SM 187. Body Fluid Compartments, Osmosis, & Basic Kidney Functions

Question 1

Proportions of total body fluid in ECF, ICF, interstitium, plasma

What are the predominant cations in ICF and ECF?

Answer 1

ICF: 2/3 TBW; K+ predominant cation
ECF: 1/3 TBW; Na+ predominant cation
Interstitium: 3/4 ECF
Plasma: 1/4 ECF

Question 2

What are the direction and driving force for osmosis?

Answer 2

Direction: water flows from low solute conc. to high solute conc.

Driving force: osmotic pressure proportional to number of particles in solution (flows from low to high osmotic pressure)

Question 3

Difference between osmolality and osmolarity

Answer 3

Osmolality: moles of solute per kg of water (clinically used, easier to measure, not sensitive to changes in temperature)

Osmolarity: moles of solute per L of water (temperature sensitive, less commonly used)

Question 4

Definitions of Tonicity, Hypertonicity, and Hypotonicity

Answer 4

Tonicity: influence of a solution’s osmolarity of cell volume

Hypertonicity: solution is hyperosmolar, water flows out of cells; cell shrinkage

Hypotonicity: solution is hypoosmolar, water flows into cells; cell swelling, lysis

Question 5

Equation to calculate (estimate) serum osmolality

Answer 5

Serum Osmolality = [2*S(Na)] + [Glucose/18] + [BUN/2.8]

Question 6

How does VECF, VICF, Osm-ECF, Osm-ICF change in response to:

1. Isoosmotic Fluid Loss
2. Pure Water Loss
3. Hypoosmotic Fluid Loss

and give examples of each

Answer 6

1. VECF down, no change to anything else
   (secretory diarrhea)
2. VECF down, VICF more down, both OsM increase
   (insensible losses, dehydration, diabetes insipidus)
3. VECF more down, VICF down, both OsM increase
   (sweating, diuresis)

Question 7

How does VECF, VICF, Osm-ECF, Osm-ICF change in response to:

1. Isoosmotic Fluid Gain
2. Hypoosmotic Fluid Gain
3. Acute Water Intoxication
4. Hyperosmotic Fluid Gain

Answer 7

1. VECF increases; no change to anything else (IV NS)
2. VECF more up, VICF less up, Both OsM down (Half-NS)
3. VECF up, VICF more up, both OsM down (oral water)
   * may cause brain herniation due to extreme cerebral edema*
4. VECF more up, VICF down, both OsM up (3% NS)

Question 8

List the 5 basic functions of the kidney

Answer 8

1. Waste Management (Nitrogenous Wastes - protein degradation; Acid Wastes - H+)
2. Fluid + Electrolyte Balance (Water, Acid-base, K+, Ca2+, Mg2+)
3. Regulation of ECF Volume (BP Regulation, Na Balance -balance more important in VECF than electrolyte balance)
4. Endocrine (RAAS, ANP, PTH, Vitamin D, Erythropoietin)
5. Drug/Hormone Elimination (small peptide elimination, clearance of insulin and other hormones, drug clearancE)