Physiology-Body Fluid Composition Flashcards
What are the major sources of water intake? What are the major ways that water is excreted?
*
What percentage of the body is water? What percentage of that water intracellular? What percentage of that water is extracellular? What percentage of extracellular water is in the interstitial fluid and plasma?
60% of your body is water. 40% of water is intracellular. 20% is extracellular. 75% of extracellular fluid is ISF. 25% of extracellular fluid is plasma.
What deviates in body water compositions exist?
Females have < 60%. Newborns have > 60%. Obesity have less water in adipocytes.
What causes osmotic flow of water in this solution to stop?
The water/solute concentrations is the same on both sides of the semipermeable membrane or the hydrostatic pressure = osmotic pressure.
How do you calculate osmolarity?
*
What are the different tonicities that result from different osmolarities?
Isotonic solution: cells don’t swell or shrink. Hypotonic solution: cells will swell. Hypertonic solution: cells will shrink.
Red blood cells have intracellular osmolarity of 300mOsm/L. If they are added to a solution of 150mM NaCl, will the cells shrink, swell or stay the same?
(150mOsm/L Na) + (150mOsm/L Cl) = 300mOsm/L. This is equal to the intracellular osmolarity of the RBC. Since Na and Cl don’t cross the membrane they are effective osmoles and the solution is both isosmotic and isotonic.
Red blood cells have intracellular osmolarity of 300mOsm/L. If they are added to a solution of 300mM NaCl, will the cells shrink, swell or stay the same?
(300mOsm/L Na) + (300mOsm/L Cl) = 600mOsm/L. This is double the osmolarity of the RBC. Since Na and Cl don’t cross the membrane, water will and the cell will shrink because it is a hypertonic and hyperosmotic solution.
Red blood cells have intracellular osmolarity of 300mOsm/L. If they are added to a solution of 300mM Urea, will the cells shrink, swell of stay the same?
Urea osmolarity = RBC osmolarity. However, urea is not an effective osmole and will enter the cell down its concentration gradient. This will make the solution hypotonic and the cells will swell. Thus, the urea solution is isosmotic and hypotonic.
Roughly, what are the concentrations of these solutes in the intracellular and extracellular fluids?
Note that urea diffuses freely across membranes and thus there is no difference in concentration in the two fluids. Also note that CO2 is a good indicator of bicarbonate levels.
What is in plasma? How do you calculate its osmolarity?
H20, electrolytes, glucose, urea, proteins and lipids.
What equation can you do to get a safe estimate of plasma osmolarity? Why?
Plasma osmolarity = 2 * PNa. This is because sodium salts make up the largest fraction (95%) of the effective osmoles in the plasma.
What makes up the majority of plasma?
Water (93%) and Protein/Lipids (7%) in suspension. The Electrolytes, glucose and urea are all in solution with the water.
How do you calculate a physiologically relevant sodium concentration in plasma?
The physiologically relevant sodium concentration is in water.
Why is it important to measure the physiologically relevant sodium concentration in plasma?
Pseudohyponatremia. If a patient were hyperlipidemic with a “normal” [Na] in plasma water, they would appear to be hyponatremic in lab testing when really it is just because lipids take up a greater fraction of the plasma.
What assumptions are made about how water moves in the body?
All changes to body fluid occur in extracellular compartments. Water moves between the compartments, but the solutes do not.
What happens to volume and osmolarity in the ICF/ECF if you give water in an IV? 0.9% NaCl? Straight NaCl?
Adding water decreases the osmolarity of the ECF and increases the amount of fluid. This causes fluid to move into the ICF. Adding 0.9% NaCl does not change the osmolarity and only increases the volume in the ECF. Adding NaCl increases the osmolarity of the ECF and pulls water out of the ICF.
How do you replace fluids in someone who is hemorrhaging?
This is isotonic fluid loss, so you just add isotonic 0.9% NaCl to add back what the ECF lost.
How do you replace fluids in someone who has excess vomiting?
They are losing more water than ions and the ECF will have a higher osmolarity. This will cause a loss in ECF and ICF fluid levels. You treat this by adding 5% dextrose (hypotonic solution) to replace water.
What factors favor development of interstitial edema?
*
What causes the interstitial edema seen in histamine release, infections and burns?
Increased capillary permeability.
What causes the interstitial edema seen with lymphatic blockage?
*
