Mammals are ureotelic organisms. In these organisms the ammonia deposited in the mitochondria of liver cells is converted into urea in the Ornithine cycle or urea cycle. Hans Kreb discovered this cycle in 1932 (who later also discovered citric acid cycle). Urea production is occurred almost exclusively in liver and it passes into blood stream and then to kidneys. During excretion of this urea through kidneys, $\mathrm{CO}_2$ is also excreted.