Chapter 25: Digestive System Learning Outcomes
Identify the layers of the digestive tract and describe its relationship to the peritoneum.
Most of digestive tract follows a basic structural plan with the digestive tract wall consisting of layers: -Mucosa •Inner epithelium -Simple columnar in most of digestive tract -Stratified squamous from mouth through esophagus, and in lower anal canal. •Lamina propria: loose connective tissue layer •Muscularis mucosa: thin layer of smooth muscle -Tenses mucosa creating grooves and ridges that enhance surface area and contact with food. -Improves efficiency of digestion and nutrient absorption. •Mucosa-associated lymphatic tissue (MALT): the mucosa exhibits an abundance of lymphocytes and lymphatic nodules -Submucosa •Contains blood vessels, lymphatic vessels, a nerve plexus, and in some places mucus-secreting glands that dump lubricating mucus into the lumen •MALT extends into the submucosa in some parts of the GI tract -Muscularis externa •Inner circular layer -In some places, this layer thickens to form valves (sphincters) that regulate the passage of material through the tract •Outer longitudinal layer -Responsible for the motility that propels food and residue through the tract. -Serosa •Composed of a thin layer of areolar tissue topped by simple squamous mesothelium. •Begins in the lower 3 to 4 cm of the esophagus •Ends just before the rectum •Adventitia: fibrous connective tissue layer that binds and blends the pharynx, most of the esophagus, and the rectum into adjacent connective tissue of other organs Relationship to the Peritoneum •Mesenteries—connective tissue sheets that suspend stomach and intestines from abdominal wall -Looseness allows stomach and intestines to undergo strenuous contractions with freedom of movement in the abdominal cavity -Hold abdominal viscera in proper relationship to each other -Prevent intestines from becoming twisted and tangled by changes in body position and by its own contractions -Provide passage of blood vessels and nerves that supply digestive tract -Contain many lymph nodes and lymphatic vessels •Parietal peritoneum—a serous membrane that lines the wall of the abdominal cavity -Turns inward along posterior midline -Forms dorsal mesentery: a translucent two-layered membrane extending to the digestive tract -The two layers of the mesentery separate and pass around opposite sides of the organ forming the serosa -Come together on the far side of the organ and continue as another sheet of tissue, called the anterior (ventral) mesentery •May hang freely in the abdominal cavity •May attach to the anterior abdominal wall or other organs •Lesser omentum—a ventral mesentery that extends from the lesser curvature of the stomach to the liver •Greater omentum—hangs from the greater curvature of the stomach (its left inferior margin)-Covers small intestine like an apron -The inferior margin turns back on itself and passes upward -Forming a deep pouch between its deep and superficial layers -Inner superior margin forms serous membranes around the spleen and transverse colon —mesocolon •Mesocolon—extension of the mesentery that anchors the colon to the abdominal wall •Intraperitoneal—when an organ is enclosed by mesentery on both sides -Considered within the peritoneal cavity -Stomach, liver, and parts of small and large intestine •Retroperitoneal—when an organ lies against the posterior body wall and is covered by peritoneum on its anterior side only -Considered to be outside the peritoneal cavity -Duodenum, pancreas, and parts of the large intestine
Distinguish between mechanical and chemical digestion. Identify the basic chemical process that underlies all chemical digestion, and name the major substrates and products of this process.
Mechanical digestion—the physical breakdown of food into smaller particles. -Cutting and grinding action of the teeth -Churning action of stomach and small intestines -Exposes more food surface to digestive enzymes Chemical digestion—a series of hydrolysis reactions that breaks dietary macromolecules into their monomers (residues.) -Carried out by digestive enzymes produced by salivary glands, stomach, pancreas, and small intestine -Results •Polysaccharides into monosaccharides •Proteins into amino acids •Fats into monoglyceridesand fatty acids •Nucleic acids into nucleotides •Some nutrients are present in a usable form in ingested food and can be directly absorbed -Vitamins, amino acids, minerals, cholesterol, and water
Describe the gross anatomy of the digestive tract from the mouth through the esophagus.
The Mouth (Oral Cavity, Buccal Cavity) •Functions -Ingestion (food intake) -Taste and other sensory responses to food -Chewing and chemical digestion -Swallowing, speech, and respiration •Mouth enclosed by cheeks, lips, palate, and tongue •Oral fissure—anterior opening between lips •Fauces—posterior opening to the throat •Stratified squamous epithelium lines mouth -Keratinized in areas subject to food abrasion: gums and hard palate -Nonkeratinized in other areas: floor of mouth, soft palate, and insides of cheeks and lips The Cheeks and Lips •Retain food and push it between the teeth •Essential for speech •Fleshiness due to subcutaneous fat, buccinator muscle of the cheek, and orbicularis oris of the lips •Labial frenulum: median fold that attaches each lip to the gum between the anterior incisors •Vestibule: space between cheek or lips and the teeth The Tongue •Body: anterior two-thirds of tongue in oral cavity -Manipulates food and senses taste and texture of food -Covered with nonkeratinized stratified squamous epithelium and lingual papillae •Lingual frenulum: median fold that attaches the body of the tongue to the