physiology

The urinary systems main function is to eliminate waste from the body. With that being said, cells of the body get their energy from burning sugars, fats and proteins. Oxygen is required for many of the reactions. Oxygen comes from the air, into the lungs, from the lungs, into the blood stream, and then into cells (which is how the respiratory system interacts with the urinary system). The cells use this in chemical reactions. When proteins burn they produce carbon dioxide, urea (component of urine), and other chemicals. The urea and some of the other chemicals created are eliminated into the kidneys, which are then eliminated from the body by urine. Other chemicals are sent to the liver for reprocessing, or to go through the small and large intestine to be either digested or eliminated by feces.

Kidneys are the only organs that connected both system. Imagine those who had a damage of both kidneys must have a blood-washing. That because their blood contains too much toxin because the kidneys have low-capability in filtering away the poison it contains in the blood. Blood that had been filtered from kidneys will flow back to the lungs to gain more oxygen supply for heart. When it's been used by lungs, the blood will go back to the kidneys for the toxin-filtering process again.

They correlate in maintaining optimal blood pH levels. If you hyperventilate (i.e. excrete too much CO2), it will cause you plasma to increase its pH (i.e. decrease H+). We call this respiratory alkalosis. This drop in plasma H+ will cause our kidney to secrete less H+ for excretion via urine. Thus H+ again rise in plasma to bring the blood pH level to normal. In other words, we can say that the compensation is metabolic acidosis.

So in summary, to answer your question, both systems are related in maintaining blood pH levels.