Expansion
Viscosity rises steeply with haematocrit, so optimal delivery occurs at an intermediate value
Expansion
Blood is non-Newtonian: its viscosity varies with shear rate and with haematocrit.
- Raising haematocrit increases oxygen content but increases viscosity steeply and non-linearly
- By Poiseuille’s law, higher viscosity raises resistance and reduces flow
- Oxygen delivery is the product of content and flow, so it peaks at an intermediate haematocrit, around 30 to 35 per cent in most models
Clinical consequences:
- Polycythaemia, whether primary, from chronic hypoxia or from doping, raises viscosity and the risk of thrombosis and stroke; venesection improves symptoms
- Anaemia lowers viscosity, so cardiac output rises and flow is well maintained until haemoglobin falls substantially
Two flow phenomena reduce viscosity in small vessels: red cells stream axially, leaving a plasma layer at the wall, and apparent viscosity falls in vessels below about 300 micrometres (the Fahraeus-Lindqvist effect).