floor of the mouth •Vallate papillae: a V-shaped row of papillae that mark the boundary between the body and root of the tongue •Root: posterior one-third of the tongue in oropharynx •Lingual tonsils: contained in the root•Intrinsic muscles (contained entirely within the tongue) produce subtle tongue movements of speech •Extrinsic muscles: with attachments outside in the tongue •Lingual glands: serous and mucous glands amid the extrinsic muscles secrete a portion of the saliva The Palate •Separates oral cavity from nasal cavity to allow breathing while chewing food •Hard (bony) palate—anterior portion that is supported by the palatine processes of the maxillae and the palatine bones •Soft palate -Composed of skeletal muscle and glandular tissue -Uvula: conical medial projection visible at the rear of the mouth •Helps retain food in the mouth until one is ready to swallow •Pair of muscular arches on each side of the oral cavity -Palatoglossal arch: anterior arch -Palatopharyngeal arch: posterior arch -Palatine tonsils are located on the wall between the arches The Teeth •32 adult teeth -16 in mandible -16 in maxilla •2 incisors—chisel-like cutting teeth used to bite off a piece of food •1 canine—pointed and act to puncture and shred food •2 premolars—broad surface for crushing, shredding, and grinding •3 molars—even broader surface for crushing, shredding, and grinding •Alveolus—tooth socket in bone -Gomphosis joint formed between tooth and bone •Periodontal ligament—modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other -Anchors tooth firmly in alveolus but allows slight movement under pressure of chewing •Gingiva (gum)—covers the alveolar bone •Crown: portion above the gum •Root: the portion below the gum, embedded in alveolar bone •Neck: the point where crown, root, and gum meet •Gingival sulcus: space between the tooth and the gum •Dentin—hard yellowish tissue that makes up most of the tooth •Enamel—covers crown and neck -A noncellular secretion that cannot regenerate •Cementum—covers root •Cementum and dentin are living tissue and can regenerate •Root canal—space in a root leading to pulp cavity in the crown -Nerves and blood vessels -Apical foramen: pore at the basal end of each root canal •20 deciduous teeth (milk teeth or baby teeth) •Teeth develop beneath gums and erupt in a predictable order -Erupt from 6 to 30 months -Between 6 and 25 years of age, are replaced by 32 permanent teeth •Third molars (wisdom teeth) erupt from age 17 to 25 years -May be impacted: crowded against neighboring teeth and bone so they cannot erupt •Plaque—sticky residue on the teeth made up of bacteria and sugars -Calculus: calcified plaque -Bacteria metabolize sugars and release acids that dissolve the minerals of enamel and dentin to form dental caries (cavities) •Root canal therapy is necessary if cavity reaches pulp •Calculus in the gingival sulcus wedges the tooth and gum apart -Allows bacterial invasion of the sulcus -Gingivitis: inflammation of the gums -Periodontal disease: destruction of the supporting bone around the teeth which may result in tooth loss The Pharynx •Muscular funnel connecting oral cavity to esophagus and nasal cavity to larynx •Has deep layer of longitudinal skeletal muscle •Has superficial layer of circular skeletal muscles that form pharyngeal constrictors (superior, middle, and inferior) that force food downward during swallowing -When not swallowing, the inferior constrictor (upper esophageal shincter) remains contracted to exclude air from the esophagus -Disappears at the time of death when the muscles relax, so it is a physiological sphincter, not an anatomical structure The Esophagus •Straight muscular tube 25 -30 cm long •Extends from pharynx to cardial orifice of stomach passing through esophageal hiatus in diaphragm •Lower esophageal sphincter -Prevents stomach contents from regurgitating into the esophagus •Nonkeratinized stratified squamous epithelium •Esophageal glands in submucosa secrete mucus •Deeply folded into longitudinal ridges when empty •Skeletal muscle in upper one-third, mix of muscle types in middle one-third, and only smooth muscle in bottom one-third •Covered with adventitia
Define contact digestion and describe where it occurs.
•Contact digestion: chyme must contact the brush border for digestion to occur •Intestinal churning of chyme ensures contact with the mucosa
State the physiological significance of intestinal bacteria.
•Gut microbiome—about 800 species of bacteria that populate the large intestine -Bacteria digest cellulose, pectin, and other carbohydrates for which our cells lack enzymes -Help in synthesis of vitamins B and K
Discuss the types of contractions that occur in the colon.
•Haustral contractions occur every 30 minutes -Distension of a haustrum stimulates it to contract -Churns and mixes residue promoting water and salt absorption •Mass movements—stronger contractions that occur one to three times a day -Triggered by gastrocolic and duodenocolic reflexes •Filling of the stomach and duodenum stimulates motility of the colon •Move residue several centimeters
List the regions of the digestive tract and the accessory organs of the digestive system.
Digestive system has two subdivisions: digestive tract and accessory organs. •Digestive tract (alimentary canal) -30 ft long muscular tube extending from mouth to anus. -Mouth, pharynx, esophagus, stomach, small intestine, and large intestine -Gastrointestinal (GI) tract is the stomach and intestines •Accessory organs -Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Describe how each major class of nutrients is chemically digested, name the enzymes involved, and discuss the functional differences among these enzymes.
