Learning objectives - The Digestive System
•The digestive system takes in food, breaks it down into nutrient molecules, absorbs these molecules into the bloodstream, and then rids the body of the indigestible remains.
•Digestive system organs fall into two main groups: the alimentary canal and the accessory organs (pp. 852–853).
oThe alimentary canal, or gastrointestinal (GI) tract, is the continuous muscular digestive tube that winds through the body digesting and absorbing foodstuff; its organs include: the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
oAccessory digestive organs aid digestion physically and produce secretions that break down foodstuff in the GI tract; the organs involved are the teeth, tongue, gallbladder, salivary glands, liver, and pancreas.
Describe the function of the digestive system, and differentiate between organs of the alimentary canal and accessory digestive organs.
Ingestion is the simple act of putting food into the mouth (p. 853).
Propulsion moves food through the alimentary canal and includes both swallowing and peristalsis (p. 853; Fig. 23.3).
Mechanical digestion is the physical process of preparing the food for chemical digestion and involves chewing, mixing, churning, and segmentation (pp. 853–854).
Chemical digestion is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks by enzymes (p. 854).
Absorption is the passage of digested end products from the lumen of the GI tract through the mucosal cells into the blood or lymph (p. 854).
Defecation eliminates indigestible substances from the body via the anus as feces (p. 854).
List and define the major processes occurring during digestive system activity.
The digestive system creates an optimal internal environment for its functioning in the lumen of the GI tract, an area that is technically outside of the body
•Digestive activities within the GI tract are triggered by mechanical and chemical stimuli.
•Controls of the digestive activity are both extrinsic and intrinsic (nervous and hormonal).
Describe stimuli and controls of digestive activity.
•The visceral peritoneum covers the external surfaces of most of the digestive organs, and the parietal peritoneum lines the body wall of the abdominopelvic cavity.
•The peritoneal cavity is located between the visceral and parietal peritoneums and is filled with serous fluid.
•Mesentery is a double layer of peritoneum that extends to the digestive organs from the body wall. It allows blood vessels, lymphatics, and nerves to reach the digestive organs, and holds the organs in place as well as stores fat.
Describe the location and function of the peritoneum.
•Retroperitoneal organs - organs not suspended by a mesentery. include most of the pancreas and parts of the small and large intestine.
Define retroperitoneal and name the retroperitoneal organs.
The splanchnic circulation serves the digestive system and includes those arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation (p. 856).
Define splanchic circulation.
Indicate the importance of the hepatic portal system.
Histology of the Alimentary Canal
•Mucosa is the innermost, moist, epithelial membrane that lines the entire digestive tract. It secretes mucus, digestive enzymes, and hormones; absorbs digestive end products into the blood; and protects against infectious disease.
•Submucosa is a moderately dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles, and nerve fibers.
•Muscularis externa typically consists of smooth muscle and is responsible for peristalsis and segmentation.
•Serosa, the protective outer layer of the intraperitoneal organs, is the visceral peritoneum.
Describe the tissue composition and the general function of each of the four layers of the alimentary canal.
•Mouth is a stratified squamous epithelial mucosa-lined cavity with boundaries of the lips, cheeks, palate, and tongue
oThe lips and cheeks have a core of skeletal muscle covered externally by skin that helps to keep food between the teeth when we chew and plays a small role in speech.
oThe palate forms the roof of the mouth and has two parts: the hard palate anteriorly and the soft palate posteriorly.
oThe tongue is made of interlacing bundles of skeletal muscle and is used to reposition food when chewing, mix food with saliva, initiate swallowing, and help form consonants for speech.
oSalivary glands produce saliva, which cleanses the mouth, dissolves food chemicals for taste, moistens food, and contains chemicals that begin the breakdown of starches.
oThe teeth tear and grind food, breaking it into smaller pieces.
•Pharynx - oropharynx and laryngopharynx provides a common passageway for food, fluids, and air (p. 864).
•Esophagus - provides a passageway for food and fluids from the laryngopharynx to the stomach where it joins at the cardiac orifice
Describe the gross and microscopic anatomy and the basic functions of the mouth, pharynx, and esophagus.
•Saliva is largely water—97 to 99.5%—and therefore is hypoosmotic. As a rule, saliva is slightly acidic (pH 6.75 to 7.00), but its pH may vary. Its solutes include electrolytes (Na,K,Cl, PO43, and HCO3) the digestive enzymes salivary amylase and lingual lipase (both optimally active at an acid pH); the proteins mucin (mu sin), lysozyme, and IgA; and metabolic wastes (urea and uric acid). When dissolved in water, the glycoprotein mucin forms thick mucus that lubricates the oral cavity and hydrates foodstuffs.
