The Digestive System

Unlike plants, which can form organic molecules using inorganic compounds such as carbon dioxide, water, and ammonia, humans and other animals must obtain their basic organic molecules from food.

Nutrition Nutrient: substance in food used to promote growth, maintenance, and repair Major nutrients: Carbohydrates – sugars & starches Lipids – saturated/unsaturated fats Proteins – eggs, milk, meat (complete – all AA); legumes, nuts, cereals (incomplete) Vitamins – A, B, C, E, D, K Minerals – Ca, P, K, S, Na, Cl, Mg

The digestive process is hydrolysis of nutrients to monomers and their subsequent absorption Within the lumen of the gastrointestinal tract, large food molecules are hydrolyzed into their monomers (subunits). These monomers pass through the inner layer, or mucosa, of the small intestine to enter the blood or lymph in a process called absorption.

The digestion of food molecules through hydrolysis reactions.

The digestion of food molecules through hydrolysis reactions.

The digestion of food molecules through hydrolysis reactions.

Begins starch digestion. Pancreatic amylase: The Digestion of Carbohydrates Salivary amylase: Begins starch digestion. Pancreatic amylase: Digests starch to oligosaccharides. Oligosaccharides hydrolyzed by brush border enzymes. Glucose is transported by secondary active transport with Na+ into the capillaries.

Form micelles and move to brush border. The Digestion of Lipids Arrival of lipids in the duodenum serves as a stimulus for secretion of bile. Emulsification: Bile salt micelles are secreted into duodenum to break up fat droplets. Pancreatic lipase and colipase hydrolyze triglycerides to free fatty acids and monglycerides. Colipase coats the emulsification droplets and anchors the lipase enzyme to them. Form micelles and move to brush border.

The Digestion of Proteins Digestion begins in the stomach when pepsin digests proteins to form polypeptides. In the duodenum and jejunum: Endopeptidases cleave peptide bonds in the interior of the polypeptide: Trypsin. Chymotrypsin. Elastase. Exopeptidases cleave peptide bonds from the ends of the polypeptide: Carboxypeptidase. Aminopeptidase.

Motility Secretion Absorption To implement the digestive process, there are three main functions of the gastrointestinal tract. Motility Secretion Absorption

Functions of the GI Tract Motility propels ingested food from the mouth toward the rectum and mixes and reduces the size of the food. The rate at which food is propelled through the gastrointestinal tract is regulated to optimize the time for digestion and absorption.

Functions of the GI Tract Motility: Ingestion: Taking food into the mouth. Mastication: Chewing the food and mixing it with saliva. Deglutition: Swallowing the food. Peristalsis: Rhythmic wave-like contractions that move food through GI tract. Segmentation: churns material in the small intestine Defecation: eliminate indigestible residues (feces)

Longitudinal muscle Circular Contraction From mouth To anus Contraction of circular muscles behind food mass The second component of stereotyped peristaltic behavior is contraction of the circular muscle in the segment behind the advancing intraluminal contents. The longitudinal muscle layer in this segment relaxes simultaneously with contraction of the circular muscle, resulting in the conversion of this region to a propulsive segment that propels the luminal contents ahead, into the receiving segment.

Longitudinal muscle Circular Contraction From mouth To anus Contraction of circular muscles behind food mass Contraction of longitudinal muscles ahead of food mass During peristalsis, the longitudinal muscle layer in the segment ahead of the advancing intraluminal contents contracts while the circular muscle layer simultaneously relaxes. The intestinal tube behaves like a cylinder with constant surface area. The shortening of the longitudinal axis of the cylinder is accompanied by a widening of the cross-sectional diameter. The simultaneous shortening of the longitudinal muscle and relaxation of the circular muscle results in expansion of the lumen, which prepares a receiving segment for the forward-moving intraluminal contents during peristalsis.

