1
Q
respiratory system and H+ 2
A
- 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.
- If you recall from Module 03, the arterial [H+] is the primary determinant of respiratory activity.
2
Q
how changes in arterial [H+] affect the respiratory system in an unbuffered solution 3
A
- 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 increased gas exchange allows excess CO2 to be exhaled.
- This removal of CO2 means there is less H2CO3 and thus, more HCO3- and H+.
3
Q
how changes in arterial [H+] affect the respiratory system in a buffered solution 3
A
- 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+.
4
Q
respiratory system as a second line of defense 3
A
- The respiratory system is critically important for maintaining [H+] in that it removes 100 times as much H+ derived from carbonic acid than the kidneys.
- The respiratory system is considered the second line of defense because it is slower than the chemical buffer systems.
- Also, 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.
5
Q
kidneys and acid-base balance 5
A
- As already mentioned, 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.
- As we just learned, the respiratory system can also help to remove H+, but this system alone is not effective enough to eliminate all of the excess H+ from metabolic sources.
- This is where the kidneys come in.
- The kidneys are particularly important in removing the H+ produced by sulphuric, phosphoric, and lactic acid.
- they can also remove some extra H+ produced by carbonic acid.
6
Q
three ways the kidneys help to control the pH of extracellular fluid
A
1.The excretion of H+
2. The excretion/reabsorption of HCO3-
3. The secretion of ammonia
7
Q
renal H+ secretion 3
A
- Almost all of the H+ that is excreted 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.
- However, because of the secretion of H+, the pH of urine is acidic with a pH of about 6.0.
8
Q
3 steps of renal H+ secretion
A
- 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.
- Within the cells, CO2 and H2O, under the influence of intracellular carbonic anhydrase, form H2CO3, which dissociates into H+ and HCO3-.
- An energy dependent carrier on the luminal membrane will then transport H+ into the tubular fluid.
9
Q
control of the rate of H+ secretion 5
A
- 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+.
- Conversely, when the plasma [H+] is decreased, the tubular cells decrease the secretion of H+.
- As well, when plasma [CO2] rises, more H+ is secreted, while less H+ is 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.
10
Q
reabsorption of filtered HCO3 4
A
- The renal regulation of [HCO3-] is an important element of acid-base balance.
- The kidneys regulate plasma [HCO3-] in two ways: through reabsorption of HCO3- back into the plasma, and addition of “new” HCO3- to the plasma.
- We will being with the first mechanism.
- Even though HCO3- is freely filterable, the luminal membranes are impermeable to HCO3-, so its reabsorption is indirect.
11
Q
3 steps in renal HCO3- reabsorption
A
- HCO3- in the tubular fluid combines with secreted H+ to form H2CO3. H2CO3 then breaks down into CO2 and H2O, both of which can cross the luminal membranes.
- Once inside a tubular cell, carbonic anhydrase converts the CO2 and H2O back into H2CO3, which then freely dissociates into HCO3- and H+.
- HCO 3- 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.
12
Q
addition of “new” HCO3- is added to the plasma 2
A
- CO2 from the plasma and tubular cell metabolism, along with hydroxyl radicals from water breaking down, are converted into bicarbonate (HCO3-) inside the tubular cells. This bicarbonate is then transported across the basolateral membrane into the plasma.
- Hydrogen ions (H+) are released when water dissociates. These H+ ions are secreted into the tubular lumen, where they combine with urinary buffers (usually phosphate), and are then excreted from the body.
13
Q
predict how changes in plasma [H+] would alter the secretion of H+ and [HCO3-] 4
A
- 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 [H+] 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-].
14
Q
urinary buffers 4
A
- The body produces an incredible excess of H+ that must be excreted in the urine; however, 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 pH 4.5, after which it cannot actively secrete more H+.
- In the absence of urinary buffer systems, this would only allow for the secretion of 1% of all daily H+ generated by the body.
- However, urinary buffer systems can remove the free H+ from the tubular fluid so it does not contribute to tubular acidity.
15
Q
2 important urinary buffers
A
- phosphate
- ammonia
16
Q
phosphate urinary buffer 4
A
- Recall that 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.
17
Q
ammonia urinary buffer 4
A
- Under acidic conditions, when the phosphate buffer system has been overwhelmed, the tubular cells secrete ammonia (N H3) into the tubular fluid.
- The N H3 reacts with H+ to form ammonium ion (N H4+).
- NH4+ is not reabsorbed so it is excreted in the urine, thereby removing an H+ from the body.
- N H3 is actively synthesized and secreted by the tubular cells proportionally to the amount of excess H+.
