Digestive system

  • Digestive (GI) tract

    • Mouth

    • Pharynx

    • Esophagus

    • Stomach

    • Small/large intestines

    • Rectum

    • Anus


    Accessory Organs:

    • Teeth

    • Tongue

    • Salivary glands

    • Gall bladder

    • Liver

    • Pancreas


    Digestive system functions:

    • Ingestion

    • Mechanical processing

    • Digestion (duh)

    • Secretion

    • Absorption

    • Excretion



    Digestive and other systems:


    Histology of the GI tract:

    1. Mucosa

    • Consists of an inner lumen

    • Consists of the outer epithelium and the inner lamia propira

    • Stratified squamous in high-pressure areas (oral cavity, pharynx, esophagus, rectum)

    • and simple columnar in the rest, with secretory cells.

    • The muscularis mucosae moves mucosal folds.

    • Folded to increase surface area.


    4 types of epithelium:

    • Cutaneous

    • Synovial

    • Mucos

    • Subcutaneous


    1. The Submucosa:

    • Dense irregular connective tissue

    • Binds mucosa to muscularis externa

    • Innervated (continuous blood vessels and lymphatics)

    • Submucosal plexus (a neural network) on the outer border

    • Assists with the secretion of enzymes into the system


    1. Muscularis Externa:

    • Inner circular and external longitudinal smooth muscle layers

    • San, which ends at the myenteric plexus

    • Contains:

      • Parasympathetic ganglia increase activity

      • Sympathetic ganglia; decreases activity

    • Helps with churning and mixing


    1. The Serosa:

    • Covers the muscularis externa, along most of the GI tract, forming the peritoneal cavity

    Consists of

    • Mesenteries

      • specialized peritoneum stabilizes some of the GI tract

      • Innervated (contains blood vessels, nerves, and lymphatics

    • Adventitia

      • outer layer of the oral cavity, pharynx, esophagus, and rectum. (Collagen-rich)


    Oral (buccal) cavity:

    • Lined with oral mucosa

    • Senses material before swallowing

    • Mechanically process

    • Lubricates with saliva and mucus

    • Begins enzymatic digestion of carbs and lipids

    • Lateral walls: cheeks and continue to the lips

    • Roof: palate (made of the maxilla bone)

    • Floor: tongue


    • Perform cheOropharynx: starts at the base of the tongue



    The tongue:

    • Manipulates food within the oral cavity (compresses/ distorts it)

    • Senses taste, touch, and temp

    • Assists in chewing

    • Lingual tonsils: paired lymphoid nodules at the base of the tongue


    Salivary glands:

    • ~1-1.5 L saliva/day

    • 99.4% water+ mucins, ions, buffers, metabolites, enzymes.

    • Salivary amylase breaks down carbohydrates

    • parotid salivary gland:25%

    • Sublingual salivary glands:5% ( produces the least amount of saliva), merocrine secretion

    • Submandibular gland: 70% (produces the majority of saliva), apocrine secretion


    Teeth:wing or mastication

    • Made of dentin

    • Neck: area between crown and root

    • Crown: covered in enamel (crystalline form of calcium phosphate) requires Ca2+, phosphates, and vitamin D3

    • Pulp cavity: receives blood vessels and nerves through the root canal and the root of the tooth

    • The periodontal ligament with cementum binds the root of teeth to the bone


    There are 20 baby teeth.

    The bottom jaw’s incisors erupt first

    There are 32 adult teeth

    The third molar erupts last


    Pharynx

    • Food passes through the oropharynx and laryngopharynx to the esophagus

    • Mucosa is made up of stratified squamous epithelium

    • Laminate propila contains mucous glands and tonsils


    Esophagus

    • A muscular tube of ~25 cm (10 in) long and 2 cm (0.75 in) in diameter

    • The passageway of food from the pharynx to the stomach

    • Posterior to the trachea.

    • Enters abdominal cavity through the esophageal hiatus in the diaphragm.

    • Lined with stratified squamous epithelium.

