Digestion: Mechanical vs Chemical

Introduction

• Nutrients provide essential compounds to the body
• Digestion is the mechanical and chemical breakdown of food
• The gastrointestinal system (GI) facilitates both mechanical and chemical digestion

Mechanical and Chemical Digestion

• Mechanical digestion breaks food into smaller pieces without chemical changes
• Chemical digestion uses chemicals to break food into simpler compounds
• Both processes start in the pharynx

Alimentary Canal

• The alimentary canal begins at the mouth and terminates at the anus
• Organs included:
  • Mouth
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anus
• Accessory organs include:
  • Teeth
  • Tongue
  • Salivary Glands
  • Liver
  • Gallbladder
  • Pancreas
• Glandular organ secretions are composed of water, enzymes, buffers, etc.
• These secretions aid in nutrient absorption across the digestive tract's epithelium
• The small intestine in adults has a surface area of about 250 square meters

Alimentary Canal Structure

• The alimentary canal is 8 meters long
• Tube passes through thoracic/abdominopelvic cavities
• The wall of the alimentary canal has 4 layers specialized for particular functions:
  • Mucosa (Mucous Membrane)
  • Submucosa
  • Adventitia (Muscular Layer)
  • Serosa (Serous Layer)

Mucosa Layer

• The Mucosa contains: - Surface epithelium - Underlying connective tissue - Small amount of smooth muscle
• Folded regions increase absorptive surface
• The Mucosa carries out secretion and absorption

Submucosa Layer

• The Submucosa contains:
  • Loose connective tissue
  • Glands
  • Blood vessels
  • Lymphatic vessels
  • Nerves
• The Submucosa nourishes surrounding tissues and removes absorbed material

Adventitia Layer

• The Adventitia produces movement
• The Adventitia's two smooth muscle tissue coats are:
  • Circular fibers that encircle the tube to cause contraction
  • Longitudinal fibers to run lengthwise which shortens the tube

Serosa Layer

• The Serosa is composed of visceral peritoneum and connective tissue
• The Serosa protects underlying tissues
• The Serosa secretes serous fluid for abdominal organ movement

Digestive System Functions

• Functions include ingestion, mechanical processing, digestion, secretion, absorption, and excretion

Ingestion

• Ingestion happens as materials enter the digestive tract via the mouth

Mechanical Processing

• Mastication crushes and breaks materials down
• Mastication is performed by the teeth
• The stomach and intestines provide additional processing

Digestion

• The goal is chemical breakdown of food into small particles for absorption by the digestive epithelium

Secretion

• Secretion is the release of water, acids, buffers, enzymes, and salts
• Secretion is executed by the epithelium and glandular organs

Absorption

• Absorption is the movement of organic substrates, electrolytes, vitamins and water
• These substances move across the digestive tract epithelium to interstitial fluid

Excretion

• Excretion is the removal of waste products
• Waste leaves body fluids from secretions of the digestive tract and attached glandular organs
• Waste products become feces

Protection of the Digestive System

• The digestive system’s lining protects against:
  • Digestive acids/enzymes
  • Abrasion and other mechanical stress
  • Bacteria
• Bacteria in the digestive system may come from consumed food OR reside in the tract
• In lamina propria areas, immune cells attack bacteria

Digestive Material Movement

• Two basic motor functions occur in the alimentary canal:
  • Mixing movements
  • Propelling movements

Mixing Movements

• Mixing movements occur when smooth muscles contract rhythmically and mix food with digestive juices

Propelling Movements

• Segmentation involves alternating contraction and relaxation
• Peristalsis consists of wavelike movements that propel materials through the digestive tract

Abdominal Quadrants

• Abdominal quadrants are used to describe organ locations
• Two intersecting perpendicular lines form four quadrants
• The diaphragm forms top of the abdominal cavity and the the pelvis the bottom
• Quadrants include:
  • Right Upper (RUQ)
  • Left Upper (LUQ)
  • Right Lower (RLQ)
  • Left Lower (LLQ)

