9/10- Physiology of Sodium, Water and Volume Flashcards
What is the fluid/water breakdown between the various body compartments?
Water ~ 60% of the total body weight
- 50-60% in males (~40 L)
- 45-50% in females
ICF = 2/3 total water, 26L (40% body weight)
ECF = 1/3 total water, 13L
- Interstitial fluid = 16% (3/4 of ECF)
- Plasma = 4% (1/4 of ICF)
Principles of water balance and solute distribution (what maintains gradient; osmolarity between chambers, role of albumin…)
- Water is in steady state equilibrium between ECF and ICF
- Solute distribution results from action of Na-K-ATPase (Na in ECF, K in ICF)
- Albumin contributes to oncotic pressure and is found mostly in the serum
Describe ion transport of Na-K-ATPase
Pumps 3 Na out and 2 K into the cell
What changes when water is added to ECF?
Water moves from ECF to ICF until the osmolalities are equal
What changes when Na is added to ECF?
Water moves from ICF to ECF until the osmolarities are equal
How is plasma osmolality regulated?
Water balance
- Na and water are independently regulated by systems designed to maintains stable EABV and serum osmolality
How is EABV regulated?
Sodium balance
- Na and water are independently regulated by systems designed to maintains stable EABV and serum osmolality
Excess Na causes what? Clinically?
Excess Na -> expansion of ECF (edema)
Na deficit causes what? Clinically?
Na deficit -> contraction of ECF (volume depletion)
Excess water causes what?
Excess water -> hyponatremia (decreased osmolality)
Water deficit causes what?
Water deficit -> hypernatremia (increased osmolality)
Urine concentration can vary from what?
50 - 1200 mOsm/L
Urine volume can vary from what?
- As little as 0.5 L (water deficit)
- As much as 12 L (water excess)
What is body osmolality in each compartment?
287; equal in all compartments! (ICF = plasma = interstitium)
How can osmolality be measured/calculated? What is normal?
- Measured either by vapor pressure or freezing point depression
- Ca be calculated from primary solutes that contribute to it:
- P = 2Na + [glucose]/18 + [BUN]/2.8
- Normal plasma osmolality ~ 280-290 mOsm/kg
What is the osmolar gap? What is normal?
Osmolality: measured - calculated
- Normal ~ 10 mosm/L
What does an increased osmolar gap suggests what?
- Methanol
- Ethanol
- Isopropanol
- Ethylene
- Glycol
- Mannitol
- Contrast media
- Glycine
Set-point for ADH release is lower in what populations?
- Pregnant women
- Asians (some)
Mechanism for (hormonally-regulated) osmoregulation?
- Plasma osmolality is sensed by osmosensors in the hypothalamus
- This induces the release of ADH and thirst sensation
Where does ADH act in the tubule? What does it do?
- Collecting ducts (via V2 receptors)
- Enhance water permeability (uptake) through AQP2 (aquaporin2)
(- Thirst simultaneously increases water intake)
- The resultant water retention -> decrease in Na concentration
- ADH shut-off decreases water flux through AQP2 -> water clearance -> increased serum Na
What are V1 receptors? Functions?
Mediated by action of vasopression
- Vasonconstriction
- Increased PG synthesis by the kidney
- PGs reduce the action of ADH on the collecting duct
- Therefore, NSAIDs cause water retention!
What are effective solutes? Examples?
Solutes confined to one compartment; may cause water shift between ECF and ICF
- Hyperglycemia (in insulin deficiency)
- Contrast media
- Mannitol
What are non-effective solutes? Examples?
Membrane permeable and equilibrate between the ICF and ECF (cause no water shift)
- Urea
- Ethanol
- Methanol
- Isopropanol
T/F: In order for changes fluid volume (loss/addition) to be confined to ECF, it must be isotonic with plasma
True; must be 0.9% saline
What ion is the primary determinant of ECF volume?
Na
- Sodium contributes to the tonicity of ECF
- As sodium is primarily confined to the ECF (Na-K pump), adding sodium to the body primarily affects ECF & only secondarily affects ICF
T/F: Addition/subtraction of pure water affects all compartments (ECF and ICF) equally?
