Digestive System Review

Digestion Overview

• Mechanical Digestion

• Involves physical breakdown of food

• Processes that aid in the mouth include:

  • Tonsil

  • Tongue

  • Incisors

  • Soft palate

  • Uvula

Chemical Digestion

• Involves enzymes to break down organic molecules, ultimately to building blocks for absorption

• Indigestible substances (e.g. fiber) are partially broken down in the large intestine, the rest excreted as feces

Four Layers of the Alimentary Canal

• 1. Mucosa

  • Mucous membranes, provides protection, secretion, and absorption

  • Different types of epithelium:

    • Mouth/Pharynx/Esophagus: Stratified squamous

    • Stomach/Small/Large Intestine: Simple columnar

    • Anus: Stratified squamous

• 2. Submucosa

  • Rich in blood vessels, supports absorption of nutrients

• 3. Muscularis

  • Composed of skeletal and smooth muscles

  • Circular layer constricts; longitudinal layer shortens, facilitating peristalsis

• 4. Serosa

  • Outermost layer, protects and secretes serous fluid, consists of simple squamous epithelium and connective tissue

Digestive System Components

• Major organs include:

• Liver

• Mouth

• Salivary glands

• Esophagus

• Stomach

• Pancreas

• Gall bladder

• Small intestine

• Appendix

• Rectum

• Anus

Separation of Digestive Segments by Sphincters

• Sphincters separate sections to control the flow and acidity

• Cardiac sphincter (Lower Esophageal)

• Pyloric sphincter

• Ileocecal valve

• Anus (controls defecation)

Accessory Organs

• Help digestion but do not directly involve food passage

• Include: Salivary glands, liver, gall bladder, pancreas, tongue, epiglottis, rectum

Salivary Glands

• Structures involved in digestion, releasing amylase (to digest starch) and bicarbonate ions

• Also have lysozymes for antibacterial action

Role of the Stomach

• Primarily digests proteins in a highly acidic environment (pH 2)

• Pepsin is the main enzyme, activated from pepsinogen by HCl

• Intrinsic factor released for Vitamin B12 absorption

Digestion in the Small Intestine

• Completed here; absorption of 90% of nutrients

• Contains three parts: Duodenum, Jejunum, Ileum

• Specialized structures (villi) enhance absorption surface area

• Chylomicrons formed for fat absorption, enter lacteals to bypass liver initially

• Digestive enzymes from intestinal lining breakdown carbohydrates, proteins, and lipids

Large Intestine Functions

• Absorbs water and minerals; contains a rich microbial flora

• Involvement in waste digestion, fermentation, and storage until defecation

Common Problems and Hormonal Regulation

• Issues such as constipation, vomiting, GERD

• Hormones like gastrin and secretin help regulate digestive juices and processes

Digestion Overview

• Mechanical Digestion

• Involves the physical breakdown of food, increasing the surface area for enzymes to act upon.

• Processes that aid in physical digestion in the mouth include:

  • Tonsil - Helps in filtering pathogens from the food.

  • Tongue - Assists in manipulating food, mixing it with saliva, and pushing it towards the pharynx.

  • Incisors - Sharp front teeth used for cutting and shearing food.

  • Soft palate - Closes off the nasal passages during swallowing to prevent food from entering the nasal cavity.

  • Uvula - Prevents food from entering the nasal cavity and directs food toward the throat.

Chemical Digestion

• Involves enzymes that break down complex organic molecules (carbohydrates, proteins, lipids) into smaller units (monosaccharides, amino acids, fatty acids) that can be absorbed into the bloodstream.

• Saliva contains enzymes like amylase to begin starch digestion.

• In the stomach, acidic conditions activate pepsin to break down proteins.

• Indigestible substances (e.g. fiber) are partially broken down in the large intestine, with the remainder excreted as feces to maintain digestive health.

Four Layers of the Alimentary Canal

• 1. Mucosa - The innermost layer consists of mucous membranes that provide protection, secretion of enzymes, and absorption of nutrients.

  • Different types of epithelium:

    • Mouth/Pharynx/Esophagus: Stratified squamous (protects against abrasion).

    • Stomach/Small/Large Intestine: Simple columnar (facilitates absorption).

    • Anus: Stratified squamous (protects against mechanical damage).

