Integrated Control Of Breathing

Question 1

How can we get rid of carbon dioxide?

Answer 1

Increase ventilation

Question 2

What is pH proportional to?

Answer 2

pH§ [HCO3-]/PaCO2

In changes of carbon dioxide levels, unless the hydrogen carbonate ion concentration changes in proportion with the PaCO2, acidosis or alkalosis will occur.

Question 3

What causes respiratory acidosis?

Answer 3

1. Reduced ventilation/hypoventilation due to e.g disease
2. This causes PaCO2 to rise causing hypercapnia
3. This causes hydrogen carbonate ion concentration to go up, lowering the pH causing acidosis

The rise in carbon dioxide and drop in pH is detected by chemoreceptors causing increased respiratory rate causing hyperventilation to reduce carbon dioxide levels back to normal.

In chronic hypoventilation, hydrogen carbonate ion excretion must decrease to maintain normal pH.

Real real acidosis occurs when compensatory mechanism or chemoreceptor feedback doesn’t work.

Question 4

What are the causes of hypoventilation?

Answer 4

1. Airway obstruction
   a. asphyxia, choking
   b. obstructive sleep apnoea
   c. increased airway resistance e.g asthma, COPD
2. Problems with initiation of breathing (so airways are fine)
   a. drug overdose by opioids as it affects brainstem sending signals to muscles
   b. stroke
   c. neuromuscular problems such as motor neurone disease

Question 5

What causes respiratory alkalosis?

Answer 5

1. Increased ventilation/hyperventilation due to e.g disease
2. This causes lowered carbon dioxide levels called hypocapnia
3. This reduces the hydrogen ion concentration hence increasing the pH

To make up for this, chemoreceptors stop sending signals to brainstem which reduces respiratory rate which in turn reduces ventilation rate.

In chronic hyperventilation, hydrogen carbonate ions excretion increases from the kidneys to maintain normal pH.

Question 6

What causes hyperventilation?

Answer 6

Extreme emotional states from higher emotional centres e.g panic attacks.

1. Anxiety trigger causes panic attack
2. Causes increased ventilation without increased metabolic demand - this is hyperventilation, decreasing carbon dioxide levels. This causes respiratory alkalosis.

Question 7

Give an example of respiratory alkalosis?

Answer 7

Altitude

1. At high altitude, partial pressures of oxygen are lower
2. So ventilation rate increases to maintain oxygen levels in the blood (by hypoxic drive)
3. Increased ventilation causes reduced carbon dioxide levels in blood to decrease causing respiratory alkalosis

Question 8

What causes metabolic acidosis?

Answer 8

Either addition of hydrogen ions (which react with hydrogen carbonate ion) or increased excretion of hydrogen carbonate ions.

Unless carbon dioxide levels change in proportion with hydrogen carbonate ions then acidosis will occur.
Respiratory compensation occurs almost instantly but it might not be sufficient to prevent disturbances in pH.

Question 9

Give example of metabolic acidosis

Answer 9

Sepsis

1. Infection which creates an immune response
2. This leads to septic shock
3. This causes reduced oxygen delivery to tissues (hypoxia)
4. This causes lactic acid production and drop in pH

You can have respiratory compensation which increases oxygen delivery and increases lactic acid conversion.
(Septic patients hyperventilate to get rid of excess carbon dioxide.)

Question 10

Give other causes of metabolic acidosis

Answer 10

1. Diabetic ketoacidosis:
   Increased fatty acid release from liver due to insulin deficiency; increased acidic ketone body production.
2. Renal failure e.g decreased hydrogen carbonate ion reabsorption in proximal convulated tubule
3. Diarrhoea e.g decreased hydrogen carbonate ion absorption in colon.

Question 11

What causes metabolic alkalosis?

Answer 11

Removal of hydrogen ions (so less to react with hydrogencarbonate ions to form carbonic acid) or increased absorption of hydrogencarbonate ions.

Unless carbon dioxide levels change in proportion with hydrogen carbonate ions, alkalosis will occur.
Respiratory compensation will occur instantly but may not be sufficient to prevent disturbance in pH.

Question 12

Give examples of things that can cause metabolic alkalosis

Answer 12

1. Diuretics (changes to hydrogen ion concentration and hydrogen carbonate ion reabsorption) as it affects ion exchange balance in tubules causing insufficient excretion of hydrogen carbonate ion.
2. Antacids (increased hydrogen carbonate ion consumption)
3. Vomiting (loss of hydrogen ions in stomach acid)

Question 13

Look at at slideshows for example questions. Very important.

Answer 13

Look at slideshows

Question 14

What is the respiratory response to exercise?

Answer 14

Minute ventilation of lungs rapidly increase after exercise begins. In healthy individuals, partial pressure of carbon dioxide doesn’t rise in exercise. In healthy individuals, there’s increase in ventilation before the extra carbon dioxide produced by muscles can increase PaCO2. So this increases ventilation before carbon dioxide levels are high, causing more carbon dioxide to be removed, causing pH to rise.

Since ventilation is increasing before the pH of the blood gets low hence before cerebrospinal fluid pH gets low, it is not caused by the chemoreceptors.

One hypothesis:
When you start to move the muscles involved in exercise, these muscles send signals to the brainstem informing that they’ve started exercise and that removal of carbon dioxide is needed soon so it causes you to start hyperventilating now.