Secrete saliva which:
- Lubricates ingested food
- Protects the mouth from acidic food and pathogens
- Initiates carbohydrate digestion
Sites of Salivary Secretion
Major (Extrinsic) Salivary Glands
- Parotid gland
- Sublingual gland
- Submandibular gland
Minor (Intrinsic) Salivary Glands in the:
- Mostly water: hypotonic relative to plasma
- High bicarbonate and K+ concentrations: (relative to plasma), which neutralize acidic foods in the mouth.
- Low Na+ and Cl- concentrations: (relative to plasma).
- The digestive enzymes:
Salivary amylase, which initiates carbohydrate digestion, and
Lingual lipase, which initiates fat digestion.
- Mucin, which forms thick mucus to moisten and lubricate food as well as aid bolus formation.
- Lysozyme and IgA antibodies, which lyse bacteria and protect the mouth against microorganisms.
Acinar region = “secretory region”
- Secretes the initial saliva
- Water, bicarbonate, K+, Na+, and Cl- are secreted into the acinar region.
Initial salivary secretion is a plasma-like solution (isotonic relative to plasma.
Ductal region = “modifying region”
- Transport saliva to the oral cavity.
- Modify the ionic composition of the initial saliva by selectively absorbing and/or secreting water and electrolytes.
- Na+ and Cl- are reabsorbed, which makes their concentration in saliva lower relative to their plasma concentrations.
- Bicarbonate and K+ are secreted into the duct, which makes their concentration in saliva higher relative to their plasma concentration.
Final saliva that enters the oral cavity is hypotonic.
Note: Salivary ducts are nearly impermeable to water. Tight junctions between ductal cells prevent any additional water leakage into or out of salivary ducts, which makes final salivary secretion more dilute than the initial secretory product.
Flow rate of saliva through salivary ducts effects it final ionic composition because it affects the time the saliva is in contact with the surface of the ductal epithelium and, thus, the degree of absorption and secretion that occurs along the length of the duct.
Note: bicarbonate secretion is not effected by changes in flow rate; saliva is almost always rich in bicarbonate.
High Flow Rate
- Higher concentration of ions relative to water.
- Final saliva is isotonic to plasma (it’s most similar to the initial saliva secretion) because there is less time for reabsorption and secretion.
Final saliva has higher Na+ and Cl- concentrations and lower K+ concentrations than at average flow rates.
Low Flow Rate
- Water concentration is high and the ionic concentration is low.
- Final saliva is hypotonic to plasma (and the least like the initial saliva secretion); this allows more time for Na+ and Cl- reabsorption as well as K+ secretion into the ducts.
Final saliva is much more dilute than at a normal flow rate.
Although we only discussed water and electrolyte secretion from salivary acinar cells, bear in mind, they also secrete the digestive enzymes (salivary amylase and lingual lipase), mucin, lysozyme, and IgA antibodies, which are present in the final saliva.
REGULATION OF SALIVARY SECRETION
Note: Unlike other GI accessory glands, salivary secretion is not under hormonal regulation and only involves neural regulation.
Parasympathetic branches of the autonomic nervous system stimulate the majority of salivary secretion but that the sympathetic nervous system also plays a minor role in secretion, as well.
- Parasympathetic nervous system stimulates secretion of watery (serous), enzyme-rich saliva via cranial nerves IX, X and VII.
- Sight, smell, and taste of food stimulate saliva secretion via parasympathetic activation. Conditioned reflexes and nausea do, as well.
- Sympathetic activation inhibits salivary secretion.
- Sympathetic nervous system also does stimulate a viscous, mucin-rich saliva
Process of defecation (a bowel movement), in which undigested fecal waste material exits the body.
Regulate elimination → under both voluntary and involuntary control.
Internal anal sphincter:
Smooth muscle, involuntary control
External anal sphincter:
Skeletal muscle, voluntary control
Anatomical structures involved in defecation:
- Sigmoid colon → Rectum → Anal Canal → Anus
- Internal anal sphincter and external anal sphincter
Steps of Elimination:
- Mass movements push feces into the sigmoid colon and rectum.
- Rectal distension activates stretch receptors, which trigger the defecation reflex.
