Absorption of electrolytes Flashcards
Composition of intestinal fluid
Na, K, Cl, HCO3-
Na
-concentration in small intestine is relatively constant
-concentration decreases in the colon as the K concentration increases
-normal concentration: ~90-100mM in intestine
>normal extracellular is 145-150mM therefore takes energy to maintain concentration
K
-concentration in small intestine relatively constant
-concentration is higher than plasma but lower than Na concentration
-concentration increases in colon as Na decreases
Normal intestinal K concentration
~20-40mM
Extracellular is 4-5mM
Cl and bicarbonate concentrations
-relatively uniform
-Cl and HCO3 have reciprocal relationship in the small intestine (aboral anion exchange increases)= Cl high proximally, HCO3- high distally
Ion concentration of intestinal fluid during fasting
-succus entericus (duodenum secretions) is isotonic
-decrease in K, increase in Na
Ion concentration of intestinal fluid during meal
-Succus entericus (duodenum secretion) is hypertonic
-increase in K, decrease in Na =Na absorption which allows for uptake of sugars and amino acids
pH of intestinal fluid in stomach
3.5
pH of intestinal fluid in small intestine
-neutralized quickly in the small intestine due to pancreatic and intestinal bicarbonate secretion
-5.5-7.0 pH
Overall changes in pH in GIT
-generally pH will increase aborally
pH of intestinal fluid in cecum and proximal colon
-see a decrease followed by an increase in pH
>due to microbial production of Volatile fatty acids AND post gastric fermenters (horse)
Canine intestinal fluid composition
-smaller gastric fluid volume relative to humans
-within species/across breeds (range in gastric pH)
-bile acids conjugated primarily to taurine and relative to humans, higher intestinal bile salt and phospholipid content
Cat intestinal fluid composition
-lower gastric pH relative to the canine
-large variability between cats transit time
Recapturing the fluid and electrolytes in proximal small intestine
-increased excretion from the small intestinal, pancreatic, and biliary secretion
-highly permeable to water allowing it to make rapid adjustments to osmotic pressure in response to meals
Recapturing the fluid and electrolytes in distal small intestine and colon
-water moves across the mucosa by a passive process down its osmotic pressure gradient
-strong correlation between Na and H2O
Absorptive Cell role in Proximal absorption in small intestines
-cells found at apical portion of villus
-involved in absorption of Na and glucose in the proximal small intestine. Water follows through the leaky tight junctions
-occurs rapidly once free glucose is released
-requires energy
Mechanism of proximal absorption in small intestines (Glucose enhanced transport)
-driven by Na/K ATP pump creating a charge separation and concentration differences
-glucose driven into cell through SGLT 1 (sodium glucose co-transporter 1) along with 2 Na ions
-glucose either metabolized or exits the basolateral side via facilitated diffusion through GLUT 2
-once a strong gradient is established, the water follows through aquaporins or paracellularly
-the NHE (sodium hydrogen exchange) and AE (anion exchange) play a minimal role (more important with isosmotic absorption in jejunum)
-GLUT 2 can translocate to the apical membrane under high ~300mM glucose concentration
Distal small intestine absorptive cell (isosmotic drive absorption)
-SGLT 1 expression decreases in aboral direction and isosmotic absorption becomes more important
-there are slightly tighter junctions than the proximal small intestine
-minimal amount of energy needed
Colon fluid and electrolyte absorption
-strong Na absorption (epithelial Na channel)
-tighter tight junctions than small intestine (high resistance tissue)
-K secretion (apical K channels)- lowers membrane potential
-K/H ATPase present apically (K retention in distal colon to maintain homeostasis)
-SCFA (transport through SCFA/HCO3 exchange and nonionic diffusion (drives fluid absorption)
Colon absorptive cell function
-found on apical portion of colon
-involved in the absorption of Na, Cl, and VFA/SCFA
>when Na absorbed, water follows
-cells have very tight junctions (so less water flow through them) allowing for a large osmotic gradient to pull water into body
Mechanism of colon absorptive cells
-driven by Na/K ATPase
-Na enters colonocyte through epithelial sodium channel and is pumped out the basolateral side creating a large gradient RESULTING in water pulling in through aquaporin
-Anion exchanger and sodium hydrogen exchanger pull in Na and Cl resulting in pulling in water
Where is the energy, HCO3-, H+ needed to drive colon absorption coming from?
