13. Describe the intra- and extracellular ionic components and explain their physiological functions

You should know the normal values for ions and proteins in the plasma. You don’t need to know the values for the intracellular space, except for K+ and Na+.

Ion	Normal value in plasma (in mM)	Normal value in intracellular space (in mM)
Na+	136 – 146	14 – 19
K+	3.4 – 5.2	150
Ca2+	2.3 – 2.7	10-7 – 10-4
Mg2+	0.8 – 1.2	4 – 40
Cl–	96 – 111	3 – 78
HCO3–	21 – 28	7 – 18
PO43-	1.0 – 1.4	2 – 40
Osmolarity	300 mOsm/L	300 mOsm/L

The dominant ion in the extracellular space is Na+, and for the intracellular space is K+.

Sodium

Sodium plays an important role in maintaining the extracellular fluid volume.

Hyponatraemia refers to too low plasma Na+ level. Hypernatraemia refers to too high plasma Na+ level. Either of these can cause seizures or coma.

Potassium

Potassium plays an important role in maintaining the resting membrane potential of cells.

Hypokalaemia and hyperkalaemia can cause fatal cardiac arrhythmias.

Calcium

Calcium plays an important role in multiple processes, like coagulation, neuromuscular excitability, cell signalling and hormone release.

Hypercalcaemia decreases neuromuscular excitability. It can cause arrhythmias, abdominal pains and calcification of the kidney.

Hypocalcaemia increases neuromuscular excitability. It can also cause arrhythmias but also seizures, spasms and tetany. Muscle spasm of the laryngeal muscles can close the larynx and cause suffocation.