34 - Acid Base Regulation by the Lung

Question 1

What are the three ways that the body controls the pH of the blood? How long does each mechanism take to work?

Answer 1

1 - Buffering system (immediate)
2 - Respiratory compensation (fairly rapid - a few hours)
3 - Renal compensation (slow process - days)

Question 2

What is the main buffer system of the blood?

Answer 2

Bicarbonate buffer system (carbonic acid-bicarbonate)

Question 3

What is the Henderson-Hasselbalch equation?

Answer 3

It allows you to calculate the degree of dissociation of a weak acid at a given pH

pH = pKa + log [A]-/[HA]

A- =  conjugate base
HA = weak acid

Question 4

How do we apply the H-H equation to the bicarbonate buffer system in the body?

Answer 4

• Dissolved CO2 is the acid and its pKa is 6.1 under physiological conditions
• The A- is bicarb
• The HA is dissolved CO2

pH = 6.1 + log ([HCO3-]/[CO2] dissolved)

Note that the dissolved CO2 concentration will not be given to you on lab values - you will get the PCO2 or partial pressure of CO2. You then need to multiply by (0.03) in order to get the CO2 dissolved concentration. This is because of Henry’s law CO2 (d) = kPCO2 and k = 0.03 at body temperature.

Question 5

What is the normal pH, PCO2 and [HCO3-]

Answer 5

Normal

• pH = 7.4
• PCO2 = 40 torr/40 mmHg
• [HCO3-] = 24 mM

Remember that the dissolved CO2 is not reported as concentration from a clinical lab, but rather as the partial pressure

Question 6

What is the normal [HCO3-]/[CO2(d)]?

Answer 6

20

24/(0.03) x 40 = 20

Need to MAINTAIN this ratio… Either by staying at the same values, or by the buffer system responding to an increase in one by an increase in the other

Question 7

What does it mean that the body is an open system?

Answer 7

• From the Henderson-Hasselbalch equation, it is apparent that manipulation of the concentration of CO2 and HCO3 - will influence pH
• The body is an open system and the levels of carbon dioxide and HCO3 - can be varied independently of each other
• The lungs and the kidneys play pivotal roles, regulating the CO2 and HCO3 - contents, respectively
• We will only discuss lungs (not kidneys) today

Question 8

How does a closed system respond to an increased H+ concentration?

Answer 8

• Suppose sufficient H+ ions are added to the blood to increase the [H+] by 1 mmol
• Bicarb will decrease by 1 mmol because it is consumed by buffering
• minus one on top of the equation, add one on the bottom of the equation and you have a large drop in pH
• Acidic pH is dangerous

Question 9

How does a open system respond to an increased H+ concentration?

Answer 9

• Provided that the respiration is normal, the lungs will maintain the PCO2 at 40 torr
• This means there will be a decrease of one on top but no change on bottom
• There is a minimal drop in pH
• Not to the point where it is dangerous

Question 10

What is the main effect of respiration on pH?

Answer 10

As discussed above, provided that respiration is normal, changes in [H+] will not lead to changes in [CO2]dissolved.

Question 11

What is the effect of hypoventilation on PCO2?

Answer 11

• a decrease in the respiration rate (hypoventilation) will cause an increase in PCO2 and therefore an increase in the CO2 dissolved

Question 12

What does an increase in the CO2 dissolved lead to?

Answer 12

An increase in [CO2]dissolved results in an increase in the formation of H2CO3, which dissociates to yield HCO3- and H+.

Question 13

Overall, what does hypoventilation lead to?

Answer 13

Therefore, hypoventilation tends to cause a drop in blood pH. The resulting condition is known as respiratory acidosis

Question 14

What is the effect of hyperventilation on PCO2 and CO2 dissolved?

Answer 14

increase in the respiration rate (hyperventilation) will cause a decrease in PCO2, a decrease in [CO2]dissolved, and ultimately a decrease in [H+].

Question 15

Overall, what does hyperventilation lead to?

Answer 15

Hyperventilation therefore tends to cause an increase in blood pH. The resulting condition is known as respiratory alkalosis.

Question 16

What are respiratory acid/base imbalances?

Answer 16

Changes in pH that result from alterations in the PCO2 are collectively referred to as respiratory acid/base imbalances

Question 17

What are metabolic acid/base imbalances?

Answer 17

In contrast, changes in pH resulting from alterations in the [HCO3-] are referred to as metabolic acid/base imbalances

Question 18

Define respiratory acidosis

Answer 18

• Decrease in respiration
• Not blowing off enough CO2
• Blood becomes acidic
• Drop in pH

Question 19

Define respiratory alkalosis

Answer 19

• Increase in respiration
• Blowing off too much CO2
• Blood becomes basic
• Increase in pH

Question 20

Define metabolic acidosis

Answer 20

• Loss of HCO3-
• Not enough bicarb in blood
• Blood becomes acidic
• Drop in pH

Question 21

Define metabolic alkalosis

Answer 21

• Increase in HCO3- concentration
• Too much bicarb in blood
• Blood becomes basic
• Increase in pH

Question 22

What is compensation?

Answer 22

• Changes in one buffer component, either the amount of bicarb or the amount of dissolved CO2
• Compensation can be done by the respiratory system (influences dissolved CO2) or renal system (influences bicarb)

Question 23

You will be tested on pH vs HCO3- graphs

Answer 23

Go look at them!

Pg. 5-9 in handout