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Anatomy & Physiology [ SKF]

Outline for the Document: "Anatomy and Physiology of the Small Intestine"

1. Introduction

  • Overview of the small intestine
  • Functions: Nutrient absorption, water and electrolyte balance, immune barrier, endocrine secretion

2. Embryology

  • Development of the small intestine during gestation
  • Midgut formation and rotation
  • Role of the superior mesenteric artery
  • Congenital malformations (e.g., Meckel’s diverticulum, malrotation)

3. Cell Differentiation

  • Gut development along four axes
  • Recanalization and villi formation
  • Role of epithelial stem cells
  • Differentiation into four major epithelial cell types

4. Anatomy

  • Length and sections: Duodenum, jejunum, ileum
  • Layers of the small intestine: Mucosa, submucosa, muscularis propria, serosa
  • Specialized structures: Plicae circulares, villi, microvilli, Brunner glands, Peyer patches

5. Duodenum

  • Sections and relations with pancreas and other organs
  • Importance in digestive processes
  • Vascular and neural supply

6. Jejunum and Ileum

  • Differences in structure and function
  • Vascular supply: SMA branches, vasa recta
  • Role in nutrient absorption

7. Ileocecal Valve (ICV)

  • Function in preventing backflow from the colon
  • Anatomical and neural structure

8. Vascular Supply

  • Arterial supply: Celiac trunk, SMA, GDA, pancreaticoduodenal arteries
  • Venous drainage: Portal vein, superior mesenteric vein
  • Lymphatic drainage

9. Innervation

  • Autonomic Nervous System (ANS): Sympathetic and parasympathetic control
  • Enteric Nervous System (ENS): Role in motility, secretion, and local reflexes
  • Interaction between ENS and CNS

10. Small Intestinal Motility

  • Myogenic, neural, and hormonal control
  • Fed and fasting states
  • Migratory Motor Complex (MMC)

11. Hormonal Control

  • Role of hormones like Gastrin, CCK, Motilin, VIP, Secretin, and Glucagon
  • Effects on motility and digestion

12. Small Intestinal Secretion

  • Secretory functions: Mucin, bicarbonate, and water
  • Regulation by immune mediators and neural control
  • Role of Brunner glands and Peyer patches

13. Factors Affecting Small Bowel Motility

  • Systemic diseases (e.g., liver disease, NASH)
  • Colonic distention, obesity, circadian rhythm disruption
  • Impact on gastrointestinal disorders

14. Conclusion

  • Summary of key points
  • Clinical relevance of the anatomy and physiology of the small intestine

Pre-Study Material for MCQ Exam on Small Bowel Anatomy and Physiology

Key Concepts to Review:

  1. Embryology of the Small Intestine
    • Development of the gut tube
    • Midgut rotation and associated congenital malformations
  2. Anatomical Structures
    • Identification and function of the duodenum, jejunum, ileum
    • Differences between jejunum and ileum
  3. Cellular Differentiation and Function
    • Role of epithelial cells: Paneth, enteroendocrine, goblet, enterocyte
    • Stem cell origin and renewal process in intestinal crypts
  4. Vascular and Nervous Supply
    • Blood supply through SMA and its branches
    • Venous drainage via the portal system
    • ENS and ANS roles in motility and secretion
  5. Physiological Processes
    • Mechanisms of nutrient absorption and digestion
    • Motility patterns: Segmentation, peristalsis, MMC
    • Hormonal regulation (Gastrin, CCK, Secretin, etc.)
  6. Clinical Correlations
    • Impact of systemic diseases on small bowel function
    • Disorders of motility and their management
    • Surgical implications of small bowel anatomy

MCQ Exam Preparation Tips:

  • Focus on understanding the layers of the intestinal wall and their respective roles.
  • Pay close attention to the vascular supply, particularly the differences in blood flow to the jejunum and ileum.
  • Be able to distinguish between different hormones and their effects on small bowel motility.
  • Review common congenital anomalies related to the small intestine's embryological development.

Embryology of the Small Intestine

Formation and Development:

  • Primitive Gut Tube:
    • Formed at 4 weeks of gestation.
    • Lined initially by simple columnar epithelium.
    • Later it converts to Stratified Columnar
    • Composed of endoderm (epithelium) and mesoderm (muscle and connective tissue).
  • Vitelline Duct:
    • Midgut connected to the yolk sac by the vitelline duct.
    • Persistent vitelline duct results in Meckel's diverticulum.
  • Herniation and Rotation: starts at 5th week
    • 5th to 7th week: Midgut herniates and undergoes a 90-degree counterclockwise rotation.
    • 10th to 12th week: Midgut retracts with an additional 180-degree counterclockwise rotation (total 270 degrees).

