All living things have to get rid of waste. This process is called excretion and is important because a build up of waste produced as a result of processes that take place in the cells of an organism, such as respiration, can be poisonous. In humans, excretion is the output of these unwanted products such as carbon dioxide and water, which we excrete through our lungs. We also excrete nitrogen compounds, salts and water in our urine as well as salt and water when we sweat. Although it is often thought of as excretion, strictly speaking, excretion does not include the process whereby we get rid of undigested food as faeces. This is due to the fact that undigested food never passes through our cells. Plants also need to get rid of waste. During the process of photosynthesis, plants release waste oxygen and carbon dioxide from their leaves. This usually takes place during the day since it is then that the rate at which photosynthesis takes place exceeds the rate of plant respiration. At night, photosynthesis does not take place so the plant then excretes carbon dioxide.