Homeostasis of Body Fluids- Rao Flashcards
Homeostasis
Maintenance of static or constant conditions (BT, fluid volume, HR, BP, etc) in an internal environment of an animal body. All organs and tissues perform functions to help maintain constant conditions.
Kidneys play an important role in homeostasis of what?
Homeostasis of body fluid
Describe Body Fluid Homeostasis and its importance
Body fluid volume and its composition are relatively stable during steady state, and this balance between highly variable fluid intake with equal output is essential for cellular function and overall homeostasis
How do you calculate TBW?
TBW is ~60% of body weight (BW) so TBW= 0.6xBW
Tissue distribution of water:
a) %TBW? b) % of tissue itself that’s water?
Most body water is in tissues
- Muscle a) 43% b) 76%
- Skeleton a) 16% b) 22%
- Organs a) 6% b) 75%
- Adipose a) 10% b) >10%
a) Describe the relationship between body fat content and body water content:
b) why is it important to maintain a high TBW%?
a) Inverse relationship; as body fat increases, %TBW decreases
At lean BMI with little fat, TBW is as high as 72%
b) maintain high TBW% is important in order to maintain a high metabolic rate
a) How do you maintain constant body fluid volume?
b) Role of the kidney in body fluid balance?
a) You have to balance fluid intake with fluid output.
b) It’s the major regulatory organ of urine output, regulating urine concentration and volume to balance fluid output with intake
Sources of fluid intake? (3)
1/2. Drink (the major cource) and water content of food (2.2 L/day)
- Generated from oxidative metabolism in tissues (300 mL/day)
Total daily fluid intake of ~2.5 L
Routes of Fluid Output
- Urine– highly regulated but major route, amount dependent on vol of intake (minimum 0.5 L/day because this is required to get rid of metabolic wastes, up to ~20L day in cases of excess drinking)
- Insensible water loss– Lung and Skin Perspiration (normal ~700 ml/day, up to 5L/day (burn)), Sweat (100ml/day, increases with exercise)
- Feces– 100 ml/day, excessive with diarrhea
Basic Functions of Kidney (3)
- Regulation of water and inorganic ion balance
- Removal of metabolic waste products (urea, uric acid, creatinine) and foreign chemicals from blood and their excretion in urine
- Hormonal functions
Endocrine/Hormonal Functions of Kidney? (4)
- EPO production (RBC production in BM)
- Activation of inactive Vit D to 1,25-dihydroxyvitaminD (Calcium and Phosphate balance)
- Renin-Ang II production (Na balance–maintains cardiac function)
- Gluconeogenesis (during fasting– maintain blood glc levels)
TBW varies with what?
3 factors with increased fat content that result in decreased TBW:
- Decrease with age
- Sex- Lower in females
- Degree of Obesity–decreased with more obesity
a) Why are humans open systems?
b) why dont we leak things into the environment?
a) Lungs, Kidney, GI tract, and Skin are in direct contact with the external environment
b) these compartments dont leak bc of barrier functions
Fluid Compartments and their Volumes?
Total BF=40L
- ICF– 25L
- ECF–15L total
- Plasma (fluid in bl vessels)–3L
- ISF (outisde bl vessels and cells)–12L
Total Blood Volume?
8% of BW, 5L (ECF+ICF)
It’s 60% Plasma and 40% RBC (hematocrit–varies with age, sex, etc)
Barriers to body fluid movement (separate BF compartments)? Which cmpts do they separate? Characteristics?
- Capillaries– sep’s plasma from ISF; highly permeable; plasma has high protein content, ISF low protein
- Cell Membranes– sep’s ICF from ECF; highly impermeable except to H20, Cl, urea, some lipophilic molecules; strong determinant of ECF anf ICF composition
Compare Plasma and ISF composition? Why is this?
Ionic composition of plasma and ISF is similar because the capillary wall is highly permeable except to proteins.
