Anatomy and Physiology 2- Digestive System

Digestive Tract

• Also called gastrointestinal (GI) tract or alimentary canal

• Is a muscular tube that extends from oral cavity to anus

• Passes through pharynx, esophagus, stomach, and small and large intestines

Digestive System accessory organs

Teeth, tongue, various glands, liver, pancreas

Peristalsis

Smooth muscle contraction to move the food bolus forward

Oral cavity, teeth, and tongue function

Mechanical breakdown, lubricating, and mixing of food with saliva

Pharynx function

Muscular pushing of food down into the esophagus

Esophagus function

Movement of food to the stomach

Stomach function

Chemical breakdown of food by acid and enzymes; mechanical breakdown through muscular contractions

Small intestine function

Chemical digestion (enzymes) and absorption of water, organic substrates, vitamins, ions

Large intestine function

Bacterial digestion and absorption of water, organic substrates, vitamins, and ions

Salivary glands function

Secretion of fluid that lubricates and contains enzymes that break down carbohydrates

Liver function

Secretion of bile (for lipid digestion), storage of nutrients, and various vital functions

Gall bladder function

Storage, concentration, and secretion of bile

Pancreas function

Exocrine cells secrete buffers and enzymes; endocrine cells secrete hormones

Ingestion

Occurs when materials enter digestive tract via the mouth

Propulsion

Smooth muscle contraction - peristalsis

Mechanical processing

• Crushing and shearing

• Makes materials easier to propel along digestive tract

• Increases surface area

Digestion

• The chemical breakdown of food into small organic molecules that can be absorbed by digestive epithelium

• Hydrolysis reactions are aided by digestive enzymes

Secretion

Is the release of water, acids, enzymes, buffers, and salts by the epithelium of digestive tract and by glandular organs

Absorption

• Movement of organic substrates, electrolytes, vitamins, and water

• Across digestive epithelium

• Into interstitial fluid of digestive tract

Excretion

• Removal of waste products from body fluids

• Process called defecation removes feces

Protection

Mucosal enzymes, IgA, MALT, Peyer’s Patch, defensive rejections

Functions of the Tongue

• Speech

• Manipulation of food to assist in chewing and to prepare material for swallowing

• Sensory analysis of texture, temperature, and taste of food

Functions of the Oral Cavity

Sensory analysis

• Of material before swallowing

Mastication (mechanical processing)

• Through actions of teeth, tongue, and palatal surfaces

Lubrication

• Mixing with mucus and salivary gland secretions

Limited digestion

• Of carbohydrates and lipids

Three pairs of salivary glands

• Parotid salivary glands

• Sublingual salivary glands

• Submandibular salivary glands

• Each pair has distinctive cellular organization

• Each pair produces saliva with different properties

Saliva production locations and contents

• Glands produce 1.0–1.5 liters of saliva each day

• 70% by submandibular glands

• 25% by parotids

• 5% by sublingual glands

• Contents:

• 99.4% water

• 0.6% solutes: Electrolytes, Buffers, mucins, Antibodies, Enzymes

Functions of Saliva

Flushing oral surfaces to protect from bacteria buildup

• IgA, lysozyme control bacteria populations

Moistening and lubricating materials in the mouth

• Mucins: glycoproteins that give saliva lubricating property

Liberates taste chemicals that stimulate taste buds

Initiationg digestion of carbohydrates (salivary amylase)

• Polysaccharides to di and tri saccharides

• Deactivated in acid pH of stomach

Secretion of lingual lipase

• Begins lipid digestion

• pH 3-6

The Pharynx (Throat) purpose and muscle functions

-Passageway for food, liquids, and air

-Food passes through the oropharynx and laryngopharynx to the esophagus

-Pharynx has muscles that:

• Push bolus of food towards the esophagus

• Elevate larynx to force epiglottis over glottis

• Elevate soft palate and pharyngeal wall to protect nasal cavity

• At rest: bottom portion remains constricted to prevents air from entering the esophagus and stomach (upper esophageal sphincter)

