title: Red blood cell thickness is evolutionarily constrained by slow, hemoglobin-restricted diffusion in cytoplasm.
abstract: During capillary transit, red blood cells (RBCs) must exchange large quantities of CO2and O2in typically less than one second, but the degree to which this is rate-limited by diffusion through cytoplasm is not known. Gas diffusivity is intuitively assumed to be fast and this would imply that the intracellular path-length, defined by RBC shape, is not a factor that could meaningfully compromise physiology. Here, we evaluated CO2diffusivity (DCO2) in RBCs and related our results to cell shape. DCO2inside RBCs was determined by fluorescence imaging of [H+] dynamics in cells under superfusion. This method is based on the principle that H+diffusion is facilitated by CO2/HCO3-buffer and thus provides a read-out of DCO2. By imaging the spread of H+ions from a photochemically-activated source (6-nitroveratraldehyde), DCO2in human RBCs was calculated to be only 5% of the rate in water. Measurements on RBCs containing different hemoglobin concentrations demonstrated a halving of DCO2with every 75 g/L increase in mean corpuscular hemoglobin concentration (MCHC). Thus, to compensate for highly-restricted cytoplasmic diffusion, RBC thickness must be reduced as appropriate for its MCHC. This can explain the inverse relationship between MCHC and RBC thickness determined from >250 animal species.
authors: Department of Physiology, Anatomy and Genetics, Oxford OX1 3PT, European Union, United Kingdom.
authors: Sarah L
authors: SL
authors: Richardson
authors: Department of Physiology, Anatomy and Genetics, Oxford OX1 3PT, European Union, United Kingdom.
authors: Pawel
authors: P
authors: Swietach
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000097:blood*; D001692:Biological Transport; D002161:Camelids, New World; D002245:Carbon Dioxide / Q000097:blood*; D048430:Cell Shape / Q000502:physiology; D002645:Chickens; D004058:Diffusion; D004909:Erythrocyte Indices / Q000502:physiology*; D004912:Erythrocytes / Q000502:physiology*; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D010100:Oxygen / Q000097:blood; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D014982:Xenopus laevis
chemical_list: D001639:Bicarbonates; D006454:Hemoglobins; D002245:Carbon Dioxide; D010100:Oxygen