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The large intestine's main function is to transport food particles through the body and expel the indigestible parts at the other end, but it also collects waste from throughout the body. The typical brown colour of mammal waste is due to bilirubin, a breakdown product of normal heme catabolism. [1]
The liver and kidneys clear many substances from the blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the urine and feces. [ 6 ] Aquatic animals usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution.
The kidneys help maintain the water and salt level of the body. Any significant rise in plasma osmolality is detected by the hypothalamus , which communicates directly with the posterior pituitary gland .
The intestine is also called the bowel or the gut. The lower GI starts at the pyloric sphincter of the stomach and finishes at the anus. The small intestine is subdivided into the duodenum, the jejunum and the ileum. The cecum marks the division between the small and large intestine. The large intestine includes the rectum and anal canal. [2]
In the human urinary system there are two kidneys that are located between the dorsal body wall and parietal peritoneum on both the left and right sides. The formation of urine begins within the functional unit of the kidney, the nephrons. Urine then flows through the nephrons, through a system of converging tubules called collecting ducts.
The nephrons of the mammalian kidneys have loops of Henle, which are the most efficient way to reabsorb water and produce concentrated urine to conserve water in the body. [12] The mammalian kidneys combine both nephrons with a short and nephrons with a long loop of Henle. [77] The medulla is divided into outer and inner regions.
The excretion of urea is called ureotelism. Land animals, mainly amphibians and mammals, convert ammonia into urea, a process which occurs in the liver and kidney. These animals are called ureotelic. [3] Urea is a less toxic compound than ammonia; two nitrogen atoms are eliminated through it and less water is needed for its excretion.
The large intestine contains multiple types of bacteria that can break down molecules the human body cannot process alone, [40] demonstrating a symbiotic relationship. These bacteria are responsible for gas production at host–pathogen interface, which is released as flatulence. Intestinal bacteria can also participate in biosynthesis reactions.