Recanalization and Villi Formation:

  • 6th Week:
    • Proliferation of the epithelium leads to occlusion of the lumen.
  • 7th-8th Week:
    • Vacuoles develop, leading to recanalization of the lumen.
    • Mesoderm pushes into the epithelium, forming villi.
    • Mesoderm also forms the submucosa and muscular layer.

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From Speed: SKF

  • 5th Week:
    • Physiological Herniation: The midgut herniates into the umbilical cord due to rapid growth.
    • Endoderm Proliferation: The endoderm proliferates, leading to the occlusion of the gut tube.
  • 9th Week:
    • Recanalization: The previously occluded gut tube undergoes recanalization, restoring the lumen.
    • Villi Formation: The initial formation of villi begins, enhancing the absorptive surface of the gut.
  • 10th Week:
    • Return to Abdominal Cavity: The intestines retract from the umbilical cord back into the abdominal cavity.
  • 10th-12th Week:
    • Crypt Formation: The development of crypts in the small intestine begins, contributing to the overall structure and function of the intestinal lining.

Cell Differentiation:

  • Stem Cells:

    • Reside within the crypts, dividing into daughter cells.
    • One daughter cell remains anchored in the crypt, the other migrates to the villus.

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  • Peyer’s Patches = help in immunological function

  • M cells = cover the lymphoid follicles and provide a site for selective sampling of intraluminal antigens
  • Four Major Epithelial Cells:
    • Paneth Cells
      • always remain at base
      • secrete lysozyme, TNF, Cryptidins
      • Protect adjacent stem cells
    • Enteroendocrine Cells
    • Goblet Cells
    • Enterocytes
  • Digestive Function:
    • Develops only after exposure to food, essential for proper intestinal function.
  • Vitelline Duct:
    • The midgut is connected to the yolk sac by a thin stalk known as the vitelline duct.
    • If the vitelline duct persists, it results in a Meckel's diverticulum.
  • Primitive Gut Tube Formation:
    • Formed at 4 weeks of gestation.
    • Composed of endoderm (which forms the epithelium) and mesoderm (which forms muscle and connective tissue).

Answer: A) Simple columnar epithelium

Explanation: During gestation, the gastrointestinal tract of the embryo is lined by simple columnar epithelium. This epithelium eventually differentiates into specialized cells such as enterocytes, goblet cells, and Paneth cells.

Answer: A) Fetal intestinal epithelium develops digestive function by 21 weeks

Explanation: The fetal intestinal epithelium does not develop digestive function by 21 weeks of gestation. Digestive function only develops after exposure to food post-birth. The other options are accurate statements regarding the embryology of the small intestine.

Answer: C) Т2

Explanation: The superior mesenteric artery (SMA) initially originates at the T2 level during embryonic development and later migrates to the L2 level as development progresses. Initially Celiac axis is at C7, descends to T12.

MCQ Discussion: Physiological Herniation of the Intestine

Answer: A) 5 weeks

Explanation: The physiological herniation of the intestine, where the midgut temporarily herniates into the umbilical cord due to rapid growth, typically begins at 5 weeks of gestation. This herniation is a normal part of development, allowing space for the intestines to continue growing before they retract back into the abdominal cavity around the 10th week.

MCQ Discussion: Cells in the Crypts of Lieberkühn

Answer: C) Paneth cells

Explanation: Paneth cells are predominantly located at the base of the crypts of Lieberkühn in the small intestine. They play a crucial role in maintaining intestinal homeostasis by secreting antimicrobial peptides such as lysozyme and defensins, which help to regulate the gut microbiota and protect against pathogenic bacteria. The other cell types, such as enterocytes, enteroendocrine cells, and goblet cells, are present in the crypts and villi but are not predominantly found at the base.

MCQ Discussion: Function of M Cells in the Small Intestine

Answer: A) Antigen processing

Explanation: M cells (Microfold cells) are specialized epithelial cells found in the small intestine, particularly overlying Peyer's patches and other lymphoid follicles. Their primary function is antigen processing—they capture antigens from the intestinal lumen and transport them to underlying immune cells, such as dendritic cells and lymphocytes, facilitating the initiation of immune responses. They are not directly involved in digestion, lubrication, or IgA production [By B cells] , although they play a crucial role in the gut's immune surveillance system.

