Section 3

Question 1

What role does the respiratory system play in acid-base balance, and how does it achieve this?

Answer 1

The respiratory system plays an important role in acid-base balance by altering pulmonary ventilation to increase or decrease the removal of CO2. (CO2 leads to H+ generation)

Question 2

How do changes in arterial [H+] affect the respiratory system?

Answer 2

Unbuffered Solution: When arterial [H+] increases from non-respiratory sources, the respiratory center in the brain stem is stimulated to increase pulmonary ventilation, leading to increased gas exchange and the removal of excess CO2.

The removal of CO2 means there is less H2CO3 and thus less HCO3- and H+

Question 3

Why is the respiratory system critically important for maintaining [H+], in comparison to the kidneys?

Answer 3

The respiratory system removes 100 times as much H+ derived from carbonic acid than the kidneys, making it critically important for maintaining [H+].

Question 4

Why is the respiratory system considered the second line of defense in pH regulation?

Answer 4

The respiratory system is considered the second line of defense because it is slower than chemical buffer systems and less efficient. Without chemical buffers, the respiratory buffer system can only return the pH to about 50% of the way towards its normal level.

Question 5

What is the role of the kidneys in acid-base balance?

Answer 5

The kidneys play a crucial role in acid-base balance by removing excess H+ from the body fluids, particularly those produced by sulfuric, phosphoric, and lactic acid, as well as some extra H+ produced by carbonic acid.

Question 6

How do the kidneys help to control the pH of extracellular fluid? (3 ways)

Answer 6

Excretion of H+: The kidneys excrete excess H+ from the body fluids.

Excretion/Reabsorption of HCO3-: The kidneys regulate the levels of bicarbonate (HCO3-) by either excreting or reabsorbing it as needed to maintain pH balance.

Secretion of Ammonia: The kidneys secrete ammonia, which helps in buffering excess H+ ions.

Question 7

Why are the kidneys important in removing H+ from metabolic sources?

Answer 7

While the respiratory system can help remove some H+, it is not effective enough to eliminate all excess H+ from metabolic sources. The kidneys play a crucial role in removing H+ produced by metabolic processes such as the breakdown of sulfuric, phosphoric, and lactic acid.

Question 8

Where does almost all of the H+ excreted in the urine come from?

Answer 8

Almost all of the H+ excreted in the urine comes from tubular secretion in the proximal, distal, and collecting tubules of the kidney.

Question 9

Describe the process of secretion (3 steps).

Answer 9

1. 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.
2. Within the cells, CO2 and H2O, under the influence of intracellular carbonic anhydrase, form H2CO3, which dissociates into H+ and HCO3-.
3. An energy dependent carrier on the luminal membrane will then transport H+ into the tubular fluid

Question 10

How is the secretion of H+ regulated in the kidneys? Hormonal or neural?

Answer 10

The secretion of H+ in the kidneys is directly related to the acid-base status of the extracellular fluid (ECF), with no neural or hormonal control involved.

Question 11

T or F: Secretion of H+ is under neural/hormonal control.

Answer 11

FALSE, it is directly related to the acid-base state of the ECF, with no neural or hormonal control involved.

Question 12

What happens to the rate of H+ secretion when the [H+] in the peritubular capillaries is greater than normal?

Answer 12

When the [H+] passing through the peritubular capillaries is greater than normal, the tubular cells increase the secretion of H+.

Vice versa is also applicable

Question 13

How does the rate of H+ secretion change with variations in plasma [H+]?

Answer 13

Increased plasma [H+]: Tubular cells increase the secretion of H+.

Decreased plasma [H+]: Tubular cells decrease the secretion of H+.

Question 14

What effect does plasma [CO2] have on the rate of H+ secretion?

Answer 14

Increased plasma [CO2]: More H+ is secreted.
Decreased plasma [CO2]: Less H+ is secreted.

Question 15

Why is the renal regulation of [HCO3-] important for acid-base balance?

Answer 15

The renal regulation of [HCO3-] is crucial for acid-base balance as the kidneys regulate plasma [HCO3-] through two ways:

• through reabsorption of HCO3- back into the plasma
• addition of “new” HCO3- to the plasma

Question 16

T or F: HCO3- is freely filterable.

