BI 233 GI Test

What is the function of salivary amylase?

• Parotid salivary glands - Located inferior and anterior to the ears between the skin and masseter muscles – its duct empties into vestibule opposite upper second molar

  • cells secrete watery serous liquid containing salivary amylase

  • 70% of saliva

• Chemical digestion in the mouth - initiates the digestion of starches (polysaccharides), breaking them down into disaccharides

What is the function of lingual lipase?

• Secreted by glands on the dorsum of the tongue

• In newborns, it plays an important role in digesting milk fats, breaking triglycerides into free fatty acids and monoglycerides

• Has an optimal pH around 4, allowing it to remain active in the acidic environment of the stomach

• In adults, lingual lipase has limited physiological significance

What is the submucosal nerve plexus and what does it do?

• In submucosal layer

  • Regulates secretion and local blood flow

  • This autonomic nerve plexus regulates the local movements of the muscularis mucosae, which cause folding or wrinkling of the mucosal surface

    • These subtle contractions do not propel food, but rather agitate and reposition the mucosa to:

      • Enhance secretion from mucosal glands

      • Improve contact between the mucosal surface and luminal contents for absorption

What is the myenteric nerve plexus and what does it do

• In muscularis layer

  • Lies between the 2 smooth muscle layers

  • The nerve plexus consists of fibers from both autonomic divisions and functions to coordinate GI motility

    • Parasympathetic activation - increase motility

    • Sympathetic activation - decrease motility

• The submucosal plexus regulates chemical secretions and local blood flow, while the myenteric plexus controls gut movement and muscle contractions. The submucosal plexus helps change local muscle fibers in the lining (like the muscularis mucosae) to alter surface area for absorption

What are the differences between the submucosal and myenteric nerve plexuses? What nervous system structure regulates surface area in the GI tract?

The submucosal plexus regulates chemical secretions and local blood flow, while the myenteric plexus controls gut movement and muscle contractions. The submucosal plexus helps change local muscle fibers in the lining (like the muscularis mucosae) to alter surface area for absorption

What cell delivers antigen to the submucosal GI lymph nodes?

What substance is secreted by parietal cells?

Hydrochloric acid

Note: H+ and Cl- are secreted independently from the apical surface of the cell and combine in the lumen of the stomach to form HCl • H+ and K+ exchange via counter transport • HCO3 _ is secreted from the basolateral surface of the parietal cell and enters blood

What are 3 functions of HCl?

1. Converts pepsinogen → pepsin

2. Denatures/unfolds proteins by breaking hydrogen bonds

3. Stimulates secretion of hormones that promote flow of bile and pancreatic juice

4. Destroys microbes

What are at least 3 stimuli for release of the substance?

Gastrin, ACh, histamine

What substance is secreted by chief cells?

Produce and secrete pepsinogen, the inactive precursor of pepsin, which digests proteins

What is the function of pepsinogen?

Gets converted to pepsin by HCl

Pepsin helps digest proteins by breaking peptide bonds

What is the stimuli for release of pepsinogen?

ACh, gastric acid, gastrin, secretin

What substance is secreted by the ECL cells?

Secrete histamine in response to ACh and gastrin

What is the function of histamine?

Enhancing acid secretion by parietal cells - stimulate HCl by binding to H2 receptors

What are the stimuli for release of histamine?

gastrin

What substance is secreted by G cells

Gastrin

What is the function of gastrin

Stimulates both chief and parietal cells to release their respective secretions. Triggers ECL cell to secrete histamine

Causes increase of protein pump activity - increase production HCl in parietal cell

What is the stimuli for release of gastrin?

digestion of proteins by HCl, pepsin leads to increase of stomach pH

secretion happens when the pH of the stomach goes up or when theres many small peptide units

Under normal circumstances would the pH of blood leaving the stomach be: >, ~, < 7.4?

>, because as parietal cells secrete H⁺ into the stomach lumen, bicarbonate (HCO₃^-) is released into the bloodstream, making blood leaving the stomach slightly alkaline

What stimulates the release of CCK?

Released in response to fatty acids and partially digested proteins

What are the target tissues of CCK?

• Stimulates pancreatic enzyme secretion

  • Lipase, amylase, and proteases

• Triggers gallbladder contraction and relaxation of the hepatopancreatic sphincter, promoting bile and enzyme entry into the duodenum

What is the stimulus for release of GIP?

