Section 5 Flashcards
What is tubular secretion?
Tubular secretion is the process involving the movement of substances from the peritubular capillaries to the tubule lumen, opposite to tubular reabsorption.
It provides an additional pathway, alongside glomerular filtration, for the REMOVAL of substances from the body.
Major substances undergoing tubular secretion include hydrogen ions, potassium ions, and various organic anions and cations, many of which are foreign to the body.
How are hydrogen ions secreted in the kidneys, and what factors influence this process?
Hydrogen ions can be secreted in the proximal, distal, and collecting tubules of the kidneys. The extent of their secretion depends on the acidity of the plasma.
When there is an excess of hydrogen ions in the plasma, more hydrogen ions are secreted into the tubules. Conversely, when the plasma concentration of hydrogen ions is low, tubular secretion of hydrogen ions decreases. Renal hydrogen ion secretion is crucial for regulating acid-base balance in the body.
How do potassium ions undergo tubular reabsorption and secretion in the kidneys?
Potassium ions are freely filtered at the glomerulus and are actively reabsorbed in the proximal tubule.
The majority of potassium ions are reabsorbed in an unregulated manner. However, their secretion is variable and subject to regulation.
When the plasma concentration of potassium ions is elevated, they are actively secreted in the distal and collecting tubules, and vice versa.
This secretion process in the distal and collecting tubules is dependent on the Na+-K+-ATPase pump.
Since most of the potassium ions are reabsorbed in the proximal tubules, a concentration gradient is established, allowing potassium ions to move down their concentration gradient and pass through potassium channels in the luminal membrane into the tubular fluid.
Since K+ secretion is linked with Na+ reabsorption by the Na+-K+-ATPase pump, why isn’t K+ secreted throughout the Na+-reabsorbing segments of the tubule, instead of taking place only in the distal parts of the nephron?
The passive diffusion of potassium ions (K+) is facilitated by potassium channels. In the distal and collecting tubules, these channels are concentrated in the luminal membrane, allowing K+ pumped into the cell to exit into the lumen and be secreted.
However, in the reabsorbing tubular segments where Na+ reabsorption occurs, potassium channels are primarily located in the basolateral membrane. Consequently, K+ pumped into the cell from the interstitial space moves back out into the interstitial space through these channels, resulting in K+ RECYCLING and no local net effect on K+ secretion despite Na+ reabsorption.
What factors control potassium (K+) secretion?
- Plasma Na+ and K+ Levels: An increase in plasma K+ directly stimulates the release of aldosterone, leading to increased Na+ reabsorption and subsequent K+ secretion.
- Effect of H+ Secretion: The acid-base status of the body also regulates K+ secretion. The Na+-K+-ATPase pump on the basolateral membrane can substitute H+ for K+. When the plasma is too acidic, H+ ions are transported into the epithelial cell, leading to decreased K+ secretion and potentially elevated K+ levels.
What are the two types of secretory carriers in the proximal tubule?
One is for organic anions, and the other is for organic cations
What are the functions of the organic ion secretory systems in the proximal tubule?
The organic ion secretory systems in the proximal tubule serve three main functions:
- Increasing Excretion: Actively adding more organic ions to the tubular fluid increases the amount of organic ions excreted compared to glomerular filtration alone, helping to reduce or limit the biological activity of blood-borne chemical messengers.
- Excreting Poorly Soluble Organic Ions: Many organic ions are hydrophobic and circulate bound to carrier proteins in the plasma. Tubular secretion removes the unbound fraction of these ions, enhancing their removal from the body.
- Removal of Foreign Compounds: The kidneys remove foreign organic ions such as food additives, drugs, pesticides, and environmental pollutants, even though there are no regulatory mechanisms to increase their removal if necessary.
T or F: In the proximal tubule, roughly 67% of Na+ is actively reabsorbed.
True
T or F: No filtered glucose is reabsorbed within the proximal tubule.
False
T or F: All H2O is reabsorbed within the distal tubule and collecting duct.
False
T or F: There are variable amounts of PO4(3-) reabsorbed within the proximal tubule
True
