24B12: Exam Report
Describe the physiological factors that affect the arterial partial pressure of carbon dioxide.
(Measurement factors are not required).
30% of candidates passed this question.
This question was broad and required content from several areas of the syllabus. Given this breadth less detail expected.
A good answer used a structured approach discussing CO2 production, carriage in the blood, CO2 elimination and the control of ventilation, providing specific detail on how each of these affected arterial partial pressures of CO2.
“CO2 production” should include an overview of CO2 as a product of aerobic respiration, an approximation of the rate of production, how it varies with metabolic activity and finally the concept of RQ with its determinants.
“Carriage” should include an estimate of CO2 content, how PaCO2 depends upon the amount dissolved (as per Henry`s Law), how CO2 is carried in the blood enabling more total content relative to the amount dissolved, the influence of the Haldane effect, and how CO22 is more soluble at lower temperature lowering PaCO2.
“Elimination” should emphasise the importance of minute ventilation, dead space (including effective dead space due to high VQ lung units), and the effect of rebreathing CO2).
“Control of ventilation” required a brief overview of the sensors, including central and peripheral chemoreceptors, integrators and effectors that influence the minute ventilation and ultimately PaCO2.
F8iii / 24B12: Describe the physiological factors that affect the arterial partial pressure of carbon dioxide.
Definition
PaCO2 is proportional to
\( \Large \frac{CO_{2}\text{ production}}{\text {Alveolar Ventilation}} \)
Normal value: 35-45 mm Hg
CO2 production
- CO2 is a product of aerobic respiration- produced during the citric acid cycle in the cytoplasm
- Rate of production: ~ 200 milliliters per minute (ml/min)
Factors affecting rate of production
- Diet / substrate consumed:
- Respiratory quotient (RQ) is the volume of carbon dioxide released over the volume of oxygen absorbed during respiration
- RQ for fat, protein, and anaerobe is 0.7, 0.8, and 0, respectively
- Activity:
- Exercise increases CO2 production
- BMR/Temperature:
- Increased metabolic rate / hyperthermia increases CO2 production
Carriage in the blood
- Total body CO2 content- ~120L
- In arterial blood, CO2 is carried in 3 forms
- Bicarbonate- 90%
- Carbamino compounds- 5%
- Dissolved- 5%
- As per Henry`s Law, the amount of dissolved gas in a liquid is directly proportional at equilibrium to its partial pressure above the liquid.
- In other words, higher the solubility of gas > lower the partial pressure
- Other forms of carriage enable higher CO2 content compared to the dissolved form
- Influence of the Haldane effect
- Oxygen increases the ionization of nitrogen groups, reducing the capacity of the globin chain to carry carbon dioxide as carbamino compounds > deoxyhaemoglobin can carry more CO2
- Lower temperature
- Increases the solubility of CO2– lowering PaCO2
CO2 elimination
- Main route of elimination
- Respiration via lungs
- Factors increasing MV> decrease PaCO2
- Fever
- Sepsis,
- Hyperthermia
- Exercise,
- Fear
- Stress
- Pain
- Pregnancy
- Factors decreasing MV> increase PaCO2
- Increase in dead space/ high VQ lung units. Eg. Pulmonary embolism
- Rebreathing CO2
Diagram
From Deranged Physiology by Alex Yartsev
College also recommends
- “Control of ventilation” required a brief overview of the sensors, including central and peripheral chemoreceptors, integrators and effectors that influence the minute ventilation and ultimately PaCO2
- Central Controller: Respiratory Centre > drives the rate and volume of ventilation
- Sensors:
- Central Chemoreceptors – Ventral Surface of Medulla – stimulated by H+
- Peripheral Chemoreceptors – located in Carotid & Aortic Bodies – respond to paO2, paCO2 and pH
- Plus:
- Lung receptors
- J receptors
- Bronchial C fibres
- Baroreceptors
- Hormones
- Pain & Temperature
- Effectors:
- INSP: diaphragm, external intercostals
- EXPRN: internal intercostals, abdominal muscles
(Don’t know how to fit it all in 10 mins!)
Author: Nazma Navilehal Rajasab