• Glucose-Dependent Insulinotropic Peptide

• Released in response to fatty acids and glucose-rich chyme

What is the target tissue of GIP?

• Decreases gastric secretion of HCl and slows gastric emptying

• Stimulates insulin release from the pancreas in response to elevated glucose levels

What is the stimulus for release of secretin?

Released primarily in response to acidic chyme (and to a lesser extent, fatty acids, and peptides)

What are the target tissues of secretin?

• Decreases gastric secretions and motility

• Stimulates pancreatic ducts to release bicarbonate-rich (alkaline) juice, helping neutralize acid in the duodenum

What is the gastric phase of digestion?

• (“Stomach Phase”)

• Neural Mechanisms

  • Stretch of the stomach wall by incoming food activates local parasympathetic reflexes and the enteric (myenteric) plexus

  • Effect: Enhanced peristaltic contractions and increased gastric gland secretions, promoting thorough mixing of food with gastric juice

• Hormonal Mechanisms

  • The presence of partially digested proteins and buffering of stomach acid stimulates G-cells (in the pyloric antrum) to release gastrin

  • Main targets of gastrin: Parietal cells, which increase HCl secretion and Chief cells which increase pepsinogen secretion

• Feedback regulation:

  • Gastrin secretion inhibited when gastric pH < 2.0

  • Gastrin secretion stimulated when gastric pH > 2.0 (less acidic conditions)

  • Alkaline Tide: As parietal cells secrete H⁺ into the stomach lumen, bicarbonate (HCO₃⁻) is released into the bloodstream, making blood leaving the stomach slightly alkaline

What is the intestinal phase of digestion?

• (“Duodenal Phase”)  

• Initiated when chyme enters the duodenum and initiated by activation of duodenal receptors

• Functions primarily to inhibit gastric secretion and motility to prevent duodenal overload

• Purpose

  • Prevents rapid shifts in osmolarity that would draw excess water into the intestinal lumen

    • Chyme from the stomach is typically hypertonic and overload could result in osmotic water loss from the blood into the intestinal lumen leading to potential low blood volume

• Prevents large pH fluctuations that could harm intestinal mucosa

• Hormonal Regulation

What are some of the differences between the gastric phase of digestion and the intestinal phase of digestion? 

• Gastric phase

  • Takes place inside the stomach. It uses muscle movements and strong acids to turn food into a liquid mix called chyme

  • Releases gastrin to increase stomach acid and boost mixing.

• Intestinal phase

  • Takes place inside the small intestine. It focuses on finishing food breakdown and absorbing nutrients

  • Releases hormones like secretin and cholecystokinin (CCK) to slow down the stomach and help the pancreas and gallbladder work

What are the pancreatic enzymes involved in protein digestion? How are they activated?

• Enzymes released from exocrine cells

• Enzyme release is regulated by

  • Nervous: Vagus nerve

  • Hormones: released in response to chyme in duodenum

• CHO digesting: pancreatic amylase

• Triglyceride digesting: Pancreatic lipase

  • Cleaves fatty acid from glycerol

• Protein digesting

  • Trypsin: secreted in inactive form trypsinogen

    • activated by enterokinase secreted by intestinal mucosa

  • Chymotrypsin: secreted as chymotrypsinogen and activated by trypsin

  • Carboxypeptidase: secreted as procarboxypeptidase also activated by trypsin

• These enzymes are all produced in inactive form, so they don’t digest self-cells

What hormone does the beta cells of the Islets of Langerhan’s release?

Synthesize insulin - A peptide hormone composed of 51 amino acids, produced by the β-cells of the pancreatic islets

What do the beta cells of the Islets of Langerhan’s release their hormone in response to?

• Released from endocrine cells

• Primary stimulus: An increase in blood glucose concentration

• Mechanism of Insulin Secretion

  • Glucose entry into β-cells via GLUT2 transporters

  • Intracellular glucose concentration rises and glucose is metabolized to produce ATP

  • Elevated ATP levels cause ATP-sensitive K⁺ channels to close

  • The resulting decrease in K⁺ efflux depolarizes the β-cell membrane

  • Depolarization opens voltage-gated Ca²⁺ channels, allowing Ca²⁺ to enter the cell

  • Exocytosis: The rise in intracellular Ca²⁺ triggers exocytosis of insulin-containing secretory granules

What is the effect on the target tissue of the hormone released from the beta cells of the Islets of Langerhan’s?

