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OBJECTIVES

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  • Describe how the excretion of salt and water supports the function of the cardiovascular system.

  • Name the major regulators of sodium excretion.

  • Describe the systemic renin-angiotensin-aldosterone system and where its components are formed.

  • State the major actions of angiotensin II.

  • State the major actions of aldosterone.

  • Describe the three major regulators of renin secretion.

  • Describe the functions of the macula densa.

  • Define tubuloglomerular feedback and describe the mechanism for tubuloglomerular feedback and autoregulation of glomerular filtration rate.

  • State the origin of atrial natriuretic peptides, the stimulus for their secretion, and their effect on sodium reabsorption and glomerular filtration rate.

  • Describe the origin of antidiuretic hormone and the major controls of its secretion.

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THE GOALS OF REGULATION

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Total body sodium balance is crucially important to terrestrial animals. Because it is so important, overlapping and often redundant mechanisms have evolved to regulate total body sodium. As an example of this redundancy, neural input plays a significant role in regulating sodium transport, yet denervation of the kidneys or renal transplant does not abolish the ability of the kidneys to regulate sodium excretion. In addition, the specifics of how sodium is regulated depend on the simultaneous regulation of other substances. Although regulation of sodium excretion is admittedly complex, we can greatly simplify a description of this regulation by realizing there is a limited, and highly logical set of goals that these regulatory pathways accomplish. By focusing on these goals it is far easier to fit the details into the “big picture.”

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Image not available.While everything the kidneys do is important in the long run, the immediate goal of regulating sodium and water is to promote the proper functioning of the cardiovascular system. But it must do so without compromising the regulation of other substances and the excretion of organic waste. The kidneys promote functioning of the cardiovascular system by maintaining ECF volume and keeping ECF osmolality within narrow limits. Since volume is almost entirely accounted for by water, and most of the ECF solute is sodium and its anions, the task of maintaining ECF volume and osmolality boils down to the co-regulation of sodium and water. Later in this chapter we will explore the cardiovascular connection in more detail; but for now we wish to present a general principle governing how the nephron as a whole regulates sodium excretion.

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The kidneys always filter far more sodium than is excreted, and it might seem logical to control sodium excretion by varying the amount filtered, that is, vary GFR. While GFR is not absolutely constant, variation in GFR is not a major mechanism to regulate sodium excretion. The total amount of filtrate produced each day, and hence the amount of sodium entering the tubule, is kept within limits regardless of whether the body is conserving or getting rid of excess sodium. This constancy ensures that variations in sodium excretion do not interfere with the filtration of organic ...

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