The Digestive System

Digestive System

• Organ system which processes food, extracts nutrients, & eliminates residue

Stages of Digestion

• Ingestion → intake

• Propulsion → swallowing, peristalsis

• Digestion → mechanical & chemical breakdown into a usable form

• Absorption → uptake of nutrients

• Compaction → absorbing water & consolidating indigestible residue

• Defecation → elimination of feces

Digestive Function

• Mechanical Digestion → physical breakdown of food

  • Cutting/grinding w/ teeth

  • Churning within stomach & small intestines

  • Exposes more food surfaces to digestive enzymes

• Chemical Digestion → series of hydrolysis reactions

  • Breaks macromolecules into their monomers

    • Polysaccharides → monosaccharides

    • Proteins → amino acids

    • Fats → monoglycerides & fatty acids

    • Nucleic acids → nucleotides

  • Carried out by digestive enzymes

2 Subdivision/groups of Organs

• Digestive tract (alimentary canal)

  • 30 ft muscular tube from mouth to anus

  • Mouth, pharynx, esophagus, stomach, small intestine, & large intestine

• Accessory Organs

  • Teeth, tongue, salivary glands, liver, gallbladder, pancreas

Alimentary Canal

Four basic layers (tunics):

• Mucosa (innermost)

• Submucosa

  • Houses vessels, nerves, lymphatics

• Muscularis externa

  • 2 layers of smooth muscle

  • Segmentation (mix food back and forth → breakdown + absorb) & Peristalsis (wave like muscle contractions → moves food)

• Serosa (outermost)

  • Anchors to surrounding tissue

  • Replaced by the adventitia (outer support layer; hold organs/tubes in place)

Enteric Plexus

• Enteric Plexus → nervous network within the digestive tract

• Regulates motility (ability to move), secretion, blood flow

Regulation of the Digestive Tract

Motility + secretion of the digestive tract controlled by:

• Neural controls:

  • short (myenteric) reflexes → stretch or chemical stimulation

  • long (vagovagal) reflexes → parasympathetic stimulation

The Mouth Functions (Section 1)

• Ingestion

• Mechanical (mastication)

• Chemical digestion

• Absorption

• Propulsion (swallowing)

• Taste + other sensory responses

The Teeth

AKA → Dentition

• Function

  • Aid mechanical digestion

    • Mastication

  • Speeds chemical digestion

• Features

  • 32 adult teeth

  • Sharp to tear + grasp food

  • Molars crush & grind

The Tongue

• Functions:

  • Positions food between teeth

  • Senses taste + texture

  • Formation of the bolus (small rounded chewed mass of food)

  • Initiation of swallowing + speech

• Features:

  • Papillae: friction, licking, taste buds, secrete lingual lipase

Salivary Glands

• Intrinsic (minor) salivary glands: scattered in oral mucosa + keep mouth moist

  • Lingual glands, Labial glands, Palatine glands, Buccal glands

• Extrinsic (major) salivary glands: produce majority of saliva

  • Parotid glands, Submandibular gland, Sublingual gland

Saliva Functions

• Moistens mouth + (food → binds to bolus to aid swallowing)

• Cleanses teeth

• Dissolve molecules → taste buds

• Contains chemicals (IgA Abs & defensins) protect against microorganisms

• Contains enzymes (salivary amylase & lingual lipase) that breakdown starch & fat

The Pharynx

• Intersection of digestive & respiratory tracts

• Oropharynx (middle back throat) & laryngopharynx (below middle back throat)

  • Nasopharynx (upper back throat behind nose) has no digestive role

• Skeletal muscle layers: inner longitudinal, outer pharyngeal

The Esophagus

• Muscular tube from laryngopharynx to stomach

  • Cardiac orifice (upper opening of esophagus → stomach)

  • Esophageal hiatus (opening/gap) in diaphragm

  • Lower esophageal sphincter (LES) (bottom of esophagus → valve between food pipe & stomach)

• Bolus of food travels through esophagus by peristalsis

Deglutination (swallowing)

• Involves tongue, soft palate (muscular flexible tissue back roof of mouth), pharynx, esophagus, + 22 muscle groups

  • Coordinated by swallowing center, communicate with muscles via cranial nerves

• Three phases:

