GI L 3-5

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Questions and Answers

Which structure has the highest permeability for ion absorption?

Colon

Proximal large intestine

Jejunum (correct)

Ileum

What is the role of Intrinsic Factor in the absorption of Vitamin B12?

It releases calcium from food.

It facilitates magnesium absorption.

It binds to Vitamin B12 for absorption in the ileum. (correct)

It transports water across intestinal cells.

Which ion's absorption occurs predominantly through the paracellular pathway in the small intestine?

Sodium (Na+)

Potassium (K+) (correct)

Calcium (Ca++)

Magnesium (Mg++)

Which factor is essential for the conversion of ferric iron to ferrous iron for absorption?

Low pH level. Signup and view all the answers

What primarily stimulates the absorption of calcium in the small intestine?

Parathyroid hormone (PTH). Signup and view all the answers

In which phase of energy metabolism does insulin get released?

Absorptive phase. Signup and view all the answers

What is the main function of Brunner's glands in the small intestine?

Neutralizing acidic chyme Signup and view all the answers

Which cell type within the small intestinal crypts is primarily responsible for the secretion of antimicrobial enzymes?

Paneth cells Signup and view all the answers

What component of the immune system is particularly important in the intestinal surveillance within the small intestine?

Peyer’s patches Signup and view all the answers

What is the primary role of absorptive cells (enterocytes) in the small intestine?

Absorbing nutrients Signup and view all the answers

Which of the following statements about the composition of bile is true?

Bile acids are a significant component of bile Signup and view all the answers

What distinguishes phase I and phase II reactions in biotransformation?

Phase I modifies functional groups, while phase II involves further modification Signup and view all the answers

Which of the following best describes the role of enteroendocrine cells in the small intestine?

Releasing hormones such as CCK and secretin Signup and view all the answers

Which structure contributes to the large surface area of the small intestine?

Villi Signup and view all the answers

Which aspect of the regeneration of the intestinal epithelium is accurate?

Rapid regeneration occurs in intestinal crypts Signup and view all the answers

Which statement accurately describes the liver's role in carbohydrate metabolism?

It is responsible for both gluconeogenesis and glycolysis. Signup and view all the answers

Which of the following plasma proteins is primarily synthesized by the liver?

Fibrinogen Signup and view all the answers

What is the primary function of cytochrome P450 in the liver?

It is involved in phase I drug metabolism. Signup and view all the answers

Phase II of drug metabolism mainly involves what type of chemical reaction?

Conjugation with a polar molecule. Signup and view all the answers

Which statement is true regarding bile acids and their synthesis?

Cholesterol is the precursor for bile acid synthesis. Signup and view all the answers

What consequences can arise due to a lack of certain enzymes in cats during drug metabolism?

Toxic reactions to certain drugs like acetaminophen. Signup and view all the answers

How do bile salts influence cholesterol metabolism?

They inhibit cholesterol 7-alpha-hydroxylase to reduce bile acid synthesis. Signup and view all the answers

Which of the following is not a typical outcome of phase I drug metabolism?

Conversion of non-toxic to toxic metabolites. Signup and view all the answers

What is the primary role of bile in digestion?

To facilitate the absorption of fats in the intestine. Signup and view all the answers

Which factor can significantly impact drug metabolism variations among species?

Presence or absence of specific metabolic enzymes. Signup and view all the answers

What is the primary role of bile acids in the small intestine?

Emulsifying fat droplets Signup and view all the answers

Which of the following statements correctly describes the function of the gallbladder?

It stores bile when digestion is not occurring Signup and view all the answers

What initiates pancreatic secretion of digestive enzymes?

Cholecystokinin (CCK) and acetylcholine Signup and view all the answers

Which enzyme is responsible for the initial breakdown of carbohydrates in the mouth?

Amylase Signup and view all the answers

What occurs during the process of emulsification in the duodenum?

