Digestive System: Stages, Digestion, Nutrients

• The digestive system has five stages of function:
• Ingestion: food and water enter the digestive tract through the mouth.
• Propulsion: ingested food and liquids pass from one digestive organ to the next.
• Digestion: breakdown of food.
• Absorption: nutrients move through the walls of the alimentary canal via blood or lymphatic vessels.
• Defecation: elimination of feces at the terminal end of the alimentary canal.

Mechanical vs. Chemical Digestion

• Mechanical digestion involves the physical movement of food to change its physical properties.
• Chemical digestion uses enzymes to break the bonds within molecules to change the chemical properties of food.

Macronutrients vs. Micronutrients

• Macronutrients are carbohydrates, proteins, and fats/oils.
• Micronutrients are amino acids, fatty acids, mono/disaccharides, salts, vitamins, and minerals.

Alimentary Canal Segments vs. Accessory Organs

• Accessory organs provide supportive materials essential for the digestive system's proper function.
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
• The GI Tract includes the Stomach, Small Intestine (duodenum, jejunum, & ileum), Colon (Ascending, Transverse, Descending, Sigmoid), Rectum, and Anus.

Accessory Digestive Organs/Structures and Their Roles in Digestion

• Teeth are used for mechanical digestion/mastication.
• Salivary Glands produce fluid containing enzymes that begin chemical digestion of food.
• The Liver's secretory functions involves Bile and Bilirubin.
• The Gallbladder stores and concentrates bile.
• The Pancreas produces and secretes pancreatic enzymes into the duodenum.

Layers of the Digestive Tract

• Serosa: the outermost layer that is the external boundary separating the GI tract from other abdominal structures.
• Submucosa: a thick connective tissue layer containing blood vessels, lymphatics, and nerve plexuses for absorption and digestive system control.
• Muscularis Layer functions in mixing and propulsion of digestive tract contents.
• Plexuses of the Enteric Nervous System controls digestive tract motility, secretion, and blood flow.
• Lumen: the innermost layer of the GI tract, directly interacts with contents and is lined with epithelial cells that function in absorption.

Retroperitoneal Organs

• Kidneys, Pancreas, and Duodenum are retroperitoneal organs.

Omenta Function

• Omenta function in protecting organs, blood vessels and nerves.

Connective Tissue Structures of the Abdomen

• Lesser Omentum attaches to the lesser curvature of the stomach, drapes over the lesser sac, and attaches to the inferior side of the liver.
• Greater Omentum attaches to the greater curvature of the stomach, drapes over the transverse colon, and is superficial to the small intestine.
• Falciform Ligament is a vertical ligament that separates the liver into right and left lobes and attaches the liver to the anterior abdominal wall.
• Mesentery are folds of connective tissue that suspend digestive organs from the posterior abdominal wall.

Functions of the Oral Cavity

• Serves as the opening of the digestive tract allowing for food ingestion and initiation of chemical digestion with salivary gland enzymes.
• Facilitates mechanical digestion via mastication by teeth and tongue.
• Provides taste and other sensory responses to food.

Palatine Tonsils

• Palatine tonsils are located on the wall of the oral cavity between the palatoglossal & palatopharyngeal arches.
• They function in the filtration of potential pathogens that enter the nose & mouth.
• Infection of this tissue is known as Strep Throat and is caused by Group A Streptococcus bacteria.

Boundaries of the Oral Cavity

• Anterior: Oral Orifice & Lips

• Posterior: Oropharynx

• Superior: Hard and Soft Palates

• Floor: Tongue

• The lingual frenulum is implicated in a "tongue tie".

Lingual Papillae

• Lingual Papillae are located on the surface of the tongue, contain taste buds, and aid in taste sensation of ingested material.
• Lingual Papillae that contain taste buds: Foliate Papillae, Circumvallate Papillae, and Fungiform Papillae.
• Lingual Papillae that lack taste buds: Filiform Papillae.