Carbohydrates •Starch—most digestible dietary carbohydrate -Cellulose is indigestible -Starch is first digested to oligosaccharides (up to eight glucose residues long) -Oligosaccharides then digested to the disaccharide maltose -Maltose finally digested to glucose which is absorbed by the small intestine •Process begins in the mouth -Salivary amylase hydrolyzes starch into oligosaccharides -Amylase works best at pH of 6.8 to 7.0 of oral cavity -Amylase quickly denatured on contact with stomach acid and digested by pepsin -About 50% of dietary starch is digested before it reaches small intestine -Pancreatic amylase resumes starch digestion in intestine Proteins •Amino acids absorbed by the small intestine come from three sources -Dietary proteins -Digestive enzymes digested by each other -Sloughed epithelial cells digested by enzymes •Endogenous amino acids from last two sources total about 30 g/day •Exogenous amino acids from our diet total about 44 to 60 g/day •Proteases (peptidases)—enzymes that digest proteins -Begin their work in stomach in optimum pH of 1.5 to 3.5 -Pepsin hydrolyzes any peptide bond between tyrosine and phenylalanine •Pepsin digests 10% to 15% of dietary protein into shorter peptides and some free amino acids Lipids •Hydrophobicity of lipids makes their digestion and absorption complicated •Lipases—fat-digesting enzymes -Lingual lipase secreted by intrinsic salivary glands of the tongue •Active in mouth, but more active in stomach along with gastric lipase •10% to 15% of lipids digested before reaching duodenum -Before digestion in duodenum, vigorous pumping in stomach's antrum emulsifies the fat (breaks up globs) Nucleic acid -Nucleases (deoxyribonuclease and ribonuclease) of pancreatic juice hydrolyze DNA and RNA to nucleotides -Nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases -Membrane carriers allow absorption Vitamins -Absorbed unchanged -Fat-soluble vitamins: A, D, E, and K absorbed with other lipids •If ingested without fat-containing food, they are not absorbed at all, but are passed in the feces and wasted -Water-soluble vitamins, B complex and C, absorbed by simple diffusion and B12 if bound to intrinsic factor from the stomach Minerals (Electrolytes) •Absorbed all along small intestine •Na+ cotransported with sugars and amino acids •Cl− exchanged for bicarbonate reversing chloride-bicarbonate exchange that occurs in the stomach •K+ absorbed by simple diffusion •Iron and calcium absorbed as needed -Absorptive cells bind ferrous ions Fe2+ and internalize them by active transport -Unable to absorb ferric ions Fe3+ but stomach acid reduces ferric ions to absorbable ferrous ions •Calcium is absorbed throughout the intestine by different mechanisms -Transcellular absorption in the duodenum or diffusion between epithelial cells in jejunum and ileum -Parathyroid hormone—secreted in response to a drop in blood calcium levels •Stimulates kidney to synthesize vitamin D from precursors made by epidermis and liver -Vitamin D affects absorptive cells of the duodenum by increasing number of calcium channels Water •Digestive tract receives about 9 L of water/day -0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions -8 L is absorbed by small intestine and 0.8 L by large intestine -0.2 L voided in daily fecal output •Water is absorbed by osmosis following the absorption of salts and organic nutrients •Diarrhea—occurs when large intestine absorbs too little water -Feces pass through too quickly if intestine is irritated -Feces contain high concentrations of a solute (such as lactose) •Constipation—occurs when fecal movement is slow, too much water gets reabsorbed, and feces become hardened
Describe the gross and microscopic anatomy and the digestive secretions and functions of the liver.
Liver Gross Anatomy: -Reddish brown gland located immediately inferior to the diaphragm •The body's largest gland -Weighs about 1.4 kg (3 lb) •Variety of functions such as secretes bile which contributes to digestion •Four lobes—right, left, quadrate, and caudate -Falciform ligament separates left and right lobes •Sheet of mesentery that suspends the liver from the diaphragm -Round ligament (ligamentum teres)—fibrous remnant of umbilical vein •From inferior view, squarish quadrate lobe next to the gallbladder and a tail-like caudate lobe posterior to that •Hilum—irregular opening between quadrate and caudate lobes -Point of entry for hepatic portal vein and proper hepatic artery -Point of exit for the bile passages-All travel in lesser omentum •Gallbladder—adheres to a depression on the inferior surface of the liver, between right and quadrate lobes •Bare area on superior surface where it attaches to diaphragm Microscopic Anatomy •Hepatic lobules—tiny cylinders that fill the interior of the liver -About 2 mm long and 1 mm in diameter •Central vein: passes down the core •Hepatocytes: cuboidal cells surrounding central vein in radiating sheets or plates -Each plate of hepatocytes is an epithelium one or two cells thick •Hepatic sinusoids: blood-filled channels that fill spaces between the plates -Lined by a fenestrated endothelium that separates hepatocytes from blood cells •Fenestrated endothelium-Allows plasma into the space between the hepatocytes and endothelium -Hepatocytes have brush border of microvilli that project into this space -Blood filtered through the sinusoids comes directly from the stomach and intestines •Hepatic macrophages: phagocytic cells in the sinusoids that remove bacteria and debris from the blood Hepatocytes •After a meal, hepatocytes absorb from the blood: glucose, amino acids, iron, vitamins, and other nutrients for metabolism or storage •Between meals, hepatocytes break down stored glycogen and release glucose into the blood •Remove and degrade: hormones, toxins, bile pigments, and drugs •Secrete into the blood: albumin, lipoproteins, clotting factors, angiotensinogen, and other products •Hepatic lobules are separated by a sparse connective tissue called stroma •Between lobules is a hepatic triad of two vessels and a bile ductule -Other vessel: branch of hepatic portal vein -One vessel: branch of hepatic artery proper -Both vessels supply blood to sinusoids which receive a mixture of nutrient-laden venous blood from the intestines, and freshly oxygenated arterial blood from the celiac trunk •Bile canaliculi—narrow channels into which the liver secretes bile •Bile passes into bile ductules of the triads •Ultimately into the right and left hepatic ducts •Common hepatic duct: formed from convergence of right and left hepatic ducts on inferior side of the liver •Cystic duct coming from gallbladder joins common hepatic duct •Bile duct: formed from union of cystic and common hepatic ducts -Descends through lesser omentum toward the duodenum •Near duodenum, bile duct joins duct of pancreas •Forms expanded chamber: hepatopancreatic ampulla -Terminates in a fold of tissue—major duodenal papilla on duodenal wall •Major duodenal papilla contains muscular hepatopancreatic sphincter -Regulates passage of bile and pancreatic juice into duodenum -Between meals, sphincter closes and prevents release of bile into the intestines
Describe the neural control of salivation and swallowing.