• Actions of Insulin

  • Primary target cells: Skeletal muscle and adipose tissue

  • Metabolic effects:

    • Promotes glucose uptake by stimulating GLUT4 translocation to the plasma membrane

    • Inhibits glycogenolysis (breakdown of glycogen to glucose)

    • Suppresses gluconeogenesis, the conversion of amino acids or fatty acids into glucose

    • Promotes anabolic processes such as glycogen synthesis, lipid storage, and protein synthesis

  • Glucose Uptake Regulation

  • In the absence of insulin, GLUT4 transporters remain sequestered in intracellular GLUT4 storage vesicles (GSVs), limiting glucose uptake

  • When insulin binds its receptor, signaling cascades stimulate fusion of GSVs with the plasma membrane, increasing glucose transport capacity

What hormone does the alpha cells of the Islets of Langerhan’s release?

Alpha cells: Synthesize glucagon - A 29 amino acid peptide

What do the alpha cells of the Islets of Langerhan’s release their hormone in response to?

• Released from endocrine cells

• Glucagon

  • Major stimulus is for release is a decline in blood glucose levels

    • Other stimuli include increased plasma amino acid levels and the catecholamines

What is the effect on the target tissue of the hormone released from the alpha cells of the Islets of Langerhan’s?

• Major target is liver

• Glycogenolysis: conversion of glycogen to glucose via cAMP activation

  • One molecule of glucagon can cause the release of 100 million molecules of glucose into the blood

• Gluconeogenesis: formation of glucose from L.A., F.A. and Amino Acid molecules

What are the disaccharides?

Sucrose, maltose, lactose

What enzymes break disaccharides down?

• Salivary Amylase - Initiates the digestion of starches (polysaccharides), breaking them down into disaccharides

• Absorption in the small intestine

  • Brush border enzymes complete digestion of the disaccharides

    • Sucrase digest sucrose to glucose and fructose

    • Maltase digests maltose to glucose and glucose

    • Lactase digests lactose to glucose and galactose

What are the resulting monosaccharides from breaking down disaccharides

• Sucrase digest sucrose to glucose and fructose

• Maltase digests maltose to glucose and glucose

• Lactase digests lactose to glucose and galactose

How are fats absorbed from the GI into the blood?

• Forms absorbed: Monoglycerides, glycerol, and free fatty acids

  • Absorption and Transport: Inside intestinal epithelial cells (enterocytes), lipids are reassembled into triglycerides and packaged with proteins into chylomicrons

  • Chylomicrons enter lacteals (lymphatic capillaries) → travel via the thoracic duct → enter systemic venous circulation, bypassing the liver initially

  • In Circulation:  Lipoprotein lipase (LPL) – an enzyme on capillary endothelium – hydrolyzes chylomicron triglycerides into free fatty acids (FFA) and glycerol. FFA are taken up by cells (especially adipose and muscle)

  • Primary fates:

    • Immediate energy use – FFAs oxidized in mitochondria via β-oxidation → acetyl-CoA → ATP

    • Storage – When cellular ATP and glucose levels are high, FFAs are re-esterified into triglycerides and stored in adipose tissue

    • Structural roles – Used in cell membrane phospholipids and steroid synthesis.

How are carbohydrates absorbed from the GI into the blood? 

• Monosaccharides are able to cross the epithelial cells surface for absorption

  • Glucose and galactose enter the apical surface via a SGLT symporter (co-transport with Na+)

  • Fructose enters by facilitated diffusion using a GLUT 5 transporter

  • Glucose, galactose and fructose all leave the basolateral surface of the cell via a GLUT 2 transporter In the liver fructose and galactose are converted to glucose

• Primary fates:

  • Immediate energy use – Glucose is oxidized through glycolysis and the citric acid cycle to produce ATP

  • Storage as muscle glycogen – Stored locally in skeletal muscle for energy use during contraction

  • Storage as liver glycogen – Stored in hepatocytes and released during fasting to maintain blood glucose levels

  • Conversion to fat (lipogenesis) – When glycogen stores are full and energy needs are met, excess glucose is converted to triglycerides for storage in adipose tissue

How are proteins absorbed from the GI into the blood? 

