L12 Control of Body Water Flashcards
ideally total water total input = total output = ~ ___ mL
input and output sources
ideally total water total input = total output = ~ 2500 mL
input sources: ingested fluids, ingested food, metabolism
output sources: urine, feces, skin/sweat, exhaled air
distribution of body water?
distribution of body water
- 2/3 total body water is intracellular
- 1/3 is extracellular:
→ 1/4 -1/5: plasma
→ 1/3: interstitial, dense connective tissue + bone, transcellular
communication between fluid compartments?
equilibration of body water
- capillaries separate plasma from ______
- ISF in contact with ECF in connective tissue and bone
- ISF through plasma membrane of cells
water content in tissues?
- blood has largest % water
- skin has high % water
- lowest % water is fat → obese people have less water content / gram basis
how to estimate the volume of a compartment?
what are the marker for plasma? extracellular fluid? total body water?
estimating volume of a compartment using dilution of a marker
- inject specific amount of marker into compartment, allow it to equilibrate → measure its concentration
- C1V1 = C2V2
- account for any losses during equilibration period (ex. urine produced)
possible markers:
- plasma = Evan’s Blue which binds to plasma proteins, radiolabeled albumin
- extracellular = inulin, thiosulfate, sucrose, mannitol
- total body water = D2O, THO
starling’s law forces at capillary wall?
- glomerular capillaries are set up for ___ pressure filtration so different than other capillaries
- ascites: ______
starling’s law forces at capillary wall
- for distribution between different compartments
- fluid OUT capillaries: hydrostatic pressure + protein in interstitial fluid
- fluid IN capillaries: oncotic force in capillaries
- glomerular capillaries are set up for high pressure filtration so different than other capillaries
- ascites: damage in liver → pressure in portal system increases, fluid gets deposited into body cavity + hypertension in portal system causes relaxation of vascular smooth muscle → lowers BP → triggers renin-aldosterone system → increased sodium retention → even more water increase
explain the gibbs-donnan equilibrium at the capillary wall + formula
gibbs-donnan equilibrium at the capillary wall
- 2 compartments separated by semi-permeable membrane
- salts will go to Gibbs-Donnan equilibrium
- Na⁺ and Cl⁻ will travel down their concentration gradients
- proteins can’t cross membrane → will stay on same side
- product of diffusible ions on both side of membrane must be equal
- [C⁺]₁ × [A⁻]₁ = [C⁺]₂ × [A⁻]₂ ; C : extra and intracellular [permeable cation] and A : extra and intracellular [impermeant anion]
- different number of osmotically active particles on both sides → excess solute will drive water - cells have natural tendency to swell, RVD used to counteract this
osmotic equality
interstitial fluid (eg: plasma)
- cations: Na⁺ (mainly), K⁺, Ca²⁺, Mg²⁺
- anions: bicarbonate, chloride
intracellular fluid
- cations: K⁺ (mainly), little Na⁺, divalents high compared to extracellular fluid
- anions: phosphate + organic anions, much less bicarbonate
what is mEq/L determined by? to take into account what?
- mEq/L is determined by charge, not number of molecules
- anions in plasma: amount of charge on protein is quite large since proteins are multivalent – have multiple negative charges
what is the body’s response to ↓ protein in the plasma and how does it control volume consequently?
↓ protein in the plasma → forces water to move into interstitium since less oncotic force pulling it in → underfilling of vasculature → aldosterone response → trigger sodium reabsorption → expand plasma volume
ADH (antidiuretic hormone) / vasopressin
ADH / vasopressin
- released by posterior pituitary from osmosensing neurons containing the hormone in granules
- released into plasma in response to hypertonicity of plasma
- precise sensing system: mOsm ↑ or blood volume ↓ (below threshold) → ADH release → osmotic pressure ↑
how do dogs react to water deprivation? and then to water intake?
Effect of inhibiting release of Antidiuretic Hormone
- water restricted dogs → hypertonic blood (high [solutes]) → ADH acts on kidneys to increase water reabsorption → blood less hypertonic and urine is more concentrated
- then dogs drink water: ↓ in ADH release → kidneys reabsorb less and excrete more water → drop in urine concentration and ↑↑ in urine production
⇒ ADH turnover is quick
what’s the body’s response to ↓ osmolality of plasma?
↓ osmolality of plasma → osmoreceptors trigger thirst → stimulates AVP / ADH osmoreceptors → stimulate AVP / ADH neurons in hypothalamus → AVP / ADH released on kidneys → ↑ water reabsorption in collecting duct via AQP2
⇒ decrease in free water corrects osmolality
what’s the body’s response to ↓ effective circulating volume with no change in osmotic pressure in plasma?
↓ effective circulating volume with no change in osmotic pressure in plasma → brain senses ↓ arterial pressure → atrial low-pressure receptors release signal → juxtaglomerular apparatus releases renin → angiotensin II
⇒ increase in Na⁺ reabsorption + Na⁺ appetite to increase circulating volume
