Osmolarity and Osmolality

Differences between each

Fluid spaces – Relative distribution between intra- and extra-cellular spaces

Key differences between ECF and ICF in terms of cationic concentrations.

Role of Na-K ATPase activity in maintaining ionic gradients between ECF and ICF.

The Donnan equilibrium influencing movement of chloride ions.

Plasma contents – Basis of oncotic pressure (large proteins)

Curriculum

Osmolarity Vs Osmolality

Both are expressed in terms of osmoles, where 1 osmole equals 1 mole of particles, as osmolarity (Osmol/L), or osmolality (Osmol/kg H2O).

The OsmolaLity is preferred by physiologists as it is independent of temperature, though in physiological fluids the values are very similar.


Fluid spaces

Body water compartments (Covered in previous lesson already). Click to read

Electrical neutrality must be present in any compartment, i.e. the total number of positive charges and negative charges should be equal.

The ICF and ECF differ in the relative concentrations of cat-ions (Positive ions).
The K+ (potassium) concentration is much higher inside the cell than in ECF and Na+ (Sodium) concentration is higher in ECF than in ICF.
Ca2+ and Cl ions concentration is also higher in ECF.

ConstituentsPlasmaISFICF
Water
%
Litres

13%
3.5 L

22%
9.5 L

65%
27 L
Osmolality
(mOsm/kgH2O)
290290290
Cations
Na+ (mmol/L)
K+  
Free Ca

140
4
1

140
4
1

10
140
0.0001
Anions
Cl  (mmol/L)
HCO3
Proteins
Other(SO42-, PO43-)

108
26
10
3

129
26
1
0

3-30
9
50
60-88
Composition of cellular spaces Source – Medscape.

Sodium pump

Between the compartments, across the membrane, the ions keep moving. The Na+ goes in and K+ moves out of the cell following their concentration gradients.
This constant leak, if not corrected, would lead to equalization of the ions in ICF and ECF, which is prevented by the Na+-K+ ATPase, or Na+ pump.

The Na+-K+ ATPase pumps Sodium out of the cell and Potassium into the cell against the concentration gradient. (3 Sodium ions out : 2 Potassium ions in)

Most of the Calcium – Ca2+ in the cell is transported actively either out of the cell or into the endoplasmic reticulum and mitochondria. The free calcium must be under check as it can activate various cellular processes in the ICF. More on this ion in Action potential / muscle contraction topic.


Gibbs–Donnan equilibrium

The ICF has proteins the are negatively charged at physiological pH. The proteins and other large anions (such as phosphate – PO43- ) cannot cross the plasma membrane account for most of the trapped anion content of ICF.

Chloride – Cl ions can diffuse across the membrane via channels, are repelled by the fixed anions (Protein/phosphate etc). This electrical force is balanced by the chemical gradient which drives them into the cell. These 2 forces (electrical vs concentration) reach a state of equilibrium, the Gibbs–Donnan equilibrium.

Cells with high anion content will have low concentration of Cl ions and vice-versa.


Plasma contents

Plasma contains more protein than does Interstitial fluid.

Proteins are responsible for Oncotic pressure, which pulls the water into the capillaries (Against the hydrostatic pressure / blood pressure that tries to push water out of capillaries). Whether water is filtered out or absorbed into the capillaries depends on which force has higher value, Hydrostatic vs Oncotic.

Let’s look at an imaginary situation.

The hydrostatic pressure in capillary is 30mmHg and Oncotic pressure is 25mmHg
So the net pressure in capillary is 5mmHg favoring filtration out of capillary.
The tissue/interstitial space hydrostatic pressure is 5 and Oncotic pressure is 10
Net pressure on tissue side is 5mmHg pulling the water into tissue from capillary.

Combined pressure is 5+5=10mmHg towards the tissue (Filtration)

The constant filtered water will be absorbed by the Lymphatic system and will be sent back to circulation.

Transcellular fluid – fluids that are not part of any of the main compartments, but are derived from them. CSF and exocrine secretions (gastrointestinal etc) are some of the transcellular components.

Lesson tags: Osmosis
Back to: Physiology for MRCEM Primary > Basic cellular physiology