Basic Sciences - Oxygen Transport and Consumption Flashcards
Site and use of oxygen
Used in mitochondria
Produces ATP via oxidative phosphorylation in Kreb’s cycle / electron transport chain chain
Oxygen consumption calculation
Oxygen consumption = Minute volume x (FiO2 - FeO2)
FeO2 = fraction expired O2 (usually 16% at rest)
Oxygen consumption at rest
250 ml/min
From equation:
Oxygen consumption = Minute volume x (FiO2 - FeO2)
= 5000 x (0.21 - 0.16)
= 250
Partial pressure of oxygen at sea level
Approx 21 kPa
(Atmospheric partial pressure 101.3 kPa)
Solubility of Oxygen in plasma (not haemoglobin)
0.23 ml/L/kPa
Very low
Normal Hb concentration
130-150 g/L
Molecular weight of Haemoglobin
Around 68,000
Main regulator of haemoglobin production
Erythropoietin
Secreted by kidney in response to tissue O2 level
Haemoglobin structure description
4 intertwined subunits:
- 2 alpha polypeptide globin chains
- 2 beta polypeptide globin chains
Each unit contains a haem group - porphyrin ring containing Fe2+ ion
Haemoglobin structure image
How many oxygen molecules bind to each haem group
1
Driving factor for oxygen binding to Hb
Partial pressure
However relationship between PaO2 and Hb O2 binding is not proportional
Relationship between haem and O2 binding
Initial O2 binding is difficult
As first O2 molecule binds to first haem group, it alters shape of Hb molecule making other binding sites more accessible
Subsequent binding of second and third O2 molecules are easier (cooperativity)
Full saturation with fourth O2 molecule is difficult as only one remaining binding site free
Haemoglobin oxygen dissociation curve
Haemoglobin oxygen dissociation curve with arterial and venous PaO2
Venous blood oxygen saturation at rest
~ 75%
Approx 25% of available O2 extracted by tissues at rest
Venous blood PaO2 at rest
~ 5.3 kPa
P50 definition
PaO2 at which the Hb-O2 saturation 50%
Usual PaO2 of P50
~ 3.5 kPa
Use of P50
Reference point that describes position of the Hb-O2 dissociation curve and changes as the curve moves under different conditions
3 methods to describe oxygenation
PaO2
SpO2
CaO2
All related but not the same
Oxygen cascade definition
Three stage drop in arterial PaO2 to 13 kPa from atmospheric level of 21 kPa
Oxygen cascade process
1) In the upper airway humidification occurs adding water vapour
2) In the alveoli O2 is taken up in exchange for CO2
3) In the circulation a small physiological shunt caused by bronchial circulation and thebesian veins
Mechanism of saturation probe functioning
Uses different absorption characteristics of Hb and oxy-Hb for red and infrared light
Extracts only pulsatile signal to obtain arterial values
Oxygen content of blood definition
Truest measure of oxygen present but difficult to directly measure
Therefore usually estimated from other values
Oxygen content equation
CaO2 = Hb bound O2 + Plasma dissolved O2
Hb bound = [Hb] x 1.34 x O2 saturations
Dissolved = 0.23 x PaO2
(0.23 is dissolvability of O2 in plasma (from earlier flashcard)
Therefore:
CaO2 = (1.34 x [Hb] x saturations) + (0.23 x PaO2)
Volume of oxygen held by 1g of Hb when fully saturated
1.34 ml
Hence why constant of 1.34 is included in CaO2 equation
CaO2 = (1.34 x [Hb] x sats) + (0.23 x PaO2)
Approximate CaO2 in healthy adults
200 ml/L
CaO2 = (1.34 x [Hb] x sats) + 0.23 x PaO2)
= (1.34 x 150 x 0.98) + (0.23 x 13)
= 197 + 3
= 200 ml/L
Delivery rate of O2 equation
DO2 = CaO2 x CO
Usual DO2 in healthy adult
DO2 = CaO2 x CO
= 200 ml/L x 5 L/min
= 1000 ml/min
Rate of oxygen consumption at rest
VO2 only 25% of DO2 (25% of 1000 ml/min = 250 ml/min)
Also remember VO2 equation:
VO2 = minute volume x (FiO2 - FeO2)
Venous content of oxygen at rest
25% consumed at rest leaving 75% in venous blood
75% of 200 ml/L = 150 ml/L
States which increase oxygen consumption by tissues
Exercise
Shivering
Pregnancy
Childhood
States that increase metabolic rate inc pyrexia and thyrotoxicosis
Methods of physiologically providing extra oxygen for when tissue has higher oxygen consumption
Increasing:
- CO
- Minute ventilation
- Extraction of O2 from blood (reducing leftover venous blood CaO2)
How much VO2 can increase during vigorous exercise
Up to 10x the value at rest
Bohr effect description
Increase in tissue metabolism (eg exercising muscle)
More CO2 produced
Causes local acidosis
Shifts Hb-O2 curve to the right
Therefore at a given PaO2, the Hb is less saturated so gives up more O2 to the tissues
Small rise in temperature locally also contributes to the right shift of the curve
Opposite occurs at lungs with CO2 removal leading to left shift of the Hb-O2 curve, increasing affinity of Hb for O2
When is cyanosis detected clinically
When ≥ 50 g/L deoxy-Hb can be seen in the skin or mucous membranes. With a [Hb] of 150 g/L this is 33% of the total, with the remaining 67% being saturated.
In practice, patients appear cyanosed when the pulse oximetry reading is much higher, around 85%.
Cyanosis is seen in capillary blood, whilst a pulse oximeter reading is based on the arterial value, which will be substantially higher because:
* PO2 is slightly higher in the artery than the capillary
- The saturation curve in the capillary has a small right shift (Bohr effect)
