21) Integrated control of breathing

Question 1

What maintains blood pH homeostasis?

Answer 1

• Kidneys regulate HCO3- by regulating the excretion/absorption of HCO3-
• Lungs regulate PaCO2 by regulating ventilation

Question 2

How can blood pH change?

Answer 2

• Blood pH is proportional to the ratio of HCO3- to CO2
• Excessive in this ratio can cause respiratory dysfunction (CO2) or metabolic dysfunction (HCO3-)
• If ventilation is too high it can cause alkalosis (too little CO2) and if ventilation is too low it can cause acidosis (too much CO2)
• If there is a high metabolic acid production (e.g. lactic acid) or acid absorption is too high (in the kidneys) it can lead to acidosis and if there is a very low metabolic acid production or acid absorption is too low (in the kidneys) it can lead to alkalosis

Question 3

How might pH be brought back to normal if there is a dysfunction in one system?

Answer 3

• If there is a dysfunction is one system (either metabolic or respiratory) the other system may try to compensate for it
• They do this by changing their function and so brings pH back to normal

Question 4

What is acidosis and alkalosis?

Answer 4

• Acidosis: When blood pH is too low (acidic)

- Alkalosis: When blood pH is too high (alkaline)

Question 5

How does respiratory acidosis occur?

Answer 5

• When ventilation is too low in the body the amount of CO2 being expelled can not cope with the amount of CO2 being made
• This causes CO2 to build up in the alveoli which causes CO2 to build up in the blood (hypercapnia)
• This CO2 reacts with water to form HCO3- ions and H+ ions
• This increase in H+ decreases pH making it more acidic (acidosis)

Question 6

How would the respiratory system deal with respiratory acidosis?

Answer 6

• There is an increased activation of central chemoreceptors which sends more impulses to the brainstem
• This causes an increase in respiratory rate to compensate for the increase in CO2
• This corrects the decrease in ventilation which means more CO2 is removed from the alveoli which reduces the amount of CO2 in the blood resulting in blood pH decreasing

Question 7

What happens if the respiratory system is ineffective in compensating for respiratory acidosis?

Answer 7

• If the respiratory system is not functioning properly (due to pathology) the system will break down and we will not experience the compensation
• Instead the renal (kidneys) system will detect the change in pH and so will increase the absorption of HCO3- and decrease secretion
• This increases HCO3- which will increase pH

Question 8

How is blood pH maintained during chronic-hypoventilation?

Answer 8

• The renal system reduces HCO3- excretion to maintain normal pH

Question 9

How does respiratory alkalosis occur?

Answer 9

• When ventilation is too high the amount of CO2 being expelled from the body is much more than the amount of CO2 being produced
• This causes the level of CO2 in the alveoli to decrease which causes the level of CO2 in the blood to also decrease (hypocapnia)
• This will result in H2CO3 forming more CO2 and H20 causing a decrease in H2CO3
• Then H+ and HCO3- will form more H2CO3 to compensate for the loss
• This means there is a decrease in H+ concentration and so an increase in pH (alkalosis)

Question 10

How would the respiratory system deal with respiratory alkalosis?

Answer 10

• There is less activation of chemoreceptors and so less stimulations are sent to the brainstem
• This means the brainstem will slow the rate of respiration which will counteract the high ventilation rate and bring it down
• This means less CO2 is expelled allowing the levels of pH to decrease to normal

Question 11

What happens if the respiratory system is ineffective in compensating for respiratory alkalosis?

Answer 11

• If the respiratory system is not functioning properly (due to pathology) the system will break down and we will not experience the compensation
• Instead the renal (kidneys) system will detect the change in pH and so will decrease the absorption of HCO3- and increase secretion
• This decreases HCO3- which will decrease pH

Question 12

How is blood pH maintained during chronic-hyperventilation?

Answer 12

• The renal system increases HCO3- excretion to maintain normal pH

Question 13

How does anxiety cause alkalosis?

Answer 13

• When anxiety is triggered it results in panic attacks
• These panic attacks increase ventilation (tachypnoea) without increasing metabolic demand (so CO2 production stays the same)
• Hyperventilation occurs as we remove more CO2 from the body than being produced
• As a result there is a decrease in CO2 concentration in the blood leading to an increase in pH and thus respiratory alkalosis
• The reflex response via chemoreceptors would still be present and stimuli will still be sent from the chemoreceptors to the brainstem
• However due to the strength of the emotional stimulus of the anxiety and panic attack the reflex response is not sufficient to bring breathing rate down.
• As a result breathing rate remains high

Question 14

How is respiratory alkalosis through anxiety attacks treated?

