Gastroenterology: the study of the digestive system

Structures

1. Gastrointestinal (GI) Tract

aka alimentary canal

Mouth to anus

Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

2. Accessory Organs

Teeth, tongue, salivary glands, liver, gallbladder, pancreas

Food does not pass lumen of these parts

6 Basic Functions

1. Ingestion: eating/drinking

2. Secretion: ~7L/day: H2O, acid, buffers, enzymes

3. Peristalsis: smooth muscle contraction

- moves substance through body

- mix substances in stomach (“churning”)

4. Digestion

Mechanical: teeth cut/grind, smooth muscle churns

- food dissolves & mixes w/ enzymes

- physical change

Chemical: lg. organic molecules are broken down by enzymes to absorbable products

- chemical change

5. Absorption: fluids & molecules cross lining of GI tract pass into blood - circulate to cells

Food group: Digested to: protein amino acids * Carbohydrates monosaccharides* fats/lipids fatty acids * * all can be absorbed

6. Defecation: The elimination of waste (feces) from the

body

Upper GI Tract:

Mouth Through Stomach

Salivary Glands

3 main glands:

Parotid, Sublingual, Submandibular

Produce and secrete saliva

Composition of Saliva

99.5% water + 0.5% solutes

Solutes = Na, K, Cl, bacteria fighter, enzymes, mucus

Enzymes:

1. Salivary amylase

2. Lingual lipase

Bicarbonate and phosphate ions – buffers to keep pH 6.35 – 6.85

Mucus – lubrication of food

Salivation

Normal Conditions – moisten mouth and esophagus and cleanse teeth

Stress – secretions stop; mouth dries

Eating – secretions increase

Touch, taste, smell, sight, and sound of food causes increase in salivation

Mechanical Digestion in Mouth

Mastication – act of chewing

Increase surface area

Mixes food with saliva to form bolus

Involves tongue and teeth

Role of Tongue in Digestion

Moves food in mouth

Taste buds – found on papillae of tongue

Sweet, sour, bitter, salty

Gustation - taste

Teeth

Deciduous teeth (baby teeth) = 20

Adult teeth = 32 (includes wisdom)‏

Aid in Mechanical Digestion

3 types of teeth

1. Incisors – cut

2. Cuspids – tear and shred

3. Molars – crush and grind

Chemical Digestion in Mouth

Salivary amylase – acts on carbs/starches

Breaks starches into maltose (a disaccharide)

Continues in stomach for 1 hour

Stomach acid inactivates

Lingual lipase

Secreted into mouth

Inactive until it reaches acid in stomach

Pharynx and Swallowing

3 parts of Pharynx

Nasopharynx – behind nasal cavity

Oropharynx – behind oral cavity

Laryngopharynx – behind larynx

Deglutition – act of swallowing

3 stages of Swallowing

1. Voluntary Stage– bolus goes from mouth to oropharynx

2. Pharyngeal Stage – oropharynx to esophagus

Involuntary

Airways close – breathing interrupted

Soft palate/uvula close off nasopharynx

Epiglottis seals off respiratory tract

Swallowing Cont’d

3. Esophageal Stage – esophagus to stomach

Peristalsis – smooth muscle pushes food toward stomach

At distal esophagus, cardiac sphincter relaxes – bolus enters stomach

Stomach - Functions

1. Mixes saliva, food, and gastric juice to form chyme (mechanical digestion)

2. Secretes the hormone gastrin into blood

3. Secretes gastric juice

4. Breaks down food with enzymes (chemical digestion)

5. Absorbs: water, alcohol, drugs

Contents of Gastric Juice:

Mucus – protects stomach lining

Pepsinogen

inactive form of PEPSIN (enzyme)‏

Activated by HCl (low pH) or other pepsin molecules

Breaks proteins down to peptides

Gastric lipase – digests fats

HCl (hydrochloric acid)

Kills microbes

Unfolds proteins

Stimulates secretion of Gastrin

Intrinsic factor – binds to vitamin B12, so that it can be absorbed in small intestine

1. Cephalic phase

2. Gastric phase

3. Intestinal phase

3 Phases of Stomach Digestion

1. Cephalic phase

Preparatory stage; controlled by nerve stimuli

Thought, sight, smell, or taste of food causes:

Stomach to secrete gastric juice

Gastrin to be secreted into blood

Stomach to begin churning

2. Gastric phase

Active stage; controlled by stomach

Stomach stretches & pH rises: causes

further gastric secretions

Churning mechanically digests food

Gastric juice chemically digests food

pH below 2 – gastric secretions

inhibited

pH above 2 – gastric secretions

stimulated

During this phase Gastrin functions to:

