The main excretory organ of vertebrates is the kidney, and the functional unit of the kidney is the nephron, which has a blood vessel component and a tubule component. The vascular component begins with a knot of capillaries that are highly permeable and filter the blood into the tubule component. Processes of secretion and reabsorption in the tubules produce urine, which differs in composition from the extracellular fluids of the body.
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Reptiles can be more independent of water sources than amphibians can.
The nephron produces urine through filtration of blood plasma in the glomerulus and tubular processing of the filtrate by reabsorption and secretion of solutes.
Glomerular filtration involves anatomical features of glomerular capillaries and cells of Bowman’s capsule.
The liquid that enters Bowman’s capsule is called the renal filtrate and is further processed into urine as it moves through the renal tubules of the nephron.
The evolution of vertebrates is thought to have begun with a marine ancestor that moved into a freshwater habitat. The excretory system of this vertebrate ancestor would have evolved to excrete large quantities of water while conserving salts. But if the early vertebrate excretory system evolved in response to the need to excrete water, how did subsequent vertebrate lineages adapt to environments where water must be conserved and salts excreted? The answer to this question differs among vertebrate groups. Even among marine fishes, the excretory adaptations of the bony fishes differ from those of the cartilaginous fishes. Reptiles, including birds, and mammals have excretory systems that conserve water. The reptiles achieve this mainly by being uricotelic and producing a semisolid excretory product that contains little water. Mammals, in contrast, are ureotelic; they excrete a liquid waste product but have evolved the ability to produce highly concentrated urine.