Abdominal Quadrants Continued

• RUQ contains liver, gallbladder, part of large intestine, right kidney
• LUQ contains stomach, spleen, pancreas, part of large intestine, left kidney
• RLQ contains appendix, part of large intestine, right ovary, right ureter, part of uterus and urinary bladder
• LLQ contains part of large intestine, left ovary, left ureter, part of uterus and urinary bladder

The Mouth

• Digestion begins in the mouth with mastication
• Mastication is chewing of food by teeth, which prepares it for further breakdown
• Secretions from salivary glands mix with food during mastication

The Tongue

• The tongue is a muscular organ that manipulates food in the mouth to assist in chewing
• The tongue provides sensory analysis through touch, temperature, and taste receptors

Saliva

• Saliva is secreted by the salivary glands
• Saliva moistens food and begins carbohydrate chemical digestion
• Saliva dissolves foods for tasting and helps to clean mouth
• Each salivary gland has secretory serous cells and mucous cells
  • Serous cells produce watery fluid containing salivary amylase to break down starches
  • Mucous cells secrete mucus to lubricate food particles

Regulation of Saliva

• Parasympathetic nerve impulses cause saliva secretion upon sensory input or thought of food
• Unappealing food inhibits parasympathetic activity

Salivary Glands

• The three pairs of salivary glands are:
  • Parotid glands
  • Submandibular glands
  • Sublingual glands

Parotid Glands

• Parotid glands are the largest glands
• The parotid glands are located anterior and slightly inferior to each ear
• The parotid glands produce a clear and watery fluid rich in amylase

Submandibular Glands

• Submandibular glands are located on the floor of the mouth on inner lower jaw
• Submandibular glands secrete a more viscous fluid than parotid glands

Sublingual Glands

• Sublingual glands are the smallest glands
• Sublingual glands lie on the floor of the mouth inferior to the tongue
• Sublingual glands produce thick and stringy secretions

Hollow Abdominal Organs

• Abdominal organs are classified as hollow or solid organs, which are tubular structures for foodstuff movement
• Hollow organs include the esophagus, stomach, small intestine, large intestine, and gallbladder

Esophagus

• An expandable, hollow tube with a thick, muscular wall
• Transports food/liquid from mouth to stomach
• The esophagus passes through the mediastinum and the diaphragm
• The esophagus is contiguous with the stomach
• Mucous glands in the submucosa moisten and lubricate the inner lining of the tube

Esophageal Sphincters

• Circular smooth muscle fibers thicken to form the lower esophageal sphincter
• Sphincters regulate movement into/out of the esophagus

Stomach

• The stomach is a J-shaped organ in the upper left abdominal cavity inferior to the diaphragm
• Rugae are thick folds of the stomach wall to stretch the lining
• The stomach:
  • mixes food, begins protein digestion, with limited absorption
  • moves food into the small intestine

Stomach Regions

• The cardiac region is a small area near the esophageal opening
  • the cardia is the portion of the stomach that attaches to the esophagus
• The fundic region contains the Fundus which balloons superior to the cardiac portion
  • the fundus acts as a temporary storage area
• The body region is the dilated, main portion of the stomach
• The pyloric region is narrower than the rest of the stomach
  • It transforms into the pyloric antrum that connects to the pyloric canal nears the small intestine.

Pyloric Sphincter

• The muscular wall thickens at its end to form a powerful, circular pyloric sphincter
• The pyloric sphincter controls gastric emptying into the duodenum

Chyme in the Stomach

• Chyme is a semiliquid mass formed when food is churned and mixed with digestive juices
• Stomach volume increases during eating and decreases as chyme leaves
• Peristalsis pushes chyme toward the pyloric region of the stomach and relaxes the pyloric Sphincter
• Stomach contractions push chyme into the small intestine
  • Liquids pass faster than solids
  • Carbohydrates pass quickest, then proteins, then fatty foods
• Pancreas, liver, and gallbladder secretions are added in the small intestine

Stomach Mucosa

• The mucosa includes many gastric pits, which contain openings for glands

Stomach Cells

• Glands consist of three types of cells:
  • Parietal cells
  • Chief cells
  • Endocrine cells