Longitudinal muscle Circular Contraction From mouth To anus Contraction of circular muscles behind food mass Contraction of longitudinal muscles ahead of food mass Contraction of circular muscle layer forces food mass forward Intestinal segments ahead of the advancing front become receiving segments and then propulsive segments in succession as the peristaltic complex of propulsive and receiving segments travels along the intestine.

Mixing of the luminal contents occurs in the receiving segments. Mixing movements. The segmentation pattern of motility is characteristic of the digestive state. Propulsive segments separated by receiving segments occur randomly at many sites along the small intestine. Mixing of the luminal contents occurs in the receiving segments. Receiving segments convert to propulsive segments, while propulsive segments become receiving segments.

Functions of the GI Tract Secretion: Includes both exocrine and endocrine secretions. Exocrine: HCl, H20, HC03-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine are secreted into the lumen of the GI tract. Endocrine: Stomach and small intestine secrete hormones to help regulate the GI system. Gastrin, secretin, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin.

Functions of the GI Tract Absorption: Process of the passage of digestion (chemical subunits) into the blood or lymph.

Digestive System (GI) GI tract divided into: Alimentary canal. Accessory digestive organs. GI tract is 30 ft long and extends from mouth to anus. Insert fig. 18.2

Anatomy Alimentary canal Gastrointestinal (GI) tract Mouth  pharynx  esophagus  stomach  small intestine  large intestine Accessory digestive organs Teeth, tongue, digestive glands

What is the Histological Organization—Four Layers? Mucosa Epithelium Submucosa Connective tissue, nerves, blood vessels Muscularis externa Serosa (in peritoneal cavity)

Mucosa Lines the lumen of GI tract. Consists of simple columnar epithelium.

Mucosa Lamina propria: Thin layer of connective tissue containing lymph nodules.

Mucosa Muscularis mucosae: Thin layer of smooth muscle responsible for the folds. Folds increase surface area for absorption.

Mucosa Goblet cells: Secrete mucus.

Submucosa Thick, highly vascular layer of connective tissue. Absorbed molecules enter the blood and lymphatic vessels. Submucosal plexus (Meissner’s plexus): Provide autonomic nerve supply to the muscularis mucosae.

Muscularis Responsible for segmental contractions and peristaltic movement through the GI tract. Inner circular layer of smooth muscle. Outer longitudinal layer of smooth muscle. Contractions of these layers move food through the tract; pulverize and mix the food.

Muscularis Myenteric plexus located between the 2 muscle layers. Major nerve supply to GI tract. Fibers and ganglia from both sympathetic and parasympathetic nervous systems.

Serosa Binding and protective outer layer. Consists of areolar connective tissue covered with simple squamous epithelium.

Regulation of the GI Tract Extrinsic innervation: Parasympathetic nervous system: Vagus and spinal nerves: Stimulate motility and GI secretions. Sympathetic nervous system: Postganglionic sympathetic fibers that pass through submucosal and myenteric plexuses and innervate GI tract: Reduce peristalsis and secretory activity.

Regulation of the GI Tract Enteric nervous system: Sites where parasympathetic fibers synapse with postganglionic neurons that innervate smooth muscle. Submucosal and myenteric plexuses: Local regulation of the GI tract. Paracrine secretion: Molecules acting locally. Hormonal secretion: Secreted by the mucosa.

The Oral Cavity Oral cavity: mechanical, chemical digestion What are the Functions of the Oral Cavity? Mechanical processing using teeth, tongue, and palate Lubrication of food by mucus in saliva Enzymatic digestion by enzymes in saliva Sensory analysis (taste, texture)

The Oral Cavity What are the Functions of the Tongue? Mechanical processing of food Manipulation to assist chewing and swallowing Sensory analysis (taste, texture)

The Oral Cavity What are the Salivary Glands? Three pairs of glands Parotid Sublingual Submandibular Salivary glands: saliva lubricates food Saliva = mucus, salivary amylase (starch breakdown)

Teeth Participate in mastication of food

Oral phase is voluntary. From Mouth to Stomach Mastication (chewing): Mixes food with saliva which contains salivary amylase. Enzyme that can catalyze the partial digestion of starch. Oral phase Nasopharynx Soft palate Bolus Epiglottis Trachea What is Swallowing (Deglutition)—Three Steps? Oral phase Oral phase is voluntary.

