The Gastrointestinal System

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Last modified 07/03/2015

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The Gastrointestinal System


Physiologic Anatomy

1. Upper gastrointestinal (GI) tract (Figure 8-1)

a. Mouth and accessory organs

b. Pharynx

c. Esophagus

i. Transports food from the mouth to the stomach and prevents retrograde movement of the stomach contents

ii. Collapsible tube about 25 cm long that lies posterior to the trachea and the heart

(a) Begins at the level of the sixth cervical vertebra and extends through the mediastinum and diaphragm to the level of the first thoracic vertebra, where it attaches to the stomach below the level of the diaphragm

(b) Upper portion of the esophagus is striated skeletal muscle, which is gradually replaced by smooth muscle so that the lower third of the esophagus is totally smooth muscle

(c) Motor and sensory impulses for swallowing and food passage derive from the vagus nerve. Lower esophagus also innervated by splanchnic and sympathetic neurons. Food moves by the strong muscular contraction of peristalsis and by gravity. In the absence of gravity, nutrients transported by muscular contractions.

(d) Sphincters: Hypopharyngeal (proximal) prevents air from entering the esophagus during inspiration; gastroesophageal (distal) prevents gastric reflux into the esophagus

iii. Blood supply

d. Stomach

i. Food storage reservoir and site of the start of the digestive process. Normal capacity is 1000 to 1500 ml but can hold up to 6000 ml.

ii. Layers of the stomach and intestinal wall (Figure 8-2)

(a) Mucosa: Cells produce mucus that lubricates and protects the inner surface. These cells are replaced every 4 to 5 days. This layer receives the majority of the blood supply of the stomach.

(b) Submucosa: Contains connective tissue and elastic fibers, blood vessels, nerves, lymphatic vessels, and structures responsible for secreting digestive enzymes

(c) Circular and longitudinal smooth muscle layers: Continue the modification of food into a liquid consistency and move it along the GI tract. Movements are tonic and rhythmic, occurring every 20 seconds. Electrical activity is constantly present in the smooth muscle layers.

(d) Serosa: Outermost layer

iii. Gastric hormones

iv. Gastric secretion

v. Gastric emptying

(a) Is proportional to the volume of material in the stomach

(b) Depends on the character of the ingested material: Liquids, digestible solids, fats, indigestible solids

(c) Factors accelerating gastric emptying: Large volume of liquids; anger; insulin

(d) Factors inhibiting gastric emptying: Fat, protein, starch, sadness, duodenal hormones

(e) Vomiting

vi. Blood supply

vii. Innervation

(a) Intrinsic nervous system (intramural neurons) within the wall of the GI tract is independent of central nervous system controls

(b) Extrinsic system: Via the central nervous system, parasympathetic system, and sympathetic system

2. Middle GI tract: Small intestine

a. Approximately 5 m long; extends from the pylorus to the ileocecal valve

b. Consists of three divisions: Duodenum, jejunum, ileum

c. Primary function is absorption of nutrients

d. Layers of the intestinal wall (Figure 8-2)

i. Mucosa: Innermost layer; receives the majority of the blood supply; the predominant site of nutrient absorption

ii. Submucosa: Contains loose connective tissue and elastic fibers, blood vessels, lymphatic vessels, and nerves

iii. Muscularis: Muscle layer; function is involuntary and involved in motility

iv. Serosa: Outermost layer; protects and suspends intestine within the abdominal cavity

e. Peristalsis: Propulsive movements that move the intestinal contents toward the anus. Approximately 3 to 5 hours is necessary for passage through the entire small intestine.

f. Blood supply

g. Innervation: Same as for stomach

h. Small intestine digestive enzymes not secreted, but integral components of the mucosa

i. Intestinal hormones

i. Secretin: Secreted by the mucosa of the duodenum in response to acidic gastric juice from the stomach and to alcohol ingestion

ii. CCK: Secreted by the mucosa of the jejunum in response to the presence of fat, protein, and acidic contents in the intestine

iii. Gastric inhibitory peptide (GIP): Secreted by the mucosa of the upper portion of the small intestine in response to the presence of carbohydrates and fat in the intestine; inhibits gastric acid secretion and motility, slowing the rate of gastric emptying

iv. Vasoactive intestinal peptide: Secreted throughout the gut in response to acidic gastric juice in the duodenum

