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

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 + 4O 2 = HbO 8  Reversible reaction

Hb + 4O 2 = HbO 8  High partial pressure oxygen  Lungs  Oxyhaemoglobin formed  Low partial pressure oxygen  Active tissues  Oxyhaemoglobin breaks down  Oxygen released

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 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.

Effect of increased carbon dioxide

Bohr Effect  Curve shifted to right  Increased CO 2 causes 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  CO 2 + H 2 O = H 2 CO 3

This dissociates  H 2 CO 3 = H + + HCO 3 -  HCO 3 - diffuses into plasma  H + combine with oxyhaemoglobin forming haemoglobinic acid.  H + + HbO 2 = HHB + O 2  Oxygen displaced.  Haemoglobin buffering pH changes

Myoglobin -Curve shifted to left  Myoglobin binds more tightly to oxygen.  Oxygen only given up at very low PO 2

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