1. Anatomy and Physiology Part 1b: Body Fluid Ions, and Water Balance
Marieb’s Human
Anatomy and Physiology
Ninth Edition
Marieb w Hoehn
Chapter 26
Fluid, Electrolytes, and Acid-Base Balance
Lecture 17
Part 1b: Body Fluid Ions, and Water Balance
Slides 1-15; 80 min (with review of syllabus and Web sites) [Lecture 1]
Slides 16 – 38; 50 min [Lecture 2]
118 min (38 slides plus review of course Web sites and syllabus)

2. Body Fluid Ionic Composition
ECF major ions:
- sodium, chloride, and bicarbonate
ICF major ions:
- potassium, magnesium, and phosphate (plus negatively charged proteins)
Figure from: Hole’s Human A&P, 12th edition, 2010
You should know these chemical symbols and charges (valences) of ions

3. Movement of Fluids Between Compartments
Figure from: Hole’s Human A&P, 12th edition, 2010
Water moves across mesothelial surfaces: peritoneal, pleural, and pericardial cavities as well as the synovial membranes. It also moves between the blood and CSF and through the fluids of the eye and ear
Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressures

4. Fluid (Water) Balance
Figure from: Hole’s Human A&P, 12th edition, 2010
* urine production is the most important regulator of water balance (water in = water out)

5. Water Balance and ECF Osmolarity
Regulation of water intake
increase in osmotic pressure of ECF → osmoreceptors in hypothalamic thirst center → stimulates thirst and drinking (water! )
Regulation of water output
Obligatory water losses (must happen)
insensible water losses (lungs, skin)
water loss in feces
water loss in urine (min about 500 ml/day)
increase in osmotic pressure of ECF → ADH is released
small volume of concentrated urine is excreted
more water is retained
LARGE changes in blood vol/pressure → Renin and ADH release

6. Fluid Imbalance
Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

7. Dehydration Dehydration (removing only H2O)
osmotic pressure increases in extracellular fluids
water moves out of cells
osmoreceptors in hypothalamus stimulated
hypothalamus signals posterior pituitary to release ADH
urine output decreases
Severe thirst, wrinkling of skin, fall in plasma volume and decreased blood pressure, circulatory shock, death

8. Overhydration Overhydration (adding only H2O)
osmotic pressure decreases in extracellular fluids
water moves into cells
osmoreceptors inhibited in hypothalamus
hypothalamus signals posterior pituitary to decrease ADH output
urine output increases
‘Drunken’ behavior (water intoxication), confusion, hallucinations, convulsions, coma, death

9. Review
There are two major forces affecting movement of fluid between compartments
Hydrostatic Pressure
Osmotic Pressure
Fluid balance
Amount of water you take in is equal to the amount of water you lose to the environment
Intake of water in food/drink is the most important source of fluid
Kidney regulation of water is the most important regulator of water loss