Types of Teeth in Deciduous vs. Permanent Sets

• Deciduous Teeth: Central Incisor, Lateral Incisor, Canine, 1st Molar, 2nd Molar
• Permanent Teeth: Central Incisor, Lateral Incisor, Canine, 1st Premolar, 2nd Premolar, 1st Molar, 2nd Molar, 3rd Molar (Wisdom tooth)

Autonomic Nervous System Control of Salivation

• Parasympathetic stimulation stimulates salivation.
• Sympathetic stimulation decreases salivation.

Salivary Enzymes

• Salivary Amylase breaks down carbohydrates.
• Lingual Lipase breaks down lipids.

Phases of Deglutition (Swallowing)

• Oral Phase: tongue moves food upward and back against the palate into the pharynx.

• Pharyngeal Phase: skeletal muscle contracts to move soft palate upward and epiglottis downward; the upper esophageal sphincter relaxes, allowing material passage into the esophagus.

• Esophageal Phase: wave-like smooth muscle contraction moves material down the esophagus, and the lower esophageal sphincter relaxes, allowing material into the stomach.

• The only phase of deglutition that can be consciously (voluntarily) controlled: Oral Phase

Air and Food Movement

• Oropharynx: Both Food & Air.

• Laryngopharynx: Both Food & Air.

• Nasopharynx: Air only.

• Esophagus: Food only.

• Trachea: Air only.

• A sphincter is a ring-shaped opening that relaxes or contracts to open or close a passageway.

Anatomical vs. Physiological Sphincters

• Anatomical Sphincter: contains a thickened area of smooth muscle and acts as a sphincter.
• Physiological Sphincter: lacks the thickened smooth muscle of a normal sphincter but acts as a sphincter due to its anatomical relationship to other muscles.

Sphincter Functions

• Lower Esophageal Sphincter: junction of the esophagus & the stomach [Physiologic Sphincter].
• Pyloric Sphincter: controls the release of material from the stomach into the duodenum [Anatomical Sphincter].
• Ileocecal Sphincter: junction of the ileum (small intestine) & cecum (large intestine) [Anatomical Sphincter].
• Anal Sphincter: controls the release of stool at the distal end of the digestive tract [Anatomical Sphincter].

Stomach Cell Types and Secretions

• Epithelial Cells of the Stomach: mucous

• Parietal Cells: Hydrochloric Acid (HCl)

• Chief Cells: Pepsinogen

• Enteroendocrine: hormones

• Pepsinogen is activated into pepsin by the acidic environment of the stomach.

Active Pepsin Production

• Chief Cells produce & secrete pepsinogen (precursor to pepsin).
• Parietal Cells secrete HCl to produce the acidic environment in the stomach required for pepsinogen activation to pepsin.

Advantages of an Acidic Stomach Environment

• Aids in chemical digestion.

• Activates strong, degrading enzymes required for digestion.

• Provides Immune Defense against pathogens (many pathogens cannot survive in acidic environment).

• Destructive enzymes are secreted in an inactive form to avoid degrading the cells or the organ they are produced from, activating only in their intended area in a controlled space.

Duodenum Parts

• The four parts of the duodenum: Superior, Descending, Horizontal, Ascending

Liver Functions

• Metabolic Functions: roles in carb, lipid & protein homeostasis including production & storage of glycogen Storage of Vitamins & Minerals
• Secretory Function: Bile secretion, Bilirubin excretion
• Endocrine Functions: activation of Vitamin D, thyroid hormone & steroid hormone; production of IGF-1 hormone
• Filtering Functions: processing of drugs; screening & getting rid of old blood cells & bacteria (via the Hepatic Portal Vein)

