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Figure

Secretory mechanisms move substances from the plasma of the peritubular capillary into the fluid of the renal tubule.

Although most of the potassium ions in the glomerular filtrate are actively reabsorbed in the proximal convoluted tubule, some may be secreted in the distal convoluted tubule and collecting duct. During this process, the active reabsorption of sodium ions out of the tubular fluid under the influence of aldosterone produces a negative electrical charge within the tube. Because positively charged potassium ions (K+) are attracted to regions that are negatively charged, these ions move passively through the tubular epithelium and enter the tubular fluid. Potassium ions are also secreted by active processes (fig. 20.22).

To summarize, urine forms as a result of the following:

• Glomerular filtration of materials from blood plasma.

• Reabsorption of substances, including glucose; water; urea; proteins; creatine; amino, lactic, citric, and uric acids; and phosphate, sulfate, calcium, potassium, and sodium ions.

• Secretion of substances, including penicillin, histamine, phenobarbital, hydrogen ions, ammonia, and potassium ions.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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