Heat, infrared and the body's response
Red blood cell flexibility (rheology)
Rheology is the study of how fluids flow. Red blood cells are about 8 micrometres across and must fold to pass through capillaries half that width, so their flexibility — with haematocrit and plasma viscosity — sets how easily blood moves through the smallest vessels.
Also called erythrocyte deformability, haemorheology, blood viscosity
A red blood cell is a biconcave disc roughly 7 to 8 micrometres (µm) in diameter. Capillaries can be 3 to 5 µm. Red cells therefore have to deform continuously to circulate at all. Their deformability depends on the elasticity of the membrane and its spectrin cytoskeleton, the viscosity of the haemoglobin solution inside, and having more surface area than a sphere of the same volume would need.
Whole-blood viscosity is a separate but linked property, determined mainly by haematocrit (the proportion of blood that is cells), plasma viscosity, red cell aggregation, and shear rate. Blood is non-Newtonian: it thins as it flows faster.
Heat and hydration. Viscosity falls as temperature rises, so blood at 39 °C is thinner than blood at 37 °C. Working against that, fluid lost as sweat concentrates the blood and raises haematocrit, which raises viscosity. In a humid steam sauna where sweat runs off without cooling, fluid loss can be substantial. This is the mechanistic reason the hydration guidance around sessions is not just a courtesy.
Why it matters here
It is the physiology behind talk of circulation, and it is why replacing fluid before and after a session matters more than most people expect.