2 Ch 22 Human Physiology: Integrated Overview of the Digestive System

Anatomy of the Digestive System

• The digestive system is composed of two main components: the digestive tract and the accessory organs of digestion.
• The digestive tract, also known as the gastrointestinal (GI) tract, is a long, hollow, muscular tube. Its primary functions are to help digest food and drink and absorb the nutrients derived from them.
• Structures of the GI tract include:
  • Oral cavity
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
• Accessory organs facilitate the digestive process and include:
  • Teeth
  • Tongue
  • Salivary glands
  • Pancreas
  • Liver
  • Gallbladder

The Digestive Tract as an External Environment

• The lumen of the digestive tract is technically considered "outside" the body.
• Material within the lumen is separated from the body’s internal environment, specifically the extracellular fluid ($ECF$), by the epithelium of the digestive tract.
• For substances to enter the body's internal environment, they must be transported across the digestive epithelium using both passive and active transport mechanisms.
• Any material that cannot be absorbed is processed as waste and excreted through the anus.

The Digestive Process: Digestion, Absorption, and Secretion

• Digestion is the breakdown of substances into pieces small enough for the body to absorb. This occurs via two methods:
  • Mechanical Digestion: This involves physical force to break down substances. Examples include chewing by the teeth and churning by stomach muscles.
  • Chemical Digestion: This uses chemicals to break large molecules into smaller ones. An example is enzymes breaking a large carbohydrate into many glucose molecules.
• Absorption involves the movement of substances from the GI lumen into the $ECF$. The majority of absorption occurs in the small intestine.
• Secretion is the release of various substances into the lumen of the digestive tract, including mucus, acid, and enzymes.
• Motility refers to the movement of material through the tract, facilitated by the muscle tissue in the digestive tract walls.

Digestive Enzymes and Proenzymes

• Most chemical digestion is performed by digestive enzymes, which are specialized proteins.
• Enzymes may be secreted by glands into the lumen or may be attached to the apical surface of epithelial cells in the mucosa.
• To prevent the cell from digesting itself, many enzymes are produced as inactive proenzymes and are only activated once they are outside the cell.

Regional Functions of the Digestive System

• Oral Cavity: Digestion begins here with mechanical chewing and chemical enzymes in the saliva.
• Pharynx and Esophagus: Very little digestion occurs as material passes through these sections.
• Stomach: Continues mechanical digestion through contractions of the muscular layer and chemical digestion using stomach acid and enzymes. It stores food as chyme.
• Small Intestine: Performs the final steps of chemical digestion with bile and enzymes. It is the primary site for nutrient absorption.
• Large Intestine: Receives undigested material. Symbiotic microbes digest some material humans cannot. Water and ions are extracted, compacting remaining material into feces for excretion via the rectum.

Motility: GI Smooth Muscle and Contraction Patterns

• The muscle in the GI tract walls is mostly single-unit smooth muscle.
• Spontaneous contraction is triggered by pacemaker cells that produce irregular, slow-wave potentials. These do not always lead to action potentials.
• Two main types of movement exist:
  • Peristalsis: Moves a mass of food (bolus) forward. This involves a wave-style contraction similar to the movement of an earthworm. The circular layer contracts behind the bolus, while the longitudinal layer contracts in front of it.
  • Segmentation: Churns and mixes material without moving it forward. This helps mix secretions and enzymes with the lumen contents to speed up digestion.

Regulation of GI Function and the Enteric Nervous System

• GI function is regulated by the enteric nervous system ($ENS$), a complex part of the autonomic nervous system ($ANS$) in the abdominal cavity.
• The $ENS$ controls the speed of movement and stimulates glandular secretion.
• Characteristics of the $ENS$:
  • Contains interneurons.
  • Uses a wide variety of neurotransmitters, such as serotonin.
  • Contains glial cells that resemble astrocytes.
• Reflexes are categorized as:
  • Short Reflexes: Originate in the $ENS$ and are carried out entirely within the gut wall.
  • Long Reflexes: Integrated in the Central Nervous System ($CNS$). Some originate in the $ENS$, while others originate outside the GI tract.

Integrated Function: The Cephalic Phase

• The cephalic phase involves feedforward reflexes that trigger effectors in anticipation of food.
• Stimuli such as the sight, smell, or thought of food trigger saliva production and increased motility.
• Mastication (chewing) digests food into a fine paste.
• Salivary amylase begins breaking down large carbohydrates ($polysaccharides$), like starch, into disaccharides.

Integrated Function: The Gastric Phase and Secretory Cells

• The stomach mucosa contains gastric pits that lead to gastric glands. These glands contain several cell types:
  • Mucous Surface and Neck Cells: Secrete mucus (physical barrier) and Bicarbonate ($HCO_3^-$). Bicarbonate buffers gastric acid to prevent epithelial damage. Lack of protection can lead to peptic ulcers.
  • Parietal Cells: Secrete Hydrochloric Acid ($HCl$) and Intrinsic Factor. $HCl$ activates pepsinogen, denatures proteins, and kills microbes. Intrinsic factor is required for Vitamin $B_{12}$ absorption.
  • Chief Cells: Secrete the proenzyme pepsinogen and gastric lipase. Pepsinogen becomes pepsin in the presence of acid and breaks proteins into small peptides.
  • Enterochromaffin-like ($ECL$) Cells: Secrete histamine to stimulate gastric acid secretion.
  • D Cells: Secrete somatostatin to inhibit gastric acid secretion.
  • G Cells: Secrete gastrin (stimulated by Acetylcholine, peptides, and amino acids) to stimulate acid secretion.

