Although the circulatory system is not closed, and blood is definitely not an 'ideal fluid', you can still use fundamentals from the fluid mechanics of ideal fluids to help interepret the circulatory system. For example, the principle of the continuity of volume flux is a very useful concept for interpreting the circulation. Because the cross-sectional area of the aorta or venae cavae is less than the total cross-sectional area of the capillary beds, blood flow is faster in the aorta or venae cavae than the capillaries.