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Structure & Function of Digestive System Ch 38

Chapter 33 Summary of Understand pathophysiology (5th ed.) Sue E. Huether
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Gastrointestinal Tract (GI or alimentary canal)
consist of mouth, esophagus, stomach, small intestine, large intestine, rectum, and anus
Carries the digestive processes of:
(1) Ingestion of food
(2) Propulsion (push) of food and wastes from the mouth to anus
(3) Secretion of mucus, water, and enzymes
(4) Mechanical digestion of food particles (major function)
(5) Chemical digestion of food particles
(6) Absorption of digested food
(7) Elimination of waste products by defecation
Four layers of GI tract
From inside out:
-Mucosa
-Submucosa
-Muscularis
-Serosa (or adventitia)

Layers vary in thickness and have sublayers.
Digestive System
consist of GI tract and accessory organs of digestion. Breaks down ingested food, prepares it for uptake by the body's cells, provides body water, and eliminates wastes
Accessory organs of digestion
-Salivary Glands
-Liver
-Gallbladder
-Exocrine Pancreas
Gastrointestinal motibility
Movements of digestive system; controlled by hormones and the autonomic nervous system. Except for chewing, swallowing and defecation of solid wastes.
Mouth
Reservoir for the chewing and mixing of food with saliva.
Salivary glands
Secrete about 1L of saliva a day. Cholinergic parasympathetic fibers stimulate it.

three pairs:
1) Submadibular
2) Sublingual
3) Parotid glands
Saliva
Consist of mostly water with:
-mucus,
-sodium,
-bicarbonate (sustains a pH of 7.4-neutralizes bacterial acids and prevents tooth decay),
-chloride,
-potassium,
-and salivary alpha-amylase (ptyalin)

Composition depends on rate of secretion.
Also contains: mucin (provides lubrication), IgA and other antimicrobial substances, which helps prevent infection.
Exogenous fluoride (e.g. fluoride in drinking water) is also secreted, which provides additional protection against tooth decay
Salivary alpha-amylase (ptyalin)
Enzyme that initiates carbohydrate digestion in the mouth and stomach
Salivation
*Controlled by sympathetic and parasympathetic divisions of autonomic nervous system.
*Atropine (an anticholinergic agent) inhibits salivation and causes the mouth to become dry.
Swallowing
Coordinated primarily by the swallowing center in the medulla.
Food is move to the stomach by peristalsis
Esophagus
Hollow, muscular tube approx. 25 cm long that conducts substances from the oropharynx to the stomach.
Peristalsis
Coordinated sequential contraction and relaxation of outer longitudinal and inner circular layers of muscles.
Stimulated when afferent fibers distributed along the length of the esophagus sense changes in wall tension caused by stretching as food passes. The greater the tension, the greater the esophageal contraction
Sphincter
Opens and closes each end of esophagus
Upper esophageal sphincter
Keeps air from entering the esophagus during respiration
Lower esophageal sphincter (cardiac sphincter)
Prevents regurgitation from the stomach and caustic injury to the esophagus
Oropharyngeal (voluntary) phase
Takes place in less than 1 second.

The steps are:
(1) Food is segmented into a bolus by the tongue and forced posteriorly toward the pharynx
(2) The superior constrictor muscle of the pharynx contracts so the food cannot move into the nasopharynx
(3) Respiration is inhibited, and the epiglottis slides down to prevent the food from entering the larynx and trachea.
Esophageal phase
Takes place in 5-10 seconds with bolus moving 2-6cm/sec

The steps are:
(1) Bolus of food enters esophagus
(2) Waves of relaxation travel the esophagus, preparing to move the bolus
(3) Peristalsis transports the food to the lower esophageal sphincter, which is relaxed at this point
(4) Bolus enters stomach, sphincter muscles return to resting tone
Primary peristalsis
Follows the oropharyngeal phase of swallowing
Secondary peristalsis
Wave of contraction and relaxation independent of voluntary swallowing.
Happens when bolus gets stuck in esophageal lumen
Chyme
partially digested food
Stomach
--Functional areas: Fundus (upper portion), Body (middle portion), Antrum (lower portion)
--Oblique layer is most prominent
--Blood supply comes from celiac artery
--Secretes large volumes of gastric juices or secretions to include: mucus, acid, enzymes, hormoes, intrinsic factor, and gastroferrin
Gastric emptying
movement of gastric contents into the duodenum; depends of volume, osmotic pressure, and chemical composition of the gastric contents
Phases of gastric secretion
1) Cephalic phase (stimulated by thought, smell, and taste of food)
2) Gastric phase (stimulated by distention of the stomach)
3) Intestinal phase (stimulated by histamine and digested protein)