Salivation •Salivatory nuclei in medulla oblongata and pons respond to signals generated by presence of food -Excited by tactile, pressure, and taste receptors -Salivatory nuclei receive input from higher brain centers as well •Odor, sight, thought of food stimulates salivation -Send signals by way of autonomic fibers in the facial and glossopharyngeal nerves to the glands •Parasympathetic fibers stimulate the glands to produce an abundance of thin, enzyme-rich saliva •Sympathetic activity stimulates the glands to produce less, and thicker, saliva with more mucus. Swallowing •Swallowing center: pair of nuclei in medulla oblongata that coordinates swallowing -Communicates with muscles of the pharynx and esophagus by way of trigeminal, facial, glossopharyngeal, and hypoglossal nerves •Swallowing occurs in three phases: oral, pharyngeal, and esophageal Oral Phase of Swallowing •Under voluntary control •Tongue collects food, presses it against palate forming bolus, and pushes it posteriorly •Food accumulates in oropharynx in front of epiglottis •Epiglottis tips posteriorly and food bolus slides around it and into laryngopharynx Pharyngeal Phase of Swallowing •Involuntary •Prevents food and drink from reentering mouth or entering the nasal cavity •Breathing is suspended •Infrahyoid muscles pull larynx up to meet epiglottis and cover laryngeal opening •Vocal cords adduct to close airway •Upper esophagus widens •Food bolus is driven downward by constriction of the upper, then middle, and finally the lower pharyngeal constrictors Esophageal Phase of Swallowing •Peristalsis: involuntary wave of muscular contraction that pushes the bolus ahead of it •When standing or sitting upright, food and liquid drops through esophagus by gravity faster than peristalsis can keep up with it •Peristalsis ensures you can swallow regardless of body position •Liquid reaches the stomach in 1 to 2 seconds; food bolus in 4 to 8 seconds •When it reaches lower end of the esophagus, the lower esophageal sphincter relaxes to let food pass into the stomach
State the function of each type of epithelial cell in the gastric mucosa.
•Mucous cells—secrete mucus -Predominate in cardiac and pyloric glands-In gastric glands, called mucous neck cells since they are concentrated at the neck of the gland •Regenerative (stem) cells—found in base of pit and in neck of gland -Divide rapidly and produce continual supply of new cells to replace cells that die •Parietal cells—found mostly in the upper half of the gland -Secrete hydrochloric acid (HCl), intrinsic factor, and a hunger hormone called ghrelin •Chief cells—most numerous -Secrete gastric lipase and pepsinogen -Dominate lower half of gastric glands -Absent from pyloric and cardiac glands •Enteroendocrine cells—concentrated in lower end of gland -Secrete hormones and paracrine messengers that regulate digestion
State how the mucosa of the small intestine differs from that of the stomach, and explain the functional significance of the differences.
-Duodenal glands secrete bicarbonate rich mucus which neutralizes stomach acid and shields mucosa from it
Describe how each type of nutrient is absorbed by the small intestine.
Carbohydrates •When reaching small intestine, pancreatic amylase quickly converts starch to oligosaccharides and maltose •Brush border enzymes continue carbohydrate digestion: -Dextrinase and glucoamylase hydrolyze oligosaccharides -Maltase hydrolyzes maltose (a disaccharide) -Sucrase and lactase hydrolyze the disaccharides sucrose and lactose •In most people, lactase production stops in childhood -Monosaccharides produced by disaccharide hydrolysis (such as glucose) are immediately absorbed •Plasma membrane of absorptive cells has transport proteins that absorb monosaccharidesas soon as brush border enzymes release them •80% of absorbed sugar is glucose -Taken up by sodium-glucose transport (SGLT) proteins -Glucose is transported out the base of absorptive cell into ECF by facilitated diffusion -Sugar entering ECF increases its osmolarity -Draws water osmotically from lumen of intestine, through leaky tight junctions between epithelial cells -Water carries more glucose and other nutrients with it by solvent drag •SGLT also absorbs galactose •Fructose is absorbed by facilitated diffusion (by a different carrier protein) and converted to glucose •Glucose, galactose, and any remaining fructose are transported out of the base of the cell by facilitated diffusion •Absorbed by blood capillaries in the villus •Hepatic portal system delivers them to the liver Proteins •Protein digestion continues in small intestine -Pepsin inactivated when it passes into the duodenum and mixes with alkaline pancreatic juice (pH 8) -Pancreatic enzymes trypsin and chymotrypsin take over the process -Hydrolyze polypeptides into even shorter oligopeptides -Oligopeptides taken apart one amino acid at a time by three more enzymes •Carboxypeptidase—removes amino acids from -COOH end of the chain -Carboxypeptidase is a pancreatic secretion •Aminopeptidase—removes amino acids from -NH2end •Dipeptidase—splits dipeptides in the middle and release two free amino acids -Aminopeptidase and dipeptidaseare brush border enzymes •Brush border enzymes finish task, producing free amino acids that are absorbed into intestinal epithelial cells -Sodium-dependent amino acid cotransporters move amino acids into epithelial cells -Facilitated diffusion moves amino acids out into bloodstream Lipids -Before digestion in duodenum, vigorous pumping in stomach's antrum emulsifies the fat (breaks up globs) •Emulsification droplets are passed to small intestine •Emulsification droplets are broken down further by bile, lecithin, and agitation produced by intestinal segmentation -Exposes more fat surface to enzymatic action •There is enough pancreatic lipase in the small intestine after a meal to digest the average daily fat intake in as little as 1 to 2 minutes •Lipase acts on triglycerides -Removes first and third fatty acids from glycerol backbone, but leaves the middle one -The product of lipase action are two free fatty acids (FFAs) and a monoglyceride •Absorption of free fatty acids, monoglycerides, and other lipids depends on droplets in the bile called micelles -Consist of 20 to 40 bile acid molecules aggregated with their hydrophilic side groups facing outward and their hydrophobic steroid rings facing inward -Bile phospholipids and cholesterol diffuse into the center of the micelle to form its core -Micelles pass down the bile duct into the duodenum •There they absorb fat-soluble vitamins, cholesterol, and the FFAs and monoglycerides produced by fat digestion •They transport lipids to the surface of the intestinal absorptive cells •Lipids leave the micelles and diffuse through the plasma membrane into the cells •Micelles are reused, picking up another cargo of lipid,transporting them to the absorptive cells •Within the intestinal cell, free fatty acids and monoglycerides are transported to the smooth ER •Resynthesized into triglycerides •Golgi complex coats these with phospholipids and protein to form chylomicrons •Taken up by lacteal into lymph •White, fatty intestinal lymph (chyle) flows into larger and larger lymphatic vessels until it enters the bloodstream
List the functions and major physiological processes of the digestive system.
