Our daughter Aisling would like to know why we have evolved two bodily systems to excrete waste products. Why do we have to both poo and wee?
鈥� Strictly speaking, the question is misplaced. We do not 鈥渆xcrete鈥� faeces because our bodies are long fleshy tubes which can be thought of as extremely elongated doughnuts. In a doughnut, one would not consider the hole to be 鈥渋nside鈥� the cake. Similarly, the tube from our mouth through our gut to our anus is technically 鈥渙utside鈥� our living body.
鈥淎 human is an elongated doughnut: the tube from our mouth to our anus is outside our living body鈥�
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The process of 鈥渆xcretion鈥� is the passing of material from inside our bodies to the outside. Our kidneys excrete urine, our skin excretes sweat, our lungs excrete water and carbon dioxide, and the inside of our bowel tube excretes many things along its length to assist digestion, as well as disposing of waste products in our bile. The other excreta our bodies produce include tears, earwax, and various secretions associated with our reproductive processes. If young Aisling suffers (or is about to suffer) from spots, then these too are caused by excretions which have gone awry.
Our faeces, on the other hand, consist of undigested food and bacteria. It has never actually been inside our bodies. Apart from the bile and one or two other remnants from our exocrine glands, it cannot be regarded as excreta, despite the common use of that word to describe it.
Bryn Glover, Cracoe, North Yorkshire, UK
鈥� The types of waste we excrete have two different origins. Faeces contains the leftover, indigestible portion of the food we eat, plus bile from the liver, which gives excrement its brown colour.
Urine, on the other hand, is the result of blood filtration in the kidneys. Urine contains nitrogenous waste, primarily in the form of urea, which results from the metabolism of nucleic acids and proteins, separate from digestion. Furthermore, urine also contains water and solutes from the blood, and interstitial fluids 鈥� these are excreted to maintain water balance. Essentially, faeces are the result of a coarse, large-scale process of the digestive system alone, whereas urine production occurs at a much finer scale, eliminating wastes produced by all the body鈥檚 cells.
Our two excretory systems are obvious because we have a separate opening for each: the anus and the urethra. In other organisms, the distinction between the systems isn鈥檛 as obvious. Animals such as birds, reptiles, and amphibians possess a , which serves as a common opening for both liquid and solid wastes. Insects blur the line even further: rather than having a distinct urinary system, insects rely on in their digestive systems 鈥� outgrowths of the gut that perform the same filtration function as our kidneys.
Shaun Hug, La Mirada, California, US