About 90% of the filtered potassium is reabsorbed in the early regions of the nephron (mainly from the proximal tubule). In order for potassium to appear in the urine, it must be secreted into later regions of the nephron tubule. Secretion of potassium occurs in the parts of the nephron that are sensitive to aldosterone—that is, in the late distal tubule and cortical collecting duct (fig. 17.24).
As Na+ is reabsorbed in these regions of the nephron, the lumen of the tubule becomes more negatively charged (-50 mV) compared to the basolateral side. This potential difference then drives the secretion of K+ into the tubule. The transport carrier for Na+ is separate from the transport carrier for K+, so, although Na+ is reabsorbed in exchange for the secretion of K+, there is not a 1:1 exchange of these ions.
The amount of K+ secretion into the late distal tubule and cortical collecting duct depends on: (1) the amount of Na+ delivered to these regions of the nephron; and (2) the amount of aldo-sterone secreted. If the blood concentration of K+ rises, this will stimulate increased aldosterone secretion from the adrenal cortex. The aldosterone then stimulates increased reabsorption of Na+ and, as a result, increased secretion of K+.
Some diuretic drugs inhibit Na+ reabsorption in the loop of Henle and therefore increase the delivery of Na+ to the distal tubule. This results in an increased reabsorption of Na+ and se-
■ Figure 17.24 Potassium is reabsorbed and secreted. Potassium (K+) is almost completely reabsorbed in the proximal tubule, but under aldosterone stimulation it is secreted into the cortical portion of the collecting duct. All of the K+ in urine is derived from secretion rather than from filtration.
cretion of K+ in the cortical collecting duct. People who take these diuretics, therefore, tend to have excessive K+ loss in the urine. The actions of different types of diuretics are discussed in the section "Clinical Applications" at the end of this chapter.
The body cannot get rid of excess K+ in the absence of aldosterone-stimulated secretion of K+ into the cortical (j collecting ducts. Indeed, when both adrenal glands are removed from an experimental animal, the hyperkalemia (high blood K+) that results can produce fatal cardiac arrhythmias. Abnormally low plasma K+ concentrations (hypokalemia), as might result from excessive aldosterone secretion or from diuretic drugs, can produce arrhythmias as well as muscle weakness.
Cortical portion of collecting duct
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.