Body water compartments - Water Ion movement

Body water compartments - Water / ion movement

Introduction

The body is mainly composed of water
Water is divided into physiological compartments which communicate with each other at varying rates
There is a major difference in composition between intracellular (ICF) and extracellular (ECF) fluid compartments
ICF is characterised by high K+, protein-, calcium and organic phosphates. ECF is mainly Na+ and Cl- (except for albumin in the vascular space)
Other compartments - transcellular e.g. eye, joint fluid, bone
Water movement is restricted by cell and capillary membranes
The regulation of serum osmolality controls fluxes between ICF and ECF
Alterations in water volume has various clinical effects - v vascular (hypovolaemic shock), ^ ECF (pulmonary and peripheral oedema, ^v ICF (cellular dysfunction, cellular swelling, haemolysis)
The most life-threatening conditions are shock and pulmonary oedema

see The consequence of not maintaining serum electrolyte levels

Important concepts that maintain ICF/ECF difference

Na/K+ ATPase pump
Permeablity of membranes to ions
Gibb's-Donnan Effect - non-diffusible ions

A disturbance of serum osmolality results in significant alterations in cell volume (and hence function)

Body water values

60% of body weight is water (NB 1L of water weighs 1kg)
2/3 intracellular, 1/3 extracellular (3/4 interstitial, 1/4 vascular)
Based on 70kg weight gives TBW 42L = ICF 28L + ECF 14L (Interstitial 10.5 L + Vascular 3.5L)
Blood 5.5L (3.5L water + 2L cells, plasma proteins)

Water movement

Limited by cell membranes (low permeability but high net surface area! - see Fick's law of diffusion)
Occurs via different mechanisms
Diffusion - see Fick's law
Osmosis - based on difference in concentration of non-permeable particles across membrane
ICF <> ECF - based on diffusion, osmosis
Interstitial <> Vascular (across capillary endothelium) - via above mechanisms + Starling's forces + water channels (aquaporins) + pinocytosis

NB water movement across capillary membranes between vascular and interstitial compartments occurs faster than between interstitial (extracellular) and intracellular compartments due to gaps between capillary endothelial cells

The practical outcome is that every 3L of crystalloid fluid injected into the vascular space, only 1L will remain after equilibrium is reached.

Fick's law of diffusion (across cell membranes)

Flux proportional to:

Concentration (or pressure) difference
Surface area
Temperature

inversely proportional to:

Thickness of membrane

Starling's forces (of filtration) - DO NOT confuse Starling's equation with Starling's law

Along hydrostatic (pressure) gradient
Against colloid osmotic (oncotic) pressure [more important than ionic osmolar differences since electrolyte composition similar between interstitial and vascular compartments]
Plasma albumin plays an important role in preventing water movement out of the vascular space.

Ion movement

Depends on

Permeability of membrane to a specific ion
Presence of ion-channels (protein) in membrane

Types of ion transport

Ions will generally follow a chemical or electrical gradient
Ions require energy to go against the gradient

Passive (does not require energy)

Simple diffusion (via cell membrane)
Facilitated diffusion (via ion-channels)

Active (requires energy)

Simple active - single ion
Co-transport (symporter) - with another ion (e.g. Na+/Cl-)
Counter transport - in exchange with another ion (e.g. Na+/K+)

Clinical note - the 'chanellopathies'

Genetic abnormalities in ion channel structure can lead to a number of diseases that result in altered ionic fluxes and affect the function in renal tubules, excitable tissues (e.g. skeletal muscle, cardiac muscle and nerve) and other tissues

Examples

Cardiac - Brugada syndrome
Skeletal muscle - Periodic paralysis
Other -Cystic fibrosis

Definitions

Osmoles = number of osmotically active particles in a solution
Osmolality = osmoles per litre of solution
Osmolarity = osmoles per kg of solution
Osmosis = diffusion of water across a semi-permeable membrane divided by solutions of different osmolarity
Osmotic pressure = the hydraulic pressure required to counter the osmotic force across a membrane
Tonicity = the 'effective' osmolality due to the fact that the biological membrane allows some movement of solute across it ('osmolality in real life')

Osmolarity (qv osmolality)

Independent of temperature (water volume increases with temperature)
Measured by osmometer using freezing point depression in lab - can be by vapour pressure depression, boiling point elevation or osmotic pressure.
Less than but approximates osmolality in vivo due to low mass of solutes

NB Serum osmolarity determines if cells swell or shrink due to water movement)