Osmolarity, Osmotic Pressure

Osmolarity (Osmotic concentration)

The concentration of all solutes in a solution

Units of osmolarity

Osm/L (OsmolaRity: Osmoles per LitRe)

Units of osmolality

Osm/kg

Selectively semipermeable

- Allow hydrophobic (lipid soluble) substances to cross easily
- Allow some small hydrophilic (polar) substances to cross easily
- Prevent the free passage of large polar substances e.g. glucose, ions

How does water pass through membrane?

Small amount through lipid bilayer by passive diffusion, aquaporins

What are aquaporins?

water channel protein in a cell

What is osmosis?

The diffusion of water from an area of high concentration of water molecules (high water potential) to an area of low concentration of water (low water potential) across a partially permeable membrane

What type of transport is osmosis?

passive transport

high solute concentration means

low water concentration

Low solute concentration means

high water concentration

What are the requirements of osmosis?

• Semi-permeable membrane- Permeable to water and Impermeable to at least one solute
• Concentration gradient
• Driving force for osmosis is osmotic pressure
- i.e. dilute to concentrated

What is Osmotic pressure?

pressure required to prevent osmosis

In terms of osmosis, greater the solute concentration means

the greater the osmotic pressure

isosmotic

Solutions having the same concentration of solute particles and the same osmotic pressure

A solution containing 300mOsm/L of solute, regardless of its concentration of membrane-penetrating and non penetrating solutes

hypoosmotic

A term describing organisms with body fluids with a higher concentration of water and lower solute concentration than the external environment.

A solution containing less than 300mOsm/L of solute, regardless of its concentration of membrane-penetrating and non penetrating solutes

Hyperosmotic

A term describing organisms with body fluids with a lower concentration of water and higher solute concentration than the external environment.

A solution containing greater than 300mOsm/L of solute, regardless of its concentration of membrane-penetrating and non penetrating solutes

What is tonicity?

the ability of a surrounding solution to cause a cell to gain or lose water

What is tonicity used to describe?

Describes the behaviour of cells in solutions

What is the difference between Osmolarity and Tonicity?

Osmolarity is affected by all solutes in a solution, while tonicity is only affected by solutes that CANNOT cross the membrane

Isotonic

when the concentration of two solutions is the same

A solution that does not cause a change in cell volume, one that contains 300mOsm/L of non penetrating solutes, regardless of its concentration of membrane-penetrating solutes present

What is the effect of an isotonic solution?

cause no change in cell volume- Equal solutes on both sides of membrane

Hypotonic

when comparing two solutions, the solution with the lesser concentration of solutes

A solution that causes cells to swell, one that contains less than 300mOsm/L of non penetrating solutes, regardless of its concentration of membrane-penetrating solutes present

What is the effect of a hypotonic solution?

- Less solutes outside cell, so water enters cell, oedema

Hypertonic

when comparing two solutions, the solution with the greater concentration of solutes

A solution that causes cells to shrink, one that contains greater than 300mOsm/L of non penetrating solutes, regardless of its concentration of membrane-penetrating solutes present

What is the effect of a hypertonic solution?

cause cells to shrink- More solutes outside cell, so water exits cell, cell shrinks

RBC placed in isotonic solution

e.g. 0.9 % NaCl (Normal saline NS)

RBC placed in hypotonic solution

e.g. water

RBC placed in hypertonic solution

2% NaCl- shrinks

What happens if a red blood cell swells in hypotonic solution?

Haemolysis 1.6x

What contributes to the Osmolarity of body fluids?

Electrolytes

What does movement across membranes depend on?

Depends on solute concentration in ECF and ICF

What is Body fluid osmolarity?

270-300 mOsm/L

Oedema (UK) / Edema (US)

swelling due to loss of fluid from intravascular space to
the interstitial fluid

What is the function of plasma proteins?

provide the osmotic pressure to keep fluid in blood vessels, such as albumin

What happens if plasma proteins are low?

fluid leaves the plasma and moves into the tissues

What is Hypoproteinaemia?

abnormally low levels of circulating plasma protein leading to liver liver &/or kidney disease caused by malabsorption,
nutritional lack

What is Cerebral Oedema?

Accumulation of fluid in the intra/ extracellular spaces of the brain - increases ICP

What causes cerebral oedema?

Stroke, tumours, trauma

What are the effects of cerebral oedema?

Intracranial pressure increases
- Hypoxia, damage.
- Can lead to death

What is the treatment for cerebral oedema?

Osmotherapy- e.g. hypertonic solution of mannitol