Oxygen transport Flashcards
What colours do oxyhaemoglobin and deoxyhaemoglobin appear respectively?
Oxyhaemoglobin - Red
Dexoyhaemoglobin - Blue
The relative concentration of these 2 proteins determines the colour of blood and innervated tissues
What is cyanosis?
Purple discolouration of skin + tissue that occurs when deoxyhaemoglobin becomes excessive
What is central cyanosis? What is it caused by?
- Blue discolouration of core, mucous membranes and extremities
- Inadequate oxygenation of blood (e.g. hypoventilation, V/Q mismatch, gas exchange defects)
What is peripheral cyanosis?
- Bluish coloration confined to extremities (e.g. fingers)
- Inadequate O2 supply to extremities (e.g. fingers)
Why can hypoxia occur despite adequate ventilation and perfusion?
Hypoxia will still occur if the blood is not able to carry sufficient oxygen (anaemia) to meet tissue demands.
What are causes of anaemia?
What would clinical results be like for somebody who is anaemic?
- Iron deficiency (⬇️production)
- Haemorrhage (⬆️RBC loss) - A decrease in RBC density will result in a reduction in the concentration of Hb, total oxygen binding sites, and oxygen carrying capacity. The affinity of Hb is unchanged.
- Therefore, in anaemia, clinically:
- ⬇️O2 content
- Normal saturation
- Normal PaO2 (same amount is dissolved in plasma)
- ⬇️Hb & O2-Hb binding
Describe carbon monoxide poisoning
What would be observed clinically with somebody affected by carbon monoxide poisoning?
Hb has a much, much higher affinity for CO than O2, and competes for the same binding site as O2.
For this reason, if somebody inhales CO, CO will bind to Hb instead of oxygen, forming carboxyhaemoglobin
Carboxyhaemoglobin has a cherry red pigment, hence hypoxia occurs in the absence of cyanosis
In CO poisoning, the following values would be observed:
- Decreased O2 content (due to displacement by CO)
- Decreased SaO2 (if measured)
- SpO2 = normal
- PaO2 = normal
- Decreased oxygen-Hb binding
- Hb = Normal
- CO inhibits glycolysis in RBCs, hence ⬇️DPG = shifts curve to left (⬇️unloading)
What is erythropoietin, and what is its function?
Hormone secreted by kidney in response to hypoxia. Induces production of RBCs within bone marrow.
As O2 transport from lungs to tissues is dependent on Hb concentration and PaO2, a method of compensating for chronic hypoxia is to increase Hb concentration; although Hb saturation will decrease due to reduced PaO2, this can be slightly compensated for by an overall greater number of Hb.
Cardiac output increases via increased HR in response to hypoxia in order to increase O2 transport (the same number of Hb molecules are cycled from the lungs to tissue and back again more rapidly, ⬆️total volume of O2 transported per unit time).
The increased RBC production per unit plasma is termed polycythaemia.
Why is haemoglobin critical to O2 transport?
- Oxygen has low solubility in plasma. To dissolve the amount of O2 needed to supply tissues, an unachievable high alveolar PO2 would be needed.
- Hb overcomes this problem as it allows O2 to be concentrated within the blood (increasing the carrying capacity) at gas exchange surfaces and then released at respiring tissues
- Vast majority of O2 transported by blood is bound to Hb
How is oxygen content of blood measured/defined?
- O2 partial pressure (PaO2) expressed as kPa - The partial pressure of O2 within a gas phase (at a gas-liquid interface) that would yield this much O2 in the plasma at equilibrium
- Total O2 content (CaO2) expressed as mL of O2 per L of blood (ml/L) - The volume of O2 carried in each litre of blood, including O2 dissolved in the plasma and O2 bound to Hb
- O2 saturation (SaO2 = measured directly in arterial blood, SpO2 = estimated by pulse oximetry) expressed as % - The % of total Hb binding sites that are occupied by oxygen
Describe why an oxygen-haemoglobin curve has a sigmoid shape
Initially (near 0) the gradient is less steep, due to cooperative binding of Hb and O2. The first O2 binding to Hb is the most difficult to bind, but after this initial binding, Hb undergoes conformational changes that allows for much easier O2 binding.
The gradient becomes steeper, but then begins to plateau due to increasing saturation of oxygen binding sites - The binding sites become filled, so it becomes harder to bind further oxygen
When does Hb release Oxygen?
When the surrounding oxygen pressure decreases, such as in respiring tissues.
In normal circumstances this means Hb readily takes larger quantities of oxygen at respiratory surfaces and give it up to respiring tissues.
Why is haemoglobin 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) is achieved at relatively low PO2
- The concentration of heme groups and Hb contained in RBCs enables high carrying capacity
- There are 4 heme groups per Hb molecule
What does it mean if the oxygen-haemoglobin curve shifts left or right?
- Leftward shift - Higher Hb-O2 affinity = Hb binds more O2 at a given PO2
- Rightward shift - Lower Hb-O2 affinity = Hb binds less O2 at a given PO2
What do changes in Hb-O2 affinity depend on?
Local environment, enabling O2 delivery to be coupled to demand
- At the lung = ⬆️PO2, ⬇️PCO2, ⬆️pH, therefore ⬆️O2 saturation
- At resting tissues = ⬇️PO2 therefore ⬇️O2 saturation, so O2 moves from Hb to tissue
- At working tissue = ⬇️PO2, however anaerobic respiration and hypoxia also produces lactic acid (H+), CO2, and 2,3-DPG, therefore ⬆️O2 demand = ⬆️CO2, ⬇️pH and ⬆️2,3 DPG = ⬇️Hb-O2 affinity + binding therefore ⬇️O2 saturation, ⬆️O2 moves from Hb to tissue
