How do the excretory and respiratory systems regulate blood pH?

Respiration controls the amount of carbon dioxide in the blood. If respiration slows, CO2 increases, causing a respiratory acidosis. If respiration quickens or deepens, CO2 decreases, promoting a respiratory alkalosis. This is helpful if there is another process going on that impacts the pH of the blood. For instance, in diabetic ketoacidosis, the pH decreases in the blood due to the production of ketoacids. The respiratory system responds by increasing respiration and decreasing CO2 to help bring the pH of the blood up toward normal.

You're right about the kidneys. The kidneys help regulate the blood levels of several ions, sodium ions (Na), potassium ions (K), calcium ions (Ca2+), chloride ions (Cl), and phosphate ions (H(PO4)2).

Regulation of blood pH. The kidneys excrete a variable amount of hydrogen ions (H) into the urine and conserve bicarbonate ions (HCO3), which are an important buffer of H in the blood. Both of these activities help regulate blood pH. Regulation of blood volume. The kidneys adjust blood volume by conserving or eliminating water in the urine. An increase in blood volume increases blood pressure; a decrease in blood volume decreases blood pressure.