Esophagus

Muscular tube that brings food and liquid to the stomach

Histology of the esophagus

Mucosa consists of stratified squamous epithelium

Esophageal glands

• Located in the submucosa

• Secretes mucous onto the mucosa

• Reduces friction between bolus and esophageal lining

Muscularis externa

• Peristalsis - smooth rhythmic contraction of the esophagus and rest of GI tract

• Lower esophageal sphincter (LES)

Stomach functions

• Storage of ingested food

• Mechanical breakdown of ingested food to produce semi- liquid Chyme

• Chemical breakdown of food

• Disruption of chemical bonds by acid

• Chemical digestion of proteins

• Production of intrinsic factor

Intrinsic factor

A glycoprotein required for absorption of vitamin B12 in ilium of the small intestine

Histology of the stomach

Simple columnar epithelium forms mucosa of stomach

Gastric pits of stomach

• Gastric pits are shallow depressions that open onto the gastric surface

• Each gastric pit communicates with several gastric glands

• Contain cells that make gastric secretions

• Mucous cells produce mucous that covers interior surface

• Mucous neck cells (in gastric pits)

• Protects epithelium from acidic chyme and digestive enzymes

Gastric glands of stomach

• Secrete 1500 mL of gastric juice / day

• Dominated by 4 types of cells

• Parietal cells

• Chief cells

• Goblet cells

• G-Cells

All stimulated by: Ach (PNS), histamine, and gastrin

Parietal cells

Secrete intrinsic factor and hydrochloric acid (HCl)

Chief cells

Secrete pepsinogen (inactive proenzyme) and gastric lipase

Goblet cells

Mucous producing

G-Cells

Secrete Gastrin (stimulates gastric secretion, intestinal motility, relaxation of ileocecal valve)

Acidic environment of stomach

• Kills microbes that are ingested

• Breaks down plant cell walls and animal connective tissues

• Activates pepsinogen to pepsin

• Stomach performs preliminary digestion of proteins by pepsin (proteins into smaller peptides)

• Lingual lipase remains active and gastric lipase digests lipids

• Salivary amylase is deactivated

• Although digestion occurs in the stomach, nutrients are not absorbed there

The Duodenum

• The segment of small intestine closest to stomach

• 25 cm (10 in.) long

• “Mixing bowl” that receives chyme from stomach and digestive secretions from pancreas and bile from liver

The Duodenum functions

• To receive chyme from stomach

• To neutralize acids before they can damage the absorptive surfaces of the small intestine

• NaHCO3 is released by the pancreas into the duodenum

The Jejunum

• Is the middle segment of small intestine

• 2.5 meters (8.2 ft) long

• Is the location of most chemical digestion and nutrient absorption

The Ileum

• The final segment of small intestine

• 3.5 meters (11.48 ft) long

• Ends at the ileocecal valve

Ileocecal valve

• Controls flow of material from the ileum into the cecum

• Prevents feces from backing up into ileum

Muscosa

• Consists of simple columnar epithelium (absorptive cells)

• Folded into Villi to increase surface area

• Lacteal extends up the center of each villus

• Mucous (Goblet) cells between columnar epithelial cells

• Brunner glands in duodenum: submucosal glands

• Enteroendocrine cells release hormones (Gastrin, Secretin, CCK)

Intestinal glands

• Depressions at bases of villi

• Secrete 1-2 L intestinal juices

• Dividing cells contain brush border enzymes

Brush border enzymes function

• Integral membrane proteins

• Located on microvilli on the surface of absorptive cells (the brush border)

• Break down materials in contact with brush border

• Break down materials in central lumen after cells are shed from villus

Small Intestine Movements

• Segmentation contractions mix chyme with digestive secretions

• Weak peristaltic contractions move it slowly toward jejunum

Gastroenteric (enterogastric) Reflex

• Stimulates motility along small intestine

• Initially, Enteroendocrine cells release gastrin, secretin, CCK

• Inhibits gastrin secretion, causes pyloric sphincter contraction

The Gastroileal Reflex

Gastrin: Relaxation of ileocecal valve allowing materials to pass from small intestine into large intestine

