The role of the lungs in blood circulation primarily involves the process of oxygenation of the blood. As blood flows through the pulmonary circulation, it travels from the right side of the heart into the lungs. Here, oxygen from the inhaled air diffuses across the alveolar membrane into the bloodstream, while carbon dioxide in the blood moves in the opposite direction to be exhaled. This exchange is crucial for supplying oxygen to the body's tissues and organs, which is essential for cellular metabolism and overall health.
The other options do not accurately reflect the primary physiological function of the lungs in terms of blood circulation. Nutrient absorption occurs in the digestive system, not the lungs. While the heart is responsible for pumping blood throughout the body, the lungs do not have a pumping mechanism; they facilitate gas exchange. Although the lungs can help expel some toxins through breathing out, this function is secondary compared to their main role of oxygenating the blood. Therefore, the correct choice highlights the vital function of the lungs in ensuring that the blood is rich in oxygen, providing essential support for bodily functions.