SM_133b-134b: Water & Electrolyte Absorption, GI Motility Flashcards
Absorption is ____
Absorption is transfer of material across mucosa into intestinal space and then into capillaries or lymphatics
(material must cross 2 membranes)
Material moves ____ or ____
Material moves transcellularly or paracellularly
- Transcellular: across luminal membrane and serosal (basolateral) membrane of epithelial cells, hydrophobic or specific carriers
- Paracellular: crosses mucosa between epithelial cells, restricted by tight junctions of pore sizes, negative charges favor passage of cations
Absorption is influenced by the ____ of the membrane and ____, ____, and ____ of the substance
Absorption is influenced by the nature (lipoid) and surface area (large) of the membrane and the molecular size, shape, and solubility of the substance
- Uncharged molecules less than 180 MW ppass easily
- Effectiveness of transport mechanisms
- Length of time of contact of substance with absorbing surface
- Most nutrient absorption occurs in small intestine
- Significant amount of water is absorbed in colon
- No significant absorption in mouth or stomach
Water absorption is driven by ____, mainly ____
Water absorption is driven by electrolyte absorption, mainly Na_
- Na+ and Cl- movements are coupled
- Absorption of water driven by NaCl transport
- All absorption must involve apical and basolateral steps
Describe absorption of water and electrolytes in small intestine and colon
Absorption of water and electrolytes in small intestine and colon
- Small intestine: net flux out of lumen, water moves passively by osmotic forces
- Upper duodenum: net water movement into lumen due to hyperosmolarity of chyme, breakdown of polymers cuases increased osmolarity, water moves through tight junctions
- Jejunum: large amounts of solutes are being absorbed, so net water movement out of lumen through paracellular pathways
NaCl is absorbed primarily by ____
NaCl is absorbed primarily by active transport of Na+ with Cl- moving passively
- Na+ transported both by itself or with a variety of other ions or molecules
Describe mechanisms of NaCl absorption
Mechanisms of NaCl absorption
- Na+/K+ ATPase on basolateral surface provides major force for Na+ reabsorption in small intestine and colon
- Na+/K+ pump is electrogenic, causing interior of the cell to become negative with respect to lumen and cell [Na+] is low
- Na+ absorption occurs via coupled transport systems
- NaCl co-transport in mucosal surface allows electrically neutral transport in cell: removal of Na+ from lumen inhibits Cl- absorption and vice versa
- NaCl transport also occurs via combination of Na+/H+ exchange and Cl-/HCO3- exchange (ileum)
K+ absorption is mostly ____
K+ absorption is mostly passive
- K+ equilibrates electrochemically with blood by diffusing through tight junctions: hypertonic as water reabsorbed from lumen -> leaves lumen
- Active transport also via K+/H+ exchange
- Colon is activr participant in K+ homeostasis
Ca2+ is absorbed via an ____
Ca2+ is absorbed via an active mechanism
- Controlled so that Ca2+ deficiency increases absorption
- Ca2+ solubilized by gastric acid
- Ca2+ enters cell by carrier mediated diffusion by binding to calcium binding protein
- CBP sequesters intracellular Ca2+
- Ca2+ is transported out of cell by Ca2+ ATPase on basolateral surface
- CBP and Ca2+ ATPase synthesis stimulated by 1,25-dihydroxy-Vitamin D3
Iron is absorbed via ____
Iron is absorbed via an active mechanism which is influenced by body levels of the metal
- Iron taken up by mucosal cells -> stored -> transferred to plasma
- Ferrous iron is taken into mucosal cells by active transport -> binds apoferritin -> ferritin -> stored
- Iron removed from epithelial cells by binding to a plasma iron carrier (transferrin): when iron demands high, increases transferrin, leading to increased apoferritin for uptake
Absorption of water soluble vitamins can be ___, ___, or ___
Absorption of water soluble vitamins can be passive, facilitated, or active transport
- Active transport involves co-transport with Na+
- B12 absorption involves binding to intrinsic factor -> binds to specific carrier in distal ileum -> taken up
Absorption of fat soluble vitamins involves incorporation into ____ and absorption via ____
Absorption of fat soluble vitamins involves incorporation into micelles and absorption via simple diffusion
- Enter general circulation via lymphatics
____ are the waves that determine the frequency at which GI contractions rhythmically occur
Slow waves are the waves that determine the frequency at which GI contractions rhythmically occur
(slow undulating changes in resting membrane potential)
____ are responsible for slow waves
Cajal cells are responsible for slow waves
Describe slow waves in GI smooth muscle
Slow undulating changes in membrane potential
(NOT action potentials)
GI muscle contraction is stimulated by ____, ____, and ____