Pancreas location

• Lies posterior to stomach

• Starts at curve of duodenum, goes toward spleen

Pancreas functions

• Endocrine: Cells of the pancreatic islets secrete hormones into bloodstream

• Exocrine: Acinar cells and epithelial cells of duct system secrete pancreatic juice into lumen of duodenum

Exocrine function of the Pancreas

• Secretes 1000 mL (1 L) pancreatic juice per day

• Contains pancreatic enzymes

• Pancreatic alpha-amylase: Starches to di/trisaccharides

• Pancreatic lipase: Triglycerides to fatty acids and monoglycerides that are easily absorbed

• Nucleases: RNA or DNA to nucleotides

• Proteolytic enzymes: Proteins to peptides and amino acids

• Secreted as inactive proenzymes

• Activated after reaching small intestine

Liver functions

• Is the largest glandular organ (1.5 kg; 3.3 lb)

• Performs essential metabolic and hematologic functions

• Carb, lipid, and amino acid metabolism

• Waste product removal

• Drug inactivation and disposal

• Phagocytosis and antigen presentation

• Synthesis of plasma proteins

• Removal of circulating hormones, antibodies, toxins

• Digestive function:

• Secretes bile

• Bile is stored and concentrated by the gallbladder

Bile

• Bile has bile salts that aid in lipid digestion

• Dietary lipids are not water soluble

• Mechanical processing in stomach creates large drops containing lipids

• Pancreatic lipase is not lipid soluble

• Interacts only at surface of lipid drop

• Bile salts break drops apart (emulsification)

• Increases surface area exposed to enzymatic attack

• Creates tiny droplets coated with bile salts

• Keeps droplets in solution

• Aids in absorption by intestinal epithelia

Intestinal absorption: monosaccharides and amino acids

Enter absorptive cells by facilitated diffusion and cotransport

Intestinal absorption: lipids

• Fatty acids diffuse into absorptive cells

• Re-formed into triglycerides and packaged into chylomicrons inside intestinal cells

• Chylomicrons undergo exocytosis then enter lacteal

Intestinal absorption: vitamins

• Most enter cells by diffusion, fat soluble enter with fatty acids

• Vitamin B12 must bind to intrinsic factor before active transport can occur

Intestinal absorption: water

Flows along osmotic gradient (Not active)

Functions of the large intestine

• Reabsorption of water

• Compaction of intestinal contents into feces

• Absorption of important vitamins produced by intestinal bacteria

• Storage of fecal material prior to defecation

Histology of the large intestine

• Lacks villi

• Presence of distinctive intestinal glands

• Deeper than glands of small intestine

• Do NOT produce digestive enzymes

• Dominated by mucous cells

• Mucous provides lubrication as fecal material becomes drier

• The longitudinal layer of the muscularis externa is reduced to the muscular bands of taeniae coli

Reabsorption: Large Intestine

• Large Intestine: Less than 10% of nutrient absorption

• Reabsorption of water

• Reabsorption of bile salts

• In the cecum

• Transported in blood to liver for recycling

• Absorption of vitamins produced by bacteria

• Vitamin K

• Vitamin B5

• Biotin

• Absorption of organic wastes (urobilinogens to be excreted by kidneys)

Gastroileal and gastroenteric reflexes

Move materials into the cecum while you eat

Movement from cecum to transverse colon

Is very slow, allowing hours for water absorption

Peristaltic waves

Move material forward along length of colon

Segmentation movements (Haustral Churning)

Mix contents of adjacent haustra

Movements of large intestine

• Movement from transverse colon through rest of large intestine results from powerful peristaltic contractions (mass movements)

• Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexuses

• When contents get to rectum, distension of the rectal wall triggers defecation reflex

Elimination of feces

• Requires relaxation of internal and external anal sphincters

• Reflexes open internal sphincter, close external sphincter

• Opening external sphincter requires conscious effort

Carbohydrate absorption

Lipid absorption

Protein absorption