•The intrinsic salivary glands secrete saliva continuously in amounts just sufficient to keep the mouth moist.But when food enters the mouth, the extrinsic glands are activated and copious amounts of saliva pour out. The average output of saliva is 1000–1500 ml per day. Salivation is controlled primarily by the parasympathetic division of the autonomic nervous system.When we ingest foods, chemoreceptors and mechanoreceptors in the mouth send signals to the salivatory nuclei in the brain stem
Describe the composition and functions of saliva, and explain how salivation is regulated.
The dental formula is a shorthand way of indicating the numbers and relative positions of the different types of teeth in the mouth. This formula is written as a ratio, uppers over lowers, for one-half of the mouth.
(2I,1C,2PM,3M)/(2I,1C,2PM,3M) X 2=32 teeth
Primary Dentition consists of the deciduous teeth (decid = falling off), also called milk or baby teeth.
Permanent Teeth enlarge and develop, the roots of the milk teeth are resorbed from below
Explain the dental formula and differentiate clearly between deciduous and permanent teeth.
•Mastication, or chewing, begins the mechanical breakdown of food and mixes the food with saliva (p. 866).
•Deglutition, or swallowing, is a complicated process that involves two major phases (p. 866; Fig. 23.13).
1.The buccal phase is voluntary and occurs in the mouth where the bolus is forced into the oropharynx.
2.The pharyngeal-esophageal phase is involuntary and occurs when food is squeezed through the pharynx and into the esophagus.
Describe the mechanisms of chewing and swallowing.
•contains four tunics
•fibers run obliquely; allows to mix, churn, move food along but pummel the food, physically breaking down into smaller fragments and to ram the food into the small intestine
•lining epithelium is simple columnar epithelium composed entirely of mucous cells
Identify structural modifications of the wall of the stomach that enhance the digestive process.
Mucous neck cells, found in the upper, or “neck,” regions of the glands, produce a thin, soluble mucus
Parietal cells - found mainly in the middle region of the glands, simultaneously secrete hydrochloric acid (HCl) and intrinsic factor. Although the parietal cells appear spherical when viewed with a light microscope, they actually have three prongs that bear dense microvilli (they look like fuzzy pitchforks!). This structure provides a huge surface area for secreting H and Cl into the stomach lumen. HCl makes the stomach contents extremely acidic (pH 1.5–3.5), a condition necessary for activation and optimal activity of pepsin. The acidity also helps in food digestion by denaturing proteins and breaking down cell walls of plant foods, and is harsh enough to kill many of the bacteria ingested with foods. Intrinsic factor is a glycoprotein required for vitamin B12 absorption in the small intestine.
Chief cells occur mainly in the basal regions of the gastric glands. The chief cells produce pepsinogen, the inactive form of the protein-digesting enzyme pepsin. Chief cells also secrete insignificant amounts of lipases (fat-digesting enzymes).
Enteroendocrine cells - (“gut endocrine”), typically located deep in the gastric glands, release a variety of chemical messengers directly into the interstitial fluid of the lamina propria.
Name the cell types responsible for secreting the various components of gastric juice and indicate the importance of each component in stomach activity.
Describe stomach structure and indicate changes in the basic alimentary canal structure that aid its digestive function.
•Neural and hormonal mechanisms
•Stimulatory and inhibitory events occur in three phases:
1.Reflex phase (cephalic): few minutes prior to food entry: smell/taste
2.Gastric phase: 3–4 hours after food enters the stomach
3.Intestinal phase: brief stimulatory effect as chyme enters the duodenum, followed by inhibitory effects (enterogastric reflex and enterogastrones): not too much now!
Explain how gastric secretion and stomach motility are regulated.
As H is pumped from the cell and HCO3 (bicarbonate ion) accumulates within the cell,HCO3 is ejected through the basal cell membrane into the capillary blood. As a result, blood draining from the stomach is more alkaline than the blood serving it. This phenomenon is called the alkaline tide.
Define and account for the alkaline tide.
•the mucousa and submucousa are modified to reflect the intestines functions in the digestive pathway
•epithelium= simple columnar absorptive cells bound by tight junctions (responsible for nutrient and electrolyte absorption)
intestinal crypts= secretory cells the secrete intestinal juice (serve as a carrier fluid for absorbing nutrients from chyme)
Identify and describe structural modifications of the wall of the small intestine that enhance the digestive process.