- Defecation reflex:
- Rectal smooth muscle contraction → further pushes feces from the rectum to the anal canal
- Internal anal sphincter relaxation and opening → allow feces to pass through the anal canal and exit via the anus.
- Defecation: both the external and internal anal sphincters relax.
Abdominal contraction also aids defecation → increases intra-abdominal pressure and pushes feces through the distal GI tract.
Voluntary external anal sphincter contraction prevents defecation.
The distended rectal wall relaxes and the urge to eliminate dissipates.
Subsequent mass movements reactivate rectal stretch receptors and defecation reflex initiation.
Clinical Correlation: Diverticular Disease
- Lower portion of the large intestine.
- Pouches (diverticula) form at weak areas of colon wall → form outpockets of the mucosa and submucosa → bulge through, disrupt the smooth muscle layer.
- Diverticula form following straining during a bowel movement (i.e. during constipation) → causes high pressures in the colon, weakening the colon wall.
- Smooth muscle layer thickens over time at areas of diverticula formation. The muscle contracts more strongly to eliminate feces.
Other contributors to diverticular disease are:
- Age, more common in elderly.
- Diets low in fiber, which cause hardened feces.
- Defects in GI motility.
- Defects in wall strength.
Diverticular disease = asymptomatic
Diverticulitis: diverticula become infected and progress to an inflammatory state, which causes severe pain.
(Definition): Slow, segmenting movements that further mix chyme.
- About every 30 minutes.
- Occur in haustra: small pouches caused by the teniae coli (longitudinal smooth muscle ribbons that run along outside the entire length of the colon). Because they are shorter than the large intestine, the large intestine tucks between the teniae and form sacs
- Primarily occur in ascending and transverse colons.
- Produced by contractions of smooth muscle layer
- Chyme fills a haustrum
- Distension in the haustrum.
- Smooth muscle layer contracts
- Contractions move chyme into the next haustrum and subsequent haustra, where the sequence begins again.
#Note that haustral contractions play a relatively minor role in propelling fecal waste through the large intestine; their main function to further mix waste.
Contractions also bring chyme in close contact with the large intestine mucosal layer to maximize water and electrolyte absorption
- Hasutral contractions also occur in the descending and sigmoid colon to further concentrate stored fecal waste prior to elimination.
(Definition): slow, but powerful contractions of the large intestine that move undigested waste to the rectum for defecation via the anus.
- Much like stronger and sustained peristaltic contractions.
- 3-4 times a day.
- Mainly in the transverse, descending, and sigmoid colons.
- Produced by circular layer (smooth muscle) contractions
- Undigested waste in the transverse colon.
- Triggered by the gastrocolic reflex (initiated following ingestion of a meal when food enters the stomach causes its distension)
- Circular layer contracts in the transverse colon
- Contractions move waste towards the rectum.
Note: Unlike peristalsis, the circular remains contracted for some time following its initial trigger mass movement.
- Fecal waste moves down the descending colon and into the sigmoid colon toward the rectum
#Prepares for elimination (defecation)
(Definition) uni-directional propulsion of digested food forward through the digestive tract.
- Peristaltic contractions continue from the upper GI and the small intestine to gradually move undigested waste through the remainder of the GI tract.
- Rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.
- Circular layer contracts behinds the chyme, decreases the diameter of the large intestine to propel chyme forward.
Longitudinal layer relaxes.
- Longitudinal layer contracts, which shortens the small intestine and decreases the distance the chyme must travel.
Circular layer relaxes.
- Circular layer contracts and pinches the large intestine further distally, which propels chyme forward through the large intestine towards the rectum.
Longitudinal layer relaxes.
MUSCULAR MOVEMENTS OF SMALL INTESTINE
Uni-directional propulsion of digested food forward through the digestive tract.
- Activated following a meal (postprandial).
- Rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.
Key Steps of Peristalsis
- Circular layer contracts and pinches behind the chyme
– Decreases diameter of the lumen
– Propels chyme forward
- Longitudinal layer relaxes around the chyme
– Dilates the small intestine to easily receive the chyme.
