Digestive Systems and Nutrition Notes

Learning Outcomes

• After completion of this session, students should be able to:

  • Define Brush Border Membrane (BBM) and its critical role in digestion and nutrient absorption within the intestine, emphasizing its structural uniqueness that optimizes enzymatic activities.

  • Differentiate comprehensively between luminal and membranous digestion, elucidating the distinct sites and mechanisms by which these processes occur.

  • Explain the intricate processes of digestion and absorption for carbohydrates, proteins, and lipids, detailing the specific enzymes involved and the physiological significance of each step.

  • Analyze the secretion and absorption processes of electrolytes and water, including the transport mechanisms and their importance in maintaining homeostasis.

  • Describe the multifunctional roles of the large intestine in digestion, focusing on water absorption, microbial fermentation, and fecal formation.

  • Explain in detail the two defecation reflexes—myenteric and parasympathetic—and their roles in gastrointestinal function and coordination.

  • Assess the consequences of damage to the pelvic and pudendal nerves on gastrointestinal (GI) functions, identifying potential clinical manifestations and implications for treatment.

Digestion of Carbohydrates

• Types of Carbohydrates: Starch and Glycogen, which are polysaccharides made of glucose, are important carbohydrates with α-1-4 and α-1-6 glycosidic bonds essential for energy supply.

• Luminal Phase:

  • Enzymes such as salivary amylase (from saliva) and pancreatic amylase hydrolyze α-1-4 bonds in starch and glycogen. This hydrolysis produces intermediate products, including oligosaccharides and some disaccharides (notably maltose), which remain non-absorbable until further digested in the BBM.

• Brush Border Phase:

  • Enzymes such as oligo- and disaccharidases (e.g., maltase) located within the BBM further hydrolyze these carbohydrates into monosaccharides, notably glucose, galactose, and fructose.

  • The end products are absorbable monosaccharides that play crucial roles in energy metabolism.

• Dietary Disaccharides:

  • Require specific disaccharidases located in BBM for complete digestion:

  • Sucrose (table sugar) is digested by Sucrase into glucose and fructose, crucial for energy.

  • Lactose (milk sugar) is hydrolyzed by Lactase into glucose and galactose, where a significant percentage of adults may develop lactose intolerance post-weaning, affecting their ability to digest dairy products.

Absorption of Monosaccharides

• Mechanisms:

  • Glucose and Galactose absorption occurs through a secondary active transport mechanism using Na+-cotransport, which relies on the sodium gradient created by the Na+/K+ pump. This ensures efficient uptake of these vital energy sources.

  • Fructose absorption relies on facilitated diffusion through the GLUT5 transporter, with a distinct pathway due to its structure.

• Exit from Enterocyte:

  • Absorbed monosaccharides exit the enterocytes by facilitated diffusion, entering the interstitial fluid before being transported into the bloodstream, significantly contributing to overall glucose homeostasis.

Digestion of Proteins

• Proteins must be broken down into tri- or dipeptides or amino acids before absorption to ensure bioavailability.

• Luminal Phase:

  • The digestive process begins in the stomach, where HCl denatures proteins and activates pepsin, leading to the breakdown of these proteins into polypeptides.

  • Pancreatic endopeptidases (such as trypsin) and exopeptidases then further hydrolyze these polypeptides into smaller oligopeptides, enhancing their digestibility in the subsequent phase.

• Brush Border Phase:

  • Brush border enzymes efficiently convert oligopeptides into tri- and dipeptides, and free amino acids, ensuring that the fractions are suitable for absorption.

• In Newborns:

  • Newborn infants can absorb immunoglobulins from colostrum through endocytosis, due to a period of limited HCl and pro-chymosin secretion, providing crucial passive immunity shortly after birth.

Absorption of Amino Acids and Dipeptides

• Both amino acids and dipeptides are absorbed via cotransport mechanisms with Na+ or H+, emphasizing the energy-intensive effort required for proper absorption.

• Effective absorption necessitates intact enterocytes equipped with specific transporters; Na+ absorption is essential for energy and nutrient transport as well as maintaining osmotic balance in the intestinal environment.

Digestion/Absorption of Fats & Free Fatty Acids

• The digestion of dietary fats relies on pancreatic enzymes and bile salts instead of brush border enzymes, underlining the importance of emulsification in fat digestion.

• Emulsification: Bile salts play a critical role by breaking down large fat droplets, significantly enhancing the efficiency of fat digestion by pancreatic lipase/colipase, which hydrolyzes triglycerides into monoglycerides and free fatty acids.

• Micelle Formation: Following hydrolysis, monoglycerides and fatty acids complex with bile salts to form micelles, facilitating their delivery to the BBM for absorption by simple diffusion into enterocytes.

• Post-Absorption: Inside enterocytes, fatty acids and monoglycerides are reassembled into triglycerides and packaged into chylomicrons for transport via the lymphatic system, which is vital for fat metabolism.

Absorption and Reabsorption of Electrolytes & Water

• Active Secretion: The gastrointestinal tract secretes large amounts of electrolytes and fluids into the gut lumen; the resultant osmotic pressure from nutrient hydrolysis draws water into the gut, raising the importance of maintaining fluid balance in digestion.

• Water Absorption: Primarily follows sodium and other osmotic gradients, allowing for efficient reabsorption back into the bloodstream, crucial for homeostasis.

• Bicarbonate Absorption: Involves sodium gradients, facilitating the buffering of gastric acid to maintain pH neutrality necessary for optimal enzymatic function.

Large Intestine Functions

• The large intestine plays key roles in processing indigestible materials, which include:

  1. Propulsion of residual chyme towards the rectum for elimination.

  2. Absorption of remaining water and key electrolytes that are crucial in preventing dehydration.

  3. Microbial fermentation of undigested matter, particularly in nonruminant herbivores, fostering a unique gut microbiome that aids digestion.

  4. Storage and subsequent elimination of feces, highlighting the importance of this organ in waste management.

Large Intestinal Motility

• Controlled by the ileocecal sphincter, the movements of the large intestine include:

  • Mixing and segmentation to enhance nutrient absorption while aiding in the formation of fecal matter.

  • Peristaltic movements driven by gastro-colic reflexes, which stimulate mass movements in response to food intake as well as to colon irritation, ensuring efficient material transit.

Secretions and Absorption in the Large Intestine

• The mucosa of the large intestine is structurally simpler than that of the small intestine, lacking villi, which reduces the surface area available for absorption. Fluid secretion, fermentation, and absorption predominantly occur in the crypts of the colon, underscoring its distinct physiological role.

Defecation Process

• Myenteric Reflex: Initiates mass movements which stimulate peristalsis and relaxation of the internal sphincter in response to rectal filling.

• Parasympathetic Reflexes: Act to enhance signals for increased peristalsis, especially after rectal distension, contributing to the coordination of defecation.

• Damage to pelvic nerves may disrupt this reflex, potentially causing constipation, while injury to the pudendal nerves can lead to fecal incontinence, illustrating the critical need for intact neural pathways in GI function.

Case Study Example

• A 6-year-old male Boxer experiencing fecal incontinence likely suffers from pelvic nerve damage, as indicated by clinical signs and diagnostic imaging results, illustrating how nerve function is pivotal for proper GI control and health.