    • Sphincters are present at both ends, made of a circular pattern of smooth muscle cells.


    Deglutition

    • Initiated voluntarily

    Proceeds automatically


    The Stomach:

    • Functions:

      • Temporary storage of ingested food.

      • Mechanical breakdown of ingested food

      • Chemical digestion by acids and enzymes

      • Production of intrinsic factor, a compound necessary for the absorption of Vit B12.

    • Chyme: Ingested material mixed with glandular secretions, ns producing a viscous, highly acidic soup. (Moves faster when the meal contains fewer promises and more carbs, caffeine, and alcohol)


    Stomach Anatomy:

    1. Cardia: connects with the esophagus

    2. Fungus: bulge of the superior stomach

    3. Body: a large area between the fungus and the pylorus

    4. Pylorus: distal, connects the stomach to the small intestine. Regulates the flow of chyme to the small intestine

    The Gastric Wall:

    • Lined with simple columnar epithelium with numerous mucous cells.

    • Mucous cells secrete alkaline mucus that protects the epithelium.

    • Gastric Pits: Shallow depressions that open to the surface

    • Neck cells undergo active mitosis, replacing mucosa cells.

    • Gastric glands:

      • Parietal cells

        • Intrinsic factor for VitB12 absorption.  HCl lowers the pH of gastric juices to 1.5-2

        • Kills microorganisms and activates enzymes.

        • Pepsinogen+ HCL= Pepsin

      • Chief cells

        • Kills microorganisms and activates enzymes.

      • G cells: Gastrin

      • Mucous cells: life spans ~ 3-7 days

    Parietal and chief cells produce ~1.5L of gastric juice in a day


    Digestion:

    • Pepsin initiates protein digestion into small peptides

    • Salivary amylase will digest carbohydrates until pH ~4.5. (initiated in the mouth

    • No nutrients are absorbed in the stomach because,

      • Mucosa covered by alkaline mucous.

      • Epithelial cells lack transport mechanisms.

      • Gastric lining is impermeable to water.

      • Digestion is incomplete, and nutrients are still complex.


    Small intestines:

    • intestinal wall: 3 levels of increased surface area

      1. Circular folds: (plicae circulares) transverse ridges

      2. Villi: finger-like projections of mucosa covered with columnar epithelium

      3. Microvilli: (brush border) modified apical surface of columnar cells


    intestinal movement:

    • Weak peristaltic contractions: initiated by a rural network independent of the CNS

  • Gastroenteric reflex: initiated by distension of the stomach. Increased peristaltic contractions in the duodenum

  • Gastrointestinal reflex: triggered by gastrin, reflexes in the ileocecal valve

  • ~5hrs from duodenum to end of ileum


Juices: mostly water from mucosa, rest by intestinal glands (~1.8 L/day)

Hormones:

  • Gastrin: stimulates the production of acid and enzymes

  • Secretin: increases buffer and bile secretions

  • CCK: increases enzyme production (pancreas) and ejection of bile (gall bladder)

  • GIP: Inhibits the gastric actI it’s and causes release of insulin. (Inhibits increased gas production and releases insulin)


The Pancreas:

  • Location: behind the stomach, extending from the duodenum towards the spleen ~80 g. Secretes ~1 L of pancreatic juices mostly (Na2HCO2)

  • Major tissues and function:

    • Pancreatic islets: endocrine type secretes insulin, glucagon

    • Pancreatic acinar cells: exocrine type, produce digestive enzymes, juices, and buffers


Histology:

  • Pancreatic acini: pouches of acinar cells

  • Pancreatic ducts: formed by the convergence of many smaller ducts

  • Islets ~1% contains:

    • Alpha cells that secrete glucagon

    • Beta cells that secrete insulin


Pancreatic secretions: CCK stimulates the production of 4 classes of enzymes:

  • Carbohydrase: digests sugars and starches (ex. amylase)

  • Lipase: breaks down lipases

  • Nucleases: break down nucleic acids

  • Proteases: breakdown of proteins, ~ 70% ( ex. trypsin, chymotrypsin)


High acid chyme triggers secretin release, which increases bicarbonate levels to buffer chyme.


