of stomach & chemical content of food  Gastrin secretion (especially when pH is raised when food enters) Increases gastric and pancreatic secretion Parasympathetic and gastrin stimulation of pancreatic secretion Protein (amino acids) and other chemicals stimulate stomach secretion Gastrocolic reflex (parasympathetic) Increased motility of lower small intestine and colon Stimulus: high pH in intestine (before gastric emptying)  Increased stomach contractions and pyloric tone Stimulus: gastric emptying  Opening of sphincter from small intestine into large intestine

Intestinal phase Triggered by food entering small intestine Stimulus: distension (stretch) Enterogastric reflex (parasympathetic) Decreased gastric motility Stimulus: low pH (stomach acid in chyme, the fluid from the stomach) & high fat Secretin secretion by duodenal cells Increased bicarbonate secretion by pancreas (raises pH) Decreased stomach motility Decreased pepsin secretion Decreased gastrin, thus HCl secretion

Digestive enzymes are released as proenzymes (zymogens)

Example: Pepsinogen ---> Pepsin

Digestive enzymes are hydrolases     ANIM

Promote hydrolysis of the substrate molecule

Enzymes are specific     FIG

Work only with certain substrates

Enzymes have an optimum temperature

Peak activity near 37° C

Enzymes have an optimum pH

	Low in stomach
	Neutral elsewhere

Enzymes may be free-floating or attached

	Enzymes in "digestive juices" are free-floating
	Brush-border enzymes in intestine are attached Contact digestion

No need for digestion

Water absorbed by osmosis

Minerals absorbed by active transport and facilitated diffusion

Polysaccharides are digested by amylase in the saliva (yeah, right) and in the intestines (pancreatic amylase) into disaccharides

Disaccharides are digested by "contact digestion" with enzymes imbedded in the brush border

Monosaccharides are absorbed by facilitated diffusion linked to active transport of sodium ("sodium cotransport")     ANIM

Glucose absorption sometimes limited by availability of sodium (and vice versa)

Proteins are denatured (unfolded) by HCl in gastric juice

Denatured proteins are broken into shorter peptides by pepsin in gastric juice

Peptides are further digested by trypsin & chymotrypsin in small intestine (pancreatic juice)

Amino acids are freed by contact digestion in brush border

Absorbed by sodium cotransport (facilitated diffusion linked to active transport of sodium)

Lipids don't get along well in watery fluids as do other nutrients and thus tend to form big "globs" of fat that prevent enzymes from getting to most of the lipid molecules

	Lipids must first be emulsified (broken into tiny droplets that stay separated) by bile salts (phospholipids such as lecithin)
	Form coated droplets called micelles Micelles are like a spherical version of a phospholipid bilayer

Lipases from pancreatic juice in the small intestine break down fats into

	Fatty acids
	Monoglycerides
	Glycerol
	Cholesterol

Simple molecules are absorbed by simple diffusion directly across plasma membranes of brush border cells of small intestine

Fats are immediately rebuilt by the cell and packaged into vesicles called chylomicrons

Chylomicrons of fat are released by exocytosis and are carried away in the lacteals

Metabolic states     PP

	The absorptive state  During and for several hours after a meal Insulin Increased glucose uptake by cells (from blood)      ANIM Increased glycogenesis in liver cells Increased lipogenesis (fat synthesis) Decreased lipolysis (fat breakdown) in fat cells Note: insulin can make you feel sluggish/sleepy, which explain the urge for a nap after a holiday meal Continues until...
	The postabsorptive state Starts hours after the last meal Glucagon Increased glycogenolysis (breakdown of glycogen into glucose) Increased gluconeogenesis (prepares lipids & amino acids for citric acid cycle) Increased lipolysis Decreased lipogenesis  Continues to next meal; may lead to starvation if a meal is not taken

Vitamins and minerals

	Vitamins are cofactors, assisting enzymes throughout the body
	Some vitamins may act as antioxidants, removing oxygen free radicals and thus their harmful effects
	Minerals form ions such as sodium, potassium, calcium, etc. Used in a variety of ways in the body