Both: Na is major cation and Cl- is major anion, a little HCO3, very low/no K
Plasma has Protein, so it’s more negative than ISF, so it also has higher Na and lower Cl
ISF has slightly more HCO3
Composition of ICF vs. ECF:
Their ionic compositions are fundamentally different:
ICF: major cation K; major anions– protein, bicarb, and PO4; much higher Mg2+, PO4, and protein; no Ca2+
ECF: major cation Na; major anions–Cl- and Bicarb; has Ca2+, higher bicarb
How do you measure volume of BF cmpts? How does it work?
By applying the Dilution Principle. Insert a known amt/ [ ] of substance (probe) into an unknown amount of fluid, allow the sunbstance to disperse evenly throughout the fluid cmpt. The concentration will be reduced by the unknown volume of the fluid.
Volume of fl cmpt = Quantity Injected/Concentration = Q/(Q/Vinj); then subtract volume of dye injected
Criteria for probe for BF measurements (5)
- Non-toxic at concentration used 2. Neither synth’d or metab’d i body 3. Disperses evenly thruout the fluid 4. Disperses only in the cmpt of interest 5. Does not influence fluid cmpt volume
How do you measure plasma volume (PV)? Blood volume (if you know hematocrit)?
Probe: 131-I-Albumin, Evans Blue Dye
- IV injection of small volume, known amount (Q)
- Withdraw blood and prepare plasma
- Measure conc of probe in plasma (Q/V)
PV= Q/(Q/V); Blood Vol = PV/(1-Hematocrit)
Measuring ECFV (PV and ISFV)?
Probe: Inulin, Thiosulfate, Na (these all enter plasma and ISF but not ICF)
Same process as for plasma
ECFV=Q/(Q/V)
ISFV=ECFV-PV
How do you correct this if probe is lost in urine excretion?
Corrected Q= original Q minus amount excreted in urine
then plug corrected Q into the top part of the equartion
How do you measure TBW? ICFV?
Probe: labeled water, or antipyrene (lipid soluble)– need to be permeable thru capillary wall and cell membrane
Same process as with PV
TBW=Q/(Q/V)
ICFV=TBW-ECFV
Factors that determine fluid movement between plasma and ISF?
This movement is filtration thru capillary wall
Driven by Starling forces: 1. Hydrostatic Pressure–mvmt from more full cmpt to less full, out of plasma into ISF 2. Oncotic (colloidal osmotic) pressure–mvmt of fluid into plasma due to impermeable molecules like proteins that cant pass thru cap wall
Factors that determine fluid mvmts between ICF and ECF:
This movement is by Osmosis because the cell membrane is impermeable to most molecules. So, relative solute concentration determines movement. Changing the concentration of impermeable solutes (Na) will induce osmotic pressure, causing water to move.
Sodium–impermeable, Urea and Water– permeable, Glycerol– slowly permeable (initially generates osmotic pressure, but it diminishes due to its gradual diffusion)
Osmosis?
Net diffusion of water from cmpt with low solute concentration to cmpt with higher solute concentration.
If the solute causing the concentration gradient is impermeable, one cmpt will lose water and the other will gain water.
Moles vs Osmoles examples: Glc, NaCl, Na2SO4
I mole Glc in L soln = 1 Osmole Glc in L soln
I mole NaCl in L soln = 2 Osmole salt in L soln
1 mole Na2SO4 in L soln = 3 Osmole in L soln
Osmolarity vs. Osmolality
Osmolarity= Osmoles solute per L soln
Osmolality=Osmoles solute per kg of water
What is the precise amount of pressure required to prevent osmosis (net diffusion of water thru the membrane) called?
Osmotic Pressure
Osmotic pressure of a solution is directly proportional to what ?
directly proportional to the concentration of osmotically active (impermeable) PARTICLES in that solution.
1 mOsmole of gradient = 19.3 mm Hg of pressure
NOTE: One particle of Glc and one particle of albumin have the same osmotic pressure despite size difference.
Compare the osmolarity of different body fluid compartments. Why is this so?