Fat droplets are reduced in size to increase surface area Signup and view all the answers

What happens to short-chain fatty acids during lipid absorption?

They are absorbed directly into the bloodstream Signup and view all the answers

What is the clinical manifestation of pancreatic insufficiency?

Greasy and oily stools Signup and view all the answers

What occurs to triacylglycerols inside enterocytes after absorption?

They undergo re-esterification into complex lipids Signup and view all the answers

Which factor influences the secretion of secretin in the digestive process?

Decreased pH in the duodenum Signup and view all the answers

Which of the following describes the role of chylomicrons in the body?

Carrying triglycerides and fat-soluble vitamins through the lymphatic system Signup and view all the answers

What primarily influences the length of the villi in the small intestine?

The balance between cell loss at the tips and cell replication in the crypts Signup and view all the answers

Which cells migrate downward into the crypts of the small intestine?

Paneth cells Signup and view all the answers

What is the primary mechanism through which water is secreted in the intestines?

Osmosis Signup and view all the answers

How does most water reabsorption occur in the small intestine?

Through osmosis following solute absorption Signup and view all the answers

Which statement best describes the structure of the large intestine's mucosa?

It is composed of crypts lined predominantly by goblet cells Signup and view all the answers

Where do absorbed nutrients enter after moving through the interstitial space?

Blood capillaries Signup and view all the answers

Which components are included in the portal triad found in the liver lobule?

Portal venule, arteriole of hepatic artery, and bile duct Signup and view all the answers

What role do hepatocytes play within the liver lobule structure?

They produce bile and perform metabolic functions Signup and view all the answers

What occurs to venous blood from the gastrointestinal tract before it returns to the heart?

It passes through the liver via the hepatic portal vein Signup and view all the answers

What function does the large intestine serve in relation to vitamins?

It absorbs vitamins and minerals efficiently Signup and view all the answers

Study Notes

Small Intestine

Large Surface Area: The small intestine has a large surface area to allow for reabsorption of nutrients.
Three Levels of Surface Convolutions:
- Plicae Circulares: Spiral or circular folds lined by villi and microvilli.
- Villi: Finger-like epithelial projections.
- Microvilli: Small projections on the surface of villi.
Structure of Small Intestine:
- Crypts of Lieberkuhn: Gland-like structures found at the base of villi.
- Crypts Cell Types:
  - Paneth cells: Secrete antimicrobial enzymes and peptides.
  - Endocrine cells: Produce hormones.
  - Stem cells: Responsible for regeneration of intestinal epithelium.
  - Goblet cells: Secrete mucus.
Cell Types of Small Intestine:
- Absorptive cells (Enterocytes): Absorb nutrients.
  - Secrete digestive enzymes, water, chloride ions (Cl-), and bicarbonate ions (HCO3-).
- Goblet cells: Secrete mucus.
- Enteroendocrine cells: Produce hormones like CCK, secretin, and GIP.
- Paneth cells: Secrete antimicrobial enzymes and peptides.
Brunner's Glands:
- Located in the submucosa.
- Tubuloalveolar glands that produce mucus.
- Secrete glycoproteins and bicarbonate ions.
- Secretion has a pH of 8.1-9.3.
- Function: Protect small intestine mucosa by neutralizing acidic chyme from the stomach.
Small Intestine Immune System:
- Peyer's Patches: Important in immune surveillance of the intestinal lumen.
  - Facilitate the generation of immune responses within the mucosa.
Cell Replacement in Small Intestine:
- Occurs within the intestinal crypts.
- Enterocytes divide and replicate.
- Crypt cells are among the most rapidly regenerating cells in the body.
- Enterocyte turnover time: 4 to 7 days.
- As cells multiply, they migrate upward along the villi (except Paneth cells).
- The length of villi is determined by the balance between cell loss at the tips and cell replacement.
Cells of the Crypt and Regeneration:
- Progenitor cells differentiate into absorptive enterocytes, goblet cells, enteroendocrine cells, and Paneth cells.
Water Absorption in Small Intestine:
- All water secretion in the intestine is mediated by osmosis.
- Food entering the intestine may be hyperosmotic, especially after digestion.
- Water moves via the paracellular pathway, following the movement of solutes.
- Most water reabsorption occurs in the small intestine due to:
  - Larger solute reabsorption.
  - Larger surface area (villi & microvilli).
  - Larger openings in tight junctions.
  - Extensive capillary network in the villi.