- Longitudinal layer contracts in front of pinched region
– Shortens region of the small intestine (much like when we bunch up a tube sock to get our foot into it)
– This step does not actively move the chyme forward → it shortens the distance the chyme must travel and prepares for its propulsion forward through the small intestine.
- Circular layer contracts again, pinches the small intestine farther distally
– Propels chyme further through the small intestine
Migrating Motility Complex (MMC)
Produces specific periodic peristaltic contractions that sweep digested contents, cellular debris, and bacteria through the small intestine during fasting.
- Occur every 90 minutes to two hours (during interdigestive periods – when a meal has been digested and absorbed).
- Regulated by the candidate digestive hormone motilin.
Alternating contractions of the circular layer between intestinal segments to breakdown food and mix chyme.
- Activated following a meal (postprandial).
Key Steps of Segmentation
- Mixture of chyme enters the small intestine, surrounded by digestive juices: duodenal secretions, pancreatic secretions, and bile
- Circular layer segmentally contracts → chyme separated into these segments
- Circular layer contracts and pinches the middle of these segments to re-segment them.
– Chyme from each segment moves in both directions (forward and backward) from the contraction site, into the other segments.
- Circular layer contracts again in the original site and again re-segments the small intestine.
– Chyme from the different segments mix.
– Also mix chyme with the duodenal and pancreatic secretions and bile.
- The circular layer promotes stationary mixing of partially digested chyme with small intestine and pancreatic secretions.
Other key features of segmentation:
- Actively brings digested chyme into close contact with intestinal epithelium → efficient absorption.
- Does not contribute to the movement of chyme down the small intestine.
- Contributes to mechanical digestion of food → further breakdown of ingested macromolecules to simpler, absorbable forms.
- Segmentation contractions cease following meal absorption.
- Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.
Small intestine (specifically the duodenum)
- Major site of nutrient absorption.
- Three folded mucosal structures maximize surface area for absorption
1 – Plicae circulares: wavy, folds on the inner walls of small intestine → form circular folds → increase surface area 3-fold.
2 – Villi: finger-like projections that protrude from the plicae circulares → surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.
3 – Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) → increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase
Nutrients are absorbed in the small intestine
- Monosaccharides (digested carbohydrate products)
- Amino acids, di-peptides and tri-peptides (digested protein products)
- Intact proteins
- Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
- Water and electrolytes
Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.
- Apical surface: interfaces the intestinal lumen and epithelium.
- Basolateral surface: opposite to the apical surface, lines the inside of the villi.
- Capillaries and lacteals inside villi
– Most nutrients cross the basolateral surface and pass directly into circulation.
- Fats, however, pass directly into lacteals (lymphatic system).
Key Transport Mechanisms
- Apical Surface
– Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
– Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
– Simple diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
– Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.
- Basolateral Surface
– Facilitated diffusion.
– Simple diffusion.
– Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.
Most relevant to the function of the large intestine but also occurs in the small intestine.
– Large intestine stores and concentrates fecal material before elimination.
– Mainly absorbs water and electrolytes to do so
– Also absorbs bacterial byproducts.
- Three General Steps of Water Absorption:
- Sodium-potassium pump on the basolateral surface
– Pumps K+ into cell and Na+ out of cell (pump utilizes ATP to move sodium and potassium against their concentration gradients).
- Na+ passively enters via general sodium ion transporter on the apical surface (possible because of the active transport of sodium out of the cell)
- Net positive charge in the cell
- Chloride enters the cell through a general chloride ion transporter on the apical surfacen via facilitated diffusion down the electrical gradient.
- Higher solute concentration inside the cell relative to the lumen.
- Water crosses the apical surface via osmosis.
FIVE MAJOR PROCESSES OF THE DIGESTIVE TRACT
- Motility of digestive products through the digestive tract.
- Secretion of enzymes and fluids into the digestive tract.
- Digestion of food breakdown.
- Absorption of nutrients and water.
- Barrier from the external environment and microorganisms.
- Elimination of waste and undigested material from the body.
Alimentary canal and glands/organs
- Alimentary canal
– Runs from the mouth to the anus.
– We can imagine the alimentary canal as a donut hole within a giant donut.