Integrated Function: The Intestinal Phase and the Brush Border

• The small intestine mucosa features the brush border, consisting of circular folds, villi, and microvilli to increase surface area for digestion and absorption.
• Brush border enzymes are attached directly to this surface.
• The pancreas and liver release substances into the duodenum via the duodenal ampulla:
  • Pancreatic juice: Contains water, bicarbonate ions (to neutralize stomach acid), and enzymes.
  • Bile: Secreted by hepatocytes ($liver cells$), stored in the gallbladder, and contains bile salts, bile pigments ($bilirubin$), cholesterol, and water.

Pancreatic Enzymes and Bile in Digestion

• The exocrine pancreas secretes enzymes that are activated by the brush border:
  • Trypsin and Chymotrypsin: Break proteins into peptides.
  • Pancreatic Amylase: Breaks carbohydrates into disaccharides.
  • Pancreatic Lipase: Breaks lipids into fatty acids.
• Lipid digestion requires emulsification because lipids are hydrophobic and form large droplets.
• Bile salts are amphipathic (possessing both hydrophobic and hydrophilic sides). They coat lipid droplets to reduce surface tension, breaking them into smaller droplets called micelles.
• Micelles contain phospholipids, glycerides, cholesterol, and free fatty acids.

Absorption of Macronutrients

• Lipids: Fatty acids and monoglycerides diffuse into intestinal mucosa, are packaged into vesicles ($chylomicrons$), and enter lymphatic vessels called lacteals in the villi. Bile salts are recycled in the ileum $2$ to $5$ times per meal via the hepatic portal system.
• Carbohydrates: Amylase breaks polysaccharides into disaccharides. Brush border enzymes (sucrase, lactase, maltase) break these into monosaccharides (glucose, fructose, galactose). Glucose transporters move them into the blood. Lactose intolerance occurs when adults stop producing lactase.
• Proteins: Proteases come in two varieties:
  • Endopeptidases: Break proteins in the middle (e.g., pepsin, trypsin, chymotrypsin).
  • Exopeptidases: Break amino acids off the ends (e.g., aminopeptidases, carboxypeptidases).
  • Proteins (length $50+$ amino acids) and peptides (length $2$ to $50$ amino acids) are broken down into amino acids, dipeptides, or tripeptides for transport. Large peptides may use transcytosis.
• Nucleic Acids: Nucleases break DNA/RNA into nucleotides; nucleosidases break nucleotides into nitrogenous bases and ribose.

Absorption of Vitamins, Ions, and Water

• Vitamins: Fat-soluble vitamins are absorbed with lipids. Water-soluble vitamins use transport proteins. Vitamin $B_{12}$ requires intrinsic factor.
• Iron ($Fe$): Absorption is active and regulated by the liver.
• Calcium ($Ca$): Mostly paracellular and unregulated, but calcitriol (from the kidneys) stimulates active transport.
• Ions and Water: $Na^+$, $K^+$, and $Cl^-$ are absorbed via passive and active transport. This creates an osmotic gradient that pulls water into the $ECF$.

Regulation and Integrated Reflexes

• Enteric reflexes inhibit stomach motility when chyme enters the duodenum to allow time for digestion.
• Acid in chyme triggers bicarbonate secretion.
• Nutrient-specific responses:
  • Carbohydrate-rich meals: Trigger Glucagon-like peptide-1 ($GLP-1$) and Gastric inhibitory peptide ($GIP$), stimulating insulin release and satiety.
  • Lipid-rich meals: Trigger Cholecystokinin ($CCK$), which slows gastric motility and stimulates bile release.

The Large Intestine and Microbes

• By the time material reaches the large intestine, most nutrients are gone.
• Symbiotic microbes produce Vitamin $K$ (essential for blood clotting).
• Microbes produce hydrogen sulfide ($H_2S$) gas, creating flatulence.
• Gastrocolic Reflex: Food in the stomach triggers mass movements ($3$ to $4$ times daily) in the large intestine.
• Defecation Reflex: Rectal distension relaxes the internal anal sphincter.

Statistics and Numerical Data

• The liver weighs approximately $1.5\,kg$ ($3.3\,lb$).
• Bile salts can be recycled $2$ to $5$ times during a single meal.
• Mass movements in the large intestine typically occur $3$ to $4$ times a day.
• Approximately $70\%$ of the surface area of a hepatocyte faces the sinusoids for exchange.
• Peptides are defined as being $2$ to $50$ amino acids in length; proteins are $50+$ amino acids.