1. Fluids and Electrolytes
Cristina E. Pinal, DO PGY4
Nephrology Fellow

2. Why is this important?
Because you and your patients are mostly water…and electrolytes!

3. Key concepts to understand
Total Body Water is approximately 60% of body weight
Percentage highest in babies and adult males
Percentage lowest in adult females and obese people
How is this distributed?
Intracellular fluid (ICF) = 2/3 TBW
Extracellular fluid (ECF) = 1/3 TBW

4. TBW ICF ECF Major cations are k+ and MG2+
Major anions are protein and organic phosphates (adp, atp, amp)
ECF
Interstitial fluid and plasma
Major cation is NA+
Major anions are cl- and HCO3-
Plasma is ¼ of ECF and 1/12 of of TBW (1/4 x 1/3 = 1/12)
Plasma proteins – albumin and globulins
Interstitial fluid is ¾ of ECF and ¼ of TBW (3/4 x 1/3 = 3/12 = ¼)
Composition is similar to plasma but has little protein

5. Shifts of water between compartments

6. When you give isotonic NaCL
ECF volume increased
Plasma proteins and hematocrit may decrease because of dilution
Arterial blood pressure increases (yay!) because ECF is increasing
Isosmotic expansion
Uses:
For fluid losses, volume contraction (dehydration, hypovolemia), etc

7. diarrhea ECF decreases; osmolarity is unchaged
Plasma protein/hematocrit increase (concentration of protein and RBCs)
Arterial blood pressure decreases as ECF volume decreases
Isosmotic volume contraction
fluid is secreted from the GI tract with same osmolarity as that of the ECF; thus, no change in the ECF osmolarity. So no shift in body fluid between the ICF and the ECF. So, only total volume of the ECF contracts due to its loss within in the diarrheal fluid.
Give isotonic fluid back

8. Alteration of fluids

9. Fluid states in disease

10. Types of fluids
1) Crystalloid – contains chemicals of simple ionic structure that can pass semi permeable membrane (water follows crystalline component by osmosis)
2) Colloid – contain large molecules that cannot pass semi permeable membrane (increase oncotic pressure)

11. Types of fluids

12. Fluid distribution

13. Sodium homeostasis
-sodium is major cation that determines serum osmolality
This regulates water flow
Total amount of sodium in the body is a component of water balance
Hyponatremia, hypernatremia
Hyponatremia- for assessment you will need: Urine sodium, urine osmolality, serum osmolality, urine potassium and volume status
TSH
Uric acid

14. Potassium homeostasis
Most abundant intracellular cation
Hypokalemia (GI losses, diuretics, metabolic alkalosis, hypomagnesemia)
Hyperkalemia (renal failure, metabolic acidosis, rhabdomyolysis, digoxin)

15. Potassium stores in body

16. Magnesium homeostasis
Intracellular cation
Associated with potassium and calcium homeostasis
A decrease in intracellular magnesium, (via deficiency), releases the magnesium- mediated inhibition of ROMK channels and increases potassium secretion. 
(hypomagnesemia associated with loop and thiazide diuretics, GI losses, trauma, burns, sepsis)
Hypermagnesemia (renal failure or iatrogenic)

17. Magnesium absorption and elimination

18. Hyperkalemia