GI muscle contraction is stimulated by stretch, ACh, and parasympathetics
GI muscle hyperpolarization is stimulated by ____ and ____
GI muscle hyperpolarization is stimulated by norepinephrine and sympathetics
In small intestine, low wave pacemaker frequency is greatest in ____ and slowest in ____
In small intestine, low wave pacemaker frequency is greatest in duodenum and slowest in ileum
- Frequency decreases as go farther down intestine
Migrating motor complexes are found in the ____ and ____
Migrating motor complexes are found in the duodenum and jejunum
Vagal excitatory fibers excite ____ which then excite ____
Vagal excitatory fibers excite excitatory myenteric neurons which then excite LES smooth muscle
Pacemaker zone of stomach is between the ____ and ____
Pacemaker zone of stomach is between the fundus and body
Regions of the stomach involved in motility are ____, ____, ____, and ____
Regions of the stomach involved in motility are
- Lower esophageal sphincter
- Reservoir (tonic contractions)
- Antral pump (phasic contractions)
- Pylorus
Functional regions of the stomach are the ____, ____, ____, and ____
Functional regions of the stomach are the
- Lower esophageal sphincter: prevention of reflux
- Secretion reservoir: fundus and body
- Mixing grinding: antrum
- Pylorus: control of emptying
Gastric pacemaker projects to the ____ and ____
Gastric pacemaker projects to the body and antrum
Gastric pacemaker establishes a ____
Gastric pacemaker establishes a basal electrical rhythm
- Waves of depolarization initiated by the pacemaker are insufficient to trigger contractions unless these are superimposed with a contractile stimulus
____ and ____ are involved in receptive regulation of the stomach
Intrinsic and vago-vagal reflexes are involved in receptive regulation of the stomach
- Signals triggered by nutrients in the duodenal lumen or duodenal distention also result in relaxation of the gastric fundus
Gastric motility patterns contribute to ____, ____, and ____ of gastric contents
Gastric motility patterns contribute to mixing, grinding, and sieving of gastric contents
Gastric contractions include ____, ____, and ____
Gastric contractions include propulsion, grinding, and retropulsion
- Propulsion: bolus is pushed toward the closed pylorus
- Grinding: antrum churns the trapped material
- Retropulsion: bolus is pushed back into proximal stomach
Gastric emptying: solid meal ___ protein solution ___ glucose solution
Gastric emptying: solid meal > protein solution > glucose solution
Describe gastric accomodation
Gastric accomodation
- Baseline intraluminal pressure low under normal circumstances but at higher pressure after vagotomy
- Food in stomach at low intraluminal pressure under normal circumstances but at higher pressure after vagotomy
Gastric emptying is controlled in the ___ phase
Gastric emptying is controlled in the intestinal phase
Describe mechanics of peristalsis
Peristalsis mechanics
- Initial state: longitudinal muscles and circular muscles relaxed
- Contraction of circular muscles behind bolus
- Contraction of longitudinal muscles ahead of bolus
- Contraction in circular muscle layer forces bolus forward
Describe control of peristalsis by enteric nervous system
Control of peristalsis by enteric nervous system
- Contraction of circular muscle
- Excitatory motor neuron to myenteric plexus
- Through myenteric plexus
- To inhibitory motor neuron secreting NO and VIP
- Relaxation of circular muscle
____ mixes food from nonadjacent segments in the peristaltic wave
Segmentation mixes food from nonadjacent segments in the peristaltic wave
- Mechanoreceptor -> tonic-type neuron -> tonic-type interneuron -> burst-type oscillator and burst-type follower
___ is when adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally
Peristalsis is when adjacent segments of alimentary tract organs alternately contract and relax, moving food along the tract distally
____ is when nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward, causing food mixing and slow food propulsion
Segmentation is when nonadjacent segments of alimentary tract organs alternately contract and relax, moving the food forward then backward, causing food mixing and slow food propulsion
Ileocecal sphincter is a ____
Ileocecal sphincter is a one-way valve
- Pressure or chemical irritation inhibits peristalsis of ileum and excites sphincter
- Pressure and chemical irritation relax sphincter and excite peristalsis
- Fluidity of contents promotes emptying
Haustra are ___ and are formed by ____
Haustra are small segmented pouches of bowel separated by haustral folds and are formed by circumferential contraction of the inner muscular layer of the colon
- The outer longitudinal muscular layers are organized into three bands (tanieae coli) which run from cecum to rectum
Describr the anatomy of the rectum / anal canal and esophagogastric junction
Anatomy of the rectum / anal canal and esophagogastric junction
