6: Basic Renal Processes for Sodium, Chloride, and Water

OBJECTIVES

• ▸ 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.

OVERVIEW

This and Chapter 7 are devoted entirely to the renal handling of sodium, chloride, and water. Sodium and chloride are crucial substances because they account for most of the osmotic content of the extracellular fluid while water constitutes the major part of the body volume. And water is the solvent for all dissolved solutes.

As described in Chapter 7, these substances play a huge role in the function of the cardiovascular system and are subject to complex regulation.

Body Fluid Compartments

Approximately 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 and blood plasma. 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 2 volumes varies with gain and loss of water, whereas the relative proportion in each compartment is influenced by gain and loss of sodium. Additions or losses of sodium from the body are mostly to or from the ECF because the actions of cellular Na-K-ATPases prevent major changes in intracellular sodium concentration.¹ If the addition or loss of fluid is isotonic sodium, only the volume of the ECF is ...