v. Somatostatin: Secreted throughout the intestine in response to vagal stimulation, ingestion of food, and release of CCK, GIP, glucagon, and secretin

vi. Serotonin: Secreted throughout the intestine in response to vagal stimulation, increased luminal pressure, and the presence of acid or fat in the duodenum; inhibits gastric acid secretion and mucin production

j. Functions: Almost all absorption occurs in the small intestine via four mechanisms: Active transport, passive diffusion, facilitated diffusion, and nonionic transport

i. Vitamins are absorbed primarily in the intestine by passive diffusion, except for the fat-soluble vitamins, which require bile salts for absorption, and vitamin B12, which requires intrinsic factor

ii. Water absorption: Approximately 8 L of water per day is absorbed by the small intestine

iii. Electrolyte absorption: Most occurs in the proximal small intestine

iv. Iron absorption: Absorbed in the ferrous form in the duodenum

v. Carbohydrate absorption: Complex carbohydrates are broken down into monosaccharides or basic sugars (fructose, glucose, galactose) by specific enzymes (e.g., amylase, maltase)

vi. Protein absorption: Protein is broken down into amino acids and small peptides; essential amino acids are lysine, phenylalanine, isoleucine, valine, methionine, leucine, threonine, and tryptophan

vii. Fat absorption

3. Lower GI tract

a. Colon

b. Divisions of the colon

c. Layers of the large intestine wall (Figure 8-2): No villi and no secretion of digestive enzymes. Layers similar to those of the middle GI tract with exceptions:

d. Blood supply (Figure 8-3)

e. Colonic functions

i. Absorption of water and electrolytes: Approximately 500 ml of chyme (the byproduct of digestion) enters the colon per day and, of this, 400 ml of water and electrolytes are reabsorbed

ii. Breakdown of cellulose by enteric bacteria

iii. Synthesis of vitamins (folic acid, vitamin K, riboflavin, nicotinic acid) by enteric bacteria

iv. Storage of fecal mass until it can be expelled from the body

v. Motility

f. Innervation: Same as for the stomach and small intestine

g. Gut defenses

i. The gut encounters a variety of potentially harmful substances daily; these can include natural toxins in food, insecticides, preservatives, chemical waste products, and airborne particulate matter that is swallowed.

ii. Mechanisms exist within the GI tract to protect the integrity of the gut and thus the individual

iii. Fluid and cellular layers

(a) Aqueous layer: Stationary layer immediately adjacent to the microvillus border of the enterocytes; consists of acids, digestive enzymes, and bacteria depending on the location in GI lumen

(b) Mucosal barrier: Physical and chemical barriers that protect the wall of the gut from harmful substances. Surfaces of the stomach, intestine, biliary and pancreatic ducts, and gallbladder have cells that synthesize and release mucus.

(c) Epithelial cells: Tight junctions between cells regulated by hormones and cytokines make them relatively impervious to large molecules and bacteria; rapid proliferation of cells minimizes the adherence of flora. The level of permeability varies within the various segments of the GI tract.

(d) Mucus-bicarbonate barrier: Forms a layer of alkalinity between the epithelium and luminal acids that neutralizes the pH and protects against surface shear

iv. Motility: Prevents bacteria in the distal small intestine from migrating proximally into the sterile parts of the upper GI system

v. Gut immunity: Necessary because the gut is a reservoir of potentially pathogenic bacteria

vi. Gastric acid: Intragastric pH below 4.0 is essential

vii. Commensal bacteria: Natural gut flora are stable and protective in a healthy person by competing with pathogenic species for nutrients and attachment sites, and produce inhibitory substances against pathogenic species

viii. Impaired gut barrier function facilitates bacterial translocation, which is the egress of bacteria and/or their toxins across the mucosal barrier and into the lymphatic vessels and portal circulation

4. Accessory organs of digestion (Figure 8-4)

a. Liver

i. Largest solid organ, weighing approximately 3 lb (1500 g), located in the right upper quadrant, beneath the diaphragm

ii. Consists of three lobes divided into eight independent segments, each of which has its own vascular inflow, outflow, and biliary drainage. Because of this division into self-contained units, each can be resected without damaging those remaining.

iii. Microscopically the liver consists of functional units called lobules composed of portal triads in which the bile ducts, hepatocytes, and artery are located. The portal triads are then bounded by sinusoids and a central vein. A cross section of a classic lobule or acinus is hexagonal.

