text: Blood rheology and ischaemia.
  Ischaemia is essentially the failure of tissue to obtain a sufficient oxygen supply for its function. In the context of rheology, which is the study of the deformation and flow of materials, this implies failure to deliver the blood, rather than failure to oxygenate the blood or extract oxygen from it. Resistance to the delivery of blood is generally considered to have vascular and rheological components. Vascular effects on resistance may often be dominant, and there is wide appreciation of the ischaemic consequences of vascular obstruction and narrowing, for example in atherosclerotic disease. However, rheological factors can vary widely between individuals and in disease, and such variations have the potential to influence oxygen supply. Here, the rheological factors which affect blood flow are reviewed and their role in the development of ischaemia is discussed, with particular reference to the eye where possible.