-metabolism of SCFAs produced by digestion/fermentation of fibre
SCFA role in colon absorption
-SCFA stimulate electroneutral uptake of Na through the acidification of colonocytes and activation of apical Na/H exchangers
-Metabolism of SCFA via TCA cycle and carbonic anhydrase results in H+ and HCO3- production
Cl uptake in colon
-stimulated by increased HCO3- production during SCFA metabolism and stimulation of apical Cl/HCO3- exchanger
Alternative colon absorptive model
-butyrate taken up by nonionic diffusion or SCFA/HCO3- exchanger or monocarboxylate transporter
Why is resistant starch added to oral rehydration?
-colonic bacteria use it and produce short chain fatty acids which results in increases fluid absorption
Absorption abilities of colon aborally
-increase aborally
Electrolyte levels and net fluid and electrolyte movements in GI tract of swine
-used PEG (will not be absorbed by gut) and observed change in ratio between PEG and other nutrients.
>If PEG increases, then assume other nutrients decrease
-pH increased dramatically in colon (due to SCFA being synthesized by microbiome)
*more gradual increase in nutrient absorption (start absorbing earlier) in humans, dogs, horses, cattle compared to pigs that absorb more later on, closer to colon
Pig water and electrolyte absorption
-unlike other species, the weaning pig relies largely on its large intestine for absorption of fluid and electrolytes with small changes in net fluid movement occurring along jejunal and ileal segments
Water absorption in dog
-Water in= 3050
-Water out (absorbed)=3015
**small intestine absorbs more water than colon
Water absorption in horse
-water in:144
-water out (absorbed)= 137
*similar water absorption in both small intestine and colon… small intestine slightly higher
Forces used to absorb water
1.osmotic pressure
2.hydrostatic pressure (from increase in pressure due to water in a confined area)
Absorption of water steps
1.in villous tip, countercurrent flow of arteriole and venule causes solute in the descending venule to diffuse into the ascending arteriole to be carried up to tip
2. water uses aquaporin channels and osmotic gradient created by Na and AA absorption into the extracellular fluid
3. Hydrostatic pressure drive water into capillaries which have lower resistance than the tight junctions
Thirst
-excessive water excretion without intake
-extracellular fluid hyperosmolarity- causing either an increase in secretion of ADH (causes decreased excretion of urine) and sensation of thirst
*regions of brain identified
Oral rehydration therapy
-used extensively in human and vet med
*diarrhea-associated hypovolemia (dehydration) is the second leading cause of death among children under 5 yrs
-this therapy first introduced in 1960s and is gold standard for treating fluid loss as a result of acute diarrhea
Early Oral rehydration therapy success
-fluids in 1980s used hyper-osmotic/hypertonic glucose solution ; now we use hypo-osmotic/hypotonic solution
>important for the SGLT 1 glucose mediated active transportation (which is generally not significantly affected during diarrhea states)
-advised glucose to be used as an organic solvent carrier for Na transport. Other options include probiotics, zinc, prebiotics, glucose polymers, amino acids
Current oral rehydration therapy
-hypoosmotic/hypotonic solutions recommended
-results in reduced fecal fluid loss, need for IV therapy, duration of illness
Hyper vs. hypoosmotic fluid therapy solutions for calf vs. pigs
- Hyperosmotic for calves because less access to them during the day/can only give them 3-4x a day
- Hypoosmotic for pigs because can access them throughout the day/all day
Oral rehydration therapy in vet med
-contains Na, K, Cl; typically isotonic, or hypertonic if animal handling is an issue
-NaHCO3- often added to help combat metabolic acidosis which comes with diarrhea
-Glucose and AA added to increase Na/glucose and Na/AA transporter mechanisms and provide energy
-SCFA and probiotics used to increase water reabsorption in colon
