Notes on the Human Digestive System

Overview of the Human Digestive System

  • The human digestive system is a complex and multi-part mechanism that has evolved over millions of years.

  • Key components include:

    • Rectum

    • Large Intestine

    • Small Intestine

    • Pancreas

    • Stomach (also referred to as gastro and ventriculus)

    • Liver with the gallbladder

    • Esophagus

    • Salivary Glands (located near the mouth)

Initial Breakdown of Food

  • The digestive process begins in the mouth:

    • Teeth break up food, increasing its surface area.

    • Salivary Glands produce saliva for moistening and initial digestion.

  • Saliva contains the digestive enzyme amylase, which starts breaking down carbohydrates into smaller units.

  • The mixture of food and saliva forms a bolus, which is propelled by the tongue into the throat and subsequently into the esophagus.

Esophagus

  • The esophagus propels the bolus to the stomach:

    • Characteristics of the esophagus include:

    • Flexible lumen allows transport of boluses of varying sizes.

    • Comprised of several layers, consistent throughout the digestive tract.

  • The two outer muscle layers facilitate peristalsis, enabling the bolus to move effectively even while inverted.

Stomach

  • The stomach is organized into six sections and features:

    • Similar structure to the esophagus, containing:

    • Longitudinal muscle layer externally

    • Circular muscle fibers beneath

    • An oblique muscle layer overlaying the mucosa

    • Internal rouge that allow expansion upon food intake.

  • Contains gastric glands, which secrete:

    • Mucus for wall protection against gastric acid.

    • Gastric Acid, produced upon:

    • Smelling, seeing food, or spice

    • Stomach stretching

  • Production of approximately 1 to 2 liters of gastric juice per day.

  • The stomach and esophagus are separated by a sphincter:

    • Function: Relaxes to allow bolus entry, contracts to prevent reflux.

  • Gastric juice composition includes:

    • Hydrochloric Acid

    • Pepsin: Enzyme that breaks down proteins;

    • Intrinsic Factor: Required for Vitamin B12 absorption;

    • Lipase: Aids in fat digestion.

  • Hydrochloric acid functions to:

    • Destroy harmful bacteria.

    • Convert pepsinogen to pepsin, which aids protein digestion.

    • Intrinsic Factor is essential for Vitamin B12 absorption, crucial for nerve health, blood cell formation, and DNA synthesis.

    • Gastric lipase splits triglycerides into:

    • Free fatty acids

    • Diglycerides (only free fatty acids can be absorbed).

Chime Formation and Movement

  • Gastric juice and movements about every 20 seconds create chime.

  • The pyloric sphincter controls the exit of chime to the duodenum:

    • Only allows small amounts to prevent larger food pieces from leaving the stomach.

Small Intestine

  • The small intestine comprises three sections:

    • Duodenum

    • Jejunum

    • Ileum

  • Function in the duodenum includes:

    • Mixing of bile and pancreatic secretions via the ampulla of Vater.

  • Pancreatic juice contains:

    • Digestive proenzymes and enzymes requiring a higher pH than that of the stomach.

    • The presence of sodium hydrogen carbonate neutralizes stomach acids, creating an optimal pH of 7 or 8.

  • Activation of proenzymes occurs via enterokinase released by the duodenum wall:

    • Converts trypsinogen into trypsin, which aids in protein digestion and the activation of other trypsinogens.

  • Functions of other enzymes include:

    • Alpha amylase (completes carbohydrate digestion by converting them into maltose and isomaltose).

    • Pancreatic lipase, which efficiently splits triglycerides into two free fatty acids due to the emulsification process performed by bile.

Bile Production and Transport

  • Bile is produced by liver cells and stored in the gallbladder.

  • Released into the duodenum via the ampulla of Vater, aiding in fat emulsification.

  • The structure of the small intestine aids in absorption with:

    • Circular folds that enhance surface area.

    • Villi (1mm long finger-like projections) with blood capillaries and lymphatic capillaries (lacteal) that absorb nutrients.

  • Nutrient absorption mechanism:

    • Some nutrients like glucose are transported freely in the bloodstream.

    • Others, like iron, require transport proteins (e.g. transferrin).

    • Fats utilize chylomicrons, which are lipoproteins that transport triglycerides enclosed within them.

Microvilli and Nutrient Absorption

  • Microvilli, smaller projections on each villus, further increase absorption surface.

  • Processes in the ileum:

    • Absorbs electrolytes (e.g., calcium, zinc), vitamins (e.g., Vitamin B12), and remaining bile acids returning to the liver.

Large Intestine

  • The large intestine is about one meter long and surrounds the small intestine:

    • Connected to the small intestine via the Bowheen's valve, which opens to allow chime passage.

  • The composition does not include villi since most nutrients would have already been absorbed.

  • Contains an enormous population of bacteria (approx. 100 billion), which:

    • Produce vitamins

    • Decompose fiber for energy

    • Play a role in the immune system by attacking harmful germs.

  • Peristalsis transports chime through the large intestine:

    • Movement from ascending colon to transverse colon to descending colon while removing water and adding mucus.

  • Final excretion occurs in the rectum, where indigestible substances accumulate before exiting through the anus.