Large Intestine

Structure:
- One layer of cylindrical epithelium.
- Crypts are present, but villi are absent.
Functions:
- Minimal water absorption (about 14%).
- Vitamin absorption.
- Goblet cells produce mucus in small volumes, which is isotonic to plasma.
- Microbial metabolism is particularly significant in horses and rabbits.

Liver

Liver Lobule:
- Hepatocytes: Liver cells arranged in a hexagonal pattern.
- Central vein: Located at the center of the hexagonal structure.
- Portal triad: Found at the vertices of the hexagon, containing:
  - Portal venule.
  - Arteriole of hepatic artery.
  - Bile duct.
- Sinusoids: Low-pressure vascular channels that receive blood from the hepatic artery and portal vein.
- Bile canaliculi: Tiny tissue spaces between hepatocytes that form an intralobular network of channels.
  - Drain bile produced by hepatocytes.
Liver Functions:
- Carbohydrate metabolism: Gluconeogenesis, glycolysis.
- Amino acid and protein metabolism: Synthesis of plasma proteins.
- Lipid metabolism: Fatty acids oxidation, ketone bodies synthesis.
- Storage: Glycogen, lipids, vitamins, copper, iron.
- Synthesis and secretion of bile acids.
- Biotransformation of drugs, xenobiotics, and metabolic byproducts.
- Synthesis of hormones and mediators.
- Synthesis of components of the immune system
Synthesis of Plasma Proteins:
- The liver produces most of the body's circulating plasma proteins, including:
  - Albumins.
  - Lipoproteins (VLDLs, LDLs, HDLs).
  - Glycoproteins (haptoglobin, transferrin).
  - Prothrombin and fibrinogen.
  - Nonimmune alpha- and beta-globulins.
Synthesis of Hormones by the Liver:
- Angiotensinogen: A prohormone.
- Thrombopoietin: A hormone (growth factor).
- Insulin-like growth factors (IGF1 and IGF2).
- Hepcidin: A small peptide hormone involved in iron homeostasis.

Biotransformation

Definition: A group of reactions that convert toxic molecules into less toxic, less active, and water-soluble forms.
Essential for drug metabolism, termination of drug action, and elimination.
Cytochrome P450: Microsomal enzymes primarily located in the liver.
Drug metabolism increases polarity, making drugs and their metabolites more water-soluble.
Phase I: Oxidation reactions (hydroxylation, carboxylation).
Phase II: Conjugation reactions (glucuronic acid, glycine, taurine).

Bile Acid Production

Bile: Produced by hepatocytes, modified by gallbladder epithelial cells.
Gallbladder: Stores and concentrates bile by resorption of electrolytes and water.
Bile acids are synthesized from cholesterol.
Bile salts are bile acids conjugated with amino acids (glycine, taurine).
Bile salts inhibit cholesterol 7-alpha-hydroxylase, reducing the synthesis of bile acids.
Bile acids are amphipathic molecules and the most important components of bile.
Secretion:
- Secreted into bile canaliculi.
- Canaliculi merge into larger canals and ultimately the common bile duct.
- The sphincter of Oddi regulates bile flow into the duodenum.
Enterohepatic Circulation: Continous recycling of bile acids between the liver, small intestine, and gallbladder.