•simple columnar absorptive cells=nutrient and electrolyte absorption
•crypt epithelial cells= secretory cells the secrete intestinal juice
•specialized cell types: absorptive cells, goblet cells, enteroendocrine cells, paneth cells
Differentiate between the roles of the various cell types of the intestinal mucosa.
Describe the function of local intestinal hormones and paracrines.
The liver is composed of sesame seed–sized structural and functional units called liver lobules. Each lobule is a roughly hexagonal (six-sided) structure consisting of plates of liver cells, or hepatocytes, organized like bricks in a garden wall. The hepatocyte plates radiate outward from a central vein running in the longitudinal axis of the lobule.
Describe the histologic anatomy of the liver.
Bile is a fat emulsifier. Bile does not usually enter the small intestine until the gallbladder contracts when stimulated by cholecystokinin.
State the role of bile in digestion and describe how its entry into the small intestine is regulated
The gallbladder stores and concentrates bile that is not needed immediately for digestion.
Describe the role of the gallbladder.
•Pancreatic juice consists mainly of water and contains enzymes that break down all categories of foodstuffs and electrolytes.
•Secretion of pancreatic juice is regulated by local hormones and the parasympathetic nervous system.
State the role of pancreatic juice in digestion.
Pancreatic juice is regulated by neural stimuli and, more importantly, hormones (cholecystokinin and secretin).
When no digestion is occurring, the hepatopancreatic sphincter (guarding the entry of bile and pancreatic juice into the duodenum) is closed and the released bile backs up the cystic duct into the gallbladder, where it is stored until needed
Describe how entry of pancreatic juice into the small intestine is regulated.
The large intestine absorbs water from indigestible food residues and eliminates the latter as feces
List the major functions of the large intestine.
This spinal cord–mediated parasympathetic reflex causes the sigmoid colon and the rectum to contract, and the internal anal sphincter to relax. As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should be opened or remain constricted to stop feces passage temporarily. If defecation is delayed reflex contractions end within a few seconds, and the rectal walls relax.With the next mass movement, the defecation reflex is initiated again—and so on, until the person chooses to defecate or the urge to defecate becomes unavoidable.
Describe the regulation of defecation.
Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex.
•salivary amylase - Chemical digestion of carbohydrates begins in the mouth, where salivary amylase breaks large polysaccharides into smaller fragments.
•Pepsin - secreted by the chief cells, begins the chemical digestion of proteins in the stomach.
•Rennin - is produced in infants and breaks down milk proteins.
•Pancreatic enzymes - such as trypsin and chymotrypsin, further break down proteins in the small intestine.
•The brush border enzymes - carboxypeptidase, aminopeptidase, and dipeptidase work on freeing single amino acids in the small intestine.
•Lipases are secreted by the pancreas and are the enzymes that digest fats after they have been pretreated with bile.
List the enzymes involved in chemical digestion; name the foodstuffs on which they act.
oAmino acids are absorbed by cotransport with sodium ions.
oSome dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells.
oAmino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.
oFatty acids and monoglycerides enter the intestinal cells via diffusion.
oFatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis.
oThe chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct.
oSome short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein.
oMonosaccharides (simple sugars), the monomers of carbohydrates, are absorbed immediately
oGlucose and galactose are absorbed via cotransport with sodium ions.
oFructose passes via facilitated diffusion.
oAll monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein.
oUnits enter intestinal cells by active transport via membrane carriers.
oUnits are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein.
List the end products of protein, fat, carbohydrate, and nucleic acid digestion.
1.Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum.
2.Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa.
3.Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons.
4.Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph.
Describe the process of absorption of breakdown products of foodstuffs that occurs in the small intestine.
1.The epithelial lining of the developing alimentary canal forms from the endoderm with the rest of the wall arising from the mesoderm.
2.The anteriormost endoderm touches the depressed area of the surface ectoderm where the membranes fuse to form the oral membrane and ultimately the mouth.
3.The end of the hindgut fuses with an ectodermal depression, called the proctodeum, to form the cloacal membrane and ultimately the anus.
4.By week 8 the alimentary canal is a continuous tube stretching from the mouth to the anus.
Describe embryonic development of the digestive system.
•GI tract motility declines, digestive juice production decreases, absorption is less efficient, and peristalsis slows, resulting in less frequent bowel movements and often constipation.
•Diverticulosis, fecal incontinence, and cancer of the GI tract are fairly common problems in the elderly.
Describe abnormalities of the gastrointestinal tract at different stages of life.