Digestive system—organ system that processes food, extracts nutrients, and eliminates residue Five stages of digestion 1.Ingestion: selective intake of food 2.Digestion: mechanical and chemical breakdown of food into a form usable by the body 3.Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph 4.Compaction: absorbing water and consolidating the indigestible residue into feces 5.Defecation: elimination of feces
Describe the gross and microscopic anatomy and the digestive secretions and functions of the gallbladder and bile duct system.
Gallbladder •Gallbladder—a pear-shaped sac on underside of liver •Serves to store and concentrate bile by absorbing water and electrolytes •Internally lined by highly folded mucosa with simple columnar epithelium •Head (fundus) usually projects slightly beyond inferior margin of liver •Neck (cervix) leads into the cystic duct •Bile—yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids •Bilirubin: principal pigment derived from the decomposition of hemoglobin •Bacteria in large intestine metabolize bilirubin to urobilinogen -Stercobilin responsible for the brown color of feces -Urobilin responsible for yellow color of urine •Bile acids (bile salts): steroids synthesized from cholesterol -Bile acids and lecithin, a phospholipid, aid in fat digestion and absorption •Bile gets to the gallbladder by first filling the bile duct then overflowing into the gallbladder •80% of bile acids are reabsorbed in the ileum and returned to the liver -Hepatocytes absorb and resecrete them -Enterohepatic circulation—route of secretion, reabsorption, and resecretion of bile acids two or more times during digestion of an average meal •20% of the bile acids are excreted in the feces -Body's only way of eliminating excess cholesterol -Liver synthesizes new bile acids from cholesterol to replace those lost in feces
Describe the gross and microscopic anatomy of the stomach.
Gross Anatomy: •Divided into four regions -Cardial part (cardia)—small area within about 3 cm of the cardial orifice -Fundus (fundus)—dome-shaped portion superior to esophageal attachment -Body (corpus)—makes up the greatest part of stomach -Pyloric part—narrower pouch at the inferior end •Subdivided into the funnel-like antrum •Narrower pyloric canal that terminates at pylorus •Pylorus: narrow passage to duodenum •Pyloric sphincter—regulates the passage of chyme into the duodenum •Stomach has greater and lesser curvatures -Greater curvature is about 40 cm long, on inferolateral surface •Greater omentum hangs from greater curvature-Lesser curvature is about 10 cm long, on superomedial margin •Lesser omentum connects lesser curvature of stomach to liver •Stomach receives: -Parasympathetic fibers from vagus -Sympathetic fibers from celiac ganglia •Supplied with blood by branches of the celiac trunk •Blood drained from stomach and intestines enters hepatic portal circulation and is filtered through liver before returning to heart Microscopic •Stomach mucosa has simple columnar epithelium -Apical regions of its surface cells are filled with mucin -Mucin swells with water and becomes mucus after it is secreted •Mucosa and submucosa are flat when stomach is full, but form longitudinal wrinkles called gastric rugae when empty •Muscularis externa has three layers (instead of the two seen elsewhere) -Outer longitudinal, middle circular, and inner oblique layers •Gastric pits—depressions in gastric mucosa•Lined with simple columnar epithelium •Two or three tubular glands open into the bottom of each gastric pit -Cardiac glands in cardial part -Pyloric glands in pyloric parts -Gastric glands in the rest of the stomach
Explain how the stomach produces hydrochloric acid and pepsin.
HCl •Parietal cells produce HCl and contain carbonic anhydrase (CAH) •H+is pumped into gastric gland lumen by H+- K+-ATPase pump •HCO3− exchanged for Cl−(chloride shift) from blood plasma •Cl−(chloride ion) pumped into the lumen of gastric gland to join H+forming HCl •Elevated HCO3− (bicarbonate ion) in blood causes alkaline tide (high pH of blood leaving stomach) when digestion is occurring •HCl activates pepsin and lingual lipase •Breaks up connective tissues and plant cell walls -Helps liquefy food to form chyme •Converts ingested ferric ions Fe3+ to ferrous ions Fe2+-Fe2+absorbed and used for hemoglobin synthesis •Contributes to nonspecific disease resistance by destroying most ingested pathogens Pepsin •Zymogens—digestive enzymes secreted as inactive proteins -Converted to active enzymes by removing some of their amino acids •Pepsinogen—zymogen secreted by chief cells -Hydrochloric acid removes some of its amino acids and forms pepsin that digests proteins -Autocatalytic effect—as some pepsin is formed, it converts more pepsinogen into more pepsin •Pepsin digests dietary proteins into shorter peptides -Protein digestion is completed in the small intestine
Describe the gross and microscopic anatomy and the digestive secretions and functions of the pancreas.
