The cardiovascular system consists of a pump and a complex series of tubes. Its function is to supply every cell of the body with its needs (O 2 , glucose, amino acids, lipids, vitamins, hormones, heat, inorganic ions, H 2 O, . etc.) and to remove waste products (CO 2 , H 2 O, lactate, protons, urea, heat, etc. The supply to each cell should be in accordance with its needs.
There are ~10 14 cells in the body so supplying each with its needs, which are different for each cell type and change with cell activity, is a formidable task. While this complex task generally continues successfully without any conscious activity on our part, inevitably there are conflicts from time to time and priorities have been set (by evolution) which determine which cells are highest priority and have a protected circulation during times of stress. If the blood supply to the brain ceases for > 10s we lose consciousness (potentially a disaster in evolutionary terms). Consequently supplying blood to the brain is the highest priority and, because blood supply to the brain requires the heart, blood supply to the heart is equally high priority. If blood supply to the brain ceases for more than ~ 10 min there is likely to be permanent brain damage even if flow is restored.
To understand the circulation some understanding of the underlying physical principles is valuable. Two processes are central; diffusion and flow through tubes. Diffusion is the process by which molecules move down their concentration gradient powered by the random thermal motion of molecules. Thus if a cell uses O 2 , the concentration of O 2 in the cell decreases, and the increasing inward concentration gradient causes additional O 2 to diffuse into the cell. In general the rate of diffusion is proportional to the concentration gradient, to the area involved and to the diffusion coefficient of the molecule and inversely proportional to the square of the distance involved.
