Osmolarity

Concentration

number of solutes / volume

Solutes

anything that can dissolve in a given volume of liquid

What causes diffusion of solutes?

Brownian motion: solutes are antisocial and need room for each other —> so solutes will distribute evenly for equilibrium (both sides are equal concentration) 

If you have more concentration, will receptor bind more or less to solute?

More ; more concentrated will activate receptor more because there is more chance to bind to receptor 

1 solute

A —> 5 solutes / 5 volume = 1 solute/volume 

B —> 1 solute / 1 volume = 1 solute/volume

How do nonpermebale solutes adjust to reach equilibrium?

Water can still diffuse across in order to reach equals concentration (osmosis)

Osmosis

water moves from low solute to high NP solute concentration

water helps equilibrate solute concentrations when they are nonpenetrating

Types of solutes 

Penetrating: urea

Non penetrating: ions (Na+, K+) 

Partially penetrating: glucose (can penetrate the membrane, but once inside gets trapped because of phosphorylation) 

Osmoles

takes into consideration the dissolution of the solute in solution

Osmolarity

compares any 2 solutions and describes a solution concentration

Mechanisms of equilibrium:: diffusion, osmosis

does not matter nature of solute

Units: OsM

Hyperosmotic

Hypoosmotic

Isoosmotic

Hyperosmotic: a solution has more osmolarity than another solution

Hypoosmotic: a solution has lower osmolarity than another solution

Isoosmotic: two solutions have same osmolarity

Tonicity 

Uses: Describes a solution compared to a cell only

Describes how that solution will affect behavior of cell - whether a solution will cause water to move out of cell or not (shrink, swell, etc)

Mechanism of equilibrium: osmosis

Nature of solute matter: Yes - only NP solute (as NP allows water to equilibrate to create equilibrium)

Units: None

Hypertonic

Isotonic

Hypotonic

• Hypertonic solution:  water moves out of cell → causes cell to shrink

• Isotonic: no net movement of water → normal cells

• Hypotonic: water moves into the cell → cells swell and eventually burst 

If there is no NP solute in solution

cell will burst if its hypotonic

Total body water

33% is extracellular fluid: plasma (8%) and interstitial fluid (25%)

67% is intracellular fluid

Markers 

indicate volumes of body compartments

D20: measure TBW

Insulin: measure ECF

Evan’s blue: measure plasma 

Physiology reference man

155 lb or 70 kg

TBW: 60% of 70 L = 42 L

Plasma Osmolarity: 300 mOsm

Volumes of distribution: 14 ECF and 28 ICF

Dehydration

Loss of hypoosmotic solution = lose more water than solute

concentration of ECF has increased, water move from ICF to ECF to equilibrate

Hemorrhage

extreme blood loss: losing same proportional amount of water and solid

Lose isosmotic solution: no change in concentration

Priorities to treat dehydration 

1. Regain volume loss for circulatory system (ex blood pressure) in ECF

2. Rehydrate cells to regain proper cell function

3. Then lower osmolarity

How to assess initial cell response

1. Calculate concentration

2. Look at osmolarity If hyperosmotic: cell will shrink

If hypoosmotic: cell will swell

If isosmotic cell will not change

3. Now calculate the concentration of non penetrating solutes outside compared to inside

Hypertonic = NP outside greater NP inside

Cell shrinks

Hypotonic = NP outside less than NP inside

Cell swells

Cell burst/hemolyze if NP outside = 0

Isotonic = NP outside = NP inside

Cell no change

IV therapy

replenish fluid loss 

Normal Saline

0.9% saline

Isomotic

Isotonic

Normal saline sits at 300 mOsm which reflects osmolarity of the body

Best for hemorrhage patient

D5 normal saline

5% dextrose in 0.9% saline

Hyperosmotic

Isotonic

D5W

5% dextrose in water

Isomotic

Hypotonic

Half normal saline 

0.45% saline

Hyposmotic

Hypotonic

D5 half normal saline

5% dextrose in 0.45% saline

Hyperosmotic

Hypotonic

What solutions is isotonic?

• Normal saline

• D5 normal saline

What solutions is hypotonic

D5W

½ normal saline

D5 ½ normal saline

Treat dehydration 

Hyposmotic fluid loss from ECF and ICF

Hyposmotic, hypotonic solution with NP solutes

Hydrates cells but keeps some fluid in plasma

Treat hemorrhage

Isomotic fluid loss from ECF only

Isotonic solution