Pancreas •Retroperitoneal gland posterior to greater curvature of stomach •Head encircled by duodenum, a body (midportion), and a tail on the left •Both an endocrine and exocrine gland •Endocrine portion—pancreatic islets that secrete insulin and glucagon •Exocrine portion—99% of pancreas that secretes 1,200 to 1,500 mL of pancreatic juice per day -Secretory acini release their secretion into small ducts that converge on the main pancreatic duct. •Pancreatic duct runs lengthwise through middle of the gland -Joins the bile duct at the hepatopancreatic ampulla -Hepatopancreatic sphincter controls release of both bile and pancreatic juice into the duodenum •Accessory pancreatic duct: smaller duct that branches from the main pancreatic duct -Opens independently into the duodenum -Bypasses the sphincter and allows pancreatic juice to be released into duodenum even when bile is not Pancreatic Juice •Alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes -Acini secrete the enzymes and zymogens -Ducts secrete bicarbonate •Bicarbonate buffers HCl arriving from the stomach •Pancreatic zymogens are: -Trypsinogen •Secreted into intestinal lumen •Converted to trypsin by enteropeptidase that is secreted by mucosa of small intestine •Trypsin is autocatalytic—converts trypsinogen into still more trypsin -Chymotrypsinogen: converted to trypsinogen by trypsin -Procarboxypeptidase: converted to carboxypeptidaseby trypsin •Other pancreatic enzymes -Pancreatic amylase: digests starch -Pancreatic lipase: digests fat -Ribonucleaseand deoxyribonuclease: digest RNA and DNA respectively
Describe the types of movement that occur in the small intestine
•Contractions of small intestine serve three functions -To mix chyme with intestinal juice, bile, and pancreatic juice •To neutralize acid •Digest nutrients more effectively -To churn chyme and bring it in contact with the mucosa for contact digestion and nutrient absorption -To move residue toward large intestine Segmentation •Movement in which stationary ring-like constrictions appear in several places along the intestine •They relax and new constrictions form elsewhere •Most common kind of intestinal contraction •Enteric pacemaker cells in muscularis externa set rhythm of segmentation -Contractions about 12 times per minute in the duodenum -8 to 9 times per minute in the ileum -When most nutrients have been absorbed and little remains but undigested residue, segmentation declines and peristalsis begins Peristalsis •Peristalsis moves contents of small intestine toward colon •Peristaltic wave (triggered by motilin) begins in duodenum, travels 10 to 70 cm and dies out •Followed by another wave starting further down the tract •Migrating motor complex—successive, overlapping waves of contraction •Milk chyme toward colon over a period of 2 hours •Ileocecal valve usually closed -Food in stomach triggers gastroileal reflex that enhances segmentation in the ileum and relaxes the valve -As cecum fills with residue, pressure pinches the valve shut •Prevents reflux of cecal contents into the ileum
Identify the secretions of the stomach and state their functions.
•Gastric juice—2 to 3 L per day produced by the gastric glands •Mainly a mixture of water, hydrochloric acid, and pepsin •Gastric lipase produced by chief cells and lingual lipase play a minor role in digesting dietary fats -Digests 10% to 15% of dietary fats in the stomach -Rest digested in the small intestine •Intrinsic factor—a glycoprotein secreted by parietal cells •Essential to absorption of vitamin B12 by the small intestine -Binds vitamin B12 and then intestinal cells absorb this complex by receptor-mediated endocytosis •Vitamin B12 is needed to synthesize hemoglobin -Deficiency causes anemia •Secretion of intrinsic factor is the only indispensable function of the stomach -Digestion can continue if stomach is removed (gastrectomy), but B12 supplements will be needed •Gastric and pyloric glands have a variety of cells that produce a variety of chemical messengers •Most are hormones that enter blood and stimulate distant cells •Some are paracrine secretions that stimulate neighboring cells •Several are peptides produced in both the digestive tract and the central nervous system: gut-brain peptides -Substance P, vasoactive intestinal peptide (VIP), secretin, gastric inhibitory peptide (GIP), cholecystokinin, and neuropeptide Y (NPY)
Describe the composition and functions of saliva.
•Intrinsic salivary glands- small glands dispersed amid oral tissues -Secrete saliva at a fairly constant rate •Extrinsic salivary glands- three pairs connected to oral cavity by ducts 1.Parotid: located beneath the skin anterior to the earlobe 2.Submandibular gland: located halfway along the body of the mandible •Its duct empties at the side of the lingual frenulum, near the lower central incisors 3.Sublingual gland: located in the floor of the mouth •Has multiple ducts that empty posterior to the papilla of the submandibular duct •Compound tubuloacinar glands -Branched ducts ending in acini •Mucous cells secrete mucus •Serous cells secrete thin fluid rich in enzymes and electrolytes •Mixed acinus has both mucous and serous cells •Moistens mouth •Moistens food and binds it together into bolus to aid in swallowing -Mucus lubricates bolus and aids in swallowing •Dissolves molecules so they can stimulate the taste buds •Begins starch and fat digestion -Salivary amylase: enzyme that begins starch digestion in the mouth -Lingual lipase: enzyme that is activated by stomach acid and digests fat after food is swallowed •Inhibits bacterial growth -Lysozyme: enzyme that kills bacteria -Immunoglobulin A (IgA) •Extrinsic salivary glands secrete about 1 -1.5 L of saliva per day •Cells of acini filter water and electrolytes from blood and add amylase, mucin, and lysozyme
Summarize the functions of the large intestine.
•Large intestine takes about 36 to 48 hours to reduce residue of a meal to feces -Most time in transverse colon -Does not chemically change the residue -Reabsorbs water and electrolytes •Feces consist of about 75% water and 25% solids -Solids: 30% bacteria, 30% undigested fiber, 10% to 20% fat, small amount of mucus, proteins, salts, digestive secretions, and sloughed epithelial cells
Describe the microscopic anatomy of the small intestine.
