Body Fluid Compartments

and

Fluid Balance

Water gain must = water loss

Rate of formation of metabolic water is not regulated

Source of metabolicallyformed water

Osmolarity of plasma is about 300 mOsm/L

(Measured by a method called “Freezing Point Depression)

TONICITY is Osmolarity in relation to that of plasma. It has the ability to change the VOLUME of a cell

Isotonic solution: same osmolarity as of plasma

Hypertonic solution

Hypotonic solution?

ECF IS ISOTONICOsmotic equilibrium is maintained rapidly between ICF and ECF

Isotonic fluid gain: e.g Intravenous infusion of isotonic saline (0.9 % NaCl, 5% glucose solution)

Isotonic fluid loss: Hemorrhage No net fluid shift occur

ECF IS HYPOTONICWhat happens?

Water moves into the cells from ECF until osmotic equilibrium is achieved. Pronounced swelling of neurons leads to brain dysfunction

Swelling of muscle leads to muscle weakness.Hypertension and edema caused by expansion of plasma volume.

What Causes this?Over hydration:

Renal failure with inability

to excrete diluted urine

Excessive drinking (transient)

Increase in ADH secretion

Water intoxication:

condition of over hydration,

hypotonicity and cellular swelling

from excess water.

ECF IS HYPERTONICWhat happens?

Water moves out of the cells to ECF until osmotic equilibrium is achieved Cells shrink as water leaves them

Neurons dehydration causes disturbances in brain function manifested as mental confusion to coma.

What Causes this?Dehydration due to: Insufficient

water intake Excessive

water loss (vomiting or

diarrhea) Diabetes

Insipidus (deficiency of

ADH)

Accumulation of high levels of osmotically active solutes (urea in kidney

failure)

Tonicity & RBC

Regulation of body water gain

Rate of formation of

metabolicwater is not regulated to

maintain homeostasis

Water gain is regulated mainly

by drinking through thirst

mechanism2% decrease in body mass due

to fluid loss causes mild dehydration

Baroreceptor

27_table_02

27_table_01

Measurement of fluid volumes in the different body

compartmentsDye-dilution metod (Indicator-dilution)

Measurements of various body volumes

Total body wateruse of a substance which when inserted in to the blood, will spread evenly through out all the body compartments.

Substances used i. Radio active water ii. Antipyrine –because it is lipid

soluble

Extra cellular volume Substance should disperse easily through water/plasma but NOT enter cells

Substances used I Radioactive Na, Inulin These rapidly spread through out extra cellular space30-60

min Extra cellular volume is also called SODIUM space or INULIN

space

ICF = TBW-Extra cellular volume

Plasma & blood volumes Plasma Volume Substance must stay INSIDE blood

compartment after it is injected in to the blood stream

Evans Blue (Dye) is the best choice

Blood volume = Plasma volume + hematocrit (cell volume)

Then BV = Plasma Volume/ (1-

Hct)

Given Plasma vol as 3 litres, and

Hct as 0.45 BV = 3/(1-0.45) = 3/0.55 = 5.45 L

Measurement of various body fluid compartments

Summary 1. Osmolarity, its normal value, and osmotic pressure

2. Hydrostatic pressure

3. Distribution of body water

4. Intra and extra cellular fluids and their composition

5. Tonicity of plasma and learnt what is iso, hypo and hyper tonicity

6. Measurements of body fluid compartments