The kidneys FILTER the blood and remove things that should not be there or that there is too much of. UREA, GLUCOSE,WATER, AND MINERAL IONS diffuse across the partially permeable membrane. The urea is all filtered out of the blood and is URINATED out along with any EXCESS water and mineral particles, but the GLUCOSE is all reabsorbed into the blood by active transport because it is needed for respiration. Some water and mineral ions are also reabsorbed to be used by the body.
RED BLOOD CELLS and PROTEINS are too big to fit through the walls of the nephrons that the capillaries are near to, so they cannot diffuse out of the blood like urea, glucose, water and minerals can :)
UREA Bad stuff that is toxic to the body, all of it is removed and urinate
PARTIALLY PERMEABLE MEMBRANE - wall that has very small holes in, only very small molecules can fit through using diffusion, osmosis or active transport
Transplant and Dialysis
Dialysis 'cleans' a persons blood and is used when someones kidneys cannot effectively remove toxins and excess ions. A person’s blood leaves their body and flows into the machine, through partially permeable membranes. After the membranes comes the dialysis fluid, which contains the same concentration of glucose so that none diffuses out of the blood, a lower concentration of urea so that it diffuses from the higher concentration to the lower (the blood into the fluid) and carefully controlled concentrations of water and minearl ions so that just the right amount diffuses out.
The kidney is replaced with a new, healthy one from a donor, this means that dialysis is no longer needed :) However, there is a risk of rejection, so the tissue types of the donor and recipient must be as close as possible, and the recipient has to take immunosuppresant drugs to stop the body attacking the 'foreign' organ, this lowers their immune system, so they are more vunerable to disease.