Flashcards in Physiology: gas pressures / HB Deck (14):
What is pressure?
force/unit area = N/m²
1 pascal = 1 N/m²
1kPa = 1000N/m²
Commonly expressed in mmHg (close to Torr)
1mmHg = 0.133kPa
1kPa = 7.55mmHg
Atmospheric pressure = 101.3kPa (760mmHg)
What is partial pressure?
Individual pressure exerted by a gas
In a mixture of gases, each gas exerts individual partial pressure (partial pressure independent of other gases in mixture)
Total pressure = sum of component partial pressures
Dalton's Law = PT =P₁+P₂+P₃+P₄
PATM = PN2 +PO2 +PCO2 +PCO +PH2O
O₂ constitutes 21% of air, so if atmospheric pressure ~100kPa, PO₂ = 21%
How is concentration of a gas in solution determined?
[gas] = partial pressure x solubility coefficient
Each gas has individual coefficient (e.g. CO2 ~ 20x more soluble than O2)
Solubility also dependent on factors e.g temperature
What are the values of PaO₂, Hb saturation and O₂ content in arteriole and mixed blood?
PaO₂: 13.3kPa (100mmHg), 5.3kPa (40mmHg)
Hb sat: 97-98%, 75%
O₂ content: 200ml/l, 150ml/l
What are the two forms that oxygen is carried in?
Dissolved in plasma: described by Henry's law - directly related to PaO₂ (PaO₂ = arteriole partial pressure)
Bound to Hb: normal range =
How much O₂ is consumed at rest? [O₂] dissolved in arteriole blood?
How does O₂ enter blood/reach tissues?
Diffuses from alveolar air (PO₂ ~13.3kPa) to blood (venous blood PO₂ = 5.3kPa)
Diffuses from arteriole blood (PO₂ ~13.3kPa) to interstitial fluid (PO₂ ~5.3kPa) to mitochondria (PO₂ ~ 0.1kPa)
Outline the structure of haemoglobin
4 sub-units, each ~17kDa
each subunit = a haem group (porphyrn ring), a globin chain (peptide)
each subunit will bind 1 molecule of O₂
saturated Hb therefore will bind 4O₂
How much oxygen does 1g Hb bind? If [Hb] = 15g/100ml - what is O₂ content?
15 x 1.36 = 24ml/100ml = 204ml/l
Outline the types of globin chains in Hb
Adult haemoglobin (HbA)
2 alpha (141 amino acids), 2 beta (146 amino acids)
HbA₂ ~ 2% of adult Hb
2 alpha, 2 delta
Fetal haemoglobin (HbF) ~ 1% adult Hb
2 alpha, 2 gamma
What is fetal Hb?
2a and 2 gamma chains
embryonic forms replaced by HbF from 13 weeks at gestation
At birth the majority of Hb is still HbF
HbA = 20%, HbF = 80%
Switch to adult forms occurs between 3-6 months old
HbF not fully replaced as approximately 1% of adult Hb
What genetic defects can occur in globin chains?
Thalassaemias: defect in the synthesis of the globin chains
Sickle cell disease: defect in the structure of chains
What is thalassaemia?
Inherited defects in globin chain synthesis
Either a or beta-thalassaemia
Leads to imbalance of chains - decreased erythropoiesis
Increased haemolysis of mature cells