A+P Ch. 25: Digestive System

Swallowing (Deglutition): 1. Voluntary Phase

Bolus of food is consciously moved to the back of the oral cavity by the tongue.

Peritoneum

Largest serous membrane of the body. 3 layers: - Parietal. - Visceral. - Retroperitoneal organs.

Swallowing (Deglutition)

Series of muscular contractions coordinated by brain. 3 phases: voluntary, pharyngeal and esophageal.

Layers of GI Tract: 4. Serosa

Superficial layer. Areolar tissue.

Absorption in the Stomach

Very few substances. Water, ions, short-chain fatty acids, some drugs like aspirin & alcohol.

Chemical Digestion of the Stomach

*Salivary enzymes: Even before fully mixed with gastric juice, carbohydrates continue to be digested by salivary amylase. Once the acid gastric juice mixes with the chyme, salivary amylase in inactivated and lingual lipase is activated, digesting fats. *Acid: Kills many microbes and denatures, or unfolds proteins from food. *Pepsin: Secreted by chief cells and hydrolytically breaks peptide bonds in proteins. Functions at very acidic pH and is inactive at more basic pH. *Gastric Lipase: Also secreted by chief cells and breaks the chemical bonds between fatty acids and glycerol in fats to produce free fatty acids and monoglycerides.

Accessory Digestive Organs

- Teeth. - Tongue. - Salivary glands. - Liver. - Gallbladder. - Pancreas.

Large Intestine: Colon

1. Ascending (right side of abd, turns left @ liver). 2. Transverse (crosses the abd from right to left). 3. Descending (turns downward at the spleen and heads south to the sigmoid). 4. Sigmoid (S-shaped portion that terminates at the rectum. 5. Rectum (about 20cm). 6. Anal canal (2-3 cm. Last).

Functions of Liver

1. Carbohydrate Metabolism: Maintaining normal blood glucose levels. When blood glucose is low, the liver breaks down stored glycogen. When blood glucose is high, the liver converts it to glycogen and fat for storage. 2. Lipid Metabolism: Store triglycerides, break down fatty acids for ATP production, synthesize cholesterol etc. 3. Protein Metabolism: Can use amino acids for ATP production. The ammonia removed from amino acids toxic and converted to urea, which is less toxic and secreted in urine. 4. Processing of drugs and hormones. 5. Excretion of bilirubin: Degraded heme from hemoglobin. Secreted in bile and eliminated via the intestines. 6. Synthesis of bile salts: made from cholesterol. 7. Storage: Glycogen, vits and minerals. 8. Phagocytosis: Taking up RBCs and recycling. Macrophages eat dead RBCs. 9. Activation of vit D: Participate in vitD synthesis, which is required for calcium metabolism and bone health.

Small Intestine: Surface Area

1. Circular Folds: (plicae circularis) up to 10mm tall. Involve only mucosa and submucosa. Chyme flows in spiral path causing more contact. Folds of mucos and submucosa. Increase the surface area of the small intestine. 2. Villi: Fingerlike projection 1mm tall. Contain blood vessels and lymphatics (lacteal). Nutrients are absorbed by the epithelium and so have blood and lymph vessels closely associated. 3. Microvilli: 1micron tall, cover surface. Brush border on cells. Brush border enzymes for final stages of digestion. Projections on individual absorptive cells. Increase the surface area for absorption.

Small Intestine: 3 Regions

1. Duodenum: Curves around head of pancreas (25 cm long). Receives stomach contents, pancreatic juice and bile. Neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase, pancreatic enzymes. 2. Jejunum: 1 meter long (in upper abd). Large tall circular folds; walls are thick; muscular. Most digestion and nutrient absorption occur here. 3. Ileum: Last 2 meters (lower abd), longest. Ends at ileocecal junction with large intestine.

6 Basic Processes of The Digestive System

1. Ingestion: Food intake. 2. Secretion: Water, acid, buffers enzymes. 3. Mixing and propulsion: Mobility of food and secretions. 4. Digestion: Breakdown of molecules (mechincal and chemical). 5. Absorption: Uptake nutrients into blood/lymph. 6. Defecation: Undigested material leaves body.

Mechanical Digestion in the Small Intestine

1. Mixed chyme with intestinal juice, bile and pancreatic juice. 2. Churns chyme to increase contact with mucosa for absorption and digestion. 3. Moves residue towards large intestine (segmentation and peristaltic waves begin in duodenum but each one moves further down. Pushed chyme down for 2 hours, suppressed by refilling of stomach).