Answer 14

• Getting the patient to breathe into a paper bag.
• This allows CO2 being expired to be recycled and they are breathing in the air they just expelled
• This keeps the CO2 levels at a relatively normal level which prevents alkalosis

Question 15

How does altitude cause respiratory alkalosis?

Answer 15

• As altitude increases there is less oxygen in the air
• This decrease in oxygen means at regular respiratory rates a person can become hypoxaemic
• This hypoxaemia is detected by peripheral chemoreceptors and will increase alveolar ventilation to try and maintain normal levels of O2 in the alveoli and blood
• By increasing ventilation the body decreases levels of CO2 causing the blood pH to decrease and hence leads to respiratory alkalosis

Question 16

How is respiratory alkalosis through altitude increase prevented?

Answer 16

• Climbing altitude at a slower speed

Question 17

How does metabolic acidosis occur?

Answer 17

• Metabolic acid production is excessive or HCO3- levels are too low
• This can be due to the formation of too much lactic acid or other types of acid
• This can also be not enough absorption of HCO3- in the diet or increased excretion from the kidney
• These factors cause HCO3- to decrease leading to an increase in H+
• There is a decrease in pH and hence acidosis
• Respiratory compensation will occur where ventilation increases which causes the pH to return to the normal range

Question 18

How does the body deal with metabolic acidosis?

Answer 18

• A decrease in HCO3- ions would lead to a decrease in pH as the number of H+ ions in the blood is higher than normal
• This decrease in pH will cause CO2 levels to increase
• This will lead to the increased activation of chemoreceptors in the reflex compensation and so increases respiratory rate and ventilation
• This will decrease the amount of CO2 in the blood and so will eventually return pH back to the normal range

Question 19

How does metabolic alkalosis occur?

Answer 19

• Cuased by lower levels of metabolic acid or when levels of HCO3- are excessive
• This includes excessive excretion/removal of metabolic acid
• This also includes excessive absorption of HCO3- or decreased levels of excretion of HCO3- in the kidneys
• These factors cause HCO3- to increase leading to an decrease in H+
• There is an increase in pH and hence alkalosis
• Respiratory compensation will occur where ventilation decreases which causes the pH to return to the normal range

Question 20

How does the body deal with metabolic alkalosis?

Answer 20

• An increase in HCO3- ions would lead to an increase in pH as the number of H+ ions in the blood is lower than normal
• This increase in pH will cause CO2 levels to decrease
• This will lead to the decreased activation of chemoreceptors in the reflex compensation and so decreases respiratory rate and ventilation
• This will increase the amount of CO2 in the blood and so will eventually return pH back to the normal range

Question 21

What is hyperkalaemia?

Answer 21

• A condition where potassium levels in the blood are higher than normal

Question 22

How does an imbalance in acid-base equilibrium cause hyperkalaemia?

Answer 22

• During acidosis there is an increase in extracellular H+ concentration.
• This causes the H+ concentration gradient between the extracellular and intracellular compartments of the cells to decrease
• This means there will be a decreased influx of H+ into cells which will hinder ion movement mechanisms
• This results in excess potassium being moved into the extracellular fluid and in the blood (called acidosis-induced hyperkalaemia)

Question 23

What are the effects of hyperkalaemia on the body?

Answer 23

• Cardiac arrhythmias

- Muscle weakness

Question 24

How does an imbalance in acid-base equilibrium cause cerebral vasoconstriction?

Answer 24

• CO2 (as it is converted into H+) acts as a vasodilator in blood vessels
• Cerebral arteries are more sensitive to CO2 as a vasodilator
• During hyperventilation there are reduced CO2 levels and as a result reduced H+ levels (leading to alkalosis)
• Alkalosis causes vasoconstriction of cerebral arteries and so there is less cerebral blood flow
• A decrease in cerebral blood flow can cause headache, light headedness, confusion and seizures

Question 25

How is increased ventilation during exercise mediated/controlled?

Answer 25

• Amount of CO2 in the blood does not increase during exercise (it may even decrease slightly).
• This is because ventilation increases before extra CO2 is produced by the muscles