M4 S5

Question 1

tubular secretion 2

Answer 1

• Tubular secretion involves the movement of substances from the
  peritubular capillaries to the tubule lumen, and thus is an additional pathway, other than glomerular
  filtration, for the removal of substances from the body.
• Various substances undergo tubular secretion, the most important being hydrogen ions, potassium
  ions, and organic anions and cations, many of which are foreign to the body.

Question 2

hydrogen ion secretion 5

Answer 2

• Hydrogen ions can be secreted in the proximal, distal, and collecting tubules.
• The extent to which
  they are secreted in the tubules depends on the acidity of the plasma.
• When too much H+ is present in the plasma, more H+ undergoes tubular secretion.
• Conversely, when
  plasma H+ is low, tubular secretion of H+ decreases.
• Renal H+ secretion plays a key role in the regulation of acid-base balance

Question 3

potassium ion secretion 7

Answer 3

• Potassium ions actually undergo both tubular reabsorption and tubular secretion.
• Potassium is freely
  filtered at the glomerulus, but then actively reabsorbed in the proximal tubule.
• The majority of the K+ is reabsorbed in an unregulated fashion.
• However, its secretion is variable and subject to regulation.
• If the plasma concentration of K+ is elevated, K+ is actively secreted in the distal
  and collecting tubules, and vice versa.
• In this manner, the kidneys are actively involved in regulating
  plasma K+ levels.
• Potassium ion secretion in the distal and collecting tubules is an active process dependent upon the Na
  + - K+ -ATP ase pump.

Question 4

potassium ion secretion in relation to Na+ reabsorption 2

Answer 4

• Na+ concentration gradient was maintained by
  pumping Na+ out across the basolateral membrane in exchange for K+.
• Since most of the K+ was
  reabsorbed in the proximal tubules, this makes a concentration gradient such that K+ moves down its
  concentration gradient and passes through K+ channels in the luminal membrane into the tubular fluid

Question 5

Since K+ secretion is linked with Na+ reabsorption by the N a+ - 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? 5

Answer 5

• The answer lies in the location of the passive K+ channels.
• In the distal and collecting tubules, the K+ channels through which K+ passively diffuses are concentrated in the luminal membrane, providing a route for K+ pumped into the cell to exit into the lumen, thus being secreted.
• However, in the reabsorbing tubular segments, these channels are located primarily in the basolateral membrane.
• As a result, K+ pumped into the cell from the interstitial space simply moves back out into the interstitial space through these channels.
• This K+ recycling permits the ongoing operation of the Na+ - K+ -ATPase pump to accomplish reabsorption with no local net effect on K+

Question 6

2 factors that can alter the rate of K+ secretion

Answer 6

1. Na+ and K+
2. effect of H+ secretion

Question 7

Na+ and K+ in K+ secretion control 3

Answer 7

• A rise in plasma K+ directly stimulates the release of aldosterone from the adrenal cortex.
• aldosterone will increase Na+ reabsorption, which in turn means more K+ will be secreted.
• This relationship between Na+ reabsorption and K+ secretion means that stimuli such as a decreased plasma Na+, decreased ECF volume, or a decrease in arterial blood pressure can inadvertently stimulate abnormal K+ secretion to the extent that K+ depletion occurs.

Question 8

effect of H+ secretion in K+ secretion control 5

Answer 8

• Another important factor that regulates K+ secretion is the acid-base status of the body.
• The Na+ -K+ -ATP ase pump on the basolateral membrane of the distal sections of the nephron can readily substitute H+ for K+.
• Because there is a limited number of pumps, when the interstitial concentration of one of these ions increases, it can decrease the rate at which the other is transported.
• When the plasma is too acidic, H+ move into the interstitial space and are transported into the epithelial cell where they passively move into tubular fluid, as already discussed.
• The consequence is that less K+ is secreted, which can lead to an inappropriate high levels of K+

Question 9

2 secretory carriers in the proximal tubule

Answer 9

1. organic anions
2. organic cations

Question 10

3 reasons why the organic ion secretory systems in the proximal tubule are important

Answer 10

1. increasing excretion
2. excrete poorly soluble organic ions
3. removal of foreign compounds

Question 11

Increasing Excretion 2

Answer 11

• Actively adding more organic ions to the tubular fluid, in addition to that which was filtered, can increase the amount of the organic ion excreted compared to glomerular filtration alone.
• This is particularly important for blood-borne chemical messengers such as norepinephrine, histamine, and prostaglandins in order to reduce or limit their biological activity.

Question 12

Excrete Poorly Soluble Organic Ions 4

Answer 12

• Many organic ions are not very soluble (they are hydrophobic) and circulate within the plasma bound to carrier proteins.
• Since large proteins aren’t filtered, only the fraction that isn’t carrier-bound can enter the glomerular filtrate.
• Tubular secretion further removes the small, unbound fraction of the organic ions, which causes even more “unloading” of the organic ion from its carrier molecule.
• In this manner, organic ions highly bound to carrier proteins can be excreted.

Question 13

Removal of Foreign Compounds 2

Answer 13

• In addition to endogenous organic ions, many foreign organic ions such as food additives, drugs, pesticides, environmental pollutants, and more, need to be removed from the body.
• The kidneys routinely remove these compounds, but there are no regulatory mechanisms in place to increase their removal if necessary.