Exocrine Pancreas

Function: Secrete digestive enzymes and bicarbonate ions.
Secretion:
- Pancreatic Juice: Containing digestive enzymes and bicarbonate ions.
- Zymogens: Inactive forms of digestive enzymes.
Regulation of Pancreatic Secretion:
- Cephalic Phase: Stimulated by sight, smell, or thought of food.
- Gastric Phase: Stimulated by distention of the stomach.
- Intestinal Phase:
  - ↓pH: Stimulates secretion of secretin.
  - ↑ amino acids (AA), free fatty acids (FA): Stimulates secretion of CCK.
Effects of Regulators:
- Secretin: Stimulates production of bicarbonate-rich pancreatic juice.
- Ach (acetylcholine) and CCK: Stimulate secretion of enzyme-rich and chloride-rich fluid.
- Pancreatic Enzymes: Released into acini via exocytosis.
Pancreatic Insufficiency:
- Insufficient production of digestive enzymes by the exocrine pancreas.
- Leads to reduced digestion.
- Clinical signs: Greasy stools (steatorrhea), polyphagia, rapid weight loss.
Pancreatitis:
- Inflammation of the pancreas.
- Commonly seen in older, obese dogs.
- Risk factors: High-fat diet, human food, garbage.
- Pathophysiology: Acini are destroyed and replaced by connective tissue due to auto-digestion.

Carbohydrates

Functions:
- Energy source.
- Storage form of energy.
- Cell membrane component.
- Structural component.
Monosaccharides: Simple sugars with the general formula (CH2O)n.
Disaccharides: Two monosaccharides linked by glycosidic bonds.
Oligosaccharides: 3-10 monosaccharides.
Polysaccharides: More than 10 monosaccharides.
Digestion:
- Begins in the mouth (salivary amylase, in species that produce it).
- Continues in the small intestine with pancreatic enzymes and intestinal mucosal enzymes.
Absorption:
- Occurs in the duodenum and upper jejunum.
- Mediated by:
  - Sodium-dependent glucose transporter 1 (SGLT1) at the apical membrane.
  - Glucose transporters (GLUT5 and GLUT2) at both the apical and basolateral membranes.

Protein Digestion

Begins in the stomach:
- Pepsin partially hydrolyzes proteins (12-15%).
- Denaturation of proteins.
Continues in the small intestine:
- Pancreatic enzymes further digest polypeptides and oligopeptides.
- Brush border peptidases break down oligopeptides into tri- and dipeptides.
- Amino acids are transported into the cell and eventually to the liver via the portal vein.

Lipids

Functions:
- Prostaglandins.
- Steroid hormones.
- Phospholipids.
- Platelet-activating factor (PAF).
- Sphingomyelin.
Dietary Lipids:
- Cholesterol esters, phospholipids, triglycerides.
- Short-chain and medium-chain fatty acids (<12 carbons).
- Long-chain fatty acids (>12 carbons).
Digestion and Utilization:
- Emulsification:
  - Reduces droplet size.
  - Increases surface area of hydrophobic lipid droplets.
- Bile acids: Emulsify fat droplets.
- Pancreatic lipases: Hydrolyze triglycerides into monoglycerides and free fatty acids (FFA) in the duodenum.
Mixed Micelles:
- Formed by monoglycerides, FFA, cholesterol, and fat-soluble vitamins.
- Carry lipids to the brush border membrane for absorption.
Absorption:
- Long-chain fatty acids (LCFA) are re-esterified into triglycerides in the endoplasmic reticulum.
- Chylomicrons (CMs): Triglyceride-rich lipoproteins that transport lipids, primarily through lymphatic vessels.
- Chylomicrons enter the bloodstream via the thoracic duct, bypassing the liver.