Parietal Cells

• Parietal cells produce hydrochloric acid for digestion
• Parietal cells produce intrinsic factor for absorption of vitamin B12

Chief Cells

• Chief cells produce pepsinogen, an important inactive enzyme
• Pepsin is the active form of pepsinogen, formed when pepsinogen is exposed to hydrochloric acid
• Pepsin breaks down proteins

Endocrine Cells

• Endocrine cells produce regulatory hormones
• Endocrine cell hormones significantly affect the motility of stomach substances
• Gastrin increases stomach secretions/gastric emptying rate
• Secretin inhibits gastric secretion and stimulates alkaline pancreatic secretions and inhibits gastric motility

Hormones Impacting Stomach

• Cholecystokinin stimulates pancreatic secretions and gallbladder contractions and inhibits gastric motility
• Gastric inhibitory peptide inhibits both gastric secretion and motility

Small Intestine

• The longest part of the digestive tract and the major site for food digestion and nutrient absorption
• Pancreas/liver secretions lubricate and protect from acidic chyme

Hormones Impacting Secretion

• Secretin and cholecystokinin stimulate liver/pancreas digestive enzymes
• The small intestine has three sections:
  • Duodenum
  • Jejunum
  • Ileum

Duodenum

• First portion of the small intestine, which forms a 180º arch
• The Duodenum is located posterior to the parietal peritoneum and follows a C-shaped path anterior to the right kidney
• The Duodenum has four sections, two of which are retroperitoneal, then re-enter the peritoneal cavity at the duodenojejunal flexure, or ligament of Treitz
• The pancreas is within the C-shaped loop
• The Duodenum functions as a mixing area that receives chyme from the stomach
• Digestive secretions come from the pancreas and liver
• The remaining small intestine is more mobile and lies free inside the peritoneal cavity
• At the duodenojejunal flexure, the duodenum bends sharply and continues into the jejunum

Jejunum

• Proximal two fifths of small intestine (2.5 m)
• Thicker walls with more folds than other portions

Ileum

• The Ileum is the last section of the small intestine and continues to the large intestine
• It has a smaller diameter, thinner, less vascular, and less active wall
• The Ileum is the longest portion with Peyer patches

Plicae Circulares

• Circular folds that run perpendicular to the long axis of the digestive tract
• Increase surface area for absorption
• Each fold contains villi that are fingerlike projections (0.5 to 1.5 mm long)
  • Villi contain a capillary and lacteal
  • Lacteals are lymph capillaries.
  • Absorptive cells produce digestive enzymes and absorb digested food
    • Disaccharidases break down sugars
    • Peptidases break down proteins
  • Goblet cells produces protective mucous lining
  • Endocrine cells produce regulatory hormones

Ducts in the Duodenum

• Common bile duct and hepatic duct drain into the lumen of the duodenum at the ampulla of Vater
• Lumen is the opening of a vessel
• The diameter of the small intestine gradually decreases from the duodenum to the jejunum and ileum ,which are smaller structures

Ileum

• The Ileum contains intermittent lymph nodules called Peyer patches
• The Ileocecal junction is the junction between the ileum and large intestine
• The Ileocecal valve prevents backflow of intestinal contents

Small Intestine Role

• The small intestine is the most important absorbing organ of the alimentary canal
  • Carbohydrate digestion starts in the mouth
• Digestion is completed by enzymes from the intestinal mucosa and pancreas
• Monosaccharides are absorbed by the villi
• Simple sugars are absorbed by active transport or facilitated diffusion

Peristaltic Rush

• Results from overdistension/irritation of small intestine
• Moves contents into the large intestine quickly
• Normal absorption of nutrients, water, and electrolytes does not occur
• Results in diarrhea, which results in water/electrolyte imbalances

Connection to Large Intestine

• The small intestine connects the ileocecal sphincter/junction to the large intestine
• The ileocecal sphincter is constricted to keep contents separate and it relaxes after eating

Large Intestine

• The Cecum is the beginning of the large intestine
• Typically, the Cecum forms a blind sac with the vermiform appendix attached
• The Vermiform appendix is an appendage attached to the end of the Cecum containing lymph nodules