Anatomy of the Small Intestine

1. General Overview

  • Length: Approximately 7 meters, extending from the pylorus to the ileocecal valve.
  • Divisions:
    • Duodenum: ~25 cm, retroperitoneal.
    • Jejunum: ~40% of the remaining length.
    • Ileum: ~60% of the remaining length.
  • Rapid Turnover of Epithelium:
    • The epithelium of the small intestine is replaced every 3 to 6 days and can be influenced by a variety of factors.
    • The rapid turnover and high mitotic rate of the cells makes the intestinal lining susceptible to the effects of radiation and chemotherapy.

2. Layers of the Small Intestine

  • Mucosa:
    • Innermost layer responsible for nutrient and water absorption.
    • Composed of three sub-layers:
      • Epithelium: Single layer of columnar cells.
      • Lamina propria: Loose connective tissue.
      • Muscularis mucosae: Thin layer of smooth muscle.
  • Submucosa: = Strongest layer
    • Dense connective tissue providing structural support.
    • Contains blood vessels, lymphatics (including Peyer patches), and nerves (Meissner plexus).
    • Important for ensuring the integrity of bowel anastomosis during surgery.
  • Muscularis Propria:
    • Two layers of smooth muscle:
      • Outer longitudinal layer
      • Inner circular layer
    • Contains the Auerbach (myenteric) plexus, essential for controlling bowel motility.
  • Serosa:
    • Outermost layer composed of a single layer of mesothelial cells.
    • Provides a smooth, frictionless surface for the small intestine within the abdominal cavity.

3. Specialized Structures

  • Plicae Circulares:
    • Transverse folds of the mucosa and submucosa.
    • Increase the surface area threefold for enhanced nutrient absorption.
    • Visible on radiographic imaging.
  • Villi:
    • Finger-like projections of the mucosa, present along the entire length.
    • Longest in the distal duodenum and proximal jejunum, shortest in the distal ileum.
    • Coated with enterocytes and interspersed with goblet cells.
    • Increase the absorptive area tenfold.
  • Microvilli:
    • Tiny projections on the apical border of enterocytes.
    • Coated with glycocalyx, containing brush border enzymes like nucleosidases, peptidases, and disaccharidases.
    • Increase the absorptive area twentyfold.

4. Vascular Supply

  • Arterial Supply:

    • Duodenum: Receives blood from both the celiac artery (via the GDA) and the superior mesenteric artery (SMA).
    • Jejunum and Ileum: Supplied by branches of the SMA.
    • Vasa Recta: Long and straight in the jejunum, shorter and more branched in the ileum.

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  • Venous Drainage:

    • Drains into the portal vein via the superior mesenteric vein.

5. Lymphatics

  • Lymphatic Drainage: Follows the blood vessels, draining into nodes along the mesenteric arcade and eventually into the cisterna chyli.

6. Innervation

  • Autonomic Nervous System (ANS):
    • Sympathetic Innervation: Originates from T5-L2 spinal segments; inhibits motility and secretion.
    • Parasympathetic Innervation: Provided by the vagus nerve; stimulates motility and secretion.

  • Enteric Nervous System (ENS):

    • Auerbach (Myenteric) Plexus: Controls motor activity.
    • Meissner (Submucosal) Plexus: Regulates secretion and local blood flow.

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MCQ Discussion: Anatomy of the Small Intestine

Answer: D) Vasa recta of ileum are longer than that of jejunum

Explanation: The statement in option D is false. The vasa recta in the jejunum are longer and straighter compared to those in the ileum, where the vasa recta are shorter and more branched. The other statements accurately describe the anatomy of the small intestine.

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MCQ Discussion: Villi in the Small Intestine

Answer: A) Duodenum

Explanation: The longest villi in the small intestine are found in the duodenum. The villi progressively decrease in length as you move along the small intestine towards the ileum, where they are shortest. The villi in the duodenum are longer to maximize the absorption of nutrients as chyme enters from the stomach.

MCQ Discussion: Anatomy of the Duodenum

Answer: C) Third

Explanation: The third part of the duodenum (also known as the horizontal or transverse part) is the longest section. It runs horizontally across the vertebral column, anterior to the aorta and inferior vena cava, and is approximately 8-10 cm in length. The other parts are generally shorter in comparison.

