Digestive System

Concepts and Significance of Digestion and Absorption

Digestion: Breakdown of food into small molecular substances within the digestive tract.

Absorption: Passage of digested small molecular substances through the digestive tract mucosa into the bloodstream and lymphatic circulation.

Digestion Modes: Mechanical digestion (physical - digestive tract movement grinds and propels food), chemical digestion (enzymes secreted by digestive glands break down food into small molecules - chemical).

Digestive Organs

Physiological Characteristics of Smooth Muscle in the Digestive Tract / Characteristics of Digestive Tract Smooth Muscle Activity

General Characteristics: Low excitability, slow contraction; self-regulation; tonicity (tonic contraction); high distensibility; functional syncytium characteristics

Electrophysiological Characteristics:

Resting Membrane Potential K+ Efflux
Basic electrical rhythm (slow wave) Mechanical threshold - Ca2+ slight influx
Action potentials-occur on the basis of slow waves depolarization Electric threshold -Ca2+ influx, Repolarization-K+ efflux

Digestive Glands and their Secretion Characteristics

Alter pH; digestive action; dilute food; secrete mucus, antibodies, and fluids for protection.

Gastrointestinal Hormones ——Fluid Regulation of Digestive Organ Movement

Concept: Hormones secreted by gastrointestinal endocrine cells.

Functions: 1, regulate digestive gland secretion and digestive tract movement; 2, regulate the secretion of other hormones, nutritional functions.

Gastric Digestion

Gastric acid, pepsinogen, intrinsic factor, mucus-carbonic acid barrier

Tonic contraction, receptive relaxation (increased storage capacity - unique), peristalsis, emptying, vomiting

Intestinal Digestion

Pancreatic juice (alkaline): High HCO3−HCO_3^- content (neutralizes gastric acid, provides alkaline pH environment), digestive enzymes (proteins, fats, starch), etc.

Bile: Digestion and absorption of fats

Intestinal juice: Protective function (neutralize gastric acid), digestive function (protein), dilution function (facilitate absorption)

Movement: Tonic contraction, segmentation (unique), peristalsis

Digestion and Absorption

Bile drainage pathway: Liver → bile ducts → liver lobules → hepatic ducts → common bile duct → gallbladder (storage) → common bile duct → duodenal papilla → duodenum

Liver: Liver lobule is the basic structural unit of the liver, right lobe is larger than the left.

Gastric emptying rate (fast → slow): sugar → protein → fat

Small intestine: Main site of digestion and absorption. Duodenal papilla: duct terminal opening

Structure of the digestive tract wall: mucosa, submucosa, muscular layer, serosa

Digestion and Absorption of Food

Main absorption sites: Small intestine (duodenum and jejunum are primary, ileum serves as a reservoir)

Structural adaptation of the small intestine to function:

  1. Large absorption area: longest, villi, folds, microvilli
  2. Assurance of motility: smooth muscle-villi rhythmic swinging
  3. Time guarantee: prolonged residence time
  4. Already digested into small molecular substances
Sugars -Absorption rate: glucose, galactose are highest -Active transport process against concentration gradient, energy from sodium pump, secondary active transport process
Fats Combined with bile
Proteins -Secondary active transport, Na+​​Na^+​​-amino acid transporter, Na+Na^+​​-peptide transporter -Neutral amino acids have the fastest transport rate, peptides > amino acids
Water -Passive absorption, absorbed through transcellular and paracellular pathways. -Force: osmotic pressure of various solutes
Inorganic salts Absorption rate: monovalent alkaline > divalent alkaline, cannot be absorbed when combined with calcium Sodium: active absorption - easy diffusion - small intestine Iron: active absorption - active transport - duodenum and jejunum Calcium: active absorption - soluble state - duodenum

Gastric Acid and Gastric Mucosal Barrier

Gastric acid - Hydrochloric acid: Forms (free acid - multiple and bound acid)

Functions of gastric acid: 1) kill bacteria; 2) provide acidic environment (facilitate calcium and iron absorption, promote secretion of digestive juice, activate pepsinogen); 3) protein denaturation

Mucus: Constituents - glycoproteins, forms a gel protective layer, reduces mechanical damage

Mucus-carbonic acid barrier: Formed by HCO3−HCO_3^- binding with gastric mucus, Functions: 1) Protects gastric mucosa from erosion by H^+, 2) pH=7 environment inactivates gastric protease

Mucosal epithelium: Prevents high-concentration hydrochloric acid and gastric protease in gastric juice from eroding the mucosa

Reflex Regulation of Digestive Organ Activity

Nerves that control digestive organs: Extrinsic nerves (sympathetic nervous system + parasympathetic nervous system) + intrinsic nerves (submucosal plexus + myenteric plexus)

Extrinsic nerves: Sympathetic-NE-inhibition; Parasympathetic-vagus-ACh-excitation (refer to the figure below, note the differences)

Intrinsic nerves (wall-inner plexus): Contain sensory neurons, motor neurons, interneurons

Stimulus of food on the digestive tract wall directly causes a response through the inner plexus — local reflex

Reflex Regulation of Digestive Organ Activity: Food stimulus causes reflex — conditioned reflex and unconditioned reflex

Conditioned Reflex: “Thinking of plums quenches thirst”

Unconditioned Reflex: Mechanical, temperature, taste receptor stimulation on the oral mucosa by food

“Most”

Small Intestine: The longest section of the digestive tract

Stomach: The most expanded part of the digestive tract

Liver: The largest gland in the human body

The digestive tract is the largest and most complex endocrine organ in the body

Gastric juice is the liquid with the lowest pH in the body

Pancreatic juice: The most important digestive fluid

Intestinal digestion is the most crucial stage in the digestive process

Small intestine: The most powerful, with the most types

Homework Questions

  1. Which absorbs and digests faster, which absorbs and digests slower?
  2. Starch/protein/fat
  3. The main absorption location
  4. “The relationship between gastric mucosal barrier and gastric acid” to “Why use pepsinogen?”
  5. Characteristics of digestive tract smooth muscles

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