Gas Transport in the Blood Flashcards
What is the alveolar PO2?
100 mmHg or 13.3 kPa
What is the alveolar PCO2?
40 mmHg or 5.3 kPa
How does hypoventilation affect the alveolar partial pressures of O2 and CO2?
- PO2 decreases 30 mmHg,
- PCO2 increases to approximately 100 mmHg
How does hyperventilation affect the alveolar partial pressures of O2 and CO2?
- PO2 increases to approximately 120 mmHg
- PCO2 decreases to approximately 20 mmHg
Why are there only small fluctuations in alveolar partial pressure during normal breathing, rather than large fluctuations?
- Amount of oxygen that enters the alveoli with each breath is roughly equal to the amount of oxygen which enters the blood
- Amount of air that enters the lung with each breath is only about 10% of the total lung volume at the end of inspiration
Why does the partial pressure of oxygen change from 160 mmHg (in air) to 100 mmHg in the lungs?
- Dilution via water vapour - reduces the concentration of oxygen by displacing oxygen molecules per unit volume of air
- Anatomical Dead Space
- Residual Volume - Air that has experienced a certain level of gas exchange then mixes with fresh air, thus reducing (slightly) the partial pressure of O2.
- Gas exchange at the alveoli
What is the arterial PO2 after oxygenation?
95 mmHg (can vary from 85-100 mmHg)
What is the arterial PCO2 after gas exchange in the lungs?
40 mmHg (can vary from 35-45 mmHg)
What is the PO2 in peripheral cells?
40 mmHg - facilitates partial pressure gradient
What is the PCO2 in peripheral cells?
46 mmHg - facilitates diffusion of CO2 out of the cells
What is the venous PO2?
46 mmHg
What is the venous PCO2?
46 mmHg
How is oxygen transported in the blood?
Oxygen dissolved in plasma (approx 3 ml/L) and oxygen bound to haemoglobin (approx 197 ml/L).
What determines the amount of oxygen that binds to haemoglobin?
- PO2 of surrounding plasma
- Number of Hb binding sites
What determines the binding site saturation on haemoglobin?
Plasma PO2 - the higher the PO2, the more oxygen binds to Hb
What is SpO2 when plasma PO2 is 100 mmHg?
Approximately 98% - haemoglobin can pick up maximum amount it can carry when the alveoli are properly ventilated
What does the oxygen dissociation curve demonstrate in terms of falling levels of PO2?
SpO2 is still maintained at high concentrations down to PO2 levels of 60 mmHg (still approximately 90%). After this, saturation begins to decline more sharply, meaning that small decreases in PO2 result in larger decrements in saturation.
At 40 mmHg, saturations are sitting at about 75%
What is the structure of haemoglobin?
Haemoglobin is a multi sub-unit globular protein, composed of four alpha-helical chains. Each chain is folded in a particular conformation due to hydrogen bonding within and between the chains.
Each protein subunit is closely associated with a non-protein haem group, which has a central iron ion held in a heterocyclic ring known as porphyrin. The iron ion is where oxygen reversibly binds to.
What volume of O2 does normal plasma hold?
3 ml/L
What happens to the haemoglobin molecule as oxygen binds to it?
It alters the conformation of the Hb subunit next to it, thus increasing its affinity to bind O2 (sequential binding; allosteric)
How is the partial pressure gradient between plasma and alveoli maintained?
Haemoglobin sequesters O2 from plasma