Physiological Processes in the Small Intestine

1. Nutrient Absorption

  • Primary Function: The small intestine is the major site for the absorption of nutrients, including carbohydrates, proteins, fats, vitamins, and minerals.
  • Enhancement of Surface Area:
    • Plicae Circulares: Permanent circular folds of the mucosa and submucosa that increase the surface area for absorption.
    • Villi: Finger-like projections extending into the lumen, covered by epithelial cells.
    • Microvilli: Tiny hair-like projections on the surface of enterocytes, forming the brush border. These structures increase the surface area by 600 to 1000 times, making nutrient absorption highly efficient.
  • Villus Structure:
    • Each villus contains a central lacteal (lymphatic vessel) that absorbs fats in the form of chylomicrons.
    • A dense network of capillaries absorbs amino acids, monosaccharides, and other nutrients directly into the bloodstream.

2. Digestive Function

  • Brush Border Enzymes:
    • Embedded in the microvilli, these enzymes include:
      • Disaccharidases (e.g., lactase, sucrase): Break down disaccharides into monosaccharides.
      • Peptidases: Hydrolyze peptides into amino acids.
      • Enterokinase: Activates pancreatic trypsinogen to trypsin, crucial for protein digestion.
  • Lipid Absorption:
    • Emulsification: Bile salts from the liver emulsify fats into micelles, increasing the surface area for pancreatic lipase action.
    • Pancreatic Lipase: Breaks down triglycerides into free fatty acids and monoglycerides.
    • Absorption: Free fatty acids and monoglycerides diffuse into enterocytes, where they are re-esterified into triglycerides and packaged into chylomicrons.
    • Chylomicrons: These are transported via lacteals into the lymphatic system, eventually entering the bloodstream.

3. Motility Patterns

  • Fed State:
    • Segmentation:
      • Involves localized, rhythmic contractions of the circular muscles, which mix chyme and enhance contact with the absorptive surface of the mucosa.
      • This mixing action ensures that nutrients are effectively absorbed.
    • Peristalsis:
      • Coordinated, wave-like contractions that move chyme forward along the length of the small intestine.
      • Peristalsis is crucial for propelling food from the duodenum to the ileum.
  • Fasting State:
    • Migrating Motor Complex (MMC):
      • A cyclic motility pattern that occurs approximately every 90-120 minutes during fasting.
      • Phase I: A period of quiescence with minimal electrical and contractile activity.
      • Phase II: Characterized by intermittent, irregular contractions that begin to sweep through the stomach and intestine.
      • Phase III: The "housekeeper" phase, where strong, regular contractions move indigestible residues and bacteria out of the small intestine.
      • Phase IV: A brief transition back to Phase I.
      • Function: The MMC helps to clear the small intestine of residual food, secretions, and bacteria, preventing bacterial overgrowth.

4. Hormonal Regulation

  • Gastrin:
    • Produced by G cells in the stomach and duodenum.
    • Increases gastric acid secretion and gastric motility, indirectly promoting small intestine motility.
  • Cholecystokinin (CCK):
    • Secreted by I cells in the duodenum and jejunum in response to the presence of fats and proteins.
    • Stimulates gallbladder contraction to release bile and pancreatic enzyme secretion.
    • Slows gastric emptying to allow more time for nutrient digestion and absorption in the small intestine.
  • Secretin:
    • Released by S cells in the duodenum in response to acidic chyme.
    • Stimulates the pancreas to secrete bicarbonate-rich fluid to neutralize the acid, protecting the mucosa and optimizing the pH for digestive enzymes.
  • Motilin:
    • Secreted by M cells in the duodenum and jejunum during fasting.
    • Initiates the MMC, promoting the cleansing of the small intestine between meals.
  • Vasoactive Intestinal Peptide (VIP):
    • Promotes relaxation of smooth muscle, increases water and electrolyte secretion into the lumen, and inhibits gastric acid secretion.
    • Plays a role in enhancing the local environment for digestion and absorption.

