The regulation of water and sodium balance is a crucial function of the kidneys. They maintain the osmolality and volume of body fluids within a narrow range by controlling the excretion of water and NaCl. The body is composed of two major compartments of water - intracellular fluid (ICF) and extracellular fluid (ECF), which are in osmotic equilibrium. The kidneys play a key role in regulating water balance and are primarily responsible for eliminating water from the body. Other routes of water loss include evaporation from the skin and respiratory passages, sweating, and water loss through the gastrointestinal tract. However, the renal excretion of water is the only route that can be tightly regulated to maintain overall water balance. When water intake is low or water losses increase, the kidneys conserve water by producing a small volume of concentrated urine. Conversely, when water intake is high, a large volume of diluted urine is produced. The kidneys have the ability to regulate water excretion separately from the excretion of solutes, which is essential for maintaining homeostasis.
What is the effective circulating volume, how is it influenced by changes in Na+ balance, and how does it influence renal Na+ excretion?
10.
What are the mechanisms by which the body monitors the effective circulating volume?
11.
What are the major signals acting on the kidneys to alter their excretion of Na+?
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How do changes in extracellular fluid volume alter Na+ transport in the different segments of the nephron, and how do these changes in transport regulate renal Na+ excretion?
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What are the mechanisms involved in formation of edema, and what role do the kidneys play in this process?
he kidneys maintain the osmolality and volume of the body fluids within a narrow range by regulating excretion of water and NaCl, respectively. This chapter discusses the regulation of renal water excretion (urine concentration and dilution) and NaCl excretion. The composition and volumes of the various body fluid compartments are reviewed in
Control of Body Fluid Osmolality: Urine Concentration and Dilution , water constitutes approximately 60% of the healthy adult human body. Body water is divided into two major compartments—intracellular fluid (ICF) and extracellular fluid (ECF)—that are in osmotic equilibrium because of the high permeability of most cell membranes to water via aquaporins (e.g., AQP1). Water intake into the body generally occurs orally. This may be water contained in beverages as well as water generated during metabolism of ingested foods (e.g., carbohydrates). In many clinical situations, intravenous infusion is an important route of water entry.
The kidneys are responsible for regulating water balance and under most conditions are the major route for elimination of water from the body (
Table 35.1 ). Other routes of water loss from the body include evaporation from cells of the skin and respiratory passages. Collectively, water loss by these routes is termed insensible water loss because the individual is unaware of its occurrence. Production of sweat accounts for the loss of additional water. Water loss by this mechanism can increase dramatically in a hot environment, with exercise, or in the presence of fever ( 35.2 ). Finally, water can be lost from the gastrointestinal tract. Fecal water loss is normally small (≈100 mL/day) but can increase dramatically with diarrhea (e.g., 20 L/ day with cholera). Vomiting can also cause gastrointestinal water losses.
Although water loss from sweating, defecation, and evaporation from the lungs and skin can vary depending on the environmental conditions or during pathological conditions, loss of water by these routes cannot be regulated. In contrast, renal excretion of water is tightly regulated to maintain whole-body water balance. Maintenance of water balance requires that water intake and loss from the body be precisely matched. If intake exceeds losses, positive water balance exists. Conversely, when intake is less than losses, for review of (with its anions Cl− and HCO3 −), these disorders also result steady-state balance). in alterations in plasma or serum [Na+] (
Fig. 35.2 ). One
When water intake is low or water losses increase, the kidneys conserve water by producing a small volume of urine that is hyperosmotic with respect to plasma. When water intake is high, a large volume of hypoosmotic urine is produced. In a normal individual, urine osmolality (Uosm) can vary from approximately 50 to 1200 mOsm/kg H2O, and the corresponding urine volume can vary from approximately 18 L/day to 0.5 L/day. Importantly the kidneys can regulate excretion of water separately from excretion of total solute (
Fig. 35.1 ). The ability to regulate water excretion separate from excretion of solutes (e.g., Na+ , K+ , urea, etc.) is necessary for survival because it allows water balance to be achieved without upsetting the other homeostatic functions of the kidneys.