The human lungs are the organs of respiration. Humans have two lungs, a right lung and a left lung. The right lung consists of three lobes while the left lung is slightly smaller consisting of only two lobes. Together, the lungs contain approximately 2,400 kilometres of airways and 300 to 500 million alveoli. Estimates of the total surface area of lungs vary from 30-50 square metres up to 70-100 square metres in adults — which might be roughly the same area as one side of a tennis court. However, such estimates may be of limited use unless qualified by a statement of scale at which they are taken. Furthermore, if all of the capillaries that surround the alveoli were unwound and laid end to end, they would extend for about 992 kilometres. The lungs together weigh approximately 1.3 kilograms, with the right lung weighing more than the left. The pleural cavity is the potential space between the two serous membranes, of the lungs; the parietal pleura, lining the inner wall of the thoracic cage, and the visceral pleura, lining the organs themselves–the lungs. The respiratory system includes the conducting zone, which is part of the respiratory tract, that conducts air into the lungs.