The small molecules can cross in and out of the capillaries through facilitated or simple diffusion. However, the majority of the flow of capillary and tissue fluid takes place through the process of filtration and reabsorption. Filtration refers to the movement of the fluid out of the capillaries and is mediated by the capillary hydrostatic pressure.
After many generations, only those with genes suitable to live in harsh environments can live. These genes are passed onto their offspring and then we see a rise in antibiotic resistant strains of bacteria such as Methicillin Resistant Staphylococcus Aureus (MRSA).