The kidneys are the major route for excretion of NaCl from the body. Only about 10% of the Na+ lost from the body each day does so by nonrenal routes (e.g., in perspiration and feces). As such, the kidneys are critically important in regulating ECFV. Under normal conditions the kidneys keep ECFV constant (a state termed euvolemia) by adjusting the excretion of NaCl to match the amount ingested in the diet. If ingestion exceeds excretion, ECFV increases above normal (volume expansion), whereas the opposite occurs if excretion exceeds ingestion (volume contraction).
The typical diet contains approximately 140 mEq/day of Na+ (8 g of NaCl), and thus daily Na+ excretion in urine is also about 140 mEq/day. However, the kidneys can vary excretion of Na+ over a wide range. Excretion rates as low as 10 mEq/day can be attained when individuals are placed on a low-salt diet. Conversely, the kidneys can increase their excretion rate to more than 1000 mEq/day when challenged by ingestion of a high-salt diet. These changes in Na+ excretion can occur with only modest changes in the ECFV and Na+ content of the body.
The response of the kidneys to abrupt changes in NaCl intake typically takes several hours to several days, depending on the magnitude of the change. During this transition period the intake and excretion of Na+ are not matched as they are in the steady state. Thus the individual experiences either positive Na+ balance (intake > excretion) or negative Na+ balance (intake < excretion). However, by the end of the transition period a new steady state is established and intake once again equals excretion.
This section reviews the physiology of the receptors that monitor ECFV and explains the various signals that act on the kidneys to regulate NaCl excretion and thereby ECFV. In addition, responses of the various portions of the nephron to these signals is considered.
Concept of Effective Circulating Volume , the ECF is subdivided into two compartments: blood plasma and interstitial fluid. Plasma volume is a determinant of vascular volume and thus blood pressure and cardiac output. Maintenance of Na+ balance, and thus ECFV, involves a complex system of sensors and effector signals that act primarily on the kidneys to regulate NaCl excretion. As can be appreciated from the dependency of vascular volume, blood pressure, and cardiac output on ECFV, this complex system is designed to ensure adequate tissue perfusion. Because the primary sensors of this system are located in the large vessels of the vascular system, changes in vascular volume, blood pressure, and cardiac output are the principal factors regulating renal NaCl excretion (discussed later). In a healthy individual, changes in ECFV result in parallel changes in vascular volume, blood pressure, and cardiac output. Thus a decrease in ECFV results in reduced vascular volume, blood pressure, and cardiac output. Conversely, an increase in ECFV results in increased vascular volume, blood pressure, and cardiac output. The degree to which these cardiovascular parameters
CHAPTER 35 Control of Body Fluid Osmolality and Volume change is dependent upon the degree of ECFV contraction or expansion and the effectiveness of cardiovascular reflex mechanisms (see ). When a person is in negative Na+ balance, ECFV is decreased and renal NaCl excretion is reduced. Conversely, with positive Na+ balance there is an increase in ECFV, which results in enhanced renal NaCl excretion (i.e., natriuresis).