Keep in mind that the pH of the blood is also maintained by buffers. Consider the equlibrium between hydrogen phosphate and dihydrogen phosphate. If acid is added to such a buffer, it will react with the hydrogen phosphate. Modest amounts of acid will not change the ratio much and the pH will change only slightly. We have focussed on the role of blood in transporting the necessities of catabolism to cells and on its role in removing CO 2 .
Blood also transports the products of the urea cycle to the kidneys for removal.
The first function of the kidney is as a filter. Since only water and small molecules can pass the filter membrane, proteins and blood cells are retained in the blood. In some instances, it is also necessary to secrete specific materials into the filtrate. After filtration, though, the body must reclaim almost all of the water and most of the ions and small molecules. This is done through reabsorption, the capture of water and solutes from the filtrate by moving them back into the blood.