Liver vs. Gallbladder

• Liver: bile production
• Gallbladder: storage & concentration of bile

Ducts of the Digestive System

• Cystic Duct: bile stored in the gallbladder; empties into Bile Duct
• Hepatic Duct: new bile just produced by the liver; to the gallbladder for storage/concentration
• Pancreatic Duct: pancreatic enzymes to the duodenum
• Hepatopancreatic Duct: bile from the gallbladder + pancreatic enzymes from the pancreas to the 2nd part (Descending) Duodenum

Bile Components

• Bile Salts: function in the emulsification of fats

• Bilirubin: responsible for bile pigment; result of hemoglobin destruction by the liver

• Cholesterol: structural component of plasma membrane

• Emulsification is using surfactant to produce a mixture of hydrophobic & hydrophilic substances, like converting oil & vinegar into salad dressing.

Jaundice

• Jaundice is caused by the buildup of bilirubin in the blood (secondary to impaired liver function).
• Fluorescent light is used to degrade the excess bilirubin.

Pancreas Functions

• Endocrine Pancreas: secretes hormones into the blood that function in maintaining blood glucose level
• Exocrine Pancreas: secretes pancreatic enzymes into the duodenum to function in digestion

Pancreatic Enzymes

• Carbohydrates: Pancreatic Amylase
• Proteins: Trypsin (Trypsinogen), Chymotrypsin (Chymotrypsinogen), Carboxypeptidase (Procarboxypeptidase)
• Lipids: Pancreatic Lipase

Enteropeptidase

• Entropeptidase is a weak protein digesting enzyme that is always present in the duodenum.
• It functions in cleaving zymogens, producing active forms of the peptidase enzymes.

Pancreatic Bicarbonate

• Pancreatic bicarbonate is released into the duodenum to neutralize acidic chyme entering from the stomach.

Jejunum vs. Ileum

• Diameter: the diameter of the jejunum is greater than that of the ileum
• Mesenteric Windows: present in jejunum; absent in ileum
• Vascularity: the jejunum is more vascular that the ileum
• Lymphatic Tissue Concentration: the ileum has more lymphatic tissue than the jejunum

Jejunum/Ileum & Colon

• Jejunum/lleum: Villi & Microvilli to maximize nutrient absorption
• Colon: Taenia Coli muscle constriction forms Haustra to allow for circulation of the feces, maximizing water absorption

Intestinal Villi Structures

• Lacteals: absorption and transport of lipids
• Goblet Cells: secrete mucus
• Blood Capillaries: absorption & transport of proteins & glucose
• Microvilli: contain brush border enzymes that function in the final stages of chemical digestion

Small Intestine Microanatomy

• The lumen of the small intestine appears to have coils due to the circular folds of the submucosal & mucosal layers.

Small Intestine Microanatomy Significance

• Increase Surface Area for absorption

Organization of the Small Intestine

• Circular Folds: increase the surface area & slow the progression of chyme to maximize absorption

• Villi: finger like projections covered in Columnar Enterocytes & Goblet Cells

• Columnar Enterocytes & Goblet Cells: Columnar Enterocytes: absorption units & Goblet Cells: mucus secretion

• Villi composed of many enterocytes & Goblet Cells, Circular Folds composed of many Villi

• Not all macromolecules are absorbed by the same structures of the digestive system & share a common destination.

Structures that Absorb Fats, Proteins, & Carbohydrates

• Lipids: absorbed by lymphatic vessels (lacteals) within the core of the intestinal villi and transported into the lymphatic system.
• Proteins/Carbs: absorbed by intestinal capillaries within the core of the intestinal villi and transported into the hepatic portal system for screening & metabolism by the liver.

