tonicity (from the Greek tonos = voltage) describes the osmotic pressure between two, separated from a semipermeable membrane, liquids. In this context one speaks also of outside and inside medium. The latter usually refers to the intracellular space of a cell, hence the cytoplasm. Whereas the external medium is synonymous with the extracellular space. Between the outer and inner medium is a semi-permeable membrane through which only certain ions can diffuse.
Water-loosened substances, i.a. the electrolytes calcium, magnesium, sodium, potassium and chloride always strive to balance the concentrations (balance between internal medium and external medium). The osmotic pressure is responsible for the diffusion of the loosened particles through the membrane. The greater the difference in the number of particles liberated between the cell fluid and the external medium, the greater the effect of the osmotic pressure. In a concentration balance of both fluids is an isotonic state. Depending on whether more or less loosened particles are present in the internal medium, hypertonic or hypotonic conditions are present accordingly. From this it can be concluded:
A liquid is isotonicif it has as many dissolved substances as the comparison medium.
A liquid is hypertonicif she has more loosened substances, such as the comparison medium.
A liquid is hypotonicif it has less loosened substances, such as the comparison medium.