The normal fluid requirements need to take into consideration the measurable fluid losses that occur from the body, plus those from routes such as sweating and respiration (termed insensible losses).

Fluid Losses

Termed	From	As	ml/24 h
Insensible losses	Skin Lungs	Sweat Moisture from humidified air	500 300
Measured losses	Renal Gut	Urine Faeces	500-1200 500
		Total fluid loss	1800-2500

Table 1 Fluid Losses

These fluids all need to be replaced appropriately to maintain the volume and composition of the body’s fluids. The requirements vary in times of ill health, increasing markedly in patients either post-operatively or with sepsis and an increased metabolic rate.