Integrated Control of Ventilation

Question 1

How do the lung and kidneys maintain blood pH?

Answer 1

The lungs and kidneys maintain blood pH homeostasis by regulating PaCO2 and [HCO3-] respectively

Question 2

What disorders can arise from acid-base balance?

Answer 2

As blood pH is proportional to the ratio of [HCO3-] to PaCO2, disruptions in pH homeostasis may result from respiratory (CO2) or metabolic (HCO3-) dysfunction (↓pH= acidosis, ↑pH= alkalosis)
Low pH accompanied by high PaCO2 is termed respiratory acidosis, as hypoventilation is responsible for the increase in CO2 levels and decrease pH
Low ventilation= high PACO2= high PaCO2 (hypercapnia)= high [H+] which = low pH (acidosis)
High pH accompanied by low PaCO2 is termed respiratory alkalosis, as hyperventilation is responsible for the decrease in CO2 levels and increased pH
High ventilation= low PACO2= PaCO2 (hypocapnia)= low [H+] which= high pH (alkalosis)
Low pH accompanies by low [HCO3-] is termed metabolic acidosis, as excessive metabolic acid production and/or increased excretion of HCO3- is responsible for decreased pH
High pH accompanied by high [HCO3-] is termed metabolic alkalosis, as reduced metabolic acid production or reduced excretion of HCO3- is responsible for the increased pH

Question 3

What are the mechanisms underlying respiratory compensation to acid-base disturbances?

Answer 3

Acidosis involves increased [H+] within the blood, resulting in increased activation of respiratory chemoreceptors and increased respiratory activity (compensatory hyperventilation).
The resulting increase in CO2 removal from the blood/body increases blood pH until normal levels are re-established via negative feedback.
Alkalosis involves decreased [H+] within the blood, resulting in decreased activation of respiratory chemoreceptors and decreased respiratory activity (compensatory hypoventilation)
The resulting accumulation of CO2 within the blood/body decreases blood pH until normal levels are re-established via negative feedback

Question 4

What imbalances can come from the respiratory system?

Answer 4

Respiratory acidosis is associated with pathological states involving chronic respiratory failure, hypoventilation and accumulation of CO2 e.g. COPD
Respiratory alkalosis is associated with pathological states involving hyperventilation and excessive removal of CO2 e.g. anxiety- or altitude-induced hyperventilation

Question 5

What imbalances can come from the metabolic system?

Answer 5

Metabolic acidosis is associated with pathological states involving either excessive acid production (sepsis, ketoacidosis) or excessive excretion of HCO3 (renal failure)
Metabolic alkalosis is associated with pathological states involving excessive consumption of basic substances (e.g. antacid abuse), excessive excretion of acid from the body (vomiting), deficient absorption of HCO3 from the gastrointestinal tract (chronic diarrhoea), or excessive reabsorption of HCO3 (renal dysfunction)

Question 6

how does hyperventilation from anxiety cause respiratory alkalosis?

Answer 6

Causes of hyperventilation; anxiety
Anxiety trigger → panic attack
↓
Increased ventilation (tachypnoea) without increased metabolic demand
↓
Hyperventilation → Decreased PaCO2
↓
Respiratory alkalosis

Question 7

How can altitude cause respiratory alkalosis?

Answer 7

At sea level, PAtm ≈ 100kPa
100kPa x 21% O2 ≈ 21kPa, CO2 ≈ 0kPa
↓
At  8000ft, PAtm ≈ 75kPa
75kPa x 21% O2 ≈ 15kPa, CO2 ≈ 0kPa
↓
↑ alveolar ventilation to try and maintain normal PAO2 & PaO2 (via hypoxic drive)
↓
Increased ventilation = ↓PaCO2
↓
Respiratory alkalosis

Question 8

How can sepsis cause metabolic acidosis (lactic acidosis specifically)?

Answer 8

Infection
↓
Pathological immune response
↓
Septic shock
↓
↓oxygen delivery to tissues (hypoxia)
↓
↑Lactic acid production & ↓pH
↓
Respiratory compensation to ↑O2 delivery and ↑lactic acid conversion

Question 9

What is acidosis induced hyperkalaemia (how does it occur)?

Answer 9

The homeostasis of potassium levels within the blood is dependent on pH
Cells have multiple mechanisms for indirectly exchanging H+ for K+ (hydrogen efflux for potassium influx); one mechanism is via initial exchange of H+ and Na+ (hydrogen efflux, sodium influx), followed by Na+ and K+ exchange (sodium efflux, potassium influx)- the net effect is that K+ ions enter the cell in exchange for H+ leaving
The initial step relies on the presence of a H+ conc. gradient- (H+ conc. cell> than outside cell)
In the event of acidosis, this process breaks down as pH falls, representing an increse in blood and extracellular [H+]
The overall effect is that uptake of K+ is reduced in response to acidosis, leading to potassium accumulation in the extracellular fluid in blood
K+ has critical roles in membrane potential and muscle function, so cells in skeletal muscle and cardiac pacemaker cells are adversely affected, seen through the symptoms of cardiac arrhythmia and muscle weakness

Question 10

What is alkalosis-induced vasoconstriction, what are the symptoms associated with it?

Answer 10

The contractile tone of vascular smooth muscle is sensitive to changes in pH, with cerebral arteries being particularly sensitive
CO2 (via conversion to H+) acts as a vasodilator, relaxing smooth muscle and increasing blood flow.
Examples of the effects of acid-base disorders: alkalosis-induced cerebral vasoconstriction
CO2 (via H+) acts as a vasodilator in blood vessels (cerebral arteries particularly sensitive)
In the event of hyperventilation-induced alkalosis, reductions in PaCO2 and H+ (increased pH) induce vasoconstriction of cerebral arteries, reducing blood flow to brain Hyperventilation = ↓CO2 and ↓H+ (alkalosis)
Alkalosis causes vasoconstriction of cerebral arteries
↓ cerebral blood flow = headache, lightheadedness, confusion, seizures