Intestinal Absorption of Ions and Water

Sodium (Na+) Transport:
- Very efficient and drives most transport processes.
Potassium (K+) and Chloride (Cl-) Absorption:
- Primarily occurs in the small intestine through the paracellular pathway.
Water Absorption:
- 85% in the small intestine (duodenum and jejunum: 55%; ileum: 30%).
- 14% in the large intestine.
Factors Influencing Absorption:
- Increased surface area.
- Mucosa uptake mechanisms.
- High blood perfusion.
- Permeability.
- Pore size and water permeability of tight junctions vary along the intestine.

Intestinal Absorption of Essential Nutrients

Calcium (Ca++) Absorption:
- Parathyroid hormone (PTH): Stimulates vitamin D production in the kidney.
- Calcitriol (active vitamin D): Increases calcium absorption in the small intestine by:
  - Increasing apical Ca++ channels.
  - Increasing calbindin synthesis.
  - Increasing Ca++ ATPase activity.
Magnesium (Mg++) Absorption:
- Absorbed through Mg++ channels and paracellularly.
Phosphate (PO4--) Absorption:
- Absorbed through the Na+/Phosphate symporter.
Vitamin B12 Absorption:
- Haptocorrin (HC): Glycoprotein produced by salivary glands, binds to vitamin B12 in the stomach.
- Pancreatic enzymes (trypsin): Degrade HC in the duodenum.
- Intrinsic factor (IF): Produced by parietal cells in the stomach, binds to free vitamin B12.
- IF/vitamin B12 complex: Absorbed in the ileum.
- Transcobalamin: Transports vitamin B12 in the plasma.
Iron Absorption:
- Primarily occurs in the duodenum and proximal jejunum.
- Iron must be in the ferrous (Fe2+) state for absorption.
- The low pH in the proximal duodenum facilitates the conversion of ferric (Fe3+) to ferrous iron.
- Once in the bloodstream, iron binds to transferrin.

Iron Transport

Transferrin is a glycoprotein found in blood, which binds to iron and transports it throughout the body.
Absorption of vitamins occurs mainly in the small intestine for water-soluble vitamins.
Water-soluble vitamins are absorbed from the small intestine.
Vitamins produced by microbes in the large intestine are absorbed in the large intestine.
Specific carrier-mediated transport systems are responsible for the absorption of vitamins.
These transporters can either be secondary active transporters relying on the Na-K-ATPase or independent of sodium.
Fat-soluble vitamins (A, D, E, K) are absorbed through mixed micelles and enter the enterocyte by diffusion.

Catabolic Pathways

Generate energy (ATP, NADH) through the breakdown of energy-rich molecules.

Anabolic Pathways

Combine small molecules (e.g. amino acids) to form complex molecules (e.g. proteins).
Require energy (ATP→ADP) and often chemical reductions (NADH).

Phases of Energy Metabolism

Energy metabolism can be divided into three phases:

Absorptive Phase

  - During active digestion and absorption of nutrients from the gut.
  - Insulin is released.
  - Glucose is taken up by the liver and converted to glycogen and fatty acids.
  - Fatty acids are sent out of the liver as VLDL to adipose tissue or muscle.
  - Amino acids are used for protein synthesis or deaminated for gluconeogenesis.

Post-Absorptive Phase

  - Between meals when nutrients are mobilized from storage pools to tissues.
  - Glucagon is released.
  - Glycogenolysis and gluconeogenesis are stimulated
  - Amino acids are mobilized from muscle.

Prolonged Energy Deficiency or Food Deprivation

  - Glucose and amino acids are conserved.
  - Fatty acids are mobilized in the form of non-esterified fatty acids (NEFAs).
  - Formation of ketone bodies in the liver (mitochondria).

Definitions

Glycogen: The primary form of carbohydrate storage in the liver and muscle.
Glycogenolysis: The breakdown of glycogen to glucose
Gluconeogenesis: The generation of glucose.

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Explore the intricate structure and functions of the small intestine in this quiz. Learn about its specialized cells, such as enterocytes and goblet cells, as well as the significance of its large surface area for nutrient absorption. Test your knowledge on the different levels of surface convolutions and their roles in digestion.