True
Ex) If 1 L of D5W is administered IV to pts, how much goes into each compartment?
- 2/3 goes into ICF (666 mL)
- 1/3 goes into ECF (333 mL)
—- Only 1/4 goes into vascular compartment (83 mL)
—- 3/4 goes into interstitial compartment (250 mL)
… Just note, for each L infused, only 83 mL makes it into blood
Ex) If 1L of isotonic saline (0.9% saline) administered IV to pts, how much goes into each compartment?
As it is isotonic to plasma, all of it stays in the ECF
- ¼ goes into the intravascular space
- ¾ goes into the interstitial space
Ex) If 1L of half normal saline (0.45%) is adminstered IV to pts, how much goes into each compartment?
Think of as 500 mL of water + 500 mL of isotonic saline
- Water distributes 2/3 ICF , 1/3 ECF with ¼ into vascular space (42ml)
- Normal saline portion stays in ECF but ¼ in vascular space (125 ml)
What is effective arterial blood volume? Easily measured?
- Physiologic concept (not easily measurable)
- Proportion of intra-vascular volume in the arterial compartment & effectively perfusing the kidneys (body)
- Adequacy of arterial volume to fill the capacity of the vascular tree
What is effective arterial blood volume (EABV)?
Clinical conditions in which:
- cardiac output is decreased, or
- systemic arterial vasodilation exists Baro-receptors perceive underfilling Result in neurohumoral activation and renal sodium and water retention
T/F: EABV and ECF are basically the same
False!
How do ECF and EABV respond in dehydration?
Both are low
How do ECF and EABV respond in cirrhosis?
- ECF volume is expanded
- EABV is diminished
How do ECF and EABV respond in CHF?
- ECF volume is expanded
- EABV is diminished
What are some renal indicators of EABV?
- FENA
- Low urine Na
- High urine specific gravity
How is FENA calculated? What is pathologic?
FENA = (Una/Sna)/(Ucr/Scr) x 100
FENA < 1% is CW low EABV (or pre-renal state)
What are some tests and physical signs of dehydration?
- Low BP
- Shock
- Orthostasis (+ tilt)
- Decreased skin turgor
- Decreased CVP
- Tachycardia
- Hemoconcentration
What happens to the EABV (up/down/same), if we give:
- 1.2 L of water
- 400 mL of 3% saline
- 1 L of normal saline
- Water: EABV increases
- 3% NaCl: increases
- NS: increases All increase!
What happens to the serum sodium concentration (up/down/same), if we give:
- 1.2 L of water
- 400 mL of 3% saline
- 1 L of normal saline
- Water: Serum Na decreases
- 3% NaCl: increases
- NS: same
What happens to the urine sodium concentration (up/down/same), if we give:
- 2 L of water
- 400 mL of 3% saline
- 1 L of normal saline
- Water: Urine Na increases
- 3% NaCl: increases
- NS: increases
How does urine Na compare to EABV? Serum Na? ECF volume?
- Urine reflects EABV
- Does not necessarily reflect serum Na (ore even ECF volume!)
All the effectors that are induced in low EABV (including ADH) increase ____ by the kidney, directly or indirectly
All the effectors that are induced in low EABV (including ADH) increase Na by the kidney, directly or indirectly
Thus, low EABV is associated with increased ______ (____follows)
Thus, low EABV is associated with increased renal Na uptake (water follows) -
High EABV is associated with ______ Na+ uptake
High EABV is associated with decreased Na+ uptake
[Na+ retentive hormones shut off (plus ANP release)]; i.e., natriuresis and diuresis.
ANP (BNP) is produced in response to what? What does it do?
Stretch (volume expansion)
- Works on the kidney to induce natriueresis
ANP (BNP) is a marker of what?
ANP (BNP) is a reliable marker for heart failure,
Is ANP (BNP) effective in the treatment of Na retention (edema)?
Has little usefulness in the treatment of Na retention (edema ), due to the operation of compensatory renal pathways that offset its action.
Summary