•Lumen lined with simple columnar epithelium •Muscularis externais noted for a thick inner circular layer and a thinner outer longitudinal layer •Large internal surface area -great length and three types of internal folds or projections -Circular folds (plicaecirculares)—increase surface area by a factor of 2 to 3 •Involve only mucosa and submucosa •Occur from duodenum to middle of ileum -Relatively small and sparse in ileum; not found in distal half, as most nutrient absorption is completed by this point •Cause chyme flow in spiral path causing more contact with mucosa •Promote more thorough mixing and nutrient absorption •Villi—increase surface area by a factor of 10 -Make mucosa look fuzzy -Villus covered with two types of epithelial cells •Absorptive cells (enterocytes) •Goblet cells—secrete mucus -Epithelia joined by tight junctions that prevent digestive enzymes from seeping between them -Core of villus filled with areolar tissue of lamina propria •Contains arteriole, capillaries, venule, and lymphatic capillary called a lacteal •Microvilli—increase the surface area by a factor of 20 -Form a fuzzy brush border on apical surface of each absorptive cell -Brush border enzymes—contained in plasma membrane of microvilli •Carry out some of the final stages of enzymatic digestion •Not released into the lumen •Contact digestion: chyme must contact the brush border for digestion to occur •Intestinal churning of chyme ensures contact with the mucosa •Intestinal crypts—numerous pores that open into tubular glands on the floor of the small intestine between bases of the villi -Similar to gastric glands -In upper half, have enterocytes and goblet cells like the villi-In lower half, dominated by dividing stem cells •Life span of 3 to 6 days •New epithelial cells migrate up the crypt to the tip of the villus where they are sloughed off and digested -A few Paneth cells are clustered at the base of each crypt •Secrete lysozyme, phospholipase, and defensins—defensive proteins that resist bacterial invasion of the mucosa •Duodenal glands—in submucosa of duodenum -Secrete an abundance of bicarbonate-rich mucus -Neutralize stomach acid and shield the mucosa from its erosive effects •Large population of lymphocytes throughout lamina propria and submucosa of small intestine -Intercept pathogens before they can invade bloodstream -Aggregated into lymphatic nodules in ileum •Called Peyer patches
Describe the gross anatomy of the large intestine.
•Measures 1.5 m (5 ft) long and 6.5 cm (2.5 in.) in diameter in cadaver •Begins as cecum inferior to ileocecal valve •Appendix attached to lower end of cecum -Densely populated with lymphocytes—a significant source of immune cells •Ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, and descending colon frame the small intestine •Sigmoid colon is S-shaped portion leading down into pelvic cavity •Rectum: portion ending at anal canal -Has three curves and three infoldings, called the transverse rectal folds (rectal valves) •Anal canal: final 3 cm of the large intestine -Passes through levatorani muscle and pelvic floor, terminates at the anus -Anal columns and sinuses—exude mucus and lubricant into anal canal during defecation -Large hemorrhoidal veins for superficial plexus in anal columns and around orifice •Muscularis externa of colon is unusual -Taeniacoli—longitudinal fibers concentrated in three thickened, ribbon-like strips -Haustra—pouches in the colon caused by the muscle tone of the taeniacoli -Internal anal sphincter—smooth muscle of muscularis externa -External anal sphincter—skeletal muscle of pelvic diaphragm •Omental appendages—club-like, fatty pouches of peritoneum adhering to the colon; unknown function Circulation of the Large Intestine •The large intestine is served by mesenteric arteries and veins much like the small intestine -Branches of the superior mesenteric artery fan out to supply the ascending colon and most of the transverse colon -The inferior mesenteric artery supplies the rest of the transverse colon as well as the descending and sigmoid colon and the rectum •The superior and inferior mesenteric veins drain the same parts of the large intestine as the correspondingly named arteries and drain into the hepatic portal system
Contrast the mucosa of the colon with that of the small intestine.
•Mucosa—simple columnar epithelium -Anal canal has nonkeratinized stratified squamous epithelium in its lower half for abrasion resistance •No circular folds or villi in large intestine •Intestinal crypts—glands sunken deep into lamina propria with a high density of mucus-secreting goblet cells •Lamina propria and submucosal have a lot of lymphatic tissue -Provides protection from large population of bacteria in large intestine
Describe the gross anatomy of the small intestine.
•Nearly all chemical digestion and nutrient absorption occurs in the small intestine •The longest part of the digestive tract -About 5 m long in a living person -Up to 8 m long in a cadaver -no muscle tone •"Small" intestine refers to the diameter—not length -Diameter is about 2.5 cm (1 in.) •Small intestine—coiled mass filling most of the abdominal cavity inferior to stomach and liver •Small intestine divided into three regions: duodenum, jejunum, and ileum. Duodenum: first 25 cm (10 in.) -Begins at pyloric valve -Major and minor duodenal papilla distal to pyloric valve -Receives major and minor pancreatic ducts respectively •Arches around head of the pancreas •Ends at a sharp bend called the duodenojejunal flexure •Most is retroperitoneal •Stomach acid is neutralized here •Fats are physically broken up (emulsified) by bile acids •Pepsin is inactivated by increased pH •Pancreatic enzymes perform chemical digestion Jejunum: first 40% of small intestine beyond duodenum -Roughly 1.0 to 1.7 m in a living person -Has large, tall, closely spaced circular folds -Its wall is relatively thick and muscular -Especially rich blood supply which gives it a red color. -Most digestion and nutrient absorption occurs here. Ileum: forms last 60% of the postduodenal small intestine -About 1.6 to 2.7 m -Thinner, less muscular, less vascular, and paler pink color -Aggregated lymphoid nodules •Prominent lymphatic nodules in clusters on the side opposite the mesenteric attachment •Visible to naked eye; become larger near large intestine -Ileocecal junction—end of the small intestine •Where the ileum joins the cecum of the large intestine -Ileocecal valve—a sphincter formed by the thickened muscularis of the ileum •Protrudes into the cecum •Regulates passage of food residue into the large intestine -Both jejunum and ileum are intraperitoneal and covered with serosa Circulation •The small intestine receives nearly all of its blood supply from the superior mesenteric artery •Fans out through the mesentery to give rise to 12 to 15 jejunal and ileal arteries •Superior mesenteric vein -This joins the splenic vein and then flows into the hepatic portal system, headed for the liver with its load of nutrients
Describe the three phases of gastric function and how gastric activity is activated and inhibited.
