![]() The symptoms of an increase in P osm are also primarily neurological and include lethargy, weakness, seizures, coma, and even death. When P osm is increased (i.e., hyperosmolality), water is lost from cells. For example, a rapid fall in P osm can alter neurological function and thereby cause nausea, malaise, headache, confusion, lethargy, seizures, and coma. Symptoms associated with hypoosmolality are related primarily to swelling of brain cells. In the clinical setting, hypoosmolality (a reduction in plasma osmolality) shifts water into cells, and this process results in cell swelling. The control of vasopressin secretion and its important role in regulating excretion of water by the kidneys are also explained (see also Chapter 40). ![]() The following sections discuss the mechanisms by which the kidneys excrete either hypoosmotic (dilute) or hyperosmotic (concentrated) urine. Indeed, this ability is necessary for survival because it allows water balance to be achieved without upsetting the other homeostatic functions of the kidneys. Under steady-state conditions, the kidneys control water excretion independently of their ability to control the excretion of various other physiologically important substances such as Na +, K +, and urea. As described later, changes in Na + balance result in alterations in the volume of ECF, not its osmolality. However, the problem most often relates to water balance, not Na + balance. When an abnormal plasma is observed in an individual, it is tempting to suspect a problem in Na + balance. Because the major determinant of plasma osmolality is Na + (with its anions Cl − and HCO 3 −), these disorders also result in alterations in plasma. It is important to recognize that disorders in water balance are manifested by alterations in body fluid osmolality, which are usually measured by changes in plasma osmolality (P osm). In a normal individual, urine osmolality (U osm) can vary from approximately 50 to 1200 mOsm/kg H 2O, and the corresponding urine volume can vary from approximately 18 to 0.5 L/day. When water intake is high, a large volume of hypoosmotic urine is produced. When water intake is low or water loss increases, the kidneys conserve water by producing a small volume of urine that is hyperosmotic with respect to plasma. Conversely, if intake is less than loss, negative water balance exists. If intake exceeds loss, positive water balance exists. Maintenance of water balance requires that water intake and loss from the body be precisely matched. In contrast, the renal excretion of water is tightly regulated to maintain whole-body water balance. Table 34-2 Effect of Environmental Temperature and Exercise on Water Loss and Intake (mL/day) in AdultsĪlthough water loss from sweating, defecation, and evaporation from the lungs and skin can vary with environmental conditions or during pathological conditions, loss of water by these routes cannot be regulated. ![]() * Fluid intake varies widely for both social and cultural reasons. Table 34-1 Normal Routes of Water Gain and Loss in Adults at Room Temperature (23° C) Vomiting can also cause gastrointestinal water loss. Fecal water loss is normally small (≈100 mL/day) but can increase dramatically with diarrhea (e.g., 20 L/day with cholera). Finally, water can be lost from the gastrointestinal tract. Water loss by this mechanism can increase dramatically in a hot environment, with exercise, or in the presence of fever ( Table 34-2). The production of sweat accounts for the loss of additional water. Collectively, water loss by these routes is termed insensible water loss because the individual is unaware of its occurrence. Other routes of water loss from the body include evaporation from cells of the skin and respiratory passages. The kidneys are responsible for regulating water balance and under most conditions are the major route for elimination of water from the body (Table 34-1). However, in clinical situations, intravenous infusion is an important route of water entry. Water intake into the body generally occurs orally. Body water is divided into two compartments (i.e., intracellular fluid and extracellular fluid ), which are in osmotic equilibrium. ![]() CONTROL OF BODY FLUID OSMOLALITY: URINE CONCENTRATION AND DILUTIONĪs described in Chapter 2, water constitutes approximately 60% of the healthy adult human body.
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