Digestive System Notes

Main Functions

Ingestion: Taking food into the mouth is the first step in the digestive process. The body prepares to metabolize the nutrients within food.

Propulsion: Movement of food through the gastrointestinal (GI) tract is essential for digestion. This includes voluntary actions like swallowing and involuntary actions such as peristalsis.

Peristalsis: This is the involuntary constriction and relaxation of smooth muscles in the GI tract, resulting in wave-like movements that push food along from the esophagus to the rectum. It is crucial for the movement of bolus and chyme through various parts of the digestive tract.

squishes the food to make it easier for digestion, with it in smaller pieces there is more surface area for enzymes to act upon, facilitating the breakdown of macronutrients and enhancing nutrient absorption in the intestines.

Digestion: The process of breaking down food into absorbable nutrients involves both mechanical and chemical digestion.

• Mechanical digestion: Includes the physical breakdown of food into smaller pieces through processes like chewing (mastication) and the churning action of the stomach.

• Chemical digestion: Involves enzymatic reactions that convert foods into their simplest forms as follows:

  • Carbohydrates → Simple sugars (e.g., glucose, monosaccharides) by hydrolysis

  • Proteins → Amino acids

  • Lipids → Fatty acids and glycerol

Secretion: Approximately 5L of digestive secretions are produced each day, including saliva, stomach acid, bile, and pancreatic juice, which aid in digestion and absorption.

Absorption: This is the process of bringing nutrients from the digestive tract into the bloodstream, primarily occurring in the small intestine. The efficiency of nutrient absorption is influenced by the surface area provided by villi and microvilli.

Defecation: Final removal of indigestible substances and wastes from the body occurs through the anus in the form of feces.

Digestive Organs

Gastrointestinal (GI) Tract: A continuous tube that winds through the body, consisting of hollow organs that facilitate digestion.

Accessory Organs: Organs that lie outside of the GI tract but are essential for digestion, including:

• Hollow organs: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus.

• Accessory organs: Teeth, tongue, salivary glands, liver, gallbladder, pancreas.

Histology of the Alimentary Canal

Mucosa: The innermost layer, which contains epithelial tissue that varies based on location—either simple columnar or stratified squamous. It also contains glands that secrete mucus.

Submucosa: Comprising loose areolar and dense irregular connective tissue (CT), this layer contains blood and lymphatic vessels, autonomic nerve fibers, and allows for the elasticity of the GI tract.

Muscularis external: A smooth muscle layer that facilitates peristalsis through coordinated contractions.

Serosa (visceral peritoneum): The outer layer that gives structure to the GI tract, supporting organs and protecting them from friction with surrounding tissues.

Oral Cavity, Pharynx, and Esophagus

Salivary Glands:

• Produce approximately 1L of saliva per day, containing enzymes and lubricants necessary for digestion.

  • Salivary amylase: Enzyme that digests starch into maltose and dextrins.

  • Lysozyme: Provides antibacterial action to help maintain oral health.

  • Mucin: Forms mucus when mixed with water, aiding in lubrication for easier swallowing.

Chewing: Food is broken down into a bolus, a manageable mass ready for swallowing, and increasingly mixes with saliva for easier transit.

Pharynx:

• Epiglottis: A flap that prevents food from entering the trachea during swallowing, directing it towards the esophagus.

Esophagus:

• Transports the bolus from the pharynx to the stomach via involuntary peristalsis. It is lined with stratified squamous epithelium to withstand friction from food.

Stomach

Function: Acts as a storage and mixing chamber, preparing food for further digestion in the small intestine.

Capacity: Can hold between 1.5 and 4 liters of food and liquid.

Secretions:

• Produces about 1.5 L/day of gastric secretions, including mucus, hydrochloric acid (HCl), and pepsinogen, which activates to pepsin to digest proteins.

Rugae: Longitudinal folds that allow the stomach to expand and accommodate varying amounts of food.

Chyme Formation: The bolus is mixed with gastric juices to form chyme, a semi-liquid mass that is gradually released through the pyloric sphincter into the small intestine for further digestion.

Small Intestine

Three Segments: Duodenum, Jejunum, Ileum.
Function: The majority of digestion and nutrient absorption occurs here, thanks to its complex inner structure.
Structure:

• Features folds, villi, and microvilli to maximize surface area for absorption.

• Produces approximately 2L/day of enzymatic juices containing water, mucus, and digestive enzymes necessary for further breakdown of nutrients.

• Receives secretions from the liver (bile) and pancreas (digestive enzymes) to assist in digestion.

Pancreas

Function:

• Secretes approximately 1L of pancreatic juice daily, which contains around 22 enzymes as well as bicarbonate to neutralize stomach acid and optimize intestinal pH for enzyme activity.

• The pancreatic duct drains this fluid into the small intestine for digestion of all major food classes.

Liver

Function:

• Secretes bile into the gallbladder, which is then released into the small intestine to emulsify fats, increasing the surface area of lipids for digestion by enzymes.

• The liver performs over 500 functions, including:

  • Storage: Glycogen, lipids, vitamins (A, D, E, K), and iron.

  • Synthesis: Nutrients such as proteins (albumin) and clotting factors (e.g., fibrinogen).

  • Detoxification: Processing and eliminating toxins, poisons, and drugs from the bloodstream.

  • Phagocytosis: Removal of old red blood cells (RBCs) and excretion of bilirubin, a byproduct of RBC breakdown.

Gallbladder

Function:

• Stores and concentrates bile, releasing it when fats are present in the duodenum to aid in emulsification.

• Gallstones can occur, which are solid particles that form from cholesterol or pigmented substances in the bile.

Large Intestine

Components: Ileocecal sphincter, Ascending colon, Transverse colon, Descending colon, Sigmoid colon, Rectum, Anus.
Function:

• Absorbs water and salts from indigestible food matter.

• Converts undigested material into feces; E. coli bacteria synthesize vitamin K in this process.

• Feces are stored in the rectum until released through the anus during defecation.

• Conditions: Issues like hemorrhoids can arise, and procedures such as colonoscopies are used for diagnosis and treatment of colon conditions.

Key Terminology

• Chyme: Partially digested food exiting the stomach, consisting of nutrients and gastric juices.

• Bolus: A mass of food that has been chewed and mixed with saliva, ready to be swallowed into the esophagus.

• Peristalsis: Involuntary waves of muscle contraction that propel food and waste through the digestive tract, crucial for proper digestive function.