•Nervous and endocrine systems collaborate -Increase gastric secretion and motility when food is eaten; suppress them when the stomach empties •Gastric activity is divided into three phases -Cephalic phase: stomach being controlled by brain -Gastric phase: stomach controlling itself -Intestinal phase: stomach being controlled by small intestine •Phases overlap and can occur simultaneously Cephalic Phase •Stomach responds to sight, smell, taste, or thought of food •Sensory and mental inputs converge on hypothalamus •Hypothalamus relays signals to medulla oblongata •Vagus nerve fibers from medulla stimulate the enteric nervous system of stomach, stimulating gastric secretion •40% of stomach's acid secretion occurs in cephalic phase Gastric Phase •Two-thirds of gastric secretion and one-half of acid secretion occur in this phase •Ingested food stimulates gastric activity in two ways -By stretching the stomach •Activates short reflex mediated through myenteric plexus •Activates long reflex mediated through the vagus nerves and the brainstem -By increasing the pH of its contents -Gastric secretion is stimulated by three chemicals •Acetylcholine (ACh)—secreted by parasympathetic nerve fibers of both reflexes •Histamine—a paracrine secretion from enteroendocrine cells in the gastric glands •Gastrin—a hormone produced by the enteroendocrine G cells in pyloric glands Intestinal Phase •Duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous reflexes •Duodenum initially enhances gastric secretion, but soon inhibits it -Stretching of duodenum accentuates vasovagal reflex that stimulates stomach -Peptides and amino acids in chyme stimulate G cells of duodenum to secrete more gastrin, further stimulating stomach -Soon acids and fats trigger enterogastric reflex—duodenum sends inhibitory signals to stomach by way of enteric nervous system -Duodenum also signals medulla to: inhibit vagal nuclei (reducing vagal stimulation of stomach) and stimulate sympathetic neurons (sending inhibitory signals to the stomach) •Chyme also stimulates duodenal enteroendocrine cells to release secretin and cholecystokinin -They stimulate the pancreas and gallbladder -Also suppress gastric secretion •Gastrin secretion declines and pyloric sphincter contracts tightly to limit chyme entering duodenum -Gives duodenum time to work on chyme •Enteroendocrine cells also secrete glucose-dependent insulinotropic peptide (GIP) originally called gastrin-inhibiting peptide -Stimulates insulin secretion in preparation for processing nutrients about to be absorbed by small intestine
Describe the general nervous and chemical controls over digestive function.
•Nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow -Can function independently of central nervous system •But CNS usually exerts influence on its action -Often considered part of autonomic nervous system •Enteric nervous system is composed of two networks of neurons -Submucosal plexus: in submucosa •Controls glandular secretions of mucosa •Controls movements of muscularis mucosae -Myenteric plexus: parasympathetic ganglia and nerve fibers between the two layers of the muscularisexterna •Controls peristalsis and other contractions of muscularis externa •Motility and secretion of the digestive tract are controlled by neural, hormonal, and paracrine mechanisms •Neural control -Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus•Stimulates paristaltic contractions of swallowing -Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion •Hormones -Chemical messengers secreted into bloodstream that stimulate distant parts of the digestive tract -Gastrin and secretin •Paracrine secretions -Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells
Explain the neurological control of defecation.
•Stretching of rectum stimulates two defecation reflexes •Accounts for urge to defecate often felt soon after a meal •Intrinsic defecation reflex works entirely within myenteric plexus to produce relatively weak response -Stretch signals travel through plexus to the muscularis, causing it to contract and the internal sphincter to relax •Parasympathetic defecation reflex involves spinal cord -Stretching of rectum sends sensory signals to spinal cord -Pelvic nerves return signals, intensifying peristalsis and relaxing the internal anal sphincter •Defecation occurs only if external anal sphincter and puborectalis muscles are voluntarily relaxed •Abdominal contractions (Valsalva maneuver) increase abdominal pressure and compresses rectum
Describe the contractile responses of the stomach to food.
•Swallowing center of medulla oblongata signals stomach to relax •Vagus nerve relays message from medulla and activates a receptive-relaxation response in stomach -Resists stretching briefly, but relaxes to hold more food •Soon stomach shows a rhythm of peristaltic contractions controlled by enteric pacemaker cells in longitudinal layer of muscularis externa •A ring of constriction every 20 seconds •Becomes stronger contraction at pyloric part •After 30 minutes or so these contractions become quite strong -They churn the food, mix it with gastric juice, and promote its physical breakup and chemical digestion •Thick muscularis of antrum acts as a strong pump that breaks up semi-digested food and prepares it for intestine •Antral contractions come in waves that churn and break up the chyme into small particles •Only about 3 mL of chyme is squirted into the duodenum at a time; this small amount allows duodenum to: -Neutralize the stomach acid -Digest nutrients little by little •If duodenum is overfilled it inhibits gastric motility •Typical meal emptied from stomach in 4 hours -Less time if the meal is more liquid -As long as 6 hours for a high-fat meal
Explain how hormones regulate secretion by the liver and pancreas.
•Three stimuli are chiefly responsible for the release of pancreatic juice and bile: acetylcholine, cholecystokinin, and secretin -Acetylcholine (ACh): from vagus and enteric nerves •Stimulates acini to secrete enzymes during cephalic phase of gastric control even before food is swallowed -Enzymes remain in acini and ducts until chyme enters the duodenum -Cholecystokinin (CCK): secreted by mucosa of duodenum in response to arrival of fats in small intestine •Stimulates pancreatic acinito secrete enzymes •Strongly stimulates gallbladder •Induces contractions of gallbladder and relaxation of hepatopancreatic sphincter to discharge bile into duodenum -Secretin: released from duodenum in response to acidic chyme arriving from the stomach •Stimulates ducts of both liver and pancreas to secrete more sodium bicarbonate •Raises pH to the level required for activity of the pancreatic and intestinal digestive enzymes
