Chapter 26B and 27: The Digestive System - Detailed Notes

Small Intestines

• Duodenum: Approximately 10 inches.
• Jejunum: Approximately 8 feet.
• Ilium: Approximately 12 feet.
• The ileum and jejunum are suspended from the posterior abdominal wall by mesenteries.

Functions & Structural Adaptations

• Main site of nutrient absorption.
• Structural adaptations that maximize absorption:
  • Folds of the wall dramatically increase the surface area available for absorption.
  • Folds slow the progress of food through the small intestine (SI), allowing more time for digestion and absorption.

Structural Details

• Folds upon folds:
  • Serosa, longitudinal muscle, circular muscle, submucosa, and mucosa.
  • Circular folds, villi, and microvilli.
• Villus structure:
  • Capillaries, central lacteal, absorptive cells (enterocytes), goblet cells, and lacteal.
• Mucosa-associated lymphoid tissue.
• Intestinal crypt (Crypt of Lieberkühn).
• Duodenal gland.

Villi

• Simple columnar epithelium, primarily enterocytes (absorptive cells).
• Goblet cells.
• Surrounded by a network of blood vessels and a lacteal (lymphatic capillary).

Microvilli/ Brush Border

• Apical surface of enterocytes is lined with microvilli.
  • Further increases surface area.
  • Move rhythmically to mix chyme.
• Digestive enzymes are embedded in the plasma membrane of the enterocytes.

Cells of Intestinal Glands (crypts)

• Goblet cells:
  • Produce mucus
  • Unicellular glands
  • Make enteropeptidase
• Enteroendocrine cells:
  • Secrete hormones
• Immune cells:
  • T cells
  • Paneth cells

Specializations of Small Intestines

• Brunner’s glands (aka submucosal or duodenal glands):
  • In the duodenum
  • Produce alkaline mucus.
• Peyer’s patches:
  • In the ileum
  • Aggregated lymphoid nodules

Digestion and Absorption

• Most substances needed for digestion come from the liver and pancreas and are secreted into the duodenum.
• The small intestine completes digestion and absorbs nutrients.
• Absorption occurs mainly in the duodenum and jejunum.
• Tight junctions between epithelial cells of the intestinal mucosa prevent movement between cells.
• Substances must cross the plasma membranes of enterocyte cells (twice: into and out of the cell) and then pass into blood or lymphatic capillary.

Absorption Mechanisms

• Absorption requires transport across cell membranes; substances cross membranes in different ways:
  • Passive processes:
    • Simple diffusion
    • Facilitated diffusion
  • Active processes:
    • Vesicular transport
      • Endocytosis
      • Exocytosis
    • Active transport
      • Primary
      • Secondary

Crossing Cell Membranes

• Substances cross cell membranes in one of two ways:
  • Passively:
    • Diffusion across the cell membrane or through a channel.
    • From an area of higher concentration to lower concentration.
    • Does not require energy (ATP).
  • Actively:
    • Pumped across the cell membrane (active transport).
    • From an area of lower concentration to higher concentration.
    • Carried across in vesicles (vesicular transport).
    • Requires energy (ATP).

Passive Processes: Simple Diffusion

• Simple diffusion can only occur if the membrane is permeable to the substance trying to diffuse. (e.g., non-polar, lipid-soluble substances).

Passive Processes: Facilitated Diffusion

• Polar, hydrophilic, or large molecules can’t move across the membrane via simple diffusion.
• Proteins facilitate movement:
  • Carriers: Binding leads to a shape change in the protein, enabling movement of substances.
  • Channels: Selectively allows movement across the membrane.
• Movement of molecules is still down the concentration gradient, and no energy is required.

Active Processes: Active Transport

• Molecules move against the concentration gradient.
• Requires energy (ATP) to “pump” molecules across the cell membrane.
• Two types:
  • Primary active transport
  • Secondary active transport

Primary & Secondary Active Transport

• Primary active transport:
  • The ATP-driven Na^+-K^+ pump stores energy by creating a steep concentration gradient for Na^+ entry into the cell.
• Secondary active transport:
  • As Na^+ diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell.

Vesicular Transport

• Endocytosis: Transport into cell
• Exocytosis: Transport out of cell
• Transcytosis: Transport into, across, and then out of cell
• Substance (vesicular) trafficking: Transport from one area or organelle in the cell to another; cargo remains inside the cell

Nutrient Absorption

Different classes of nutrients absorbed in different manners:

• Passive processes
  • Simple diffusion
  • Facilitated diffusion
• Active processes:
  • Vesicular transport
    • Endocytosis
    • Exocytosis
  • Active transport
    • Primary
    • Secondary

Large Intestines

• Functions:
  • Processes chyme into feces and stores it until defecation
  • Absorbs most of the remaining water from chyme
  • Absorbs some B vitamins and vitamin K (synthesized by beneficial bacteria)

Secretions of the Colon: Mucus

• The mucosa is lined with mucus-secreting goblet cells.
• Mucus functions:
  • Ease the passage of feces
  • Protect the intestinal wall from irritating acids and gases released by resident bacteria
  • Provide an adherent medium for holding fecal matter together

Liver

• Largest organ
• Has 4 lobes
• Located under the diaphragm, mostly within the rib cage
• Several different functions:
  • Metabolic
  • Regulatory
  • Digestive

Liver Functions

• Secrete bile
• Detoxification & storage
• Carbohydrate metabolism & blood sugar regulation
• Fat metabolism
• Protein metabolism
• Other

Bile

• Yellow-green, alkaline solution secreted by the liver and stored in the gallbladder
• Contains:
  • Bile salts
  • Bile pigments (bilirubin)
  • Cholesterol
  • Triglycerides
  • Phospholipids
  • Electrolytes
• Bile emulsifies fat, making it easier for lipase to digest.

