Demonstrate the effect of pCO2 on plasma pH if bicarbonate concentration doesn't change

If [HCO3-] remains unchanged: - pCO2 increase = pH falls (notably) - pCO2 decrease = pH rises (notably)

S4) Chemical Control of Breathing Flashcards by shreya Arun

What is the concentration of bicarbonate in the blood?

22–26mmol/L

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What is the average PaO₂ in the blood?

9.3–13.3kPa

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What is the average PaCO₂ in the blood?

4.7–6.0kPa

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Identify 2 functions of the respiratory system

Maintain oxygen and carbon dioxide partial pressure gradients to optimise transfer
Regulate pH of ECF

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What is hypoxia?

Hypoxia is a fall in pO₂

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What is hypercapnia?

Hypercapnia is a rise in pCO₂

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What is hypocapnia?

Hypocapnia is a fall in pCO₂

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What is hyperventilation?

Hyperventilation is when ventilation increases without change in metabolism

respiratory alkalosis
if HCO3 secereted
compensated respiratory alkalosis (fully or compensated depends if pH Is back to normal)

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What is hypoventilation?

Hypoventilation is when ventilation decreases without change in metabolism

respiratory acidosis
increase Hco3
compensated respiratory acidosis

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How is hypocapnia caused?

pO₂ changes without a change in pCO₂
correction of pO₂ will cause pCO₂to drop

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Why does a control system need to avoid marked hypoxia?

Oxygen-Haemoglobin dissociation curve is flat from approx. 8kPa
Hence, pO₂ can fall considerably before saturation is markedly effected

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What equation is used to make calculations in the carbonic acid-bicarbonate buffer system?

pH = pK + log [HCO₃^-] / [H₂CO₃^-]

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Demonstrate the effect of pCO₂on plasma pH if bicarbonate concentration doesn’t change

If [HCO₃^-] remains unchanged:

pCO₂ increase = pH falls (notably)
pCO₂ decrease = pH rises (notably)

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What happens if the pH rises above 7.6?

Free calcium concentration drops which leads to tetany

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What happens if the pH falls below 7.0 ?

Enzymes become denatured

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In two steps, explain how respiratory acidosis occurs

⇒ Hypoventilation leads to an increase in pCO₂

⇒ Hypercapnia leads to a fall in plasma pH

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In two steps, explain how respiratory alkalosis occurs

⇒ Hyperventilation leads to a decrease in pCO₂

⇒ Hypocapnia leads to a rise in plasma pH

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How do the kidneys compensate for respiratory acidosis?

Respiratory acidosis is compensated by the kidneys increasing [HCO₃^-]

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How do the kidneys compensate for respiratory alkalosis?

Respiratory alkalosis is compensated by the kidneys decreasing [HCO₃^-]

How long does it take the kidney to compensate for pH changes?

2-3 days

In two steps, explain how metabolic acidosis occurs

⇒ Tissues produce acid & this reacts with HCO₃^-

⇒ The fall in [HCO₃^-] leads to a fall in pH

How can metabolic acidosis be compensated for?

Change ventilation:

Increased ventilation lowers pCO₂
pH is restored to normal

In two steps, explain how metabolic alkalosis occurs

⇒ If plasma [HCO₃^-] rises e.g. after vomiting

⇒ Plasma pH rises

How can metabolic alkalosis be compensated for?

Compensated to a degree by decreasing ventilation

State 3 principles which allow compensation for pH changes to happen?

- Plasma pH depends on the **ratio** of [HCO₃^-] to pCO₂and not their **absolute values** - **Respiratory** driven changes in pH compensated by the **kidney** - **Metabolic** changes in pH compensated by **breathing**

Compare and contrast the pH, pCO₂and HCO₃^- in the following conditions: - Metabolic acidosis - Respiratory acidosis - Metabolic alkalosis - Respiratory alkalosis

Where are the peripheral chemoreceptors located?

- Carotid bodies - Aortic bodies they detect changes in pCo2 and P02 impulse is carried via glossopharyngeal nerves and vagus nerves

Explain how peripheral chemoreceptors are sensitive to large changes in pO₂

Stimulates: - Increased breathing - Changes in heart rate - Changes in blood flow distribution (increasing flow to brain & kidneys)

Compare and contrast peripheral and central chemoreceptors in terms of pCO₂ sensitivity

- **Peripheral chemoreceptors** will detect changes but are relatively insensitive to pCO₂ - **Central chemoreceptors** in the medulla of the brain are much more sensitive to pCO₂

What do the central chemoreceptors do?

**Detect changes in arterial pCO₂:** - Small rises in pCO₂ increase ventilation - Small falls in pCO₂ decrease ventilation

Ilustrate the negative feedback control of breathing by central chemoreceptors

Central chemoreceptors respond to changes in the pH of cerebro-spinal fluid (CSF). Explain the pH control of the CSF

CSF separated from blood by the blood-brain barrier: - CSF [HCO₃^-] controlled by **choroid plexus cells** - CSF pCO₂ determined by **arterial pCO₂**

What determines the pH of CSF?

CSF pH is determined by ratio of [HCO₃^-] to pCO₂

Why is [HCO₃^-] constant in the short term?

Blood Brain Barrier is impermeable to HCO₃^-

What is the signficance of this fixed [bicarbonate]?

- Falls in pCO₂ lead to rises in CSF pH (**decreases ventilation**) - Rises in pCO₂ lead to falls in CSF pH (**increases ventilation**)

What is the role of [HCO₃^-] in pH of CSF?

- CSF [HCO₃^-] determines which pCO₂ is seen as ‘normal’ CSF pH (‘sets’ the control system) - It can be ‘reset’ by changing CSF [HCO₃^-] (choroid plexus cells)

Describe the events that lead to persistent hypoxia

- Hypoxia detected by peripheral chemoreceptors which **increases ventilation** - But pCO₂ falls further which **decreases ventilation** - So, CSF composition compensates for the altered pCO₂ and **choroid plexus cells** selectively add H⁺/ HCO₃^-into CSF - Central chemoreceptors **“accept” pCO₂ as normal**

Describe the events that lead to persistent hypercapnia

- Hypoxia and hypercapnia lead to **respiratory acidosis** - **Decreased pH of CSF** cause both chemoreceptors to stimulate breathing - Acidic pH is undesirable for neurons so **choroid plexus adjusts** pH of CSF by adding HCO₃^- - Central chemoreceptors “accept” the **high pCO₂ as normal**

which parts of the brain are responsible for the respiratory rate

* dorsal neurones → medulla * ventral neurones → ventral lateral side of medulla * pneumotaxic centre of neurones → dorsally on pons (sends inhibitory pulses to to slow down inspiration)

how do stretch receptors prevent over inflation of lungs

* stretch receptors are located in bronchi and bronchioles * they transmit info via vagus nerve to brainstem * prevent over inflation

which group of neurones set the basic respiratory rate

dorsal neurones on the medulla

which group of neurones set the basic respiratory rate

dorsal neurones on the medulla

what are baroreceptors and where are they located

they detect arteriole BP found in carotid sinus and aortic arch

what controls body response to oxygen

* carotid and aortic bodies * stimulated by decrease in oxygen * only respond to significant falls * they increase tidal volume and rate of respiration * direct more blood to the brain and they kidneys and increase heart rate

what is controls the body response to co2

aortic and carotid bodies only respond quickly and only to large changes in PcCo2

what sense the bodies change In Ph

carotid bodies

what is the order we follow to check the ABG

1. pH 2. pCo2 3. bicarb levels