Microcirculation, bioengineering and the boundary between blood and tissue in health and disease

Blood delivers oxygen by transporting adequate amounts of red blood cells, a process directly related to their concentration or hematocrit. We are generally aware of our blood pressure, but to a lesser extent of our hematocrit, which varies among individuals due to age, environment, and even subtle changes in our health condition. Hematocrit defines blood viscosity, which can affect blood pressure. In the healthy organism, hematocrit and blood pressure are not related due to autoregulatory mechanisms that translate the increase of hematocrit in vasodilation thereby keeping the blood pressure within a normal working range. Paradoxically, when healthy organisms are subjected to small increases in hematocrit, there is a decrease of blood pressure and vice versa. This unusual effect is generated at the boundary between blood and the blood vessel wall, by hydrodynamic forces that cause red blood cells to move away from the microvessel surface forming a plasma layer separating red blood cells from the vessel wall. Its width is determined by hematocrit and blood flow velocity, and affects most homeostatic systems, in particular the exchange of oxygen and the availability of nitric oxide. Changes of the plasma layer affect the regulation of nitric oxide, which when prolonged in time are a precursor to inflammation, which is the root of many systemic diseases. These conclusions are obtained using an engineering perspective on in vivo studies which span the scale from cellular, systemic to population surveys exemplifying the interdisciplinary nature of the bioengineering approach. A suggestion: if you take a blood test… ask for your hematocrit: small changes may be indicative of big eventual health differences.