5. Neural Control

  • Enteric Nervous System (ENS):
    • Known as the "second brain," the ENS is a complex network of neurons embedded within the gut wall.
    • Auerbach (Myenteric) Plexus:
      • Located between the longitudinal and circular layers of the muscularis propria.
      • Controls peristalsis and overall motor activity.
    • Meissner (Submucosal) Plexus:
      • Found within the submucosa.
      • Regulates local blood flow, secretions, and absorption processes.
    • The ENS can function independently of the central nervous system but is modulated by the autonomic nervous system.
  • Autonomic Nervous System (ANS):
    • Sympathetic Innervation:
      • Originates from the thoracolumbar spinal cord (T5-L2).
      • Norepinephrine release leads to decreased motility, secretion, and blood flow by causing vasoconstriction.
      • Prepares the body for "fight or flight" by diverting blood flow away from the gut.
    • Parasympathetic Innervation:
      • Provided by the vagus nerve (cranial nerve X).
      • Acetylcholine release promotes increased motility, secretion, and blood flow, facilitating digestion and nutrient absorption.

6. Water and Electrolyte Balance

  • Sodium Absorption:
    • Sodium is absorbed primarily through the co-transport mechanism with glucose and amino acids.
    • Na+/K+ ATPase pump on the basolateral membrane maintains the electrochemical gradient, driving sodium absorption.
  • Chloride Absorption:
    • Absorbed primarily through chloride-bicarbonate exchange mechanisms in the enterocytes.
  • Water Absorption:
    • Water moves osmotically across the intestinal wall, following the absorption of nutrients and electrolytes.
    • The majority of water absorption occurs in the small intestine, crucial for maintaining fluid balance.
  • Bicarbonate Secretion:
    • The duodenum secretes bicarbonate to neutralize the acidic chyme from the stomach, protecting the mucosa and creating an optimal pH for enzyme activity.

7. Immune Function

  • Peyer’s Patches:
    • Aggregates of lymphoid tissue primarily found in the ileum.
    • Play a crucial role in the immune surveillance of the intestinal lumen, detecting pathogens and initiating immune responses.
  • M Cells:
    • Specialized epithelial cells found in the lining of Peyer’s patches.
    • Transport antigens from the intestinal lumen to the underlying immune cells, facilitating an immune response.
  • IgA Secretion:
    • Immunoglobulin A (IgA) is secreted by plasma cells in the lamina propria and transported across the epithelium into the lumen.
    • IgA plays a critical role in neutralizing pathogens and preventing their adherence to the mucosal surface.

MCQ Discussion: Promotility Hormones

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Answer: C) Glucagon

Explanation:

  • Glucagon is not a promotility hormone; in fact, it has an inhibitory effect on gastrointestinal motility. The other options, Gastrin, VIP (Vasoactive Intestinal Peptide), and CCK (Cholecystokinin), are known to promote motility in different parts of the gastrointestinal tract. Gastrin increases the contraction rate of the stomach, VIP can stimulate intestinal motility under certain conditions, and CCK enhances intestinal motility, especially in response to the presence of fats.
  • Secretin: Primarily functions to inhibit gastric motility and decrease gastric acid secretion, which indirectly slows small intestinal motility. Its main role is to promote the secretion of bicarbonate from the pancreas to neutralize the acidic chyme entering the duodenum.

MCQ Discussion: Hormonal Control of Small Intestinal Secretion

Answer: C) Neuropeptide YY

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Explanation: According to the table provided in the image, Neuropeptide YY (Peptide YY) is listed as an antisecretory hormone. It increases the absorptive time and decreases gastric and pancreatic secretions, effectively reducing the secretion activities in the small intestine. In contrast, VIP (Vasoactive Intestinal Peptide), Serotonin, and Cholecystokinin are classified as prosecretory hormones, meaning they promote the secretion of fluids and electrolytes into the small intestine.

MCQ Discussion: Vitamin Absorption in the Small Intestine

Answer: A) B6

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Explanation: According to the table provided in the image, Vitamin B6 (pyridoxine) and Nicotinic acid are absorbed in the small intestine by passive diffusion. In contrast, Folic acid is absorbed via Na⁺-independent brush border carriers, Vitamin K (being fat-soluble) is absorbed via chylomicrons, and Vitamin B12 (cobalamin) is absorbed through translocation with intrinsic factor.

MCQ Discussion: Absorption Mechanisms in the Small Intestine

Answer: C) Chloride - Active transport

Explanation:

  • A) Iron: Absorbed primarily in the duodenum through active transport mechanisms, particularly via the divalent metal transporter 1 (DMT1).
  • B) Calcium: Absorbed in the duodenum and jejunum through active transport that is regulated by vitamin D, especially in low calcium conditions.
  • C) Chloride: Chloride is generally absorbed via passive diffusion through chloride channels and through chloride-bicarbonate exchange, not active transport.
  • D) Vitamin C: Absorbed in the small intestine via active transport through sodium-dependent vitamin C transporters (SVCT1 and SVCT2).