Detoxification & Nutrient Storage

• Hepatic portal system: Brings nutrient-rich (and toxin-rich) blood from capillaries in digestive organs and the spleen to capillaries in the liver
• Hepatic artery: Supplies the liver with O_2-rich blood

Microscopic Anatomy of Liver Lobule

• Hepatocytes
• Portal triad:
  • Portal arteriole (branched off hepatic artery)
  • Portal venule (branched off hepatic portal vein)
  • Bile duct
• Sinusoids
• Central vein
• Bile canaliculi
• Macrophages (reticuloendothelial cells = stellate macrophages = hepatic macrophages = Kupffer cells)

Carbohydrate Metabolism & Blood Sugar Regulation

• Glycogenesis: Glucose molecules converted into glycogen
• Glycogenolysis: Glycogen molecules split into glucose
• Gluconeogenesis: Glucose formed from non-carbohydrate molecules

Fat & Protein Metabolism & Other Functions

• Fat metabolism:
  • Fatty acids are used as an energy source
  • Fats are synthesized from other compounds
• Protein metabolism:
  • Breaks down & rebuilds proteins
• Other:
  • Stores vitamins, iron (ferritin)
  • Makes blood substances used in coagulation

Gallbladder

• Thin-walled, green muscular sac on the inferior surface of the liver
• Stores and concentrates bile
• Associated structures:
  • Cystic duct
  • Common bile duct

Pancreas

• Located behind the stomach
• Has endocrine and exocrine functions:
  • Endocrine: Hormones released into blood
  • Exocrine: Pancreatic juices released into the pancreatic duct, which joins the common bile duct, and then the duodenum

Pancreas Cells

• Endocrine cells in pancreatic islets:
  • Release insulin & glucagon
• Exocrine cells are acinar cells (acini):
  • Produce pancreatic juice

Pancreatic Juice

• Contains:
  • Water
  • Electrolytes
  • Sodium bicarbonate (secreted by epithelial cells lining ducts)
  • Enzymes (many secreted as zymogens - inactive forms)
• The composition of juice is partially determined by types of foods in chyme

Zymogen (proenzyme) Activation

• Inactive enzymes (zymogens) are activated in the small intestine.
• Trypsinogen is converted to trypsin by enteropeptidase (EN) on the epithelial cells of the duodenum.
• Trypsin then activates other inactive enzymes.

Control of Gastrointestinal Motility & Secretions

Three overlapping mechanisms:

• Intrinsic (local)
• Neural
• Hormonal

1. Intrinsic (local) Control

• Myogenic smooth muscle:
  • Myogenic: muscle can contract without any external stimuli
  • External stimuli can modify activity
• Enteric pacemaker cells:
  • Muscles contract rhythmically
  • Pacemaker cells generate subthreshold depolarization at a set frequency (≈3/min).
• Secretions in response to local signals:
  • Some secretions can be released in the absence of neural or hormonal signals
  • E.g., HCl secreted by parietal cells of the stomach in response to proteins in the stomach

2. Enteric Nervous System

• The GI tract has its own nervous system.
• Local nerve plexuses lie in the wall of the gut.
• Controls GI movements & secretions independently or in cooperation with the CNS.

2. Neural Control

• GI motility & secretions mediated by both:
  • Short reflexes involving only the enteric nervous system
  • Long reflexes also involve the CNS

Example of Long Reflex

• Duodenum fills with chyme, sensory stretch receptors are stimulated, sensory nerve impulses travel to the central nervous system, and nerve impulses inhibit peristalsis in the stomach wall.

Hormonal Control

• Cholecystokinin (CCK):
  • Secreted by the duodenum in response to partially digested foods (esp. fatty chyme & proteins)
  • Inhibits the stomach’s secretory activity; stimulates secretion of bile & pancreatic enzymes
• Secretin::
  • Secreted by the duodenum in response to acidic chyme
  • Stimulates release of bicarbonate-rich fluids; inhibits stomach motility
• Gastrin:
  • Secreted by the stomach in response to food in the stomach (esp. partially digested proteins) or the thought/smell/sight of food
  • Stimulates HCl secretion in the stomach and increased gut motility

Glucagon-like peptide-1

• Semaglutide (Ozempic) mimics GLP-1
  • Originally developed to treat Type 2 diabetes (insulin resistance)
• GLP-1 signals the pancreas to produce insulin & signals the brain to increase satiety and decrease food intake

Coordination of GI Control

• All three mechanisms coordinate control of GI motility & secretions.
• For example, digestive processes in the stomach involve rhythmic contractions generated by enteric pacemaker cells (intrinsic control), both long and short reflexes (neural control), and stimulation from gastrin, CCK, and secretin (hormonal control).