  • Oral/buccal phase → voluntary tongue contraction; forces bolus into oropharynx

  • Pharyngeal phase → involuntary; blocks all route except down esophagus

  • Esophageal phase → involuntary; peristalsis

The Stomach (Section 2)

• Location: LUQ, inferior to diaphragm

• Hollow, muscular sac

• Functions:

  • Food storage

  • Mechanical digestion, liquefies food

  • Begins chemical digestion of proteins & fats

  • Results in chyme (acidic, soupy mixture of semi digested food)

The Stomach Four Regions

• Four regions:

  • Cardiac region (cardia)

  • Fundic region (fundus)

  • Body

  • Pyloric Region (bottom part of stomach connecting to duodenum (first part of small intestine))

• Two margins:

  • Greater curvature

  • Lesser curvature

Circulation of the Stomach

• Receive blood by branches of the celiac trunk

• Blood drained from stomach + intestines → hepatic portal circulation (nutrient rich deoxygenated blood) filtered through liver → heart

Stomach Tissue Layers

Tissue Layers

• Muscularis externa

  • Outer longitudinal

  • Middle circular

  • Inner oblique

• Mucosa (innermost lining of digestive tract)

• Submucosa (thick layer connective tissue beneath Mucosa)

  • Gastric rugae (large folds of mucosa and submucosa layers allowing stomach to expand as it fills)

Mucosa

• Produces mucus → traps bicarbonate-rich fluid beneath it

• Gastric pits lined with specialized cells → into tubular glands

Gastric Pits

• Depressions in gastric mucosa

• 2 or 3 tubular glands open at bottom of each pit

  • Cardiac glands

  • Pyloric glands

  • Gastric glands

• Glands responsible for gastric juice production

• Cellular Composition

  • Mucous cells, Parietal cells, Chief cells, Enteroendocrine cells

Mucous Cells

• Secret mucus

• Predominate in cardiac (near entrance stomach) & pyloric glands (near exit stomach)

Parietal Cells

Secrete

• Intrinsic Factor: required for absorption of Vit B12 in small intestine

• Ghrelin: hunger hormone

• HCI: portion of gastric juice; pH 1.5-3.5

  • Denatures protein

  • Activates pepsin

  • Kills bacteria

Chief Cells

• Secrete

  • Pepsinogen: an inactive form of the enzyme pepsin

  • Gastric lipase: fat digestion

• Dominates lower half gastric glands

• Absent from pyloric & cardiac glands

G cells (AKA: Enteroendocrine cells)

• Secrete chemical messengers:

  • Paracrines (chemical signals released by one cell → affects nearby cells) (acts locally)

    • Serotonin & histamine (amines act as neurotransmitters in brain → signal molecules through body)

  • Hormones:

    • Somatostatin & gastrin (G cells)

    • Regulates secretion and mobility

Regulation of Digestive Tract

Motility + secretion of the digestive tract controlled by:

• Neural controls:

  • short (myenteric) reflexes → stretch or chemical stimulation

  • long (vagovagal) reflexes → parasympathetic stimulation

• Hormones:

  • Ex: gastrin, secretin

• Paracrine secretions

  • stimulate nearby target cells

Cephalic Phase (Stage 1)

• Stimuli from inside/outside the GI tract (muscular tube from mouth to anus)

  • Sight, small, taste, thought of food

• Involve CNS centers

  • Sensory & mental inputs → hypothalamus (pea size region in brain → homeostasis)

  • Hypothalamus → medulla oblongata (low part of brainstem → connect brain to spinal cord

  • Medulla → vagus nerve fibers (nerve pathways carry messages between brain and organs of body)

  • Vagus nerve fibers → enteric nervous system (nerves in walls of GI tract → helps control digestion)

• Stimulates gastric secretion

Gastric Phase (Stage 2)

• Mediated in GI tract by local stimuli from ingested food

  • Stretches stomach

    • Activates short reflex through myenteric plexus

    • Activates long reflex through vagus nerve + brain stem

  • Increase gastric juice pH

• Gastric secretion is stimulated by:

  • ACh

  • Histamine

  • Gastrin

Response to Stomach Filling

• Receptive-Relaxation (Reflex-Mediated) Response:

  • Coordinated by swallowing center of brain stem

  • Swallowing food → signal to medulla

  • Medulla signals stomach to relax via vagus nerve

• Gastric accomodation:

