Obligatory water loss

This volume, referred to as obligatory water loss, is 420 ml water/day. In other words, 420 ml of water will be lost in the urine each day in order to excrete metabolic waste products regardless of water intake. 

In persons with normal kidney function, sodium balance is maintained at a sodium intake of 120 to 150 mEq/day. The fractional excretion of sodium (FENa) is approximately 1% to 3%. Water balance is also maintained, with a normal range of urinary osmolality of 50 to 1200 mOsm/L. In patients with severe CKD (Stages 4 and 5), sodium balance is achieved, but results in a volume-expanded state. FENa may increase to as much as 10% to 20%, possibly due to increased concentrations of atrial natriuretic peptide. An osmotic diuresis occurs with an increase in FENa leading to obligatory water losses and impairment in the kidney s ability to dilute or concentrate urine (urinary osmolality is often fixed at that of plasma or approximately 300 mOsm/L). Nocturia is present relatively early in the course of CKD (Stage 3) secondary to the defect in urinary concentrating ability. Total renal sodium excretion decreases despite an increase in sodium excretion by remaining nephrons. Volume overload with pulmonary edema can result, but the most common manifestation of increased intravascular volume is systemic hypertension. ... 

Water is lost Irom the body as urine and as obligatory insensible losses from the skin and lungs. 

The nutritional requirement is a reflection of obligatory losses (maintenance) and the needs of growth, pregnancy, and lactation. Abnormal losses owing to disease, or in animals such as humans and horses which sweat extensively, raise the requirement. The impact of equine sweating is different from that in humans. Human sweat always contains sodium at concentrations well below plasma levels (and when aldosterone secretion is raised, levels of sweat sodium fall very low) horse sweat is hypertonic but this helps to offset the osmotic effect of the increased respiratory water loss during exertion, i.e., it may be a defense against hypernatremia, rather than a potential cause of sodium depletion. Similarly hypernatremia in many species induces dehydration natriuresis - an appropriate defense. 

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