Pharyngeal and esophageal phases are involuntary. What is Swallowing (Deglutition)—Three Steps? Oral phase Pharyngeal phase Larynx is raised. Epiglottis covers the entrance to respiratory tract. Oral phase Pharyngeal Tongue Nasopharynx Soft palate Bolus Epiglottis Trachea Pharyngeal and esophageal phases are involuntary. Cannot be stopped.

What is Swallowing (Deglutition)—Three Steps? Oral phase Pharyngeal phase Larynx is raised. Epiglottis covers the entrance to respiratory tract. Esophageal phase Peristalsis pushes bolus toward stomach Oral phase Pharyngeal Esophageal Peristalsis Esophagus Tongue Nasopharynx Soft palate Bolus Epiglottis Trachea

Connects pharynx to the stomach. Upper third contains skeletal muscle. Involuntary muscular contractions and relaxations in the mouth, pharynx, larynx, and esophagus are coordinated by the swallowing center in the medulla. Thoracic cavity Oral phase Pharyngeal Esophageal Bolus enters stomach Lower esophageal sphincter Stomach Diaphragm Peristalsis Esophagus Tongue Nasopharynx Soft palate Bolus Epiglottis Trachea Esophagus: Connects pharynx to the stomach. Upper third contains skeletal muscle. Middle third contains a mixture of skeletal and smooth muscle. Terminal portion contains only smooth muscle.

Esophagus Peristalsis: Produced by a series of localized reflexes in response to distention of wall by bolus. Wave-like muscular contractions: Circular smooth muscle contract behind, relaxes in front of the bolus. Followed by longitudinal contraction (shortening) of smooth muscle. Rate of 2-4 cm/sec. After food passes into stomach, LES constricts. Esophagus Insert 18.4a

Stomach Most distensible part of GI tract. Empties into the duodenum.

The Stomach What are the Functions of the Stomach? Temporary storage of ingested food Mechanical breakdown of food Forms chyme, a liquid suspension Breakage of chemical bonds in food by action of acid and enzymes Initiates digestion of proteins. Kills bacteria. Moves food (chyme) into intestine.

What are the Regions of the Stomach? Cardia Closest to heart Fundus “Hump” on top Body Shaped like the letter “C” Pylorus Pyloric sphincter guards the exit

There are three components of gastric motility: (1) relaxation of the orad region of the stomach to receive the food bolus from the esophagus, (2) contractions that reduce the size of the bolus and mix it with gastric secretions to initiate digestion, and (3) gastric emptying that propels chyme into the small intestine. The rate of delivery of chyme to the small intestine is hormonally regulated to ensure adequate time for digestion and absorption of nutrients in the small intestine.

Stomach Gastric mucosa has gastric pits in the folds. Cells that line the folds deeper in the mucosa, are gastric glands. Insert fig. 18.7

Stomach Gastric juice: converts meal to acidic chyme HCl: pH 2, kills bacteria, denatures proteins Pepsin: enzyme breaks down proteins Mucus = protects lining of stomach

The Stomach What are the Secretions of the Gastric Glands? Parietal cells Secrete HCl (strong acid), intrinsic factor Chief cells Produce pepsinogen, an inactive enzyme HCl activates pepsinogen to pepsin Goblet cells Produce mucus

Gastric Hormone secretion Enterochromaffin-like cells (ECL): histamine and serotonin. G cells: gastrin. D cells: somatostatin. Stomach: ghrelin.