Liver:

Lobes are divided into 100,000 lobules (functional unit)

Liver cells:

  • Hepatocytes: major type, arranged within lobules in the form of plates like spokes of a wheel, secrete bile, synthesize plasma proteins, serve as a transporter

  • Kupffer cells: phagocytic type provides immunity/ protection


Bile: secreted by hepatocytes within bile canaliculi - common bile duct- duodenum

Additional bile is returned to the gall bladder

Bile is a mixture of water, inconsistent, bilirubin from destroyed RBS, cholesterol, and bile salt.s

Bile salts are synthesized from cholesterol in the liver and are required for normal digestion and absorption of fats


Blood circulation:

  • Blood arrives via the hepatic artery and hepatic portal vein

  • At the sinusoids, hepatocytes absorb nutrients, toxins, and secrete plasma proteins

  • High blood glucose triggers the synthesis of glycogen

  • Low blood glucose triggers glycogen breakdown

  • Blood leaves sinusoids into the central veins, the hepatic vein, and then the IVC


Functions:

  • Metabolic: extract nutrients/ toxins from the blood

  • monitors and adjusts organic nutrients


The large intestine:

  • Horse-shoe shaped, frames around the small intestine

  • Ileocecal valve to anus

  • consists of the cecum, colon, and rectum

Functions:

  • reabsorption of water, compacting chyme into feces

  • absorption of vitamins freed by bacterial action

  • storage of feces prior to defecation

Cecum:

  • AN expanded pouch

  • Chyme enters through the ileocecal valve

  • compaction begins here

  • Appendix: found on the posteromedial surface

  • contains lymphoid nodules for immune function

  • Inflammation is called appendicitis (removal is called appendectomy)


The Colon:

  • Large Diameter, no villi, abundant mucous ccells. The external surface had


  • Haustra: Elongated pouches

  • Taenia coli: 3-longitudinal bands of smooth muscle

  • Segments:

    • Ascending colon

    • Descending colon

    • transverse colon

    • transverse colon

    • sigmoid colon

The rectum:

  • An expandable organ for storing feces

  • Anal canal is the last portion

  • anus consists of-

    • internal anal sphincter- made of smooth muscularis externa, under ANS control

    • external anal sphincter: made of skeletal muscle under voluntary control

The large intestine (functions cont.)

  • Main function: absorption/conversion

    • Water (1300ml of total 1500ml a day)

    • bile salts

    • Vit K (fat-soluble) for the production of clotting factor

    • Vitamin B5 (water-soluble)synthesis of neurotransmitters and steroid hormones

    • Vitamin H (Biotin) essential for glucose metabolism

    • conversion bilirubinnn to an absorbable form that is eliminated via urine

    • Bacterial breakdown of peptides into ammonia, nitrogen compounds, and hydrogen sulfide

    • Ammonia and other toxins are absorbed. The liver converts them into harmless forms and eliminates them through the kidneys

    • Indigestible carbs serve as a source of nutrients for bacteria that form gas

    • All non-absorbable forms remain in feces, giving a brown color

Defecation: Involves two feedback loops,

  1. Short loop:

  • Stretch receptors in the rectal walls stimulate local peristalsis

  • moves feces towards the anus

  1. Long loop:

  • Parasympathetic reflexes trigger the involuntary relaxation of the internal sphincter

  • conscious relaxation of the external sphincter to eliminate feces


Aging Effects:

  • Chief cells of a baby’s stomach produce renmate (a coagulation factor). It gives more time to help digest milk's nutrients.

  • Increase in ulcer, dehydration

  • Increase in cancer rate (colon, stomach)

  • Decline in osmoreceptor sensitivity leads to dehydration

  • Bone loss leads to tooth loss, cavities, and gingivitis (osteopenia, not osteoarthritis)

  • loss of taste and olfactory sensations