- Accessory glands/organs
– Secrete digestive juices into the alimentary canal
– To imagine the accessory glands, we draw a donut and show jelly ooze into the donut hole to represent the accessory gland secretions.
- Salivary glands secrete saliva
- Liver secretes bile into the gallbladder
- Gallbladder secretes stored bile into the duodenum after a meal
- Pancreas secretes pancreatic juice into the duodenum
Liver, gallbladder, and pancreatic secretions pass through individual ducts
– All merge at duodenum → secretions secrete into duodenum
Additional features of alimentary canal:
- Smooth muscle lining allows peristalsis
– Peristalsis is rhythmic contractions that push food down the alimentary canal.
– Sphincters are modified, one-way valves that comprise smooth muscle. They contract and relax to regulate movement through the alimentary canal.
DIGESTIVE TRACT PATHWAY
- Site of mastication (chewing).
- Transports the food bolus to the stomach (via peristalsis).
- Gastroesophageal sphincter: regulates the movement of food into the stomach
– Prevents “heartburn”: the reflux of food back into the esophagus.
- Churns and converts food into liquid chyme.
- Pyloric sphincter regulates the movement of chyme into our small intestine; it releases chyme slowly to allow the duodenum time to absorb food nutrients.
- The major site of nutrient absorption.
- Three major divisions: duodenum, jejunum, ileum.
- Most nutrient absorption in duodenum.
- Absorbs water
- Three major divisions: cecum, colon, rectum.
DIGESTION OF THE FOUR MAJOR MACROMOLECULES
Aided by luminal secretions/enzymes – promote digestion of macromolecules to absorbable nutrients
- Luminal secretions: Saliva = amylase, mucus, lysozyme
- Digests: Polysaccharides → smaller polysaccharides, maltose
– Enzyme is salivary amylase
- Luminal secretions: Mucus, which lubricates passage of food to the stomach. NO digestion
- Luminal secretions: Gastric juice = HCl, pepsin, mucus
- Digests: Polypeptides → small polypeptides
– Enzyme is pepsin
- Luminal secretions are:
– Pancreatic enzymes: trypsin, chymotrypsin, carboxypeptidase, amylase, lipase and nuclease
– Brush border enzymes (not secreted): dissaccharidases, aminopeptidases
– Polysaccharides → disaccharides, maltose. Enzyme: pancreatic amylase
– Disaccharides → monosaccharides. Enzyme: Disaccharidases (brush border)
– Polypeptides → smaller polypeptides. Enzyme: trypsin & chymotripsin
– Smaller polypeptides → amino acids. Enzymes: carboxypeptidases & aminopeptidases (brush border)
– DNA/RNA → nucleotides. Enzyme: nuclease
– Fat globules → glycerol, fatty acids. Enzyme: lipase and bile salts
- Luminal secretions: mucus. NO digestion. Instead absorbs water and electrolytes.
- Branches course within the pelvis to carry oxygenated blood to the pelvic viscera and musculoskeletal structures, the gluteal and medial thigh regions, and the perineum.
- Umbilical artery, which becomes patent after birth.
- Superior vesicular artery, which travels to the urinary bladder.
- Obturator artery, which travels through the obturator canal of the pelvis.
- Inferior vesicular/Vaginal artery, which serves reproductive structures,
- Uterine artery, which, in females, travels to the uterus.
- Middle rectal artery, which travels to the rectum.
- Internal pudendal artery, which supplies the external genitalia.
- Inferior gluteal artery, which exits the pelvis to travel to the gluteal muscles of the posterior hip.
This vessel may also arise from the posterior division of the internal iliac artery.
- Iliolumbar artery, which, as its name suggests, travels along the ilium and lumbar vertebrae.
- Lateral sacral arteries, which travel along the sacrum.
- Superior gluteal artery, which exits the pelvis to serve the gluteal muscles.
Blood loss from post-partum uterine hemorrhage can be so great as to be life-threatening, so surgeons are trained to seek out and ligate (tie off) the anterior division of the internal iliac artery to stop the blood loss. Ligation may be temporary, or permanent; ischemia of the pelvic viscera is uncommon, due to collateral arterial supply.