Digestive Processes (3)

1. Motility: Muscular contractions that break up food, mix it with enzymes and move it along. 2. Secretion: Digestive enzymes and hormones. 3: Membrane transport: absorption of nutrients.

Composition & Function of Pancreatic Juice: 4 Classes of Enzymes

1. Pancreatic Amylase: Starch/Carbohydrate digestion. 2. Proteases: Protein digestion. 3. Pancreatic Lipase: Triglyceride digestion. Digests fats into fatty acids and monoglycerides. 4. Nucleases: DNA and RNA digestion.

Gallbladder & Bile

500-1000mL bile are secreted daily from liver. Gallbladder stores and concentrates bile: Bile backs up into gallbladder from a filled bile duct. Between meals, bile is concentrated by factor of 20. Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments and phospholipids. Composition: water, bile salts, cholesterol, etc. Bile Salts: Formed from cholesterol, bile salts are required for the emulsification of fats. Liver itself makes bile, gallbladder stores.

Composition of Saliva

99.5% water with 0.5% solutes. *Lysozymes is an important components in the solutes: kill bacteria. *Salivary amylase: Begins starch digestion. Breaks starch into smaller units, such as disaccharides, trisaccharides and short glucose polymers. Digestive enzyme that acts on start. *Lingual lipase: Digests fat. Activated by stomach acid. *Immunoglobin A: Inhibits bacterial growth. Specific immune response (parents chewing for baby). *Lytes: Na+, K+, Cl-, phosphate and bicarbonate. pH 6.8-7.

Large Intestine: Cecum

A small pouch after the ileocecal valve (sphincter).

The Gallbladder Anatomy

A small sac located in a depression on the posterior surface of the liver.

Teeth

AKA "dentes". Located in the sockets of the alveolar processes of the mandible and maxillae. *ONLY need to know it's used for mechanical digestion.

Mechanical Digestion

AKA Chewing/Mastication. Physical breakdown of food into smaller particles. Teeth and churning action of stomach and intestines. Bolus: Result of the initial digestive processes in the mouth. Soft, flexible, easily swallowed.

Mouth

AKA Oral/Buccal cavity. Formed by the cheeks, hard and soft palate and tongue. Salvary glands, Tongue, Teeth & Mechanical/Chemical Digestion in the mouth.

Chemical Digestion of the Stomach: Functions of Hydrochloric Acid

Activates pepsin and lingual lipase. Breaks up connective tissues and plant cell walls (liquefies food to form chyme). Converts indigested ferric ions to ferrous ions via acid in stomach. *Destroys ingested bacteria and pathogens.

Ducts of Gallbladder, Liver and Pancreas

All you need to know is secretions from gallbladder and pancreas end up in the duodenum.

Tongue

An accessory digestive organ. Skeletal muscle covered with mucous membrane. Together with its associated muscles, it forms the floor of the oral cavity. *Papillae: Small projection covering the dorsal surface of the tongue that contain taste buds (receptors for taste called gustation). *Lingual Glands: Secrete both mucus and lingual lipase (enzyme that acts on triglycerides aka fat).

Major Folds of The Peritoneum: 2. Mesentery

Binds the small intestine to the posterior abdominal wall. Holds many blood vessels. A lot of immune cells pratrolling/scanning.

Mouth Features

Cheeks and lips: Keeps food between teeth for chewing; essential for speech and suckling in infants. Vestibule: Space between teeth and cheeks. Lips: Cutaneous area vs. red area (vermillion). Tongue: Sensitive, muscular manipulator of food. Papillae and taste buds on dorsal surface. Lingual (big glands) glands secrete saliva, tonsils in root. Hard & Soft (closes off nasal cavity) palate: Allow breathing and chewing @ same time. Palatoglossal and palatopharyngeal arches.

Pancreas: Anatomy

Closely associated with the stomach, connected to the small intestine by 2 ducts. Pancreatic juices are produced in exocrine cells of the pancreas and secreted into the ducts. One of the ducts joins with the duct from the gall bladder and pancreatic juices are dumped into the small intestine with bile. Retroperitoneal gland posterior to stomach. Endocrine and exocrine gland: Secretes insulin and glucagon into the blood. Secretes 1500mL pancreatic juice into duodenum (water, enzymes, zymogens and sodium bicarb). Pancreatic duct runs length of gland to open at sphincter of Oddi (accessory duct opens independently on duodenum).

Stomach

Connects with the esophagus and distally to the duodenum (first part of the small intestine). Functions as a mixing chamber for food and digestive secretions, but also acts as a storage vessel so that relatively large amounts of food can be eaten in a short time. Digestion of starch continues, digestion of proteins and triglycerides begins, the semi-solid bolus is converted to liquid and some substances are absorbed. Muscular sac: internal volume from 50mL to 4L.

