Module 5 Section 3

Question 1

respiratory system and [H+]

Answer 1

• because CO2 leads to H+ generation, the respiratory system plays an important role in acid-base balance by altering pulmonary ventilation to increase or decrease the removal of CO2
• the arterial [H+] is the primary determinnt of respiratory acitivty

Question 2

unbuffered soltuion effect on respiratory system

Answer 2

• when arterial [H+] increases from non-respiratory sources (i.e. metabolic) the respiratory centre in the brain stem is stimulated to increase pulmonary ventilation
• this removal of CO2 means there is less H2CO3 and this, less HCO3- and H+

Question 3

buffered solution effect on respiratory system

Answer 3

• when arterial [H+] decreases, pulmonary ventilation is reduced
• the slower, shallower breathing decreases the exhalation of CO2, allowing it to accumulate in the blood
• this excess CO2 means there is more H2CO3 and thus, more HCO3- and H+

Question 4

respiratory system maintenance of H+

Answer 4

• important for maintaining [H+] in that it removes 100 times as much H+ dervied carbonic acid than the kidneys
• it is considered the second line of defense because it is slower than the chemical buffer systems
• it is not as efficient because in the absence of chemical buffers, the respiratory buffer system can only return the pH to about 50% of the way towards its normal level

Question 5

kidneys and acid-base balance

Answer 5

• the buffer systems are effective at preventing free H+ from contributing to the fluid pH, but this is not the same as removing H+ from the body fluids
• the respiratory system can also help to remove H+, but this system alone is not effective enough to eliminate all the excess H+ from metabolic sources
• the kidneys are particuarly important in removing the H+ produced by suphuric, phosphoric, and lactic acid
• they can also remove some extra H+ produced by carbonic acid

Question 6

3 ways kidneys help to control the pH of ECF

Answer 6

1. the excretion of H+
2. the excretion/reabsorption of HCO3-
3. the secretion of ammonia

Question 7

renal H+ secretion

Answer 7

• almsot all of the H+ that is secreted in the urine comes from tubular secretion in the proximal, distal, and collecting tubules
• at a plasma pH of 7.4, the [H+] is very low so little H+ is actually filtered, and what does get filtered is excreted
• because of the secretion of H+, the pH of urine is acidic with a pH of 6.0

Question 8

step 1 renal H+ secretion

Answer 8

• the secretion of H+ all begins with CO2
• CO2 enters the tubular cells either from the plasma, the tubular fluid, or metabolically produced within the tubular cells

Question 9

step 2 renal H+ secretion

Answer 9

• within the cells, CO2 and H2O, under the influence of intracellular carbonic anhydrase, form H2CO3 which dissociates into H+ and HCO3-

Question 10

step 3 renal H+ secretion

Answer 10

an energy dependent carrier on the luminal membrane will then transport H+ into the tubular fluid

Question 11

control of the rate of H+ secretion

Answer 11

• the secretion of H+ is directly related to the acid-base status of the ECF, there is no neural or hormonal control
• when the [H+] passing through the peritubular capillaries is greater than normal, the tubular cells increase the secretion of H+
• when plasma [CO2] rises, more H+ is secreted, while less H+ s secreted when plasma [CO2] decreases
• because of this dual regulation, the kidneys are able to adjust H+ secretion both from carbonic and non-carbonic acid sources

Question 12

reabsorption of filtered HCO3-

Answer 12

• the renal regulation of [HCO3-] is an important element of acid-base balance
• the kidneys regulate plasma [HCO3-] in 2 ways

1. through reabsorption of HCO3- back into the plasma
2. addition of “new” HCO3- to the plasma

• even though HCO3- is freely filterable, the luminal membranes are impermeable to HCO3-, so its reabsorption is indirect

Question 13

renal HCO3- reabsorption - step 1

Answer 13

HCO3- in the tubular fluid combines with secreted H+ to form H2CO3 then breaks down into CO2 and H2O, both of which can cross the luminal membranes

Question 14

renal HCO3- reabsorption - step 2

Answer 14

once inside a tubular cell, carbonic anhydrase converts the CO2 and H2O back into H2CO3, which then freely dissociates into HCO3- and H+

Question 15

renal HCO3- reabsorption - step 3

Answer 15

• HCO3- can cross the basolateral membrane so it leaves the cell and H+ is again secreted
• a greater amount of H+ is secreted than HCO3- filtered
• this means all of the filtered HCO3- is normally reabsorbed, as H+ is available to combine with it to form highly absorbable CO2

Question 16

additional of “new” HCO3- to the plasma

Answer 16

• tubular cells can also produce “new” HCO3-, meaning that the H+ that is excreted is coupled with the addition of new HCO3- to the plasma
• this is in contrast to the process where secreted H+ is coupled with HCO3- reabsorption and not excreted

Question 17

how “new” HCO3- is added to the plasma - step 1

Answer 17

• CO2 from the plasma and tubular cell metabolism, along with hydroxyl radical from the dissociation of H2O is converted into HCO3- within the tubular cells where it is transpsorted across the basolateral membrane into the plasma

Question 18

how “new” HCO3- is added to the plasma - step 2

Answer 18

H+ is released from the dissociation of water and this H+ is then secreted into the tubular lumen where it combines with the urinary buffers, usually basic phosphate, and then excreted from the body

Question 19

predict how changes in plasma [H+] would alter the secretion of H+ and [HCO3-]

Answer 19

• when plasma [H+] increases above normal, there is an increased secretion of H+ along with complete reabsorption of HCO3- and the formation of new HCO3-
• this results in a decreased plasma [H+] and an increased plasma [HCO3-]
• when plasma decreases below normal, there is a decreased secretion of H+ and a partial reabsorption of HCO3- with excess excretion in the urine
• this results in increased plasma [H+] and decreased plasma [HCO3-]

Question 20

urinary buffers

Answer 20

• the body produces an incredible excess of H+ that must be excreted in the urine
• there are limits to how much H+ can be excreted into the tubular fluid
• the tubular cells can secrete H+ until the tubular fluid is around pH4.5 after which it cannot actively secrete more H+
• in the absense of urinary buffer systems, this would only allow for the secretion of 1% of all daily H+ generated by the body
• urinary buffer systems can remove the free H+ from the tubular fluid so it does not contribute to tubular acidity

Question 21

2 important urinary buffers

Answer 21

1. phosphate
2. ammonia

Question 22

phosphate

Answer 22

• dietary basic phosphate is freely filtered for elimination
• once in the tubular fluid, any H+ that it buffers will be excreted from the body
• because the primary purpose of basic phosphate filtration is to remove excess phosphate from the body, it is not really regulating tubular acidity
• since there is no mechanism to increase the amount of basic phosphate added to the tubular fluid, it has a limited buffering capacity

Question 23

ammonia

Answer 23

• under acidic conditions, when the phosphate buffer system has been overhwelmed, the tubular cells secret ammonia (NH3) into the tubular fluid
• the NH3 reacts with H+ to form ammonium ion (NH4+)
• NH4+ is not reabsorbed so it is excreted in the urine, thereby removing an H+ from the body
• NH3 is actively synthesized and secreted by the tubular cells proportionally to the amount of excess H+