• Digestion and Absorption: 

  • In the intestinal lumen

    • Endopeptidases (e.g., trypsin, chymotrypsin) break proteins into smaller peptides

    • Exopeptidases (e.g., carboxypeptidase, aminopeptidase) release individual amino acids

  • Peptides and amino acids are absorbed across the luminal membrane of enterocytes:

    • As dipeptides, tripeptides, or amino acids

    • Inside the cell, peptides are further hydrolyzed to amino acids

  • Amino acids enter capillary blood and travel via the hepatic portal vein to the liver

    • Note: Some larger peptides can cross the enterocyte intact via transcytosis (especially in infants, allowing passive immunity from maternal antibodies)

  • Primary fates:

    • Protein synthesis – Taken up by hepatocytes and other body cells for synthesis of enzymes, contractile proteins, and plasma proteins

    • Energy production – Deamination removes the amino group, leaving a keto acid that enters the citric acid cycle for ATP production

    • Conversion to fat or glucose – Keto acids can be converted to fatty acids or used in gluconeogenesis when energy demands require

What are the primary functions of the liver 

• CHO metabolism

  • Glycogenesis: conversion of glucose to glycogen when blood sugar is high

    • if glycogen stores are full then glucose is converted into triglycerides

  • Glycogenolysis: Breakdown of glycogen to glucose, when blood glucose is low

  • Gluconeogenesis: Synthesis of glucose from non-carbohydrate sources such as lactic acid and amino acids

• Lipid metabolism  

  • β-Oxidation: Fatty acids are broken down into acetyl-CoA, which enters the citric acid cycle for ATP production

  • Lipid synthesis: Hepatocytes synthesize cholesterol, phospholipids, and lipoproteins (VLDL, HDL) for transport

  • Ketogenesis: During fasting, acetyl-CoA is converted to ketone bodies as an alternate energy source

• Protein metabolism

  • Deamination: Removal of the amine (–NH₂) group from amino acids, producing ammonia (NH₃)

  • Ammonia is converted to urea in the urea cycle and excreted by the kidneys

  • Protein synthesis: Hepatic cells produce most plasma proteins, including

    • Albumin – maintains oncotic pressure

    • Fibrinogen and clotting factors (II, VII, IX, X) – essential for coagulation

    • Transport proteins for hormones, lipids, and metals

  • Exception: The liver does not synthesize γ-globulins (antibodies)

• Detoxification of Drugs and Hormones

  • Cytochrome P450 enzymes chemically modify and inactivate drugs and toxins

  • Steroid and thyroid hormones (e.g., aldosterone, estrogen, cortisol) are metabolized and excreted after conjugation

  • Alcohol and ammonia are detoxified via specific hepatic pathways

• Excretion of bile pigments:  

  • Bilirubin (from breakdown of heme in worn-out RBCs) is absorbed, conjugated, and excreted in bile

  • Conjugated bilirubin gives bile its yellow-green color

• Synthesis: Bile salts used for emulsification and absorption of dietary fats

• Phagocytosis: Kupffer cells destroy aged erythrocytes and microbes

• Storage: Glycogen, Vitamins A, B12, D, E, K; minerals copper and iron

What is the primary function of the gallbladder

• Stores and concentrates bile up to 10-fold (until it is needed in the small intestine)

• H2O and ions reabsorbed by gallbladder mucosa during concentration

• CCK stimulates smooth muscle in wall of gallbladder to contract forcing bile into cystic duct → common bile duct → small intestine

• Bile flow: Canaliculi empty into small bile ductulus → empty into the R and L hepatic duct → unite to form the common hepatic duct  

  • The common hepatic duct joins with the cystic duct from the gallbladder to form the common bile duct  

  • The common bile duct and pancreatic duct enter the duodenum via the hepatopancreatic ampulla

• Bile: Consists of H2O; bile salts; bile acids; cholesterol; lecithin; bile pigment; several ions

  • Bile salts

    • sodium and potassium salts of bile acids play a role in emulsification

    • breakdown of large fat globules into small fat droplets so that pancreatic lipase can act quickly on it

  • Principle bile pigment is bilirubin derived from the heme of worn-out RBC's