This chapter presents three methods to apply the exergy analysis in human lungs to transport oxygen, carbon dioxide, and carbon monoxide. The models presented in the literature provide a first approach of gases dissolved as ideal gases in the blood. Then we increased complexity, considering the connection of the gases in the blood with hemoglobin, and finally, only the irreversibilities associated with mass transfer. To better evaluate the different levels of CO intoxication and hemoglobin concentration (which is a function of acclimatization time and some pathologies, such as anemia), we proposed a model calculating exergy efficiency for the lungs. From this model, it was possible to conclude that a higher level of intoxication is associated with lower exergy efficiency values. When associated with carbon monoxide intoxication, higher hemoglobin levels also result in lower efficiency. Eventually, we compared previous and current studies regarding the method employed to calculate the exergy destroyed in the lungs, considering gas transport and hemoglobin concentration, and their reaction with the gases from a second law perspective. The impacts of carbon monoxide are especially dangerous for pregnant women, fetuses, and newborn babies. From the literature, a carbon monoxide transport model is modified to simulate a pregnant woman (the original model was a male), changing some parameters to express the adjusted respiratory system. It was considered the gas exchange in the placenta, to evaluate the concentration of these different gases in the fetal arterial and venous blood. Three methods of exergy analysis are implemented for both the mother’s and the fetus’s respiratory systems, aiming to compare them with the respiratory system of a male adult. The destroyed exergy of the literature did not have the same trend as the models proposed in this article, taking into consideration the hemoglobin reactions. The entropy generation associated only with the diffusion transport phenomena was lower than that of the other methods. The placenta destroyed exergy rate is significantly higher compared to the irreversibilities of the mother’s respiratory system. One possible explanation is that the placenta has other physiological functions than gas transportation.