Body Fluid Homeostasis SS - Rao Flashcards
Total body water (TBW) is what percent of a person’s weight?
60%
What are the major sources of fluid intake in the body?
Drinking water, followed by water in food and oxidative metabolic production of water
What are the major sources of fluid output?
Urine, ‘insensible’ water from skin/lung/sweat, and a little in feces
Insensible water loss is high in burn victims and fecal loss high in diarrhea
What is the absolute minimum daily urine output?
500 mL
What is the volume of ICF in an average person?
25L
What is the volume of ECF in an average person?
15L
What is the volume of fluid in interstitial space?
12L
What is the volume in a person’s plasma and total blood?
3L in plasma; 5L total blood volume; blood is 60% plasma and 40% hematocrit
What is the major cation and anion in the interstitium/plasma?
Na, Cl
What is the major anion and cation in the intracellular fluid?
K+, Protein-
What is the Gibbs-Donnan effect in relation to fluid compartments?
Explains the behavior of charged particles near a semi-permeable membrane; Na+ is greater in plasma by 2% and Cl- lower by 2% because the capillaries are more permeable than the cellular membranes, allowing negatively charged proteins to retain the Na+ through electrical interactions
Where is bicarbonate in highest concentration
Plasma and interstitium
Where is phosphate in highest concentration
intracellular
Where is magnesium in highest concentration/
intracellular
What are the criteria for probes that measure body fluid?
Non-toxic at used concentration Neither synthesized or metabolized Disperses evenly Disperses only in compartment of interest Doesn't influence volume itself
What probes are used to measure plasma volume?
Iodinated albumin, Evans blue dye, both which bind avidly to plasma proteins
What is the formula to measure volume of plasma?
Blood volume = plasma volume / (1-HCT)
What is the formula for measuring concentration of a something in a fluid compartment?
Compartment volume = Q / (Q/V), where Q/V is the concentration of the probe and Q is the amount of probe injected
Applies to intra- and extracellular compartments
What are the probes used to measure extracellular fluid volume?
Inulin, thiosulfate, and Na+
ISF=
ECF-PV
What are the probes used to measure total body water?
Radiolabelled water; antipyrene
ICF=
TBW-ECF
What forces determine the fluid movement between plasma and ISF?
Starling forces - hydrostatic and oncotic pressure
(Capillaries are more permeable than cell membranes)
NOT plasma osmolarity
What forces determine the fluid movement between the intracellular and extracellular fluid?
Osmosis, mainly due to differences in sodium concentration causing water rearrangment
How are osmoles different than moles?
Moles is amount of molecules; osmoles is amount of particles in solution
Does osmotic pressure depend on molecule size?
One particle of albumin and one particle of glucose have the same osmotic pressure
How do the osmolarities of different fluid compartments compare?
They are all the same, 295 mOsm/L, because water is freely permeable through barriers
What happens to RBCs in hypotonic Na solution?
Swelling and lysis
What happens to RBCs in hypertonic Na solution?
Shrink
What happens to RBCs in the presence of a low Na and high urea environment?
Swelling, because urea is freely permeable across membranes
Formula for estimating plasma osmolarity?
Plasma osmolarity = 2[Na+] + glucose + urea
What are the steps for estimating how much fluid to give a patient?
Calculate ECF and ICF before depletion; calculate total mOsm in ECF and ICF now, calculate volume needed to achieve 280 mOsm, then calculate amount of fluid needed to add
Giving isotonic saline to a normally hydrated person will change which compartments?
ECF only, maintain same osmolarity while increasing volume
Gaining pure water through drinking or giving 5% glucose H20 will change which compartments?
Increase volume of all compartments; decrease osmolarity of all compartments
What does excess gain of salt do to copartments?
Increase ECF osmolarity, draws water from ICF, increase ECF volume, decrease ICF volume, osmolarity of all compartments increased
What does loss of NaCl do to compartments?
Decreases ECF osmolarity, flux of water into ICF, decrease volume of ECF, increase vol of ICF, osmolarity decreasd in all compartments
This is why you should drink electrolyte-rich drinks like Gatorade after excessive sweating instead of pure water
What does infusion of isotonic urea do to compartments?
Increases volume of all compartments (water retained everywhere) without changing osmolarity of anything