GI tract or alimentary canal

Continuous tube extending from mouth to anus. Organs include: - Mouth. - Most of pharynx. - Esophagus. - Stomach. - Small & large intestine. 9-20ft.

Visceral Peritoneum

Covers some of the organs in the abdominopelvic cavity.

Chemical Digestion in the Small Intestine

Digestion of carbohydrates. Digestion of proteins. Digestion of lipids.

Water Balance

Digestive tract receives about 9L/day. 0.7L in food, 1.6L in drink, 6.7L in secretions. 8L is absorbed by small intestine and 0.8L by large intestine. Water is absorbed by osmosis following the absorption of salts and organic nutrients. Most absorption in small intestine. Some in large intestine.

Segmentation in Small Intestine

Do not push material along the intestine like peristalsis but are localized, mixing contractions that mix chyme with digestive juice.

Bile Function

Emulsification: The breakdown of large lipid globules into smaller lipid droplets. Bile salts act like dish detergent. Remember, all digestion processes are acting in an aqueous environment: Emulsification allows these aqueous process to occur with hydrophobic substances (fats). Bile allows water to interact with fats.

Absorption in the Small Intestine: Absorption of Vitamins

Fat soluble vitamins (A, D, E and K) included with lipid movement into lymph and blood. Water soluble vitamins (Bs and C) absorbed by simple diffusion. Vitamins are absorbed unchanged (A, D, E and K with other lipids. B complex and C by simple diffusion and B12 if bound to intrinsic factor). Minerals are absorbed all along small intestine.

Chemical Digestion in the Small Intestine: Digestion of Lipids

Fats must be emulsified by bile salts before enzymatic digestion can occur. Triglycerides are broken down into fatty acids and monoglycerides by lipases. These compounds can be absorbed. In lumen of S.I. 1. Emulsification. 2. Fat hydrolysis. 3. Chylomicron Formation.

Layers of GI Tract

From the lower esophagus to anus have the same layer arrangement of tissues. (Inner to outer:) 1. Mucosa. 2. Submucosa. 3. Muscularis. 4. Serosa.

Peristalsis

Gradual movement of contents towards colon. Begins after absorption occurs. Migrating motor complex controls waves of contraction (second wave begins distal to where first wave began). Kneeding material inside.

Histology of the Stomach: Mucosa

Highly folded. Glands deep in folds. Simple columnar glandular epithelium (Always sloughing and growing.) Lamina propria is filled with tubular glands (gastric pits). Muscular externa has 3 layers: 1. Outer longitudinal. 2. Middle circular. 3. Inner oblique layers.

Layers of GI Tract: 1. Mucosa

Inner lining. 3 layers: 1. Epithelium: Protective function in mouth, pharynx, esophagus and anal canal. In stomach and intestines, functions in secretion and absorption. Epithelial cells of mucosa are sealed (via tight junctions). Enteroendorcrine cells: Scattered between the epithelial cells are secretory cells, produce and secerte hormones into blood. 2. Lamina Propria: Middle. Loose connective tissue. Contains many blood and lymphatic vessels and is the route nutrients absorb in the GI tract reach other tissues. 3. Muscularis Mucosae: Absorption of nutrients. Thin layer of smooth muscle and produces small folds in the wall of the stomach and small intestine, which increases the surface area of these tissues.

Chemical Digestion of the Stomach: Gastric Enzymes and Intrinsic Factor

Intrinsic Factor: Glycoprotein. Essential for B12 absorption by small intestine. B12 is essential for RBC production (lack causes pernicious anemia). Pepsin: Protein digestion. Secreted as pepsinogen (inactive). Hcl converts it to pepsin (active). Gastric Lipase: Lipse digests butterfat of milk in infant.

Chemical Digestion

Large molecules of carbohydrates, lipids, proteins and nucleic acids are broken down into small molecules by HYDROLYSIS through the action of digestive enzymes. Digestive enzymes produced by salivary glands, stomach, pancreas and small intestine. Some of the substances in food can be absorbed without digestion including water, vits, ions and cholesterol. 2 digestive enzymes secreted into the oral cavity, salivary amylase and lingual lipase contribute to chemical digestion in the mouth. Series of hydrolysis reactions that break macromolecules into their monomers. Enzymes from saliva, stomach, pancreas and intestines. Results: Polysaccharides into monosaccharides. Proteins into amino acids. Fats into glycerol and fatty acids.

Parietal Peritoneum

Lines the wall of the abdominopelvic cavity.