Osmolarity of body fluids in all the different cmpts is the same (295 mOsmole/L) even though they have different compositions. This is because water is freely permeable thru the cmpt barriers.
Osmotic forces due to impermeable solute (Na)– What happens to a RBC in:
a) Normal/Isotonic plasma
b) Hypotonic plasma (decreased Na in plasma)
c) Hypertonic Plasma (increased Na in plasma)
a) no change
b) cell swells
c) cell shrinks
Osmotic forces due to permeable solutes:
a) Iso-osmotic plasma (low plasma Na but high urea)
b) Hyperosmotic plasma (Normal plasma Na but elevated Urea)
a) Cell swells– bc urea is freely and rapidly permeable, some goes into the cells, making plasma hypo-osmotic, so water follows it in.
b) No change in cell volume bc urea equilibriates b/t cell and plasma
Osmotic forces due to slowly permeable solutes (glycerol):
a) Hyperosmotic plasma with low plasma Na but high glycerol
a) initially cell shrinks. But, glycerol slowly enters the cells and over time, the cell ultimately swells
Dehydration: Definition? Causes?
Dehydration is the loss of water without losing solutes
Causes: water deprivation , extensive perspiration, severe diarrhea, comatose patient, infants, extraordinary circumstances (trapped in earthquake rubble)
How long can you last without water? Why?
Normally about 7 days. This is because you have to excrete a minimal amount thru kidneys to excrete waste products and also bc of insensible perspiration that cant be controlled.
Explain the dehydration mechanism and effects?
- Loss of water from ECF
- Increase in ECF osmolarity
- Draws water out of ICF
- Decrease in volume and increased osmolarity of all compartments
Steps in treating dehydration?
1 Estimation of plasma osmolarity
- Estimation of fluid infusion for treatment (how much fluid should we infuse to restore normal osmolarity)
- Considerations
Equation to Estimate Plasma Osmolarity?
Pl Osmolarity= [plasma Na x 2] + glucose + urea
Estimation of Fluid Infusion for Treatment:
Consideration:
- Calculate pt’s normal ECFV and ICFV
- Calculate total mOsmoles in ECF and ICF
- Calcuate fluid volume necessary to achieve concentration of 280 mOsm
Consideration: Because some fluid is excreted during the infusion, dont infuse all at once, instead titrate the patient.
Osmolarity Gap: Explanation? Abnormal Value? Preferred Method?
It is the difference between measured plasma osmolality and estimated/calculated plasma osmolarity.
When >10 (abnormal), it indicates that something other than the normal Na, Glc, and BUN is dissolved in the plasma.
Freezing point depression method is the preferred method bc others may miss volatile particles such as ethanol or methanol.
Infusion of Isotonic Salt soln: Causes? Net Result?
Cause: Saline infusion in the clinic
Increases ECFV without changing ECF osmolarity
No change in ICFV or ICF osmolarity
Gain of Water: Causes? Net Results?
Causes: Drinking large amts of water, infusion of fluid for nutritive purposes (glc solution)
Initially: Increase in ECFV and decrease in ECF osmolarity, so flux of water into ICF due to osmotic gradient
Net Results: Increase in volume and decreased osmolarity of all cmpts
Gain of Salt; Causes? Results?
Causes: Excessive salt consumption, Hypernatremia
Initially: Increased ECF osmolarity draws fluid out of ICF
Net Result: ECFV increases, ICFV decreases, Osmolarity increases in both
Loss of NaCl: Causes? Results?
Causes: Hyponatremia (drinking water after profuse sweating)
Initially: Decrease in ECF osmolarity, water enter ICF from ECF
Net Result: Decrease in ECFV, Increase in ICFV, Osmolarity of both decreases
Infusion of Isotonic Urea: Results?
Because urea is freely and rapidly permeable across cell membranes:
Increase in volume of both cmpts; Isotonic osmolarity of all cmpts
To treat a patient who has lost 4 L of fluid due to severe water deprivation, you would infuse 4l of what solution to restore total body fluid volume?
5% Glucose
