 Hi, my name's Broderick and I'm a science communicator. Today, I'm talking shit. Animals need systems for removing waste from digestion as well as any toxins, and to maintain a delicate balance of retaining the right amount of water and salts for the cells to operate well. Once the animals' intestines have extracted as many nutrients as they can from whatever was ingested, what is left is mostly cellulose from plant matter or fibre, some excess salts and water. The cells of the large intestine or colon reabsorb most of the water available, and the remaining waste, the feces, become more solid as they get moved along. Once they reach the end of the colon, called the rectum, the feces are stored until they can be removed at an appropriate time. So much of the transport within and between cells throughout animal bodies relies on having the right osmotic pressure or osmolarity. This is the concentration of solutes, for example salts or sugars, in water. Animal excretory systems, also called urinary systems, maintain this delicate balance. We'll take a look at how their structure enables this. Different animals have different styles of excretory systems that suit their fluid intake and environments. Marine animals are constantly taking in seawater. Humans need to drink a fair amount of water every day. Some desert reptiles and birds never drink water. Despite these differences, most animal excretory systems have three stages. First, the collection of fluids, then filtration and selective absorption of molecules. And lastly excretion. The collection and filtration of fluids requires the transport of molecules and water between structures. In most systems, this is facilitated by a network of many little tubes, or tubules, playing parallel to and covered by tiny capillary networks from the circulatory system. In earthworms, each segment of the worm has a discreet, functioning combination of lace capillaries around the excretory system that do each of the three stages. In insects, these tubes are attached to the digestive system, extracting the goods into the hemolymph, which is the insect version of blood, and return the waste into the rectum. In humans and other vertebrates, the main organs of the excretory system is the kidney. We have two kidneys, which contain many of these tubes and capillary networks, and a separate holding location for waste, the bladder. Mammal's kidneys are very compact, highly organised structures that coordinate the collection and filtration of fluids. The functional unit of the kidney is the nephron. There can be about a million nephrons per kidney. Each nephron is a coordinated system of capillary and tubule. The ingoing blood enters a special capillary network called a glomerulus, and is surrounded by the nephron's tubule end. The tubule then undergoes various twists and turns, a long loop down and up, and a collecting duct. Similar to the way the large surface area of the villi in the digestive tract allows for better uptake of nutrients, the long length of the nephron tubules allows for better transport in and out of the blood. The blood vessels and the tubules never directly connect, but they sit within a fluid, which allows for the diffusion of molecules back and forth across membranes. Different components of the specialised structure, as well as signals from the nervous system and hormones, regulate what the nephron returns to the blood and what it collects. The cells in the tubule regulate the pH, extract toxins and manage salt and water levels for the blood, by some active and some passive transport. The veins around the nephron take the filtered blood back to the heart to be re-oxygenated. The final waste fluid, urine, continues along the collecting duct into the ureter, which leads it to the bladder for storage until the animal urinates.