Logic of the Kidney

This chapter provides a short description on the anatomic and functional design of the mammalian kidney and cites examples of regulation of sodium and potassium to describe the complexity of its system. The kidney, to fulfill its homeostatic role, interfaces directly with the intravascular compartment where it modifies the composition and quantity of body fluid via its excretory and endocrine functions. Each human kidney is endowed with approximately 106 nephrons. The bisected surface of the kidney consists of the lighter-colored outer cortex and the darker-appearing inner medulla. The medulla is further divided radially into outer and inner regions with the outer medulla subdivided into outer and inner stripes. There is a drastic morphologic change from the visceral epithelium to the proximal tubule at the exact junction where filtration function ceases and resorptive and secretory functions commence. The proximal tubule can be subdivided anatomically in two ways. The initial convoluted portion (pars convolute) is entirely in the cortex, and the subsequent straight portion (pars recta) is in the cortex and the outer stripe of the outer medulla. The distal convoluted tubule (DCT) starts shortly after the macula densa. The distal convoluted tubule is heterogeneous and can be divided into an early (DCT1) and late portion (DCT2) with functionally but not so much morphologically distinct characteristics. The kidney is responsible for the bulk of potassium excretion but a modest fraction of ingested potassium may be excreted by secretion in the colon. The capacity of the latter to secrete potassium may increase when renal excretory mechanisms fail, but it is insufficient to maintain adequate external balance.