Promote secretion of acid by the stomach.
Gastric secretion
-stimulated by eating (gastric distention, hormone gastrin, paracrine pathways (histamine, gherkin, somatostatin), neurotransmitter acetylcholine and chemical (ethanol, coffee, protein)
-inhibited by somatostatin, unpleasant odors and tastes, and by rage, fear, or pain.
Small Intestine
About 5-6 meters long; divided into three segments:
Duodenum-begins at pylorus ends at ligament of Treitz
Jejunum- little more lumen tha ileum
Ileum-
ileocecal valve (sphincter)
Controls flow of digested material from ileum to larger intestine; prevents reflux into small intestine
Parasympathetic Nerves in Digestion
Mediate secretion, mobility, pain sensation, and intestinal reflexes
Sympathetic Nerves in Digestion
Inhibit mobility and produce vasoconstriction
Plica
Mucosal folds in small intestine to slow the passage of food, providing more time for digestion and absorption
The GI tract and Immunity
Major role in immunity by killing many microorganisms. The mucosal secretions produce antibodies (IgA) and enzymes.
Paneth cells- produce defensins and other antibiotic peptides and proteins important to mucosal immunity
Peyer patches (lymph nodules containing collections of lymphocytes, plasma cells, and macrophages) produce immunoglobulins in small intestine. Important for antigen processing and immune defense
Intestinal Motility
Stimulated by chyme leaving stomach and entering duodenum.
Affected by:
(1) Haustral segmentation
(2) Peristalsis
Major Nutrients absorbed in small intestine
Water and Electrolytes- about 85-90%
Carbohydrates-broken down to simplest form
Proteins-90-95%
Fats- occur in 4 stages
Minerals- calcium (in ileum), magnesium (in jejunum and ileum), phosphate, iron (in duodenum and jejunum)
Vitamins
Fats digestion and absorption
happens in four stages:
(1) Emulsification and lipolysis- agents cover small fat particles and prevent them from re-forming into fat droplets; lipolysis divides them into diglycerides, monoglycerides, free fatty acids, and glycerol.
(2) Micelle formation- made water soluble
(3) Fat absorption-absorption surface of intestinal epithelium and diffuse through resynthesis
(4) Triglycerides and phospholipids
Large intestine
About 1.5 meters long; consist of cecum, appendix, colon (ascending, transverse, descending, and sigmoid), rectum, and anal canal
Splanchnic Blood Flow
Provides blood to esophagus, stomach, small and large intestine, liver, gallbladder, pancreas, and spleen

Superior and inferior mesenteric arteries provide blood to large intestine
Liver
-Weights 1200-1600 g. ; covered by glisson capsule (contain blood vessels, lymphatics, and nerves)
-Secretes 700-1200 ml of bile per day.
-Synthesizes fat from carbs and proteins, phospholipids, and cholesterol
-Releases glucose during hypoglycemia, and absorbs glucose during hyperglycemia, stores it as glycogen or converts it into fat
-Stores certain vitamins and minerals: iron (as ferritin) , copper, B12,Vit. D, Vit. A, Vit. E, Vit. K,
Importance of Proteins in Body
Contraction
Energy
Fluid balance
Protection
Regulation
Structure
Transport
Coagulation
Tests for Liver Function
Serum Enzymes-
--Alkaline phosphatase (20-125 units/L)
--Aspartate aminotransferase [AST, SGOT](6-21 units/L)
--Alanine aminotransferase [ALT, SGPT] (0-48 units/L)
--Lactate dehydrogenase [LDH] (0-250 units/L)
--5-Nucleotidase (2-11 units/L)

Bilirubin Metabolism-
--Serum bilirubin
--Indirect [unconjugated] (0-1.0mg/dl)
--Direct [conjugated] (0-0.3mg/dl)
---TOTAL (0-1.0mg/dl)
--Urine bilirubin (0.2-1.3mg/dl)
--Urine urobilinogen (0.3-2.1mg/2hr:male)
(0.1-1.1mg/2hr: female)

