Digestive System

• The digestive and hepatobiliary systems are responsible for breaking down food so that the body can absorb nutrients for energy, growth, and repair.

Divisions of Digestive System

• The alimentary tube/canal includes the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine
• Accessory organs that aid in digestion include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Types of Digestion

• Mechanical digestion is the physical breakdown of food into smaller pieces
• Smaller pieces have more surface area exposed for action of digestive enzymes
• Chemical digestion breaks down complex molecules into simpler chemicals the body can use via digestive enzymes
• The body breaks down 3 types of complex organic molecules from food: carbohydrates, proteins, and fats

Alimentary Tube: Oral Cavity

• The oral cavity is the initial phase of mechanical breakdown of food through mastication (chewing) by teeth.
• Mastication by the teeth (chewing) is part of the initial phase of mechanical breakdown of food
• The tongue keeps food between teeth and mixes it with saliva, increasing the efficiency of chewing
• Tongue movements during chewing are reflexive
• Volitional elevation of the tongue is the first step in swallowing.
• Initial chemical digestion begins in the oral cavity via salivary amylase, which starts the chemical breakdown of carbohydrates.

Salivary Glands

• Parotid, submandibular, and sublingual glands secrete saliva, which is made from blood plasma
• Saliva contains lysozymes that inhibit bacterial growth
• Saliva also contains chemicals found in plasma
• Salivary secretion is continuous, but varies in amount, and is impacted by ANS activation

Alimentary Tube: Pharynx

• The pharynx serves as a common channel for swallowing (deglutition) and ventilation
• Constrictor muscles of the pharynx contract reflexively when the tongue pushes the food bolus backward
• The reflex center for swallowing is located in the medulla and coordinates constriction of the pharynx, cessation of breathing, elevation of the soft palate to block the nasopharynx, elevation of the larynx and closure of the epiglottis, and peristalsis of the esophagus.

Alimentary Tube: Esophagus

• The esophagus is a muscular tube connecting the pharynx to the stomach, and is composed of smooth muscle
• Peristalsis moves food down the esophagus
• The lower esophageal sphincter is non-volitional, made of smooth muscle
• The lower esophageal sphincter relaxes to allow food to enter the stomach, and constricts to prevent backflow of stomach contents

Alimentary Tube Wall Layers

• Layers of most structures in the alimentary tube in innermost to outermost order are as follow:
  • Mucosa
  • Submucosa
  • External muscle layer
  • Serosa

Mucosa

• The epithelium tissue that makes up the mucosa secrets mucus to lubricate food passage.
• In the stomach and small intestine, the mucosa layer releases digestive enzymes
• The mucosa layer is made up of areolar connective tissue, which contains lymph nodules and macrophages
• It has smooth muscle that creates folds in the mucosa to increase surface area in the stomach and small intestine.

Submucosa

• The submucosa is composed of areolar CT, blood vessels and lymphatic vessels
• The submucosa contains Meissner's plexus (submucosal plexus), a network of nerve fibers that is part of the enteric nervous system
  • The plexus innervates the mucosa to regulate secretions
  • ENS can function independent of the CNS, but is influenced by the ANS
• Sensory neurons from smooth muscle, and motor neurons to blood vessels are in the submucosa
• The submucosa regulates vessel diameter and blood flow

External Muscle Layer

• The external muscle layer helps further break up food and mix it with digestive enzymes
• One-way contractions of peristalsis move food through the tube
• Auerbach's plexus (or myenteric plexus) is the portion of the ENS in the external muscle layer
  • This receives autonomic impulses that regulate contractions

Serosa

• The layers of the serosa are fibrous connective tissues in structures above the diaphragm, such as the esophagus
• Mesentery or visceral peritoneum are in structures below the diaphragm
• Serosa contains serous membrane

Peritoneum

• Parietal peritoneum lines the abdominal cavity
• The peritoneum-mesentery is one continuous membrane
• Serous fluid between the peritoneum & mesentery prevents friction when the alimentary tube contracts & organs slide against one another