Peristalsis and Segmentation

• Peristalsis: wave-like contraction of the digestive tract smooth muscle that moves the contents through the lumen
• Segmentation: stationary ring-like constrictions appear & disappear to mix chyme & enzymes, aiding in digestion

Chemical Digestion

• Carbohydrates: Salivary amylase begins digestion in the mouth, Pancreatic amylase & brush border enzymes complete digestion into monosaccharides in the small intestine
• Proteins: Pepsin begins digestion in the stomach, pancreatic & brush border enzymes complete digestion into amino acids within the small intestine
• Lipids: Gastric Lipase begins digestion in the stomach, bile emulsifies lipid globules in the small intestine & pancreatic lipase completes digestion into free fatty acids & monoglycerides in the small intestine

Nutrient Absorption

• Carbohydrates & Proteins - absorbed into capillaries, taken to the liver via the hepatic portal vein
• Lipids - absorbed into Lacteals & processed by the lymphatic system

Enterohepatic Circulation

• The enterohepatic circulation is a unique network of blood vessels that carries all absorbed monosaccharides & amino acids into the liver for processing.

Large Intestine Structures

• Teniae Coli: 3 bands of longitudinal smooth muscle
• Haustra: pouches formed by the contraction of teniae coli
• Epiploic Appendages: fat filled pouches attached to teniae coli

Disease and Defecation

• Diarrhea: decreased duration of time chyme spends in the colon, not allowing ENOUGH water absorption to occur is stool is liquid (can result in dehydration due to loss of large quantities of water).
• Constipation: prolonged duration of time chyme spends in the colon, allowing for TOO MUCH water absorption to occur & hard stool is produced.

Digestive Tract Segments

• Stomach: pouch like sac & most superior portion of the digestive tract within the abdominal cavity
• Duodenum: retroperitoneal
• Jejunum: Mesenteric window [fat does not completely approach the junction of the mesentery & jejunum]
• Ileum: No mesenteric window [fat located at the junction of the mesentery & ileum]
• Cecum: Teniae Coli musculature produce haustra; epiploic appendages extend from the associated visceral peritoneum
• Sigmoid Colon: S-shaped tube in the left lower quadrant

Colon Digestion

• Mechanical Digestion of the Colon: peristalsis

• Chemical Digestion: activity of microbiome species produce essential ions & vitamins that are absorbed along with water

• The Levator Ani Muscle & External Anal Sphincter relax only under voluntary control.

Colon Sphincters

• Proximal Sphincter: Ileocecal Sphincter

• Distal Sphincters: Internal Anal Sphincter & External Anal Sphincter

• The dietary component that is not digested: fiber

Defecation

1. Stimulatory signal originates from stretch receptors that sense the distension of the rectal wall when mass peristalsis of the large intestine pushes feces into the rectum.
2. Parasympathetic reflex signals open the internal anal sphincter (smooth muscle).
3. Voluntary relaxation of the external anal sphincter (skeletal muscle) allows for feces to be expelled.

The Path of Food

• Food enters the mouth
• Deglutation takes place, moving the food from the oral cavity, into the pharynx and finally into the esophagus
• Gastroesophageal Junction opens, allowing food to move into the stomach
• Pyloric Sphincter opens, allowing food to move into the Duodenum
• Duodenum - Jejunum - lleum [Nutrients absorbed into Hepatic Portal System]
• Colon [Water absorbed into Hepatic Portal System]
• Rectum
• Anus

Layers of Smooth Muscle

• Stomach: 3 layers of smooth muscle muscle
• Small Intestine: 2 layers of smooth muscle
• Colon: 1 layer of smooth muscle

Enteric Nervous System

• The Enteric Nervous System is a division of the nervous system that controls the digestive system.
• The system uses the intrinsic nervous plexus to regulate the Gl tract with reflex pathways
• It maintains local control of each segment of the digestive tract
• Sympathetic Division of the Enteric Nervous System: inhibits digestion by decreasing smooth muscle activity & secretions
• Parasympathetic Division of the Enteric Nervous System: enhances digestion increasing smooth muscle activity & secretions

Hormones and the Digestive System

• Gastrin: Stimulates the activity of the stomach (mechanical movement, secretions) + increases gastric emptying
• Cholecystekinin (CCK): Inhibits the activity of the stomach + decreases gastric emptying