Weight= force acting on object by gravity, varies with atmosphere etc.

Mass= amount of matter an object is made up of, constant

Atomic unit= 1 atomic unit is defined as 1/12 the mass of the carbon-12 atom

Mole= 1 mole is the number of atoms in exactly 12g of carbon-12, which is 6 x 10^23 atoms (Avogadro’s number). 1mmol is 1/1000mole.

Molecular weight= ratio (no units) of the mass of one molecule of a substance, to the mass of 1 atomic unit. The molecular weight of a substance is numerically equal to the mass (in grams) of one mole of that substance.

Dalton= unit of mass equal to 1 atomic unit (1/12 the mass of the carbon-12 atom). Unit to express molecular weight (as it is a ratio and thus, no units)

Equivalent= moles x valence(charge) i.e. Na1+ and Ca2+ --> 1 mmol of calcium has twice the electrical effect of 1 mmol of sodium.

Gram equivalent= the mass of a substance that supplies 1 mole of reactive units e.g. Calcium: MW=40g, Charge=2+. Gram Eq= 20g

Normality= the number of gram-equivalents in 1L of solution

Oxidation: loss of an electron (e.g. loss of H+ or combination with O2)

Reduction: gain of an electron (e.g. gain of H+ or loss of O2)

What determines membrane potential?

Membrane potential is determined by:

1) Concentration gradients of ions and their Nernst potential

- Nernst equation: tells you the voltage needed to stop an ion moving down its concentration gradient i.e. the potential when there is an equilibrium between influx and efflux of ions

2) Membrane permeability to those ions and which channels are open

3) Electrogenic transporters: Na/K/ATPase

- pumps 3x Na+ out of cell for every 2x K+ moved in, it contributes to the membrane potential so is termed an electrogenic pump.

4) Fixed intracellular anions

e.g. the concentration gradient of K+ takes it out of cell, electrical gradient brings it into cell. At equilibrium there is a slight excess of cations(+) outside and anions(-) inside.

Note that the total concentrations of positive and negative ions are equal everywhere except along the membrane. Even at -70mV more than 99.99% of the cell membrane remains neutral, only a microscopic layer near the membrane differs slightly. 

What is normal osmolality? How is it calculated? How does it differ from osmolarity?

Osmosis: Defined as the diffusion of solvent/water molecules into a region where there is higher concentration of a solute, to which the membrane is impermeable.

Osmotic pressure: pressure necessary to prevent solvent/water migration

Osmoles: the concentration of osmotically active particles (1L of H2O with 1mol of NaCl has 2 Osm of NaCl because NaCl-->Na+ and Cl- ions in solution).

Osmolarity= number of osmoles per litre of solution. Note that volume changes with pressure, temperature and other dissolved substances.

Osmolarity (mOsm/L)=2Na+ glucose + urea/BUN

Osmolality= number of osmoles per kg of solvent. Since mass (kg) is more stable, osmolality is preferred to osmolarity. However since 1L water=1kg water, you may see it expressed in medicine as mOsmol/L.

The osmolality of plasma is 290mOsm/L. All but 20 of the 290mOsm is contributed by Na+ and its accompanying anions (i.e. Cl- and HCO3-). All other cations make a relatively small contribution. Because of their high molecular weights(larger in size, but low in quantity), plasma proteins contribute little to osmolality. The major non-electrolytes of plasma are glucose and urea, and they contribute about 5mOsmol/L each (changes in hyperglycaemia and uremia).

Osmolar gap= Osmolarity (measured) - osmolarity (predicted).

Osmolar gap= <10 is normal. If elevated it indicates the presence of a foreign substance like ethanol, mannitol, methanol, ethylene glycol.

What determines tonicity?

Tonicity= the osmolality of a solution relative to plasma

• 0.9% Saline is isotonic
• 3% Saline is hypertonic (higher osmolality than plasma/ECF-->cell shrinks)
• 5% glucose is hypotonic(less osmolality than plasma/ECF, because glucose is metabolised-->cell expands)

What is a buffer? Give examples

pH is a measure of H+ concentration

pH=-log10[H+], so for each unit less than 7.0, H+ increases 10 fold

Plasma is slightly alkaline (7.34-7.45), Gastric fluid is acidic (pH 3.0), Pancreatic fluid is alkaline (pH 8.0)

Weak acids/bases partially dissociate in solution and reach equilibrium, whereas strong acids/bases completely dissociates in solution

Acid= H+ donor in solution i.e. HA<-->H⁺ + A^- 

• Weak acid: acetic acid, phosphorus acid, carbonic acid)
• Strong Acids: HCl

Base= remove H+ in solution

• Weak base: ammonia
• Strong base: sodium hydroxide

Buffer: substance that can bind or release H+ in solution, keeping pH constant

• Important biological buffers: Carbonic Acid H₂CO₃, phosphates, proteins

Isohydric principle: says that all body buffer systems are linked through the same H⁺ pool, so they work together to keep pH stable.

Describe the body fluid compartments, their composition and how they are measured