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