Thus, the incorrect match is C) Chloride - Active transport, as chloride absorption primarily occurs via passive mechanisms.

MCQ Discussion: Structure of the Small Intestinal Wall

Answer: B) Submucosa

Explanation: The submucosa is considered the strongest layer of the small intestinal wall. It consists of dense connective tissue that provides structural support and contains blood vessels, nerves, and lymphatics. The strength of the submucosa is critical in maintaining the integrity of the bowel, particularly during surgical procedures such as anastomosis. The other layers, while important for their respective functions, do not contribute as significantly to the overall strength and structural integrity of the intestinal wall.

MCQ Discussion: Absorption of Digestion Products

Answer: B) Fat

Explanation: Fat is absorbed passively in the small intestine. After being broken down into fatty acids and monoglycerides by pancreatic lipases, these lipid molecules diffuse passively across the enterocyte membrane due to their lipid-soluble nature. In contrast, carbohydrates and proteins require active transport mechanisms for absorption, and starch needs to be broken down into simpler sugars before absorption can occur.

MCQ Discussion: Glucose Transport Mechanisms

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Answer: D) GLUT-3

Explanation: GLUT-3 is not involved in glucose transport in the small intestine. The primary transporters for glucose in the small intestine are:

  • SGLT-1: Sodium-glucose co-transporter 1, responsible for the active transport of glucose and galactose into enterocytes.
  • GLUT-2: Facilitates the transport of glucose from enterocytes into the bloodstream.
  • GLUT-5: Primarily involved in the transport of fructose, not glucose.

GLUT-3 is primarily found in neurons and is not involved in glucose transport in the small intestine.

MCQ Discussion: Carbohydrate Absorption

Answer: C) GLUT-2 involves active transport of glucose from cells into blood and D) GLUT-4 mediates passive transport of fructose from cells into blood

Explanation:

  • C) GLUT-2: GLUT-2 actually facilitates passive (facilitated diffusion) transport of glucose and other monosaccharides (including fructose) from enterocytes into the bloodstream. It does not involve active transport.
  • D) GLUT-4: GLUT-4 is not involved in fructose transport. GLUT-4 is an insulin-regulated glucose transporter found primarily in adipose tissue and striated muscle (skeletal and cardiac). Fructose is transported passively by GLUT-5 (into the cell) and by GLUT-2 (out of the cell), not by GLUT-4.

The correct information is that SGLT-1 mediates the active transport of glucose and galactose into enterocytes, and GLUT-5 mediates the passive absorption of fructose from the lumen into the enterocytes.

Physiological Processes of Protein and Fat Digestion:

  • Protein Digestion:
    • Location: 80-90% of protein digestion occurs in the jejunum.
    • Enzymatic Activation:
      • Trypsinogen is converted to trypsin by enterokinase.
      • Trypsin then activates other proteases like chymotrypsin and elastase.
  • Protein Absorption:
    • Proteins are absorbed through active transport mechanisms, primarily in the jejunum.
  • Fat Absorption:
    • Transport Mechanism: Fat is absorbed via passive transport.
    • Process:
      • Fats are emulsified into micelles by bile salts.
      • Micelles diffuse into enterocytes, where they are reassembled into chylomicrons.
      • Chylomicrons are then absorbed into the lacteals (lymphatic vessels) and transported through the lymphatic system.

Physiologic Functions of GI Hormones

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MCQ Discussion: Gastrointestinal Hormones

Answer: D) Gastrin Releasing Peptide

Explanation:

  • Gastrin Releasing Peptide (GRP): Also known as Bombesin, is considered the "universal on switch" in the gastrointestinal system. It stimulates the release of various GI hormones, except for secretin. GRP plays a key role in regulating gastric functions by stimulating the secretion of gastrin, which in turn promotes gastric acid secretion.
  • Somatostatin: Acts as the "universal off switch," inhibiting the release of various hormones, including gastrin, insulin, glucagon, and others.
  • Peptide YY: Primarily involved in inhibiting gastric motility and secretions, thus it is not an "on switch."
  • Neurotensin: Modulates intestinal motility and secretion but does not act as a universal "on switch."

Additionally:

  • Gastric Inhibitory Peptide (GIP): Secreted by K cells in the duodenum and jejunum, inhibits gastric acid and pepsin secretion, and stimulates insulin release in response to hyperglycemia.

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