Functions of Saliva

Moisten, begin starch & fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus. Water dissolves some components of foods so they can be tasted. Chloride ions activate salivary amylase. Buffers act on acidic foods to maintain slightly acidic saliva. Some wastes are secreted via saliva including urea and uric acid. Mucus in saliva lubricates food so it can move easily, formed into a ball & swallowed.

Pancreas: Histology

Most of the cells of the pancreas of exocrine (secretions are released into the ducts). Only 1% of the cells are endocrinne (secretions are released into the bloodstream) cells that produce hormones such as insulin, glucagon, somatostatin. Zymogens = proteases (protein digesting enzymes). Trypsinogen, chymotrypsinogen, procarboxypeptidase. Other enzymes = amylase, lipase, ribonuclease and deoxyribonuclease.

Layers of GI Tract: 3. Muscularis

Mouth, pharynx, and superior & middle parts of esophagus contain skeletal muscle, which produce voluntary swallowing. Rest of GI tract consists of smooth muscle. Involuntary contractions of smooth muscle help break down food, mix with digestive secretions and propel it along the tract.

Histology of the Stomach

Mucosa. Glastric Glands. & Gastric Juice.

Pharynx

Naso, Oro and Laryngo. Skeletal muscle lined with mucous membrane.

Chemical Digestion in the Small Intestine: Digestion of Nucleic Acids

Nucleases break down DNA and RNA into nucleotides. They are further broken down by brush-border enzymes into sugars, phosphates and nitrogenous bases for absorption.

Chemical Digestion of the Stomach: Salivary Enzymes

Once in stomach, deactivates. Salivary amylase: Continues to digest carbohydrates until acid inactivates. Lingual Lipase: Fat-digesting enzyme. Activated by stomach acid to digest triglycerides in small intestine.

Swallowing (Deglutition): 2. Pharyngeal Phase

Once the bolus moves to the oropharynx, involuntary nervous control takes over. The nasopharynx is closed off by the soft palate and uvula, preventing food from entering the nasal cavity. At the same time, the epiglottis closes off the opening to the larynx, which prevents the bolus from entering the resp tract.

Swallowing (Deglutition): 3. Esophageal Phase

Once the bolus of food is in the esophagus, a progression of smooth muscle contracts, pushes the bolus towards the stomach. *Peristalsis: Coordinated contraction & relaxation of circular and longitudinal layers of muscularis, pushing the bolus forward.

Intestinal Juices Role

Pancreatic and intestinal juices provide a liquid medium that aids absorption of substances from chyme in the small intestine.

Mechical Digestion of the Stomach

Peristaltic movements called "mixing waves" pass over the stomach every 15-25 seconds after food enters. The waves grind the food, mix it with gastric juice and change it from a bolus to a soupy mix called CHYME. Swallowing center signals stomach to relax. Food stretches stomach activating a receptive-relaxation response. Rhythm of peristalsis controlled by pacemaker (back & forth shifting from esophagus to pyloric end & squirting 3ml).

Histology of the Small Intestine

Pores opening between villi lead to intestinal glands. - Absorptive Cells: Digests and absorb nutrients. - Goblet Cells: Secretes mucus. - Intestinal Glands: Secrete intestinal juice. - At base rapidly dividing cells. Lifespan on 3-6 days as migrate up to surface and get sloughed off and digested. Secrete 1-2 L of intestinal juice/day. Water and mucus, pH 7.4-7.8.

Chemical Digestion in the Small Intestine: Digestion of Proteins

Proteases break down polypeptides (proteins) into peptides (short sequences of amino acids). Brush-border peptidases break down the peptides into amino acids, which can be absorbed. Pepsin (stops working in S.I) has optimal pH of 1.5-3.5 - Inactivated when passes into duodenum and mixes with alkaline pancreatic juice (pH 8). Pancreatic enzymes take over protein digestion by hydrolyzing polypeptides into shorter oligopeptides. Brush border enzymes finish task, producing amino acids that are absorbed into intestinal epithelial cells (amino acid cotransporters move into epithelial cells and facilitated diffusion moves amino acids out into blood stream).

Mechanical Digestion in the Large Intestine: Haustral Churning

Relaxation and contraction of the haustra, which are pouches within the large intestine.

Salivary Glands

Secrete saliva into the oral cavity. Moistens and cleans the oral cavity and when food is present, more is produced to lubricate, dissolve and begin chemical digestion. Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate. 3 pairs extrinsic glands connected to oral cavity by ducts.

