The Digestive System (Lecture)

Alimentary Canal Organs/Gastrointestinal (GI) Tract Organs

• Mouth → Pharynx → Esophagus → Stomach → Small Intestine → Large Intestine

• Nourish the Body

• Continuous Tube

Salivary Glands

• Parotid Gland

• Sublingual Gland

• Submandibular Gland

Accessory Structures

Facilitate Digestion

Cardiovascular

Blood supplies digestive organs with oxygen and processed nutrients

Endocrine

Endocrine hormones help regulate secretion in digestive glands and accessory organs

Integumentary

Skin helps protect digestive organs and synthesizes vitamin D for calcium absorption

Lymphatic

Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens; lacteals absorb lipids; and lymphatic vessels transport lipids to bloodstream

Muscular

Skeletal muscles support and protect abdominal organs

Nervous

Sensory and motor neurons help regulate secretions and muscle contractions in the digestive tract

Respiratory

Respiratory organs provide oxygen and remove carbon dioxide

Skeletal

Bones help protect and support digestive organs

Urinary

Kidneys convert vitamin D into its active form, allowing calcium absorption in the small intestine

Ingestion

Eating/Drinking

Propulsion

Swallowing, peristalsis

Mechanical Digestion

Chewing (mouth), Churning (Stomach), Segmentation (Small Intestine)

Chemical Digestion

Enzyme-driven breakdown of complex food molecules (carbohydrates, proteins, fats) into smaller, absorbable nutrients (monosaccharides, amino acids, fatty acids).

Absorption

Nutrients & water to blood vessels & lymph vessels (Small Intestine)

Defecation

Water to blood vessels (large intestine)

Peristalsis

Alternating contraction sequence of both the inner circular and outer longitudinal muscle layers

Segmentation/Mixing

Localized contractions of the circular muscle layer facilitate the blending of materials by back-and-forth movements of material

Enteric Nervous System (ENS)

• Intrinsic set of nerves

• The “Brain of the Gut”

• Submucosal plexus → controls secretions

• Myenteric plexus → GI tract mobility

Autonomic Nervous System

• Extrinsic set of nerves

• Parasympathetic → increases secretion & mobility (by stimulating ENS)

• Sympathetic → decreases secretions & mobility (by inhibiting ENS)

Layers of alimentary canal

• Serosa/Adventitia (Outermost)

• Muscularis

• Submucosa

• Mucosa (innermost)

Mucosa

• Simple/Stratified Squamous Epithelium (depending on location)

• Lamina propria: Areolar CT, MALT

• Muscularis Mucosae: Narrow bond of smooth muscle & elastic fibers

Submucosa

• Areolar/Dense Irregular CT

• Blood & Lymphatic vessels

• Submucosal plexus: Innervates the mucosa & submucosa

Muscularis

• Circular muscle

• Longitudinal muscle

• Myenteric plexus

Serosa/Adventitia

• Areolar CT with collagen/elastic fibers

• S___ (Intraperitoneal)

• A___ (Retroperitoneal)

Peritoneum

The largest serous membrane in the body:

Visceral Peritoneum

The inner layer of the serous membrane that wraps directly around abdominal and pelvic organs

Parietal Peritoneum

The outer layer of the serous membrane lining the internal abdominopelvic wall, covering the diaphragm and supporting abdominal organs

Peritoneal Cavity

The fluid-filled, potential space between the parietal and visceral peritoneum (membranes) within the abdomen

Intraperitonial Orgnas

Stomach, liver, gallbladder, spleen, jejunum, ileum, cecum, appendix, transverse colon, and sigmoid colon.

Retroperitoneal

Kidneys, ureters, adrenal glands, pancreas, aorta, and parts of the digestive tract like the duodenum and colon

5 Major Peritoneal Folds

• Greater Omentum

• Falciform Ligament

• Lesser Omentum

• Mesentery

• Mesocolon

Greater Omentum

Apron-like structure that lies superficial to the small intestine & transverse colon; a site of fat deposition in people who are overweight

Falciform Ligament

Anchors the liver to the anterior abdominal wall & inferior border of the diaphragm

Lesser Ometum

Suspends the stomach from the inferior border of the liver; provides a pathway for structures connecting to the liver

Mesentery

Vertical band of tissue anterior to the lumbar vertebrae and anchoring all of the small intestine except the initial portion (duodenum)

Mesocolon

Attaches two portions of the large intestine (the transverse & sigmoid colon) to the posterior abdominal wall

Upper GI tract organs & accessory structures

• Oral cavity & salivary glands

• Parynx

• Esophagus

• Stomach

• Duodenum (discussed with small intestine)

Salivary Glands

• Slightly acidic (pH 6.4-6.8)

• Water (~99.5%), electrolytes, digestive enzymes (Salivary Amylase, Lingual Lipase)

• Proteins: Mucin, lysozymes, defenses, IgA

• Parasympathetic → Increase Salivation

• Sympathetic → Decrease Salivation (Dry Mouth)

Teeth

• Primary (deciduous)

