Flashcards in S2: O2 Transport Deck (28):
What is the concentration of a gas dissolved in liquid dependent on?
- Partial pressure
Concentration = Partial Pressure x Solubility
How are CO2 and O2 transported in the blood?
Respiratory gases carried in the bloodstream first dissolve in the plasma (the aqueous portion of blood), but only a relatively small % remains as dissolved O2 or CO2.
O2 and CO2 are transported differently due to varying chemistry & biological adaptations.
- HCO3- = 70%
- HbCO2 = 23%
- Dissolved CO2 = 7%
- HbO2 = 98%
- Dissolved O2 = 2%
Why is Hb critical to O2 transport?
Each litre of blood needs to carry at least 50 ml of O2 to satisfy CO and oxygen consumption at rest.
The solubility of oxygen in blood plasma is low and the PO2 in ambient air is not enough to dissolve enough O2 in blood.
The presence of Hb overcomes this problem and it enables O2 to be concentrated within the blood by increasing carrying capacity. O2 can therefore be absorbed at gas exchange surfaces and released at respiring tissues.
What is Fick's Principle?
Describes the relationship between arteriovenous O2 difference, O2 consumption and cardiac output
CO = VO2/Ca - Cv
How is increased oxygen supply to tissues achieved?
Hb is 98% saturated at rest so hyperventilating has little effect on O2 delivery.
In healthy, excersizing individuals, increased O2 delivery is achieved by increasing cardiac output, not PaO2.
Describe transport of oxygen in the blood
Oxygen has low solubility in aqueous solutions – the quantity dissolved in plasma is insufficient to cope with the O2 demands of tissues.
The presence of haemoglobin, greatly increases the O2-carrying capacity of the blood, and is responsible for the vast majority of O2 transport.
Define O2 partial pressure (PaO2 - arterial plasma)
- “what partial pressure of O2 within a gas phase would be required at gas-liquid interface to yield this much O2 in the plasma?”
Define total O2 content (CaO2)
- mL of O2 per L of blood (ml/L)
- “what volume of O2 is being carried in each litre of blood, including O2 dissolved in the plasma and O2 bound to Hb?”
Define O2 saturation (SaO2 and SpO2)
- “What % of total available haemoglobin binding sites are occupied by oxygen?”
How is SaO2 and SpO2 measured?
SaO2 = measured directly in arterial blood
SpO2 = estimated by pulse oximetry
Saturation of O2 measured in %
Describe the oxygen-haemoglobin dissociation curve
It shows the relationship between O2 concentration, partial pressure and saturation
-Cooperative binding of O2 to Hb so line increases
- Plateau of line as saturation of O2 binding sites occur
How is oxygen content of blood measured?
- Blood gas analysis
- Pulse oximetry
Describe and compare blood gas analysis and pulse oximetry
Blood Gas Analysis:
- Sample of arterial blood used typically from the radial artery
- PO2, PCO2, pH, Hb, Hb-O2. Hb-CO, SaO2 measured by utilizing partially permeable membranes and selectively sensitive electrodes. E.g. more O2 in sample = more current generated by specific electrode.
· Increased accuracy versus pulse oximetry, but slower & more invasive.
Pulse oximetry only estimates SO2 + can be inaccurate in cases of CO poisoning
Why is haemoglobin so effective at transporting O2 within the body?
The structure of Hb produces high O2 affinity, therefore a high level of Hb- O2 binding (and saturation) requires relatively low PO2.
PaO2 must be really low before HbO2 saturation is substantially affected
Also, the concentration of haem groups and Hb contained in RBC's enables high carrying capacity
Explain how the oxygen haemoglobin binding curve shifts to offload oxygen to demanding tissues
Increased pH (alkalosis)
Decreased 2,3-DGP (which increases affinity of Hb for O2)
Right (e.g. in hard working tissues) - Bohr Shift:
Decreased pH (acidosis)
Increased 2,3-DPG (Hb gives up O2 more easily)
What Hb affinity changes occur for different local enviroments?
This enables O2 delivery to be coupled to demand.
1. LUNGS - Increased PO2, Decreased PCO2, Increased pH therefore increases O2 saturation in Hb
2. RESTING TISSUE - decreased PO2 which decreases O2 saturation.
O2 moves from Hb to tissue.
3. WORKING TISSUE - Decreases PO2
Anaerobic respiration and hypoxia also produces lactic acid (H+), CO2 and 2,3-DPG
Increased O2 demand therefore decreases Hb-O2 affinity and binding which decreases O2 saturation.
More O2 moves from Hb to tissue.
What does myoglobin do?
Myoglobin acts as a O2 reservoir within muscle tissue and only releases O2 at low PaO2
Compare the affinity of foetal Hb and normal Hb
Foetal haemoglobin has higher O2 affinity, and effectively 'steals' O2 from maternal Hb
Where foetal and maternal Hb come into contact in the placenta, O2 will be transferred to the relatively high affinity foetal Hb
Colour of oxyhamoglobin and deoxyhaemoglobin
Oxyhaemoglobin (Hb-O2) appears red where as deoxyhaemoglobin (Hb) appears blue the relative concentrations of O2 of determines the colour of blood
What is the definition of cyanosis?
Cyanosis = purple discoloration of the skin and tissue that occurs when the [deoxyhaemoglobin] becomes excessive.
What is central cyanosis?
- Bluish discoloration of core, mucous membranes and extremities
- Inadequate oxygenation of blood
E.g. hypoventilation, V/Q mismatch
What is peripheral cyanosis?
- Bluish coloration confined to extremities (e.g. fingers)
- Inadequate O2 supply to extremities
E.g. small vessel circulation issues
Describe anaemia (insufficient Hb) and hypoxia
Hypoxia can occur despite adequate ventilation and perfusion, if the blood is not able to carry sufficient oxygen to meet tissue demands.
Causes of anaemia (insufficient RBCs or haemoglobin):
- Iron deficiency (↓ production)
- Haemorrhage(↑ loss)
There is is decreased O2 content but normal saturation and normal PaO2 (same amount in plasma). There is decreased [Hb] and [O2-Hb]
What happens during carbon monoxide poisoning?
- Hb has >200x affinity for carbon monoxide (CO) than O2 and competes for the same binding site.
· ∴ ↑CO-Hb =↓O2 capacity
- Carboxyhaemoglobin has cherry red pigmentation, hence hypoxaemia occurs in the absence of cyanosis
In CO poisoning, there is:
Decreased O2 content (displacement by CO)
Why does CO make oxygen dissociation curve shift to left?
CO inhibits glycolysis in RBCs hence there is decreased 2,3-DPG. This increases Hb affinity for O2 so curve shifts to the left and there is decreased unloading
Signs of carbon monoxide
Loss of Consciousness
What stimulates EPO release?
Hypoxia (altitude training, EPO doping) stimulates release of EPO which causes increased RBC formation from bone marrow