639

CHAPTER

SECTION VIII RENAL PHYSIOLOGY

38

Renal Function &

Micturition

OBJECTIVES

After reading this chapter, you should be able to:

■

Describe the morphology of a typical nephron and its blood supply.

■

Define autoregulation and list the major theories advanced to explain autoregula-

tion in the kidneys.

■

Define glomerular filtration rate, describe how it can be measured, and list the

major factors affecting it.

■

Outline tubular handling of Na

+

and water.

■

Discuss tubular reabsorption and secretion of glucose and K

+

.

■

Describe how the countercurrent mechanism in the kidney operates to produce

hypertonic or hypotonic urine.

■

List the major classes of diuretics and how each operates to increase urine flow.

■

Describe the voiding reflex and draw a cystometrogram.

INTRODUCTION

In the kidneys, a fluid that resembles plasma is filtered

through the glomerular capillaries into the renal tubules
(glo-

merular filtration).
As this glomerular filtrate passes down

the tubules, its volume is reduced and its composition altered

by the processes of
tubular reabsorption
(removal of water

and solutes from the tubular fluid) and
tubular secretion

(secretion of solutes into the tubular fluid) to form the urine

that enters the renal pelvis. A comparison of the composition

of the plasma and an average urine specimen illustrates the

magnitude of some of these changes (Table 38–1). It empha-

sizes the manner by which water and important electrolytes

and metabolites are conserved while wastes are eliminated in

the urine. Furthermore, the composition of the urine can be

varied to maintain whole body fluid homeostasis (extracellu-

lar fluid [ECF]). This is achieved via many homeostatic regu-

latory mechanisms that function to change the amount of

water and solutes in the urine. From the renal pelvis, the urine

passes to the bladder and is expelled to the exterior by the

process of urination, or

micturition.

The kidneys are also

endocrine organs, making kinins (see Chapter 33) and 1, 25-

dihydroxycholecalciferol (see Chapter 23), and making and

secreting renin (see Chapter 39).