Phlegm (; Ancient Greek: φλέγμᾰ, phlégma, "inflammation", "humour caused by heat") is mucus produced by the respiratory system, excluding that produced by the nasal passages. It often refers to respiratory mucus expelled by coughing, otherwise known as sputum. Phlegm, and mucus as a whole, is in essence a water-based gel consisting of glycoproteins, immunoglobulins, lipids and other substances. Its composition varies depending on climate, genetics, and state of the immune system. Its color can vary from transparent to pale or dark yellow and green, from light to dark brown, and even to dark grey depending on the constituents. The body naturally produces about 1 quart (about 1 litre) of phlegm every day to capture and clear substances in the air and bacteria from the nose and throat.