Carriage of respiratory Gases By Y Stock
Objectives • You should be able to: • Describe the structure of erythrocytes. • Describe the role of respiratory pigments. • Understand and interpret dissociation curves.
Red blood cells • Biconcave discs • 8 micrometers diameter • No nucleus • Made in bone marrow • Destroyed by liver/spleen
Function: To carry oxygen and carbon dioxide • Adaptions: • Shape gives a large surface area, encouraging diffusion • Full of haemoglobin
Haemoglobin • Consists of 4 polypeptide chains, each with an iron containing haem group. • Each haem group combines with an oxygen molecule
Haemoglobin+oxygen=oxyhaemoglobin • Hb + 4O2 = HbO8 • Reversible reaction
High partial pressure oxygen Lungs Oxyhaemoglobin formed Low partial pressure oxygen Active tissues Oxyhaemoglobin breaks down Oxygen released Hb + 4O2 = HbO8
Oxygen dissociation curves • Found by exposing haemoglobin to air mixtures of varying oxygen partial pressures and determining its % saturation.
S shaped curve • Over the steep section a small fall in Oxygen partial pressure causes a sizeable drop in % saturation.
The curve shows that: • at relatively low oxygen concentrations there is uncombined haemoglobin in the blood and little or no oxyhaemoglobin, e.g. in body tissue • at relatively high oxygen concentrations there is little or no uncombined haemoglobin in the blood; it is in the form of oxyhaemoglobin, e.g. in the lungs.
Bohr Effect • Curve shifted to right • Increased CO2causes oxyhaemoglobin to unload some of its oxygen.
Fate of carbon dioxide • 7-10% dissolves in plasma • 20-30% in red cells as carbaminohaemoglobin (binds to amino acids) • 60-70% converted to bicarbonate in plasma
Conversion to bicarbonate • Carbon dioxide diffuses into red cells • Enzyme:carbonic anhydrase • CO2 + H2O = H2CO3
This dissociates • H2CO3 = H+ + HCO3- • HCO3- diffuses into plasma • H+ combine with oxyhaemoglobin forming haemoglobinic acid. • H+ + HbO2 = HHB + O2 • Oxygen displaced. • Haemoglobin buffering pH changes
Myoglobin -Curve shifted to left • Myoglobin binds more tightly to oxygen. • Oxygen only given up at very low PO2
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