1. Stimulate secretion of gastric juice

2. Strengthen contraction of cardiac sphincter

3. Relax pyloric and ileocecal sphincters

3. Intestinal Phase

Inhibitory stage; controlled by small intestine

Slows stomach activity

2 hormones are secreted by small intestine:

Secretin – decreases gastric secretions

CCK (cholecystokinin) – inhibits stomach emptying

Stomach empties in 2-4 hours in this order: 1. carbs, 2. proteins, 3. fats

Lower GI Tract:

Duodenum through Anus

Small Intestine – Anatomy Duodenum (10 inches)‏

Receives chyme from stomach

Pancreatic juice and bile enter here

Jejunum (3 feet)‏

Begins where duodenum turns downward

Ileum (6 feet)‏

Ends at ileocecal sphincter - prevents feces from backing into small intestine

Mesentery – thin tissue that prevents small intestines from twisting/tangling

Small intestine & digestion Characteristics that increase surface area:

Length (~10 feet long, 1” wide)‏

Villi- folds in mucosa

Microvilli – hair-like projections in villi

Circular folds (vs. straight)‏

Function:

1. Digestion

2. Absorption

Intestinal Villi

Circular Folds

Small intestine, cont’d

Secretes intestinal juice

Intestinal juice contains the following enzymes: sucrase, maltase, lactase, peptidase & enterokinase

Contents remain in intestine for 3-5 hours

Hormones of the Small Intestine

These secretions act to stimulate the pancreas:

1 Secretin

stimulates secretion of pancreatic juice rich in sodium bicarbonate ions to increase pH

2. CCK

Stimulates secretion of pancreatic juice rich in digestive enzymes

Pancreas

Produces and secretes hormone Insulin (regulates blood sugar)‏

Produces pancreatic juice which it secretes into small intestine

Pancreatic Juice

Contains water, salts, buffer, and enzymes

Pancreatic Juice

pH 7.1 – 8.2

Buffers acidic chyme from stomach

Change in pH inactivates gastric enzymes

Enzymes

Pancreatic amylase – carbohydrates

Trypsin & chymotrypsin – proteins

Pancreatic lipase – lipids (fats)‏

Pancreatic Enzymes

Protein-digesting enzymes are secreted in inactive form to protect pancreas:

Trypsinogen and chymotrypsinogen

Trypsinogen + Enterokinase* → Trypsin Chymotrypsinogen + Trypsin → Chymotrypsin *enzyme secreted by small intestine

Liver

Heaviest gland in the body

2nd largest organ

Multi-lobed organ

Liver Cells = Hepatocytes

Liver functions

Produces bile

Stores glycogen

Filters blood absorbed from small intestine (portal circulation)‏

Detoxifies blood: removes drugs, toxins, & some microbes

Gallbladder

Functions in the storage and concentration of bile

Bile – emulsifies fats

Chyme (rich in fats & proteins) causes secretion of CCK by small intestine

CCK causes secretion of bile

Summary: Digestion of Carbohydrates

Salivary amylase

secreted in mouth

Breaks starches into maltose (disaccharide)

Pancreatic amylase

secreted into small intestine

Breaks polysaccharides into disaccharides

Carbs cont’d

3 enzymes are secreted into small intestine

break disaccharides → monosaccharides

1. Sucrase

sucrose → glucose + fructose

2. Lactase

lactose → glucose and galactose

3. Maltase

maltose → glucose (2-3 molecules)‏

Summary: Digestion of Proteins Pepsin

Produced and Secreted in the stomach

Breaks proteins into peptides

Trypsin and Chymotrypsin

Secreted from pancreas into small intestine

Secreted in inactive form: trypsinogen & chymotrypsinogen

Breaks proteins into peptides

Proteins cont’d

Peptidases

Produced and secreted in the small intestine

Breaks peptides into amino acids

Summary: Digestion of Lipids

Lingual lipase Secreted in the mouth – active in stomach

Gastric lipase Produced and Secreted in the stomach

Pancreatic lipase Secreted from pancreas into small intestine

All 3 enzymes break lipids into fatty acids

Absorption in the Small Intestine

90% of all absorption occurs here

Nutrients, electrolytes, water & vitamins absorbed

Vitamin B12 must be bound to intrinsic factor to be absorbed

Large intestine & digestion

5 feet long, 2.5 inches wide No Villi present No enzymes secreted Mucus secreted for lubrication Haustra- series of pouches creates “puckered”

appearance Function: Absorb H2O Produce & excrete solid wastes (feces)

Intestinal Haustra

Formation and expulsion of feces: H2O & electrolytes are absorbed Bacteria (normal flora) break down remaining

carbs, protein & pigments Bacteria produce bi-products: - B vitamins & vitamin K - flatus (gas)‏ Feces: water, salts, sloughed off cells, bacteria,

materials (unabsorbed or indigestible) & mucus