Digestion and Absorption

Digestive System - Chapter 26 Part 2

• Small Intestine:
  • A long tube inferior to the stomach, located medially in the abdominal cavity.
  • Nutrients reside here for at least 12 hours.
  • Absorbs most nutrients, a significant portion of water, electrolytes, and vitamins.
  • Coiled, thin-walled tube extending from the pylorus to the cecum of the large intestine.
  • Consists of three segments: duodenum, jejunum, and ileum.

Small Intestine - Segments

• Duodenum:

  • First segment, originating at the pyloric sphincter.
  • C-shaped around the head of the pancreas.
  • Continuous with the jejunum at the duodenojejunal flexure.
  • Mostly retroperitoneal, with the initial portion being intraperitoneal.
  • Receives chyme from the stomach.
  • Receives accessory gland secretions from the liver, gallbladder, and pancreas.

• Jejunum:

  • Middle region of the small intestine.
  • Two-fifths of the total length of the small intestine.
  • Primary site for chemical digestion and nutrient absorption.
  • Intraperitoneal and suspended by the mesentery proper.

• Ileum:

  • Last region of the small intestine.
  • Forms three-fifths of the small intestine.
  • Terminates at the ileocecal valve.
  • Sphincter controlling entry of materials into the large intestine.
  • Intraperitoneal and suspended by mesentery proper.
  • Continues absorption of digested materials.

Small Intestine Tunics

• Circular folds:

  • Internal folds of the mucosal and submucosal tunics.
  • Increase the surface area for nutrient absorption.
  • Function as "speed bumps" to slow movement of chyme.
  • More numerous in the duodenum and jejunum, less in the ileum.

• Villi:

  • Fingerlike projections of the mucosa.
  • Larger and more numerous in the jejunum.
  • Epithelium covers the lamina propria, which increases the surface area.
  • Contains an arteriole, capillary network, and lymphatic vessel.

• Microvilli:

  • Extensions of plasma membrane of simple columnar epithelial cells.
  • Increase the surface area of the small intestine.
  • Contain brush border enzymes that aid in digestion.
  • Also contain proteins needed to transport digested products

• Lacteal:

  • Lymphatic capillary within villi.
  • Absorbs lipids and lipid-soluble vitamins.

Submucosal Gland (AKA Brunner's Gland)

• Produces alkaline mucus to protect the duodenum from chyme.
• Found in the duodenum only.

Small Intestine Motility

• Smooth muscle:

  • Mixes chyme with gland secretions, using a backward and forward motion (Segmentation).
  • Moves chyme against new areas of brush border.
  • Propels chyme through the small intestine via peristalsis.

• Segmentation:

  • More common in earlier stages than peristalsis.
  • Backward and forward motion to mix materials.

• Regulation of small intestine motility during the intestinal phase:

  • Contractions initiated by pacemaker cells and spread via gap junctions (similar to the stomach).
  • Spontaneously depolarize, more frequent in the duodenum than the ileum.
  • Force of contraction regulated by both ENS and ANS (enteric nervous system and autonomic nervous system).
  • Peristalsis prevalent later in the phase, initiated by motilin (released from duodenum).
  • Successive waves of contractions form a motility complex that continues until all contents have moved to the large intestine.

• Gastroileal reflex:

  • Involves the ileocecal sphincter relaxing, allowing contents to move from the ileum to the cecum.
  • Initiated when food enters the stomach

Accessory Digestive Organs

• Gallbladder:
  • Saclike organ attached to the inferior surface of the liver.
  • Stores, concentrates, and releases bile produced in the liver.
  • Aids in the digestion of fats.
  • Has three tunics: inner mucosa, middle muscularis, and outer serosa.
  • Connected to the common bile duct by the cystic duct.
• Pancreas:
  • Endocrine function: produces and secretes insulin and glucagon.
  • Exocrine function: produces pancreatic juice to assist in digestive activities (containing enzymes to digest starch, triglycerides, proteins, and nucleic acids)

Biliary Apparatus

• A network of ducts draining the liver's lobes (left and right) that merge to form a common hepatic duct.
• The cystic duct from the gallbladder merges with the common hepatic duct to form the common bile duct.
• The common bile duct projects inferiorly to the duodenum.

Liver

• Accessory digestive organ, largest internal organ in the body.
• Primarily responsible for bile production.
• Has a connective tissue capsule and a layer of visceral peritoneum covering most of its surface.
• Inferior vena cava is visible on posterior inferior view.