Absorption in the Small Intestine: Absorption of Lipids

Simple diffusion: No energy required. Flow down concentration gradient. Lipids dissolve in plasma membrane, so no special protein transporter required. Every time fatty acids and monoglycerides cross a membrane, they are reconstructed into triglycerides (the cytoplasm being aqueous doesn't like free fatty acids). Lipoproteins: For transport in the lymphatic system and then the blood, triglycerides must be associated with proteins and cholesterol. These structures are called lipoproteins.

Retroperitoneal Organs

Some organs lie of the posterior abd wall and are covered by peritoneum only on their anterior surfaces. Includes the kidneys and pancreas.

Chemical Digestion in the Small Intestine: Digestion of carbohydrates.

Starch broken down into smaller sugars by pancreatic amylase. Brush border enzymes further break down di and tri-saccharides into mono saccharides. The small intestine can absorb monosaccharides. Salivary amylase stops working in stomach (pH <4.5). 50% of dietary starch digested before it reaches small intestine. Pancreatic amylase completes first step in 10 mins. Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose and fructose. (lactose indigestible after age 4 in most humans).

Large Intestine

Starts @ cecum, sigmoid colon ends. The overall functions are completion of absorption, production of vitamins, formation of feces, expulsion of feces. 1.5 meters long and 6.5cm in diameter. Transit time 12-24 hrs (reabsorbs water and lytes). Feces consist of water and solids (bacteria, mucus, undigested fiber, fat and sloughed epithelial cells), haustral contractions occur every 30 minutes (distension of a hausttrum stimulates it to contract). Peristalsis is slower than upper parts of GI. 4 Major Regions: Cecum, Colon, Rectum and Anal Canal.

Esophagus

Straight muscular tube 25-30 cm long. Extends from pharynx to cardiac stomach passing through esophagus. Skeletal upper and smooth muscle bottom. Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm. Always closed and flattened unless food passes (trachea always open).

Absorption in the Small Intestine: Absorption of Monosaccharides

Sugars moved across the membrane of the absorptive cells by one of two mechanisms. Facilitated Diffusion: No energy required. Sugars flow down concentration gradient. Pass through specific channel proteins. Active Transport: Requires energy. Molecules are pumped. Specific transport proteins required.

Brush-Border Enzymes

Synthesized by the absorptive cells and inserted into the plasma membrane of the microvilli: Perform the final stages of digestion before absorption.

Pyloric Sphincter

The circular smooth muscle that controls the exit of food from the stomach to the duodenum. Distal end.

Histology of the Stomach: Gastric Glands

The epithelial cells of the mucosa extend down into the mucosa to form gastric glands. The glands have 4 cells types: 1. Mucous neck cells: Closest to lumen. Secrete mucus. 2. Parietal Cells: Secretes hydrochloric acid and instrinsic factor (needed for absorption of vit B12). 3. Chief Cells: Secretes 2 enzymes (Pepsinogen & Gastric Lipase in infancy). 4. Enteroendocrine Cells (G cells): Secretes the hormone gastrin into the bloodstream which stimulates gastric function.

Liver Anatomy

The heaviest gland in the body and second only to the skin in size. Located just below the diaphragm towards the right side. Divided into 2 principal lobes. 3lb organ located inferior to the diaphragm. 2 smaller lobes of the left lobe: quadrate and caudate. Gallbladder adheres to posterior surface between right and quadrate lGobes.

Major Folds of The Peritoneum: 1. Greater Omentum

The largest peritoneal fold, drapes over the transverse colon and coils of the small intestine. (source of beer belly)

Small intestine

The major part of digestion and absorption. Begins at the pyloric sphincter and ends at the opening to the large intestine. It's about 2.5cm in diameter and 3 meters long. Nearly all chemical digestion and nutrient absorption occurs in small intestine.

Absorption in the Small Intestine

The small molecules produced by digestion can be absorbed into blood or lymph. 1. Absorption on monosaccharides. 2. Absorption of amino acids, dipeptides, and tripeptides. 3. Absorption of Lipids. 4. Absorption of electrolytes: Transported into absorptive cells. 5. Absorption of vitamins.

Layers of GI Tract: 2. Submucosa

This layer binds mucosa to the muscularis. Composed of areolar connective tissue. Contains many blood and lymphatic vessels that receive the absorbed nutrients. In this layer: nerves, glands and lymphatic tissue.

Chemical Digestion in the Large Intestine

This stage is dependant upon the bacteria that reside in the lumen of the large intestine. They ferment remaining carbohydrates, convert proteins to amino acids. Many compounds metabolized by bacteria are absorbed. Synthesize vitamins B and K.

Composition & Function of Pancreatic Juice

Water. Salts. *Sodium Bicarb: Neutralizes the acidic chyme when it enters the duodenum. 4 classes of enzymes.