  • Stress-Relaxation Response

  • Plasticity of smooth muscle

  • Allows stretching without reflexively contracting

Intestinal Phase

• Intestines control rate of gastric emptying

  • Duodenum responds chyme arriving → moderates activity through hormones + nervous reflexes

• Stimulatory & inhibitory

  • Stimulatory:

    • Vagovagal reflex

    • Intestinal gastrin

  • Inhibitory:

    • Enterogastric reflex

    • Secretin & CCK

Vomiting (Emesis)

• Protective reflex

• Emetic Center in medulla

• Triggered by overstretching of stomach, duodenum, chemical irritants, intense pain

• Nausea and retching

• Diaphragm & abdominal muscles contract → increase intra abdominal pressure, gastroesophageal sphincter relaxes, contents forced out of stomach

Digestion and Absorption

• Mechanical Digestion

  • Gastric Motility

  • Physical breakdown of food particles; 3 layers of muscles

• Chemical Digestion

  • HCl → denatures proteins

  • Pepsin → enzymatic digestion of proteins

• Absorption:

  • Secretes intrinsic factor → small intestine absorb vit B12

• Delivers chyme to small intestine to continue digestion & absorption

The Liver (Section 3)

• Location: URQ, inferior to diaphragm

  • Largest gland in body

• Digestive function: secretes bile (break down fat + remove waste)

• Other functions:

  • stores glycogen & vitamins (fat soluble)

  • Detox of ammonia

The Liver Anatomy

• 4 Lobes

• Falciform ligament (Connects anterior liver to anterior abdominal wall + inferior surface of diaphragm)

Bile

• Yellow-Green fluid secreted by liver

• Facilitates fat & cholesterol absorption

• Bild acids (bile salts) → cholesterol derivative assists fat emulsification (forcing two unmixable liquids to combine) & absorption

• Bild pigments: bilirubin primary bile pigment; formed from breakdown of hemoglobin

The Gallbladder

• Location: posterior aspect of liver

• Anatomy: thin walled, pear shaped sac

• Functions

  • Stores + Concentrates bile

  • Releases bile via cystic duct to common bile duct

• Cholelithiasis → hardened deposits of digestive fluid (gallstones)

The Pancreas

• Location: posterior to greater curvature of stomach; head of pancreas encircled by duodenum

• Anatomy: head, body, tail, pancreatic duct

• Function: endocrine & exocrine gland

Pancreas Function

• Endocrine:

  • Secrete insulin (lowers blood sugar) & glucagon (raises blood sugar)

• Exocrine:

  • Secrete pancreatic juice (enzymes for digestion + bicarbonate ions)

Pancreas Function (Continued)

• Pancreatic juice:

  • Alkaline solution (pH 8)

  • Mixture of

    • Water + Electrolytes (primarily HCO3-)

    • Enzymes & Zymogens

      • Protease (trypsin) → protein

      • Amylase → starch

      • Lipase → lipids

      • Nucleases → nucleic acid

Regulation of Secretion

Release of bile & pancreatic secretions are regulated by:

• Acetylcholine (ACh)

• Cholecystokinin (CCK)

• Secretin

Acetylcholine (ACh)

• From vagus & enteric nerves during cephalic & gastric phases

  • Initiates enzyme production

  • Weak contractions of gallbladder

Cholecystokinin (CCK)

• Secreted by mucosa of duodenum in response to chyme entering small intestine

  • Stimulates gallbladder contraction

  • Stimulates secretion of pancreatic juice (enzymes)

Secretin

• Released by duodenum in response to acidic chyme arriving in stomach

  • Stimulates liver & pancreas to secrete more sodium bicarbonate

  • Raises pH → allows activity of pancreatic + intestinal digestive enzymes

The Small Intestine (Section 4)

• Location: abdominal cavity; inferior to stomach and liver

• Begins at pyloric sphincter (valve between lower stomach + first part small intestine) and ends at ileocecal valve (final part of small intestine - first part large intestine)

  • 2-4m long

• Subdivisions:

  • Duodenum, Jejunum, Ileum

• Digestive goal: nutrient absorption

Duodenum

• First section + shortest (25 cm; 10 in)

• Begins pyloric valve → ends duodenojejunal flexure

• Receives:

  • Stomach content / chyme via pyloric sphincter/valve

  • Pancreatic juice and bile

• Actions:

  • neutralize stomach acid

  • emulsifies fat

  • inactivates pepsin

Jejunum

• Second section

• Features:

  • Large, tall, closely spaced circular folds

  • Thick & muscular wall

  • Rich blood supply → red color

• Goal: most digestion + nutrient absorption

Ileum

• Third section + longest

• Compared to Jejunum, Ileum is thinner, less muscular, less vascular, paler

• Features: aggregated lymphoid nodules

• Goals: vitamin absorption

• Ends at the ileocecal junction

Structural Modifications (Circular Folds)

• Circular folds:

  • Occur from duodenum to middle of ileum

  • Cause chyme to flow in spiral path → more contact with mucosa

  • Promotes thorough mixing & nutrient absorption

Structural Modifications (Villi)

• Villi:

  • Motile fingerlike projections → increases surface area

  • Tight junctions

  • Villus epithelium:

    • Simple columnar absorptive cells (enterocytes)

    • Goblet cells

  • Core of villus:

    • Blood capillaries absorb most nutrients

    • Lacteals absorb most lipids

Structural Modifications (Microvilli)

• Microvilli

  • Plasma membrane extensions on apical surface of each enterocyte

• Function:

  • Increase absorptive surface area

  • Brush border enzymes:

    • carry out final stages of enzymatic digestion

Structural Modifications (Intestinal crypts)

• Intestinal crypts

  • Located between bases of villi

  • Pores open into tubular glands

  • Upper half has enterocytes & goblet cells

  • Lower half has

    • enteroendocrine cells

    • dividing stem cells

    • paneth cells

Motility of Small Intestine

• Goal:

  • Mix chyme with intestinal juice, bile, & pancreatic juice (neutralize acid + digest nutrients)

  • Churn chyme, bring it in contact with mucosa for contact digestion & nutrient absorption

  • Move residue into large intestine

• How:

  • Segmentation

    • Ring-like constrictions that mix and move contents slowly toward ileocecal valve

  • Peristalsis

    • Wave of contractions moving materials toward large intestine

Chemical Digestion and Absorption (Section 5)

Carbohydrates

• Digestive enzymes

  • Salivary amylase, Pancreatic amylase, Brush border enzymes

• Lactose Intolerant:

  • The undigested disaccharides create osmotic gradient that prevents water from being absorbed → diarrhea

  • Treatment → lactase enzyme

Proteins

• Proteases (peptidases)

  • Pepsin (stomach)

  • Pancreatic proteases: trypsin, chymotrypsin, carboxypeptidase

  • Brush border enzymes: aminopeptidases, carboxypeptidases

Lipids

• Pre treatment: Emulsification

  • Achieved by stomach antrum pumping, intestinal segmentation, as well as bile salt

• Enzymes:

  • Lingual lipase, Gastric lipase, Pancreatic lipase

• Absorption of glycerol & short chain fatty acids

The Large Intestine (Section 5)

• Located abdominal cavity

• Begins at cecum, end at anal canal

• Shorter but wider than small intestine

• Functions:

  • Absorb remaining water

Unique features of Large Intestine

• Teniae coli: three bands of longitudinal smooth muscle in muscularis

• Haustra: pouches in the colon caused by muscle tone of teniae coli

  • Epiploic appendages: fat filled pouches hanging from serosa of transverse through sigmoid

Anatomy of Large Intestine

• Cecum

  • First segment of large intestine

  • Ileal papilla (tissue projection of end of small intestine connects to large intestine)

  • Appendix

• Colon

  • Ascending colon (Right colic flexure)

  • Transverse colon (Left colic flexure)

  • Descending colon (down left side of abdomen)

Anatomy of Large Intestine

• Rectum

  • Transverse rectal folds: three curves & infoldings

  • Goblet cells

• Anal canal

  • final segment of large intestine

  • mucosa folds into anal columns with anal sinuses betwen

• Anus: regulated by two sphincters

  • Internal anal sphincter: smooth muscle of muscularis externa

  • External anal sphincter: skeletal muscle of pelvic diaphragm

Colonic Motility

• Haustral Contractions:

  • Occur every 30 minutes

  • Move material from one haustral to next

  • Slow segmenting movement

  • Churns and mixes residue → promotes water and salt absorption

• Mass Movements:

  • Stronger contractions → 1-3x a day

  • Gastrocolic & duodenocolic reflexes

  • Filling of stomach & duo stimulate motility of colon

  • Moves residue several centimeters