Body Fluid Compartments

Predict the changes in extracellular volume, extracellular osmolality, intracellular volume, and intracellular osmolality caused by infusion of three liters of 0.9% NaCl, lactated Ringers solution, 0.45% NaCl,and 3% NaCl.

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Contrast the movement of water between intracellular and extracellular compartments caused by increases or decreases in extracellular fluid osmolality.

Despite the differences in composition between the intracellular, interstitial and plasma compartments, the osmolality is essentially identical between compartments....osmolality = ratio of the NUMBER of particles in solution...if there are differences in osmolality, water moves between compartments to make them same osmolality again. aka Osmolality determines movement of water - water will move to make the osmolality the same (Oncotic pressure is the "force" that drives this water)

Identify major routes and normal ranges for water intake and loss, and predict how changes in intake and loss affect the distribution of total body water.

Water intake: drinking, eating, breathing Water loss: urination, defecation, sweating, breathing ○ Normal Intake ■Total: 2550 mL ■Beverage:1200 mL ■Food: 1000 mL ■Metabolically Produced: 300 mL ○ Normal Output ■Total: 2550 mL ■Insensible (Skin/Lungs): 900 mL ■Sweat: 50 mL ■Feces: 100 mL ■Urine: 500 mL Key Clinical Point: When you lose water, it doesn't really affect perfusion unless it is major water loss. Conversely, when someone's BP drops due to water loss, it is due to MAJOR MAJOR water loss.

Describe the ionic composition of intracellular fluids and explain how the intracellular fluid composition is maintained at a different composition than the extracellular composition.

○ Intracellular and interstitial compartments: ■ There is a large difference with the intracellular compartment having a very high concentration of potassium, phosphates, and proteins (the three P's are IN the pod /cell) as compared to the interstitial compartment. ■ The reason that this large difference can occur is due to the presence of a lipid bilayer (membrane) separating the compartments that has a low permeability for almost all solutes except those that are very small or non-polar. ■ Most solutes which move across these compartments require specific transporters to facilitate the movement 1. Na+/K+ ATPase maintains high intracellular K+ concentration while keeping the intracellular Na+ low. The impermeable proteins and organic phosphates in cells would lead to higher osmolality and cell swelling, thus the cell has to pump some ions (Na+) out of the cell to keep the osmolality of the cell equal to that of plasma ○ Plasma and interstitial compartments: ■ They are almost identical with a difference of about 5% in concentration of the diffusible ions. ■ This is accounted for by a difference in the distribution of plasma proteins between the compartments. ■ Electrolytes can flow easily across the capillary endothelium but large proteins cannot. ■ This unequal distribution of proteins leads the concentration of cations to be slightly greater in plasma (that is, they are bound to plasma proteins which are negatively charged). This is termed Gibbs-Donnan Equilibrium (just FYI we won't be directly tested on this formula the prof said)

Identify normal extracellular fluid (plasma) osmolarity and concentrations of Na+, K+, Cl-, HCO3-, proteins, creatinine, and urea, and contrast these values with those for intracellular fluids.

○ Plasma Concentration (Extracellular) ■ Na+ : the most abundant cation (145 mEq/L) ■ Cl- and HCO3-: the most abundant anions (97-107 mEq/L and 24 mEq/L, respectively) ■ Protein: Capillaries are impermeable to protein??? ○ Interstitial fluid (Extracellular) ■ Na+ : the most abundant cation ■ Cl- and HCO3-: the most abundant anions ■ No protein ○ Intracellular fluid ■ K+ : the most abundant cation (150 mEq/L) ■ Phosphate and negatively charged proteins: most abundant anions Basically the only difference between the Plasma and Interstitial contents is that Interstitium does not contain protein which make sense because capillaries are impermeable to protein (refer to next LO).

Given the body weight and percent body fat, estimate the a) total body water, b) lean body mass, c)extracellular fluid volume, d) intracellular fluid volume, e) blood volume, and f) plasma volume.

● Total body water (L) is 60% of body weight = Body weight (kg) x 0.60 ● Lean body mass (Kg) = Bodyweight (kg) - (Body weight (kg) X percent body fat) ● Extracellular fluid volume (L) is 20% of body weight = Body weight (kg) x 0.20 ● Intracellular fluid volume (L) is about 40% of body weight = Body weight (kg) x 0.40 ● Blood volume (L) is about 7% of body weight = Body weight (kg) x 0.07 ● Plasma volume (L) is about 5% of body weight = Body weight (kg) x 0.05 HTO = Tritiated water Key concept: Need to know what body compartment a particular substance is going to go into.