Intravenous fluids are used intraoperatively for several reasons:

To provide maintenance fluids
To replace fluid losses
To counteract haemodynamic changes due to the anaesthetic

Postoperative intravenous fluids are used to continue fluid replacement, and to provide daily water and electrolyte requirements, until the patient is able to drink an adequate daily volume.

The choice and volume requirements for intravenous fluid administration depends on the composition of fluid lost either before or during surgery, but must also allow for replacement of normal daily loss of water and electrolytes.

Daily requirement
Water	2.5-3 litres
Na⁺	140-160 mmol
K⁺	80-100 mmol

Table 1 Daily requirements for a normal adult