Lecture 2 – Body Fluid Compartments & Water Transport

what is homeostasis?

• the maintenance of a relatively constant internal environment

• when a change in the ECF occurs, a series of reactions are initiated to MINIMISE that change

what is the ‘internal environment’ of the body?

the ECF, specifically the ISF that directly bathes the cells

• composition must be kept constant for cell survival

• cells are protected by maintaining this stable environment

list 3 internal and 3 external stimuli that can disrupt homeostasis

internal:

• change in blood glucose levels

• change in pH

• change in blood ion concentrations

external:

• exercise

• physical insults

• psychological stresses

what is differentiation?

the process of transforming an unspecialised cell into a specialised cell for the performance of a particular function

example: a stem cell differentiating into a muscle/nerve cell

what are the two types of functions that ALL cells perform?

1. basic cellular processes: fundamental for individual cell survival

(movement across membranes, energy production/ATP, protein synthesis, maintenance of cell volume)

2. specialised activities: contribute to survival of the whole organism

(contraction, conduction, secretion)

what are the two major fluid compartments of the body and what % of TBW does each represent?

intracellular fluid: 60% of TBW

extracellular fluid: 40% of TBW

what are the 3 subdivisions of the ECF and what % of ECF does each represent?

interstitial fluid: 75% of ECF (bathes cells directly)

plasma volume: 20% of ECF (fluid portion of blood)

transcellular fluid: 5% of ECF (CSF, synovial, pleural, etc)

calculate the approximate fluid volumes for a 70kg MAN

• TBW

• ICF

• ECF

• ISF

• Plasma

• TBW: 70kg x 0.6 = 42L

• ICF: 42L x 0.6 = 25L

• ECF: 42L x 0.4 = 17L

• ISF: 17L x 0.75 = 13L

• plasma: 17L x 0.20 = 3L

calculate the approximate fluid volumes for a 70kg WOMAN

• TBW: 70 × 0.50 = 35L

• ICF: 35L x 0.60 = 21L

• ECF: 35L x 0.40 = 14L

• ISF: 14L x 0.75 = 10L

• Plasma: 14L x 0.20 = 3L

what’s the approximate concentration of Na+ in ECF vs ICF?

ECF: ~142 mM

ICF: ~15 mM

what’s the approximate concentration of K+ in ECF vs ICF?

ECF: ~4.4 mM

ICF: ~120 mM

this gradient is essential for setting the resting membrane potential

what’s the approximate concentration of Cl- in ECF vs ICF?

ECF: ~102 mM

ICF: ~16 mM

what’s the approximate concentration of proteins in ECF vs ICF?

plasma: ~7 g/dL (~1 mM)

ICF: ~30 g/dL

what’s the normal osmolality of all body fluids?

~290 mOsm/kg H2O

this includes plasma, ISF, and ICF

list 5 functions that depend on the asymmetric distribution of ions across cell membranes

1. setting the membrane potential

2. generating electrical activity

3. initiation of muscle contraction

4. energy for nutrient uptake & waste expulsion

5. generation of intracellular signalling cascades

describe the structure and function of the Na+/K+ ATPase pump

structure:

• alpha subunit (catalytic, 10 transmembrane domains)

• beta subunit (assembly & membrane location)

function:

• primary active transport

• pumps 3 Na+ OUT, 2 K+ IN per ATP molecule

• creates and maintains ion gradients

why does the Na+/K+ ATPase help regulate cell volume?

• it pumps MORE positive charges OUT than it brings IN

• this net loss of ions from the cell counteracts the osmotic tendency for water to enter the cell due to impermeant anions inside

define:

• osmole

• osmolality

• osmolarity

osmole: 1 gram molecular weight of solute particles

osmolality: osmoles per KG of water (mOsmo/kg H2O) - used physiologically

osmolarity: osmoles per LITRE of solution (mOsm/L)

what are the two determinants of cell volume?

1. the total number of osmotically active particles WITHIN the cell

2. the effective osmolarity (tonicity) of the EXTRACELLULAR fluid

define:

• isotonic

• hypotonic

• hypertonic

isotonic: same concentration of non-penetrating solutes as cell —> NO volume change

hypotonic: lower concentration of non-penetrating solutes than cell —> water moves IN —> cell SWELLS

hypertonic: higher concentration of non-penetrating solutes than cell —> water moves OUT —> cell SHRINKS (crenation)

why does urea have NO long-term effect on cell volume despite initially causing water movement?

• urea is a permanent solute that readily crosses the cell membrane

• initially it creates an osmotic gradient, but because urea can enter the cell, it rapidly equilibrates on both sides of the membrane, eliminating the gradient and restoring cell volume

what happens to cell volume if:

• ECF osmotic pressure increases

• ECF osmotic pressure decreases

• water moves OUT of cells —> cells SHRINK

• water moves INTO cells —> cells SWELL

how do you calculate blood volume (BV) form plasma volume (PV) and haematocrit (Hct)?

BV = PV / (1 - Hct)

what’s the difference between osmolality and tonicity?

osmolality: total concentration of ALL solute particles

tonicity: the EFFECT a solution has on cell volume, determined ONLY by concentration of NON-PENETRATING solutes