iv. Blood supply (Figure 8-5): Derived from both a vein and an artery

(a) 25% of cardiac output flows through the liver per minute

(b) Portal vein (after draining the mesenteric veins and pancreatic and splenic veins) and hepatic artery (off the aorta via the celiac trunk) enter the liver at the porta hepatis or hilum (a horizontal fissure in the liver, containing blood and lymph vessels, nerves, and the hepatic ducts)

(c) 75% is supplied by the portal vein; each segment receives a branch of the portal vein and 25% is supplied by the hepatic artery

(d) Small branches of each of these vessels enter the acinus at the portal triad (an area in the liver consisting of the portal vein, branches of the hepatic artery, and tributaries to the bile duct

(e) Functionally, the liver can be divided into three zones, based on oxygen supply. Zone 1 encircles the portal tracts where the oxygenated blood from hepatic arteries enters. Zone 3 is located around the central veins, where oxygenation is poor. Zone 2 is located in between.

(f) Blood from both the portal vein and the hepatic artery mixes together in the hepatic sinusoids and then flows through the sinusoids to the hepatic venules (zone 3) through the central veins, branches of the hepatic vein

(g) Sinusoids

(h) Venous drainage: Begins in the central veins in the center of the lobules; central veins empty into the hepatic veins, which empty into the inferior vena cava

v. Biliary duct system for draining bile

vi. Physiology: The liver is a metabolically complex organ with interrelated digestive, metabolic, exocrine, hematologic, and excretory functions. The many functions it performs are interwoven; each lobe is an independent functional unit, so that up to 80% of the liver can be destroyed and it will regenerate.

(a) Digestive functions: Plays a role in the synthesis, metabolism, and transport of carbohydrates, fats, and proteins

(1) Carbohydrates: Maintains normal serum glucose levels by

(2) Fats

a) Bile secretion for fat digestion plays a role in fat and lipid synthesis, metabolism, and transport

b) Principal site of synthesis and degradation of lipids (cholesterol, phospholipids, lipoprotein): Produces approximately 1000 mg of cholesterol per day

c) Exogenous lipoprotein metabolism

d) Endogenous lipoprotein metabolism: Major lipoprotein synthesized by the liver is very-low-density lipoprotein (VLDL); one third of VLDL remnants are converted to low-density lipoprotein (LDL)

e) Conversion of excess carbohydrate to triglyceride, which is stored as adipose tissue

f) Conversion of triglyceride to glycerol and fatty acids for energy

g) Storage of triglyceride and fat-soluble vitamins (A, D, E, and K)

h) Storage of fats, cholesterol, proteins, vitamin B12, and minerals

(3) Protein

(b) Endocrine functions: Metabolism of glucocorticoids, mineralocorticoids, hormones

(c) Exocrine functions

(d) Hematologic functions: Synthesis of bilirubin, coagulation factors

(e) Excretory functions

b. Gallbladder: Pear-shaped saclike organ that serves as a reservoir for bile

i. Attached to the inferior surface of the liver in the area that divides the right and left lobes (gallbladder fossa)

ii. Approximately 7 to 10 cm long; holds and concentrates approximately 30 ml of bile

iii. Blood supply: Arterial blood supply is from the cystic artery; venous drainage is via a network of small veins.

iv. Innervation: Splanchnic nerve, right branch of the vagus nerve

v. Cystic duct attaches the gallbladder to the common hepatic duct

vi. Presence of CCK in the blood (in response to chyme in the duodenum)

vii. Bile is composed of water, bile salts, and bile pigments

(a) Bile salts are responsible for the absorption and emulsification of fat and fat-soluble vitamins

(b) Bile pigments: High in cholesterol and phospholipids, give feces a brown color

(c) Bilirubin is the major bile pigment; it is a breakdown product of hemoglobin metabolism from senescent red blood cells

(d) Serum bilirubin

(1) Total: Indirect bilirubin plus direct bilirubin; when total bilirubin level is elevated and the cause is unknown, indirect and direct bilirubin fractions can be measured

(2) Indirect (unconjugated): Bilirubin bound to albumin before it binds to glucuronic acid; fat soluble. Causes of elevation of indirect bilirubin concentration in serum include the following:

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