Alimentary Tube: Stomach

• The stomach is an expansible muscular sac that serves as a reservoir for food and fluid
• The stomach has three smooth muscle layers and performs both mechanical and chemical digestion, including the initial digestion of proteins
• The fundus and body are mainly storage areas, while most digestion occurs in the pylorus
• The mucosa is wrinkled or folded when stomach is empty, and folds are called rugae
• The stomach flattens out as it is filled, permitting expansion of lining without tearing

Stomach Cells and Secretions

• Gastric juice is the collective secretions of the stomach, produced by various cells within "gastric pits" of the stomach
  • Mucous cells secrete mucus, which coats the stomach lining to help prevent erosion by the acidic gastric juice
  • Chief cells secrete pepsinogen, which is an inactive form of the enzyme pepsin
  • Parietal cells produce hydrochloric acid (HCl) and secrete intrinsic factor
  • G cells secrete the hormone gastrin

HCl and Digestion

• HCl gives gastric juice its very acidic pH (1-2) and kills most microorganisms that enter the stomach
• It converts pepsinogen to pepsin, which begins the digestion of proteins to polypeptides
• Intrinsic factor is essential for absorption of vitamin B12 in the ileum
• Gastrin stimulates the secretion of greater amounts of gastric juice
• Mechanical digestion changes food into chyme, which is a thick liquid

The Pyloric Sphincter

• The Pyloric sphincter is usually contracted as stomach churns food
• It relaxes at intervals to permit small amounts of chyme to enter the duodenum, and then contracts again to prevent backflow of intestinal contents into the stomach

Small Intestine

• The small intestine is ~ 1 inch in diameter & 20 ft long
• Digestion is completed in the small intestine: Duodenum jejunum ileum
• Nutrient absorption is the major function of the small intestine
  • Villi are folds of the mucosa
  • Microvilli are folds of the cell membranes that increase surface area for absorption

Small Intestine: Mucosa

• The mucosa of the small intestine is made of simple columnar epithelium cells with microvilli
  • Goblet cells secrete mucus
  • Enteroendocrine cells secrete hormones stimulated by good entering the duodenum
• Peyer's patches (lymph nodules) are abundant in the small intestine

Small Intestine: Absorption

• The small intestine has a large surface area for absorption, which is created by folds in the mucosa, submucosa, and microvilli of cell surfaces
• Within each villus is a capillary network and lacteal (lymphatic vessel)
• Absorption of nutrients takes place from intestine into vessels within villi
• Water-soluble nutrients are absorbed into blood in the capillary networks
• Monosaccharides, a.a., (+) ions, & water-soluble vitamins are absorbed by active transport
• (-) ions may be absorbed passively or by active transport
• Some nutrients need additional substances for their absorption
  • Vitamin B12 requires intrinsic factor
  • Efficient absorption of Ca requires parathyroid hormone & vitamin D
• Water is absorbed by osmosis following absorption of minerals, especially Na+
• Blood from capillary networks in the villi passes through the portal vein to the liver first
  • This enables the liver to regulate blood levels of glucose and amino acids, store certain vitamins, and remove potential toxins from blood
• Fat-soluble nutrients are absorbed into the lymph via lacteals
  • Bile salts are needed for efficient absorption of fatty acids & fat-soluble vitamins (A, D, E, & K)
• Once absorbed, fatty acids are recombined with glycerol to form triglycerides
  • Triglycerides form globules that include cholesterol & protein
• Most absorbed fat is transported in form of chylomicrons by lymph & eventually enters the blood