  • 20; 6-30 months

• Secondary (permanent)

  • 32; ~6-12 years

Incisor

Cutting/biting

Canine or cuspid

Tearing

Premolar & Molar

Grinding

Chemical Digestion in the mouth

• Salivary Amylase

• Lingual Lipase

Mechanical Digestion in the mouth

• Teeth

• Tounge

• Mandible

Pharynx

• Funnel-shaped muscular tube lined with mucous membrane

• Behind nose, mouth, & larynx

• Extends form base to C6 Vertebrae

Nasopharynx

Located directly behind the nasal cavity and above the soft palate

Oropharynx

Located directly behind the oral cavity, acting as a vital passageway for both air and food

Laryngopharynx

Situated between the upper epiglottis and lower cricoid cartilage (C3 to C6), acting as a crucial junction for breathing and swallowing

Esophagus

• Collapsible, muscular tube

• Posterior to trachea, connects pharynx to stomach

  • Travels through mediastinum & pierces diaphragm

• Secretes mucus, transports good

  • No enzymes produced

  • No absorption

Deglutition (Swallowing)

• Movement of food form the mouth to the stomach

• Voluntary Phase

• Pharyngeal Phase (involuntary)

• Esophageal phase (involuntary)

Voluntary Phase of Deglutition

Tongue pushes food to back of oral cavity

Pharyngeal Phase of Deglutition

Food is int eh pharynx & the airway is closed off, so it does not go the wrong way

Esophageal Phase of Deglutition

Food is going through esophagus into the stomach

4 Major Regions of the Stomach

• Cardia

• Fundus

• Body

• Pylorus

Major Functions of the Stomach

• Mixes & churns food with gastric juices to form chyme

• Begins chemical breakdown of proteins

• Releases food into the duodenum as chyme

• Absorbs some fat-soluble substances

• Possesses antimicrobial functions

Histology (Layers) of Stomach

• Serosa (Outermost)

• Muscularis

• Submucosa

• Mucosa (Innermost)

Gastric Gland Cells (Stomach)

• Surface mucous cells

• Mucous neck cells

• Parietal cells

• Chief cells

• G cells/enteronendocrine cells

Surface Mucous Cells

Mucin in an alkaline fluid

Mucous Neck Cells

Mucin in an acidic fluid

Parietal Cells

HCI & intrinsic factor

Chief Cells

Pepsinogen & lipase

G cells/enteronendocrine Cells

Gastrin

Chemical Digestion in the Stomach

Gastric Juices:

• Hydrochloric (HCl) acid

• Pepsin

Mechanical Digestion of the Stomach

Churning food with smooth muscles

Lower GI Tract Organs

• Small Intestine

• Large Intestine (colon)

• Rectum

• Anus

Small Intestine

• ~20-25 ft long; 1’ in diameter

• The majority of digestion & absorption occurs here

Duodenum

A Mixing Bowl

Jejunum

The bulk of chemical digestion & absorption

Functions of the Small Intestine

• Mixes chyme with digestive juices

• Propels food at a rate slow enough for digestion & absorption

• Absorbs breakdown products of carbohydrates, proteins, lipids, & nucleic acids, along with vitamins, minerals, & water

• Performs physical digestion via segmentation

Layers of the Small Intestine (Histology)

• Serosa (Outermost)

• Adventitia in Duodenum

• Muscularis

• Mucosa

Intestinal Gland Cells (Small Intestine)

• Absorptive

• Goblet

• Paneth

• G Cells

• I Cells

• K Cells

• M Cells

• S Cells

Absorptive Gland Cell

Location: Epithelium/intestinal Glands

Function: Digestion & absorption of nutrients in chyme

Goblet Gland Cell

Location: Epithelium/intestinal Glands

Function: Secretion of Mucus

Paneth Gland Cell

Location: Intestinal Glands

Function: Secretion of the bactericidal enzyme lysozyme; phagocytosis

G Cells

Location: Intestinal glands of duodenum

Function: Secretion of the hormone intestinal gastrin

I Cells

Location: Intestinal glands of duodenum

Function: Secretion of the hormone cholecystokinin, which stimulates release of pancreatic juices and bile

K Cells

Location: Intestinal Glands

Function: Secretion of the hormone glucose-dependent insulinotropic peptide, which stimulates the release of insulin

M Cells

Location: Intestinal glands of duodenum and Jejunum

Function: Secretion of the hormone motillin, which accelerates gastric emptying, stimulates intestinal peristalsis, and stimulates the production of pepsin

S Cells

Location: Intestinal Glands

Function: Secretion of the hormone secretin

Chemical Digestion in the Small Intestine

• Maltase, sucrase, lactase → disaccharides → monosaccharides

• Peptidases → peptides → amino acids

• Intestinal lipase → fats

Mechanical Digestion in the Small Intestine

• Segmentation (mixes food)

• Peristalsis (puch chyme foward)

Large Intestine

• ~5 ft long; 2 ½ inch in diameter

• Ileocecal sphincter