compensate for the loss. On the other hand, a positive fluid balance (where fluid gain is greater than fluid loss) might suggest a problem with either the kidney or cardiovascular system. If blood pressure is low (hypotension), the filtration rate in the kidneys will lessen, causing less fluid reabsorption and thus less urine output. An accurate measure of fluid balance is therefore an important diagnostic tool, and allows for prompt intervention to correct the imbalance. Routes of fluid loss and gain Fluid can leave the body in many ways. Fluid can enter the body as preformed water, ingested food and drink and to a lesser extent as metabolic water which is produced as a by-product of aerobic respiration (cellular respiration) and dehydration synthesis. Input A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, sweating and urination. Water generated from the biochemical metabolism of nutrients provides a significant proportion of the daily water requirements for some arthropods and desert animals, but provides only a small fraction of a human's necessary intake. In the normal resting state, input of water through ingested fluids is approximately 1200 ml/day, from ingested foods 1000 ml/day and from aerobic respiration 300 ml/day, totaling 2500 ml/day. Regulation of input Input of water is regulated mainly through ingested fluids, which, in turn, depends on thirst. An insufficiency of water results in an increased osmolarity in the extracellular fluid. This is sensed by osmoreceptors in the organum vasculosum of the lamina terminalis, which trigger thirst. Thirst can to some degree be voluntarily resisted, as during fluid restriction. The human kidneys will normally adjust to varying levels of water intake. The kidneys will require time to adjust to the new water intake level. This can cause someone who drinks a lot of water to become dehydrated more easily than someone who routinely drinks less. Output The majority of fluid output occurs via the urine, approximately 1500 ml/day (approx 1.59 qt/day) in the normal adult resting state. Some fluid is lost through perspiration (part of the body's temperature control mechanism) and as water vapor in exhaled air. These are termed "insensible fluid losses" as they cannot be easily measured. Some sources say insensible losses account for 500 to 650 ml/day (0.5 to 0.6 qt.) of water in adults, while other sources put the minimum value at 800 ml (0.8 qt.). In children, one