• 2. Submucosa - A connective tissue layer rich in blood vessels and lymphatics that supports the mucosa and provides it with nutrients while allowing absorption of digested materials.

• 3. Muscularis - Composed of an inner circular layer and an outer longitudinal layer of smooth muscle responsible for peristalsis, the rhythmic contractions that move food along the digestive tract.

  • Additional skeletal muscle may be present in parts of the esophagus for voluntary control.

• 4. Serosa - The outermost layer that protects and secretes serous fluid, reducing friction between the digestive organs and surrounding abdominal wall. It consists of simple squamous epithelium (mesothelium) and connective tissue.

Digestive System Components

• Major organs include:

  • Liver - Produces bile for emulsifying fats and plays a significant role in metabolism and detoxification.

  • Mouth - Entry point of food where mechanical digestion begins.

  • Salivary Glands - Produce saliva, containing enzymes for carbohydrate digestion and lubricating food for swallowing.

  • Esophagus - A muscular tube that connects the throat to the stomach, facilitating the movement of food through peristalsis.

  • Stomach - A hollow organ where food is mixed with gastric juices; continues protein digestion.

  • Pancreas - Produces digestive enzymes for all major food groups and secretes insulin for glucose metabolism.

  • Gall bladder - Stores bile produced by the liver, releasing it into the small intestine as needed.

  • Small intestine - The primary site for nutrient absorption; divided into the duodenum, jejunum, and ileum.

  • Appendix - A small pouch connected to the large intestine, may play a role in gut immunity.

  • Rectum - Stores feces before voluntary expulsion.

  • Anus - Controls defecation and consists of internal and external sphincters.

Separation of Digestive Segments by Sphincters

• Sphincters are muscular rings that separate sections of the digestive tract, controlling the flow of digestive contents and acidity to optimize digestion.

  • Cardiac sphincter (Lower Esophageal) - Prevents the backflow of stomach contents into the esophagus.

  • Pyloric sphincter - Regulates the passage of chyme from the stomach to the small intestine.

  • Ileocecal valve - Controls the flow from the small intestine to the large intestine and prevents backflow.

  • Anus - Controls the exit of feces, allowing for voluntary defecation.

Accessory Organs

• These organs assist in digestion but do not directly transport food.

  • Include: Salivary glands, liver, gall bladder, pancreas, tongue, epiglottis, and rectum.

Salivary Glands

• Structures involved in digestion, releasing enzymes such as amylase (to digest starch), lysozymes (to fight bacteria), and bicarbonate ions (to neutralize acids).

• Responsible for the initial breakdown of carbohydrates and hygienic functions for oral health.

Role of the Stomach

• Primarily digests proteins in a highly acidic environment (pH 2), which aids in activating digestive enzymes and killing pathogens.

• Pepsin is the main enzyme, activated from pepsinogen by hydrochloric acid (HCl).

• Intrinsic factor is also released for the absorption of Vitamin B12, essential for red blood cell production.

Digestion in the Small Intestine

• Final digestion and absorption occur here, absorbing approximately 90% of nutrients ingested.

• Duodenum - The first part where most chemical digestion occurs with the help of bile and pancreatic enzymes.

• Jejunum and Ileum - Sites of nutrient absorption, featuring specialized structures called villi that increase the surface area for nutrient uptake.

• Chylomicrons are lipid transport particles formed for fat absorption, entering lacteals to bypass the liver initially, enabling efficient lipid transport to the bloodstream.

• Digestive enzymes from the intestinal lining further breakdown carbohydrates, proteins, and lipids for absorption into the vascular system.

Large Intestine Functions

• Absorbs water and minerals from undigested food, transforming liquid waste into solid feces.

• Contains a rich microbial flora that aids in the fermentation of remaining food particles and the production of essential vitamins (e.g., vitamin K).

• Plays a role in waste digestion, fermentation, and storage until defecation, highlighting its importance in maintaining hydration and electrolytic balance in the body.

Common Problems and Hormonal Regulation

• Issues such as constipation, vomiting, gastroesophageal reflux disease (GERD), and irritable bowel syndrome can arise from digestive disturbances.

• Hormonal regulation involves hormones like gastrin (stimulates gastric juice secretion) and secretin (stimulates the pancreas to release bicarbonate), which help regulate digestive juices and processes to ensure efficient digestion and absorption.