Hematocrit and blood osmolality in developing chicken embryos (Gallus gallus): In vivo and in vitro regulation

Hematocrit (Hct) regulation is a complex process involving potentially many factors. How such regulation develops in vertebrate embryos is still poorly understood. Thus, we investigated the role of blood pH in the regulation of Hct across developmental time in chicken embryos. We hypothesized that blood pH alterations in vitro (i.e., in a test tube) would affect Hct far more than in vivo because of in vivo compensatory regulatory processes for Hct. Large changes in Hct (through mean corpuscular volume (MCV)) and blood osmolality (Osm) occur when the blood was exposed to varying ambient temperatures (T_a's) and P_CO2 in vitro alongside an experimentally induced blood pH change from ∼7.3 to 8.2. However, homeostatic regulatory mechanisms apparently limited these alterations in vivo. Changes in blood pH in vitro were accompanied by hydration or dehydration of red blood cells depending on embryonic age, resulting in changes in Hct that also were specific to developmental stage, due likely to initial blood gas and [HCO₃^-]_v values. Significant linear relationships between Hct and pH (Hct/ΔpH = −21.4%/(pH unit)), Hct and [HCO₃^-] (ΔHct/Δ[HCO₃^-] = 1.6%/(mEq L^-1)) and the mean buffer value (Δ[HCO₃^-]/ΔpH- = −13.4 (mEq L^-1)/(pH unit)) demonstrate that both pH and [HCO₃^-] likely play a role in the regulation of Hct through MCV at least in vitro. Low T_a (24 °C) resulted in relatively large changes in pH with small changes in Hct and Osm in vitro with increased T_a (42 °C) conversely resulting in larger changes in both Hct and Osm. In vivo exposure to altered T_a caused age-dependent changes in Hct, demonstrating a trend towards increased Hct at higher T_a. Further, exposing embryos to a gas mixture where P_CO2 = 5.1 kPa for > 4 h period at T_a of 37 or 42 °C also did not elicit a change in Hct or Osm. Presumably, homeostatic mechanisms ensured that in vivo Hct was stable during a 4–6 h temperature and/or hypercapnic stress. Thus, although blood pH decreases (induced by acute T_a increase and exposure to CO₂) increase MCV and, consequently, Hct in vitro, homeostatic mechanisms operating in vivo are adequate to ensure that such environmental perturbations have little effect in vivo.