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OBJECTIVES

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  • List approximate percentages of sodium reabsorbed in major tubular segments.

  • List approximate percentages of water reabsorbed in major tubular segments.

  • Describe proximal tubule sodium reabsorption, including the functions of the apical membrane sodium entry mechanisms and the basolateral Na-K-ATPase.

  • Explain why chloride reabsorption is coupled with sodium reabsorption, and list the major pathways of proximal tubule chloride reabsorption.

  • State the maximum and minimum values of urine osmolality.

  • Define osmotic diuresis and water diuresis.

  • Explain why there is always an obligatory water loss.

  • Describe the handling of sodium by the descending and ascending limbs, distal tubule, and collecting-duct system.

  • Describe the role of sodium-potassium-2 chloride symporters in the thick ascending limb.

  • Describe the handling of water by descending and ascending limbs, distal tubule, and collecting-duct system.

  • Describe the process of “separating salt from water” and why this is required to excrete either concentrated or dilute urine.

  • Describe how antidiuretic hormone affects water and urea reabsorption.

  • Describe the characteristics of the medullary osmotic gradient.

  • Explain the role of the thick ascending limb, urea recycling, and medullary blood flow in generating the medullary osmotic gradient.

  • State why the medullary osmotic gradient is partially “washed out” during a water diuresis.

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OVERVIEW

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This and Chapter 7 are devoted entirely to the renal handling of sodium, chloride, and water. We cover them as a group because their amounts in the body and transport in the kidney are interrelated. First, water constitutes the major fraction of the body volume, specifically including the blood volume, which makes it, in one sense, the most important of all the substances handled by the kidneys. Second, sodium and chloride together account for most of the osmotic content of the extracellular fluid, and therefore its osmolality. Third, the movements of sodium and chloride are mechanistically linked because electroneutrality requires that the movement of sodium, a cation, must be accompanied by the equivalent movement of an anion. As described in Chapter 7, these substances play a huge role in the function of the cardiovascular system and are subject to important, but sometimes rather involved, regulation.

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Body Fluid Compartments

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Image not available.About 60% of the body weight is made up of water, which is distributed into various aqueous spaces in proportion to their osmotic content. The collective volume of all the cells in the body is called the intracellular fluid (ICF). It contains roughly two-thirds of the body osmotic content, and therefore two-thirds of the water. The remaining one-third of the osmotic content and water is called the extracellular fluid (ECF). It is mostly interstitial fluid (about three-fourth of the ECF) and blood plasma (about one-fourth of the ECF). Because of the ease with which water crosses most cell membranes (see Chapter 4), the ECF and ICF are in osmotic equilibrium. The total of the two volumes varies with gain and loss of water, while the relative proportion of volume in each ...

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