Tubular secretion

Tubular secretion is the transfer of substances from the peritubular capillaries into the renal tubule for excretion in urine. This process is particularly important for the regulation of potassium and hydrogen ions in the body; it is also responsible for removal of many organic compounds from the body. These may include metabolic wastes as well as foreign compounds, including drugs such as penicillin. Most substances are secreted by secondary active transport.

Potassium ion secretion. Potassium ions are secreted in the distal tubule and the collecting duct. These ions diffuse down their concentration gradient from the peritubular capillaries into the interstitial fluid. They are then actively transported up their concentration gradient into the tubular epithelial cells by way of the Na+, K+ pump in the basolateral membrane. Finally, potassium ions exit the epithelial cells by passive diffusion through K+ channels in the luminal membrane and enter tubular fluid to be excreted in the urine.

Potassium secretion is enhanced by aldosterone. As the concentration of K+ ions in the extracellular fluid increases, the secretion of aldosterone from the adrenal cortex also increases. The mechanism of action of aldosterone involves an increase in the activity of the Na+, K+ pump in the basolateral membrane. Furthermore, aldosterone enhances formation of K+ channels in the luminal membrane.

Hydrogen ion secretion. Hydrogen ions are secreted in the proximal tubule, distal tubule, and collecting duct. The secretion of hydrogen ions is an important mechanism in acid-base balance. The normal pH of the arterial blood is 7.4. When the plasma becomes acidic, H+ ion secretion increases and when it becomes alkalotic, H+-ion secretion is reduced.

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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