Liver

• The liver is the "metabolic factory" of the body, and the liver lobule is its structural unit
• Liver has columns of liver cells (hepatocytes) that store nutrients and play a role in carbohydrate, protein, and fat metabolism
• Branches of the hepatic artery and portal vein run between adjacent lobules
• Capillaries of a lobule are sinusoids, and they are very permeable vessels between hepatocytes
• They receive blood from both hepatic artery and portal vein
• The hepatic artery brings oxygenated blood and the portal vein brings blood from digestive organs & spleen
• Each lobule has a central vein, and they unite to form hepatic veins that bring blood out of liver to inferior vena cava
• Only digestive function → production of bile
• Bile is mostly water and carries bilirubin and excess cholesterol to intestines to be eliminated
• Function
  • Bile salts emulsify fats in the small intestine
  • Secretin: hormone that stimulates bile production
  • Produced by duodenum when food enters the small intestine

Liver: Non-Digestive Functions

• Carbohydrate metabolism
• Regulation of blood glucose level
• Metabolism of a.a
• Regulates blood levels of a.a. for protein synthesis
• Lipid metabolism
• Forms lipoproteins for transport of fats in blood
• Synthesizes cholesterol & excretes excess into bile
• Storage
• Vitamins A, D, E, K, & B12
• Iron
• Copper
• Plasma protein synthesis
• Production of clotting factors
• Phagocytosis
• Breaks down old RBCs
• Engulfs pathogens
• Bilirubin formation
• Detoxification
• Produces enzymes to neutralize substances like medications, and alcohol
• Ammonia (produced by bacteria in the colon)

Gallbladder

• Functions to concentrate and store bile
• Cholecystokinin is secreted by duodenal cells when fatty foods enter duodenum
  • Stimulates contraction of the gallbladder
  • Forces bile into cystic duct common bile duct duodenum

Pancreas

• Exocrine functions:
  • Produces digestive enzymes for breakdown of carbs, proteins, and fats
  • Pancreatic amylase digests starch
  • Lipase converts emulsified fats to fatty acids and glycerol
  • Trypsinogen: inactive enzyme that converts to active trypsin in duodenum
    • Trypsin digests polypeptides to shorter chains of a.a. Pancreatic enzyme juice is carried to duodenum via main pancreatic duct, joining with common bile duct
• Pancreas produces bicarbonate juice (alkaline) to neutralize the gastric juice entering the duodenum
• Secretin and cholecystokinin stimulate secretion of pancreatic juices
  • Secretin stimulates production of bicarbonate juice
  • Cholecystokinin stimulates secretion of pancreatic enzymes

Digestive Enzymes

• Salivary glands secrete amylase to break down carbohydrates
• The liver produces bile to break down fats
• The Stomach produces pepsin and helps break down fats
• The Pancreas produces trypsin

Large Intestine(Colon)

• The large intestine extends from the ileum of the small intestine to the anus: cecum is the first portion.
• The ileocecal valve is a fold of intestinal mucosa surrounding the opening from ileum to cecum
  • It prevents backflow of fecal material into the small intestine
• Ascending, transverse, and descending portions encircle the small intestine
• Sigmoid colon leads to the rectum, and the anal canal
• No digestion takes place in the large intestine
  • Colonic mucosa secretes mucus to lubricate the passage of fecal material Functions include
• Absorption of water, minerals, & vitamins
  • ~ 80% of the water that enters the colon is absorbed
  • Positive & negative ions are absorbed
  • Vitamins produced by the normal flora
• Elimination of undigestible material
• Everything absorbed by colon circulates first to the liver by way of portal circulation

Elimination of Feces

• Feces include cellulose and other undigestible material, dead & living bacteria, and water
• The colon moves feces along via peristalsis
• Gastrocolic reflex: presence of food in stomach & peristalsis causes colon to contract
• Duodenocolic reflex: presence of chyme & peristalsis of duodenum causes colon to contract
• Defecation reflex is a spinal cord reflex that can be controlled volitionally
• Stretching in the wall of the rectum by the entry of feces stimulates reflex
  • Smooth muscles of the rectum contract
• Internal anal sphincter relaxes, which permits defecation to occur
• The External anal sphincter: is skeletal muscle under volitional control that can be contracted to prevent defecation