Haemodynamics (literally, "blood dynamics"), or haemorheology, is the study of the properties and flow of blood. Blood is pumped via the heart throughout the cardiovascular system. Oxygenated blood leaves the heart via a series of large arteries. As the blood travels further along these arteries the diameter becomes smaller and the arteries become arterioles. These arterioles become capillaries and eventually venules, where deoxygenated blood passes through networks of veins back to the heart. The arteriole-capillary-venule junctions compose most of the area of the vascular system, and allow the transfer of the most vitamins and nutrients.
There are many factors that influence haemodynamics: arterial/venous diameter, blood consistency, and vasculature. The study of haemodynamics with regard to neuronal function (the haemodynamic response) is the basis for functional magnetic resonance imaging (fMRI). There is growing concern among fMRI researchers regarding individual vascular differences—especially among elderly patients—and their effects on the Blood Oxygenation Level Dependent (BOLD) signal.