There are many functions that make up the GFR. These include: nitrogen based waste, sodium (salt), water, potassium, phosphate, and some medicines dissolved in the plasma such as digoxin and gentamicin. Blood pressure, the balance of acids to bases, the release of erythropoietin (a chemical that tells the body to make more red blood cells). The activation of vitamin D1, the formation of glucose in the fasting state, and the creation of peptide hormones (including insulin) all affect the GFR.