Oxygen Transport and Consumption Flashcards
How much Oxygen is used a minute and how can this be calculated?
Approximately 250ml per minute
Oxygen consumption = alveolar ventilation x (FiO2 -FeO2)
alveolar ventilation is ~ 5000 (350ml* x 14 =4900mls)
FiO2 = 0.21
FeO2 = 0.16
5000 x (0.05) = 250
*normal tidal volume ~ 500ml and anatomical dead space is ~ 150 ml therefore alveolar tidal volume =350ml
How soluble is O2 in plasma?
Very poorly soluble 0.23ml/L/KPa
How is Oxygen transported in the body?
Mainly bound to Haemoglobin but small amount is dissolved in the plasma
How is Hb production regulated?
The main regulator of Hb production is erythropoietin (EPO), which is secreted by kidney in response to tissue O2 level.
Describe the structure of a haemoglobin molecule?
The Hb molecule consists of four intertwined subunits, each of which consists of a:
Polypeptide globin chain (alpha or beta)
Haem group (porphyrin ring containing a Fe2+ ion)
O2 binds reversibly to the Fe2+ ion in the haem group, each Hb molecule holding up to four O2 (one to each Fe2+).
Describe how oxygen binds to haemoglobin?
The 1st O2 molecule is difficult to bind after this there is a conformational change making it easier for the 2nd and 3rd O2 molecules to bind (this is known as co-operative binding).
The 4th molecule then becomes more difficult to bind as the Hb reaches saturation.
Draw the oxygen dissociation curve, and explain why it has its shape?
Sigmoid shape
O2 partial pressure on x axis
Hb saturation/oxyhaemoglobin saturation on y axis
Shape is due to the co-operative binding of haemoglobin
What does P50 refer to in reference to the oxygen dissociation curve?
It is the partial pressure of O2 at which there is 50% haemoglobin oxygen saturation. It is used as a reference to describe the position of the curve (varies depending on different conditions)
What is the partial pressure of oxygen in air at sea level?
0.21 x 101Kpa ~ 21kPa
Describe the difference stages in the oxygen cascade?
Atmospheric O2 21kPa
Trachea 19.9 kPa (humidification)
Alveolus 13.3kPa (alveolar gas equation e.g dilution with CO2)
Pulmonary capillary (diffusion) to Artery 13kPa (physiological shunt)
Mitochondria 1 kPa (diffusion)
Describe how a pulse oximeter works?
Measures the absorption of red and infrared light in Hb and oxy-Hb.
By extracting only the pulsatile signal, the value for arterial blood is obtained.
It closely reflects CaO2 (arterial oxygen concentration) when the Hb concentration is the normal range.
When 1g of Hb is fully saturated how many mls of O2 does it carry?
1.34mls
What is the formula for O2 content of arterial blood (CaO2)?
CaO2 = O2 bound Hb + dissolved O2
CaO2 = (Hb* 1.34 * O2 sats) + (0.23 * PaO2)
If Hb =150g/l O2 sats = 98% and PaO2 =13Kpa then:
CaO2 = (150 1.340.98) + (0.23 *13)
CaO2 =197 +3 = 200ml/l
How is oxygen delivery calculated?
DO2 = CaO2 * CO
DO2 = 200ml * 5l/min
DO2 = 1000ml/min
What is the rate of oxygen consumption (VO2) at rest and therefore how much is used a minute?
VO2=25% at rest
DO2 =1000ml/min therefore oxygen consumption is 250ml/min