Serum Proteins-
--Albumin (4-6g/dl)
--Globulin (2-4g/dl)
---TOTAL (6-8g/dl)
--A/G ratio (1:5:1 to 2:5:1)
--Transferrin (250-300mcg/dl)
--Alpha-Fetoprotein (<10ng/ml)

Blood Clotting Functions-
--Prothrombin time [PT] (10-14 sec or 90%-100% control)
--International Normalized Ratio [INR] (0.9-1.3)
--Partial thromboplastin time [PTT] (25-40sec)
--Bromsulphalein [BSP] excretion (<6% retention in 45 min)
Exocrine Pancreas
-Approx. 20 cm long; composed of acinar cells that secrete enzymes and networks of ducts that secrete alkaline fluids
-Aquenous secretion is isotonic and contains potassium, sodium, bicarbonate, and chloride
-Neutralizes chyme in duodenum
-Secretin inhibits actions of gastrin, decreasing gastric acid secretion and motility
Tests for Exocrine Pancreatic Function
-Serum amylase (27-131 units/L)
-Serum lipase (20-180 units/L)
-Urine amylase (2-19 units/hr)
-Secretin test (Volume 1.8ml/kg/hr)
--Bicarbonate concentration: (>80mEq/L)
--Bicarbonate output (>10mEq/L/30sec)
Stool fat (2.5g/24hr)
Geriatric Considerations: Oral Cavity and Esophagus
1. Tooth enamel and dentin deteriorate, so cavities are more likely
2. Teeth are lost as a result of periodontal disease and brittle roots that break easily
3. Taste buds decline in number
4. Sense of smell diminishes
5. Salivary secretion decreases
6. Dysphagia (difficulty swallowing) is much more common
***Result******:
eating less is presumable,
appetite is reduced,
Food is not sufficiently chewed or lubricated-causing swallowing to be difficult
Geriatric Considerations: Stomach and Intestines
1. Gastric motility, blood flow, and volume and acid content of gastric juice may be reduced, particularly with gastric atrophy.
2. Protective mucosal barrier decreases
3. Change in composition of microflora and resultant increased susceptibility to disease
4. Intestinal villi shorten and more convoluted (difficult to follow), with dimished reparative capacity
5. Intestinal absorption, motility, and blood flow decrease, impairing nutrient absorption.
6. Nutritive substances are absorbed more slowly and in smaller amounts
7. Rectal muscle mass decreases, and the anal sphincter weakens
8. Constipation is common and is related to immobility, low-fiber diet, and changes in enteric nervous system functions
Geriatric Considerations: Liver
1. Decreased hepatic regeneration; size and weight of liver decrease
2. Ability to detoxify drugs decreases.
3. Blood flow decreases, influencing efficiency of drug metabolism
Geriatric Considerations: Pancreas & Gallbladder
1. Fibrosis, fatty acid deposits, and pancreatic atrophy occur
2. Secretion of digestive enzymes, particularly proteolytic enzymes, decreases.
3. No changes in gallbladder and bile ducts occur, but there is an increased prevalance of gallstones and cholecystitis (inflammation of common hepatic duct, gallbladder)
Which two hormones stimulate gastric emptying?
Gastrin and motilin
Which two hormones delay gastric emptying?
Secretin and cholecytokinin
Parietal cells secrete what? What do they do?
Hydrochloric acid and intrinsic factor
Hydorchloric acid dissolves food fibers, kills microorganisms, and activates the enzyme pepsin.
Intrinsic factor is needed for the absorption of vitamin B12
Chief cells secrete what? What does it do?
Pepsinogen. Hydrochloric acid converts pepsinogen to pepsin which begins digestion of proteins in the stomach.
What stimulates acid secretion? What inhibits it's secretion?
The vagus nerve, gastrin, and histamine stimulate acid secretion
The sympathetic nervous system and cholecystokinin inhibit acid secretion.
What stimulates pepsinogen secretion?
Acetlycholine through vagal stimulation during the cephalic and gastric phases, is the strongest stimulation for pepsin secretion.
Where does most of the digestion and absorption of all major nutrients occur?
The small intestine.
Sugars, amino acids, and fats are absorbed primarily by.....?
The duodenum and jejunum
Bile salts and vitamin B12 are absorbed by the.....?
ilium
What are the five functions of the liver?
Digestive, metabolic, hematologic, vascular, immunologic functions
Which artery supplies and which vein receives blood from the liver?
The hepatic artery supplies the liver while the portal vein receives blood from the inferior and superior mesenteric veins.