Answer 16

True

Question 17

T or F: the luminal membranes of the tubules are permeable to HCO3-

Answer 17

False, the luminal membranes are impermeable to HCO3- , so its reabsorption is indirect.

Question 18

How does renal HCO3- reabsorption occur? (3 steps)

Answer 18

1. 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.
2. Inside a tubular cell, carbonic anhydrase converts the CO2 and H2O back into H2CO3, which then freely dissociates into HCO3- and H+.
3. HCO3- can cross the basolateral membrane, leaving 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 19

Why is HCO3- reabsorption considered indirect?

Answer 19

HCO3- reabsorption is considered indirect because although HCO3- is freely filterable, the luminal membranes are impermeable to it, so its reabsorption involves conversion to CO2, which can then cross the luminal membranes.

Question 20

How does renal HCO3- reabsorption contribute to acid-base balance?

Answer 20

Renal HCO3- reabsorption ensures that all of the filtered HCO3- is normally reabsorbed, maintaining plasma [HCO3-] levels and contributing to acid-base balance by ensuring the availability of bicarbonate ions in the blood.

Question 21

What is meant by the addition of “new” HCO3- to the plasma in the kidneys?

Answer 21

The addition of “new” HCO3- to the plasma refers to the process in which H+ that is excreted by the kidneys is coupled with the addition of new HCO3- to the plasma, rather than being coupled with HCO3- reabsorption.

Question 22

How is “new” HCO3- added to the plasma in the kidneys?

Answer 22

1. Formation of HCO3-: CO2 from the plasma and tubular cell metabolism, along with hydroxyl radicals from the dissociation of H2O, are converted into HCO3- within the tubular cells.
2. Transport into Plasma: HCO3- is then transported across the basolateral membrane into the plasma.

Question 23

What happens to H+ during the process of adding “new” HCO3- to the plasma?

Answer 23

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

Question 24

How does the addition of “new” HCO3- contribute to acid-base balance?

Answer 24

The addition of “new” HCO3- to the plasma increases the plasma [HCO3-] levels, helping to maintain acid-base balance in the body by providing additional bicarbonate ions for buffering excess H+.

Question 25

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

Answer 25

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-]

Question 26

Why is it important for the body to excrete excess H+ in the urine?

Answer 26

The body produces an incredible excess of H+ that must be excreted in the urine to maintain acid-base balance.

Question 27

What is the limit to how much H+ can be actively secreted into the tubular fluid?

Answer 27

Tubular cells can actively secrete H+ into the tubular fluid until the pH of the fluid reaches around 4.5, after which they cannot actively secrete more H+.

Question 28

What is the primary purpose of urinary buffer systems?

Answer 28

The primary purpose of urinary buffer systems is to remove free H+ from the tubular fluid so it does not contribute to tubular acidity.

Question 29

What are the two important urinary buffers?

Answer 29

1. Phosphate: 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
2. Ammonia: Tubular cells secrete ammonia into the tubular fluid under acidic conditions. Ammonia reacts with H+ to form ammonium ions, which are excreted in the urine, thereby removing H+ from the body. NH3 is actively synthesized and secreted by the tubular cells proportionally to the amount of excess H+

Question 30

Which of the following has a limited capacity to resist acid/base imbalances:

a) Renal
b) Respiratory
c) Buffers

Answer 30

c) buffers

Question 31

Which of the following can prevent free H+ from contributing to fluid pH

a) Renal
b) Respiratory
c) Buffers

Answer 31

c) buffers

Question 32

Which of the following can remove excess H+ from metabolic sources:

a) Renal
b) Respiratory
c) Buffers

Answer 32

a) renal

Question 33

Which of the following can remove H+ derived from carbonic acid:

a) Renal
b) Respiratory
c) Buffers

Answer 33

b) respiratory

Question 34

Which of the following is considered the second line of defense:

a) Renal
b) Respiratory
c) Buffers

Answer 34

b) respiratory

Question 35

Which of the following involves regulating plasma [HCO3-]:

a) Renal
b) Respiratory
c) Buffers

Answer 35

a) renal