Lungs are a pair of expandable, spongy organs responsible for breathing and respiration. The are found internally, between the collarbone and diaphragm. In an adult human, each lung measures 25 to 30 cm long and has a roughly conical shape. The left lung is divided into 2 lobes; the right lung into 3 lobes, and is slightly larger. Separating the two lungs is a structure known as the mediastinum. Contained within each lung are 300 to 400 million alveoli, which are tiny air sacs and the sites of gas exchange. Having a thickness of only one cell, each alveolus is surrounded by many capillaries, also one cell thick, which carry deoxygenated blood from the heart, and oxygenated blood back to the heart to be pumped into other organs and tissues. The intake of oxygen into the lungs happens by a concentration gradient, due to the pressures of gases found internally relative to the environment. Because lungs have no muscle tissue, their movement is regulated by the rib cage and diaphragm. In a single, normal breath about 0.5L of air is taken in by an adult. However, with each breath of air, foreign substances like pollen, dust, bacteria and tobacco smoke also enter our body. This is why not only do the lungs supply oxygen, they also remove wastes and pollutants, and protect against harmful invaders. This is done with the help of at least 36 different types of cells, each with its own special function.