Osmosis

‘Osmosis is the movement of water across a semi-permeable membrane from regions of low solute concentration to those of higher concentration’.

Biological membranes are semi-permeable – they allow the free movement of water but restrict the movement of solutes.

The creation of osmotic gradients is the primary method for the movement of water in the body.

• Fluid at the same osmotic potential as plasma is hypotonic.
• Fluid at higher osmotic potential is hypertonic.
• Fluid at lower osmotic potential is hypotonic.

Osmolarity or Osmolality?

Osmotic potential is determined by the number of osmotically active particles per litre. It can be expressed as ‘osmolarity’ or ‘osmolality’:

• Osmolarity – osmotic concentration per mass of solvent (osmol/L)
• Osmolality – osmotic concentration per volume of solution (osmol/kg H₂O)

Osmolality is the preferred expression of physiologists because it is independent of temperature and pressure.

The osmolality of plasma = 290 mosmol/kg H₂O

Body Water Compartments

Water makes up 50-70% of body mass – approximately 42L in a 70kg person.

There are two main fluid compartments:

• Intracellular fluid (ICF) – the fluid within cells which accounts for 65% of total body weight.
• Extracellular fluid (ECF) – itself can be divided into:
  • Interstitial Fluid (ISF) – 65%
  • Plasma

These two compartments are separated by cell membranes, and are very different in their composition (i.e. ion concentration).

*Transcellular fluid is the name given to fluids in the body that are not contained within the ICF or ISF/plasma (ECF) compartments. It includes CSF, exocrine secretions and gastrointestinal secretions.

The biochemical balance between ICF and ECF is very tightly controlled as many crucial physiological processes depend on its composition.

Distribution of Cations

The most important difference between ICF and ECF is the relative concentration of cations:

• K⁺ concentration is much higher intracellularly
• Na⁺ concentration is much higher extracellularly

K⁺ and Na⁺ follow their concentration gradients (K⁺ out of the cell, Na⁺ in). Without intervention, this would eventually lead to equalisation of both ions on both sides of the membrane, which would be physiologically non-viable. This is prevented by the activity of the Na⁺-K⁺ ATPase pump.

The Na⁺-K⁺ ATPase pump acts to maintain the cellular environment by actively moving Na⁺ and K⁺ against their concentration gradient. It pumps 3 Na⁺ ions out of the cell, and 2 K⁺ into the cell. The Na⁺-K⁺ ATPase carrier protein has to be phosphorylated by one ATP molecule.

Intracellular Ca²⁺ is either actively transported out of the cell or into the endoplasmic reticulum/mitochondria – this leaves very low levels in the cell.

Cl^– and the Donnan Equilibrium

Cl^– ions are differentially distributed across the cell membrane due to their negative charge and because they can move freely across the cell membrane via protein channels.

Intracellular proteins are negatively charged at physiological pH. These, along with other large anions trapped in the cell (e.g. phosphate) force Cl^– out of the cell down an electrical gradient.

This creates a concentration gradient with higher concentration of Cl^– outside of the cell. Cl^– will diffuse down this concentration gradient back into the cell.

An asymmetric equilibrium is formed with the electrical and concentration gradients opposing each other – this is Donnan’s equilibrium.

Donnan’s Equilibrium: ‘The presence of a charged impermeant ion (anionic protein) on one side of a semi-permeable membrane will result in an asymmetric distribution of permeant charged ions (Cl^–)’.

Variations in intracellular large anion content mean that intracellular Cl^– concentration varies considerably.

Plasma Oncotic Pressure

The crucial difference between plasma and ISF is that plasma contains more proteins. These proteins are the only constituents of plasma that do not cross into the ISF.

These impermeant proteins create an osmotic gradient that pulls water into the plasma. This almost balances the hydostatic pressure created by the pump action of the heart that forces water out of the capillaries into the ISF. There is a small net movement of water out of the plasma.

Effects of ingesting fluids of differing osmotic potential…

Here is a fantastic explanation of osmosis and the fluid compartments from Dr. Eric.

References

Dr. Eric

Larry Keeley

Physiology at a Glance – Ward, Linden

Textbook of Medical Physiology – Guyten, Hall

MCEM Part A: MCQs – Ian Beardsell (of St. Emlyns fame)