The lungs together weigh approximately 1.3 kilograms (2.9 pounds). The right lung is bigger and heavier than the left, which shares space in the chest with the heart.
They are situated within the thoracic cavity of the chest. Humans have two lungs, one on the left and one on the right. The lungs also provide airflow that makes vocal sounds including human speech possible. In humans, the main muscle of respiration that drives breathing is the diaphragm. In earlier tetrapods, air was driven into the lungs by the pharyngeal muscles via buccal pumping, a mechanism still seen in amphibians. Mammals, reptiles and birds use their different muscles to support and foster breathing. Respiration is driven by different muscular systems in different species. The pleurae, which are thin, smooth, and moist, serve to reduce friction between the lungs and chest wall during breathing, allowing for easy and effortless movements of the lungs. Their function in the respiratory system is to extract oxygen from the air and transfer it into the bloodstream, and to release carbon dioxide from the bloodstream into the atmosphere, in a process of gas exchange. In mammals and most other vertebrates, two lungs are located near the backbone on either side of the heart. The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish.
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