Oxygen Transport and Consumption

Question 1

How much Oxygen is used a minute and how can this be calculated?

Answer 1

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

Question 2

How soluble is O2 in plasma?

Answer 2

Very poorly soluble 0.23ml/L/KPa

Question 3

How is Oxygen transported in the body?

Answer 3

Mainly bound to Haemoglobin but small amount is dissolved in the plasma

Question 4

How is Hb production regulated?

Answer 4

The main regulator of Hb production is erythropoietin (EPO), which is secreted by kidney in response to tissue O2 level.

Question 5

Describe the structure of a haemoglobin molecule?

Answer 5

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+).

Question 6

Describe how oxygen binds to haemoglobin?

Answer 6

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.

Question 7

Draw the oxygen dissociation curve, and explain why it has its shape?

Answer 7

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

Question 8

What does P50 refer to in reference to the oxygen dissociation curve?

Answer 8

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)

Question 9

What is the partial pressure of oxygen in air at sea level?

Answer 9

0.21 x 101Kpa ~ 21kPa

Question 10

Describe the difference stages in the oxygen cascade?

Answer 10

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)

Question 11

Describe how a pulse oximeter works?

Answer 11

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.

Question 12

When 1g of Hb is fully saturated how many mls of O2 does it carry?

Answer 12

1.34mls

Question 13

What is the formula for O2 content of arterial blood (CaO2)?

Answer 13

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

Question 14

How is oxygen delivery calculated?

Answer 14

DO2 = CaO2 * CO

DO2 = 200ml * 5l/min

DO2 = 1000ml/min

Question 15

What is the rate of oxygen consumption (VO2) at rest and therefore how much is used a minute?

Answer 15

VO2=25% at rest

DO2 =1000ml/min therefore oxygen consumption is 250ml/min

Question 16

Describes times when your VO2 would be increased?

Answer 16

Exercise
Childhood
Pregnancy
High metabolic states (infection, thyrotoxicosis, SIRS)

Question 17

What is the Bohr effect?

Answer 17

In the presence of CO2 the oxygen dissociation curve is shifted to the right, so that O2 is more likely to be given to tissues.

Hb oxygen binding affinity is inversely related to the acidity and CO2 concentration.

Question 18

Compare the oxygen dissociation curve in a capillary to an artery?

Answer 18

There would be a small right shift in a capillary due to the increased presence of CO2.

Question 19

What is cyanosis and when does it occur?

Answer 19

Cyanosis is the visible appearance of hypoxia; it can result from an impaired local circulation but is more importantly a marker of a global lack of oxygen.

It is clinically detectable when there is >50mg/dl of deoxy-haemoglobin aka if Hb is 150mg there is 67% capillary saturations.

Note it will occur at ~85% on an oximeter as this is based on an arterial value.

Question 20

How does anaemia affect oxygen content?

Answer 20

It causes an almost proportional decrease.

CaO2 = (Hb * 1.34 * O2 sats) + (0.23 * PaO2)