Liver Anatomy

• Liver has 2 major lobes (right and left lobes) and 2 smaller accessory lobes.
• The classic liver lobule structure features hepatocytes that perform the majority of metabolic tasks.
• The portal triad includes the hepatic artery, hepatic portal vein, and bile duct.
• Hepatic sinusoids provide nutrients and oxygen to the hepatocytes; blood flows through the sinusoids and eventually enters the hepatic veins.
• Blood and bile flow in opposite directions for efficient absorption and secretion of nutrients.
• Blood leaves by way of hepatic veins which empty into the inferior vena cava.

Clinical Views

• Gallstones (Cholelithiasis):
  • Majority asymptomatic until lodged in the cystic duct.
  • Can lead to gallbladder inflammation (cholecystitis)
  • Symptoms: severe pain in the right hypochondriac region, nausea, vomiting, indigestion, bloating.
  • May require cholecystectomy (surgical removal)
  • Risk factors: Female sex, obesity, age, and Caucasian ethnicity.
• Appendicitis: Inflammation of the appendix, often due to fecal matter obstruction.
  • Can lead to infection (peritonitis) if untreated.
  • Pain: initially referred to the umbilicus and eventually localized to the right lower quadrant.
  • Symptoms include nausea, vomiting, and abdominal tenderness.
  • Often requires surgical removal (appendectomy).

Large Intestine

• Functions:

  • Absorb water and electrolytes from remaining digested material.
  • Compacting watery chyme into feces.
  • Storing feces until eliminated by defecation.
  • Containing resident flora.

• Gastrocolic reflex:

  • Reflex that stimulates colon peristalsis in response to stomach stretch or chemical processes starting in the duodenum, leading to the urge to defecate.

• Defecation reflex:

  • Filling of the rectum initiates the urge to defecate.
  • Signals the spinal cord; increases parasympathetic output to the sigmoid colon and rectum, causes sphincter relaxation.
  • Voluntary control, learned around age 3.
  • Involves the Valsalva maneuver (a forceful exhalation against a closed glottis).

• Anal Canal:

  • Terminal centimeters of the large intestine.
  • Storage of fecal matter prior to defecation.
  • Rectal valves that retain fecal matter during gas passing.

Carbohydrate Digestion

• Monosaccharides: glucose, fructose, galactose
• Disaccharides: sucrose, maltose
• Polysaccharides: starch, cellulose
• Absorption: cotransport with Na+, facilitated diffusion
• Enzymes: Salivary amylase, pancreatic amylase, brush border enzymes

Protein Digestion

• Absorption: similar to carbohydrates (cotransport with sodium and facilitated diffusion)
• Stomach enzymes: pepsin
• Small intestine enzymes: pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase), brush border enzymes (aminopeptidases, carboxypeptidases, dipeptidases)

Lipid Digestion

• Stomach: Lingual lipase and Gastric lipase (30%) break down triglycerides into diglycerides and fatty acids.
• Small Intestine: Pancreatic lipase breaks down triglycerides into monoglycerides and fatty acids. Requires emulsification (bile salts separate large droplets into smaller droplets)

Nucleic Acid Digestion

• Absorption: Active transport via membrane carriers.
• Enzymes: Pancreatic ribonucleases and deoxyribonucleases in the small intestine.
• Absorbed in villi and transported to the liver via the hepatic portal vein.

Water and Electrolyte Absorption

• Water: Small intestine absorbs almost all ingested water; large intestine absorbs some, with the remainder passed in feces.
• Electrolytes: Small intestine absorbs almost all electrolytes; absorption unregulated, dependent on diet.
• Iron: Absorption regulated by hepcidin (hormone secreted from the liver), less absorption if iron levels are high, more absorption if iron levels are low.

Vitamin Absorption

• Fat-soluble vitamins (A, D, E, K): Absorbed in the small intestine with lipids in micelles.
• Water-soluble vitamins (B and C): Absorbed through diffusion and active transport.
• B12: Absorbed by receptor-mediated endocytosis; requires intrinsic factor produced by parietal cells in the stomach.

Test your knowledge regarding the various aspects of digestion and absorption. The questions cover enzymes, hormones, and the physiological processes involved in breaking down and absorbing nutrients in the digestive system.