Colon Portions

• The colon consists of four portions:
  • Ascending colon
  • Transverse colon
  • Descending colon
  • Sigmoid colon

Ascending Colon

• The first portion that extends upward from the Cecum
• The Ascending Colon ends at the hepatic flexure, which is the turn near the inferior border of liver

Transverse Colon

• The second portion continues across the abdomen
• The Transverse Colon transitions to the splenic flexure, which turns sharply and becomes the descending colon
• The Splenic flexure connects to the descending colon

Descending and Sigmoid Colon

• The descending colon is the third portion
• The descending colon extends from the splenic flexure to the sigmoid colon
• The sigmoid colon is the fourth portion
  • The sigmoid colon forms an S-shaped tube into the pelvis and ends at the rectum

Absence of Folds and Villi

• The colon lacks the folds and villi of the small intestine
• The colon contains numerous straight tubular glands called crypts with mucous-producing goblet cells
• Teniae coli form a longitudinal muscle layer that encircles the colon
• Contractions of it produce haustra to give the colon a puckered appearance

Rectum and Anal Canal

• The rectum is straight, muscular tube for stool storage that ends at the anus
• Stretching of rectum by stool leads to the urge to defecate
• The anal canal is very short (1-2")
• The anal canal consists of two circular sphincters (internal and external) that regulate the passage of stool
• The external sphincter is under voluntary control

Gallbladder

• The gallbladder is a pear-shaped organ on the lower surface of the liver
• The gallbladder is connected to the cystic duct that joins the common hepatic duct
• The gallbladder has epithelial lining and strong muscles for bile storage
• Bile is a digestive enzyme produced by the liver

Common Bile Duct & Cholecystokinin

• A common bile duct is formed by the hepatic and cystic duct joinings
• Leads to the duodenum
• As bile collects in the common bile duct, it backs up into the cystic duct and gallbladder
• Cholecystokinin stimulates gallbladder contraction and resulting bile enters the duodenum
• A peristaltic wave in the duodenal wall causes the hepatopancreatic sphincter to relax

Solid Abdominal Organs

• Major solid abdominal organs are the liver, spleen, and pancreas
• Solid abdominal organs bleed profusely when crushed or penetrated

Liver

• The liver is located in the RUQ
• Is the largest internal organ, weighing about 3 lbs
• The liver is enclosed in a fibrous capsule and the liver is divided into lobes by connective tissue
• The liver stores glucose, protein synthesis, and filter body wastes
• The hepatoportal system directs blood from the intestine through the liver for processing

Spleen

• The spleen is a highly vascular organ
• The spleen is located in LUQ behind the stomach
• The spleen aids in the removal of old blood cells, and in fighting infection

Pancreas

• The pancreas is an elongated organ
• Is located in the LUQ behind the stomach
• The pancreas secretes insulin, glucagon, and digestive enzymes and glucagons into the bloodstream
• Digestive enzymes travel via the main pancreatic duct into the common hepatic duct ampulla of Vater
  • Proteolytic pancreatic enzymes: trypsin, chymotrypsin, and carboxypeptidase
  • Lipases break down fats

Peritoneum

• The peritoneum is a two-layer structure of connective tissue surrounding the digestive organs

Peritoneum Layers

• Parietal peritoneum lines the abdominal cavity
• Visceral peritoneum is in close contact with organs
• The retroperitoneal space is behind the parietal peritoneum containing the kidneys, pancreas, duodenum, and major blood vessels
• Retroperitoneal organs are the kidneys and pancreas located in space
• Mesenteries are part of the peritoneum
• Mesenteries hold abdominal organs in place and provide a passageway for blood vessels and nerves

Omenta

• Omenta are folds of the peritoneum from the stomach to adjacent organs - Lesser omentum connects the lesser curvature of the stomach to the liver and diaphragm - Greater omentum connects the greater curvature of the stomach to the transverse colon and posterior body wall - Omental bursa is a cavity formed from double fold of mesentery that extends inferiorly from the stomach