Module 4 Section 5

Question 1

tubular secretion

Answer 1

• the opposite of tubular reabsorption, though it occurs via transepithelial transport
• tubular secretion involves the movement of substances from the peritubular capillaries to the tubule lumen, and this 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

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
• 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

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 K+ is reabsorbed in an unregulated fashion
• its secretion is variable and subject to regulation
• if the plasma concnetration of K+ is elevated, K+ is acitvely secreted in the distal and collecting tubules, and vice versa
• 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+-ATPase pump

Question 4

control of K+ secretion - Na+ and K+

Answer 4

• a rise in plasma K+ directly stimulates the release of aldosterone from the adrenal cortex
• for regulation of Na+ reabsorption, aldosterone will icnrease 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 5

control of K+ secretion - effect of H+ secretion

Answer 5

• the Na+-K+-ATPase pump on the basolateral membrane of the distal sections of the nephron can readily substitute H+ for K+
• becasue there is a limited number of pumps, when the interstitial concnetration of one of these ions increases, it can decrease the rate at whcih 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 pasisvely move into tubular fluid
• the consequence is that less K+ is secreted, which can lead to an inappropriate high level of K+

Question 6

secretion of organic anions and cations

Answer 6

• the proximal tubule contains two types of secretory carriers, one for organic anions and another for organic cations
• these organic ion secretory systems are important for 3 main reasons
  1. increasing excretion
  2. excrete poorly soluble organic ions
  3. removal of foreign compounds

Question 7

increasing excretion of organic ions

Answer 7

• 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 NE, histamine, and prostaglandins in order to reduce or limit their biological acitivty

Question 8

excrete poorly soluble organic ions

Answer 8

• many organic ions are not very soluble (hydrophobic) and circulate within the plasma bound to carrier protiens
• since large proteins arent filtered, only the fraction that inst carrier-bound can enter the lomerular 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
• organic ions highly bound to carrier proteins can be excreted

Question 9

removal of foreign compounds

Answer 9

• 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 kidenys routinely remove these compounds, but there are no regulatory mechnaisms in place to increase their removal if necessary