HCl Production carbonic acid 1 step In intracellular fluid, carbon dioxide (CO2) produced from aerobic metabolism combines with H2O to form H2CO3, catalyzed by carbonic anhydrase. H2CO3 dissociates into H+ and HCO3-. The H+ is secreted with Cl- into the lumen of the stomach, and the HCO3- is absorbed into the blood, as described in steps 2 and 3, respectively.

HCl Production  hydrogen potassium ATPase 2 step At the apical membrane, H+ is secreted into the lumen of the stomach via the H+-K+ ATPase. The H+-K+ ATPase is a primary active process that transports H+ and K+ against their electrochemical gradients (uphill). H+-K+ ATPase is inhibited by the drug omeprazole View drug information, which is used in the treatment of ulcers to reduce H+ secretion. Cl- follows H+ into the lumen by diffusing through Cl- channels in the apical membrane.

HCl Production 3 step At the basolateral membrane, HCO3- is absorbed from the cell into the blood via a Cl--HCO3- exchanger. The absorbed HCO3- is responsible for the "alkaline tide" (high pH) that can be observed in gastric venous blood after a meal. Eventually, this HCO3- will be secreted back into the gastrointestinal tract in pancreatic secretions.

HCl Production hydrochloric acid 4 step In combination, the events occurring at the apical and basolateral membranes of gastric parietal cells result in net secretion of HCl and net absorption of HCO3-.

Three substances stimulate H+ secretion by gastric parietal cells: ACh (a neurocrine), histamine (a paracrine), gastrin (a hormone).

Activates pepsinogen to pepsin. Pepsin is more active at pH of 2.0. HCl Functions Activates pepsinogen to pepsin. Pepsin is more active at pH of 2.0.

Makes gastric juice very acidic. HCl Functions Makes gastric juice very acidic. Denatures ingested proteins (alter tertiary structure) so become more digestible. Insert fig. 18.9

Digestion and Absorption in the Stomach Proteins partially digested by pepsin. For a variable period of time after food arrives in the stomach, starch continues the digestion that began with salivary amylase. Alcohol and aspirin are the only commonly ingested substances absorbed.

Regulation of Gastric Function Gastric motility and secretion are automatic. Waves of contraction are initiated spontaneously by pacesetter cells. Extrinsic control of gastric function is divided into 3 phases: Cephalic phase. Gastric phase. Intestinal phase.

The Phases of Gastric Secretion Stimulated by sight, smell, and taste of food. Activation of vagus: Stimulates chief cells to secrete pepsinogen. Directly stimulates G cells to secrete gastrin. Directly stimulates ECL cells to secrete histamine. Indirectly stimulates parietal cells to secrete HCl. Continues into the 1st 30 min. of a meal.

The Phases of Gastric Secretion Arrival of food in stomach stimulates the gastric phase. Gastric secretion stimulated by: Distension. Chemical nature of chyme (amino acids and short polypeptides). Stimulates G cells to secrete gastrin. Stimulates chief cells to secrete pepsinogen. Stimulates ECL cells to secrete histamine. Histamine stimulates secretin of HCl. Positive feedback effect. As more HCl and pepsinogen are secreted, more polypeptides and amino acids are released.

Gastric Phase (continued) Secretion of HCl is also regulated by a negative feedback effect: HCl secretion decreases if pH < 2.5. At pH of 1.0, gastrin secretion ceases. D cells stimulate secretion of somatostatin. Paracrine regulator to inhibit secretion of gastrin. Insert. Fig. 18.30

The Phases of Gastric Secretion Inhibits gastric activity when chyme enters the small intestine. Arrival of chyme increases osmolality and distension. Activates sensory neurons of vagus and produces an inhibitory neural reflex: Inhibits gastric motility and secretion. In the presence of fat, enterogasterone inhibits gastric motility and secretion. Hormone secretion: Inhibit gastric activity: Somatostatin, CCK, and GLP-1.