The chemical activity of cells produces metabolicwaste, which diffuses out of the cells into the tissue fluid and from there into the blood.Cellular activity can continue only if these waste products are removed from the blood, and this is one of the primary functions of the kidney. However, the role of the kidney goes far beyond the more excretion of toxic waste material.The kidney excretes in the urine any substance that becomes present in excess in the plasma, and in this way it guards the constancy of the internal environment. The kidney does not extract substances from the blood for excretion;it The chemical activity of cells produce metabolic waste, which diffuses out of the cells into the tissue fluid and from there into the blood. Cellular activity can continue from if the waste products are removed from the blood, and this is one of the primary functions of the However, the role of the kidney goes far beyond the more excretion of toxic waste material. The kidney excretes in the urine any substance that becomes present in excess in the plasma, and in this way it guards the constancy of the internal environment. The kidney does not extract substances from the blood for excretion; it