Editing Amphibian (section)

=== Digestive and excretory systems ===
[[File:Frog anatomy tags.PNG|thumb|alt=Dissected frog|Dissected frog:
{{image key
|list type=ordered
|Right atrium
|Liver
|Aorta
|Egg mass
|Colon
|Left atrium
|Ventricle
|Stomach
|Left lung
|Spleen
|Small intestine
|Cloaca
}}]]
Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws. Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by [[inertia]]. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short [[Esophagus|oesophagus]] is lined with [[Cilium|cilia]] that help to move the food to the stomach and [[mucus]] produced by glands in the mouth and [[pharynx]] eases its passage. The enzyme [[chitinase]] produced in the stomach helps digest the [[chitin]]ous cuticle of arthropod prey.{{sfn | Dorit | Walker | Barnes | 1991 | p=847 }}

Amphibians possess a [[pancreas]], [[liver]] and [[gall bladder]]. The liver is usually large with two lobes. Its size is determined by its function as a [[glycogen]] and fat storage unit, and may change with the seasons as these reserves are built or used up. [[Adipose tissue]] is another important means of storing energy and this occurs in the abdomen (in internal structures called fat bodies), under the skin and, in some salamanders, in the tail.{{sfn | Stebbins | Cohen | 1995 | p=66 }}
<!--In aquatic amphibians, the liver plays only a small role in processing nitrogen for excretion, and [[ammonia]] is diffused mainly through the skin. The liver of terrestrial amphibians converts ammonia to urea, a less toxic, water-soluble nitrogenous compound, as a means of water conservation. In some species, urea is further converted into [[uric acid]]. [[Bile]] secretions from the liver collect in the gall bladder and flow into the small intestine. In the small intestine, enzymes digest carbohydrates, fats, and proteins. Salamanders lack a valve separating the small intestine from the large intestine. Salt and water absorption occur in the large intestine, as well as mucous secretion to aid in the transport of faecal matter, which is passed out through the [[cloaca]].<ref name="Anatomy" />---Omitting this until a more reliable source can be found.--->

There are two [[kidney]]s located dorsally, near the roof of the body cavity. Their job is to filter the blood of metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.{{sfn | Dorit | Walker | Barnes | 1991 | p=849 }}
==== Urinary bladder ====
Most aquatic and semi-aquatic amphibians have a membranous skin which allows them to absorb water directly through it. Some semi-aquatic animals also have similarly permeable bladder membrane.<ref>{{Cite journal |last1=Urakabe |first1=Shigeharu |last2=Shirai |first2=Dairoku |last3=Yuasa |first3=Shigekazu |last4=Kimura |first4=Genjiro |last5=Orita |first5=Yoshimasa |last6=Abe |first6=Hiroshi |year=1976 |title=Comparative study of the effects of different diuretics on the permeability properties of the toad bladder |journal=Comparative Biochemistry and Physiology Part C: Comparative Pharmacology |language=en |volume=53 |issue=2 |pages=115–119 |doi=10.1016/0306-4492(76)90063-0 |pmid=5237}}</ref> As a result, they tend to have high rates of urine production to offset this high water intake, and have urine which is low in dissolved salts. The urinary bladder assists such animals to retain salts. Some aquatic amphibian such as ''[[Xenopus]]'' do not reabsorb water, to prevent excessive water influx.<ref>{{Cite journal |last1=Shibata |first1=Yuki |last2=Katayama |first2=Izumi |last3=Nakakura |first3=Takashi |last4=Ogushi |first4=Yuji |last5=Okada |first5=Reiko |last6=Tanaka |first6=Shigeyasu |last7=Suzuki |first7=Masakazu |year=2015 |title=Molecular and cellular characterization of urinary bladder-type aquaporin in Xenopus laevis |journal=General and Comparative Endocrinology |volume=222 |pages=11–19 |doi=10.1016/j.ygcen.2014.09.001 |pmid=25220852}}</ref> For land-dwelling amphibians, dehydration results in reduced urine output.<ref name="VittCaldwell2013">{{cite book |author1=Laurie J. Vitt |url=https://books.google.com/books?id=Gay9N_ry79kC&pg=PA184 |title=Herpetology: An Introductory Biology of Amphibians and Reptiles |author2=Janalee P. Caldwell |date=25 March 2013 |publisher=Academic |isbn=978-0-12-386920-3 |page=184}}</ref>

The amphibian bladder is usually highly distensible and among some land-dwelling species of frogs and salamanders may account for between 20% and 50% of their total body weight.<ref name="VittCaldwell2013" /> Urine flows from the kidneys through the ureters into the bladder and is periodically released from the bladder to the cloaca.<ref>{{Cite book |last1=Feder |first1=Martin E. |url=https://books.google.com/books?id=oaS-OpEjPtUC&pg=PA108 |title=Environmental Physiology of the Amphibians |last2=Burggren |first2=Warren W. |date=1992-10-15 |publisher=University of Chicago Press |isbn=978-0-226-23944-6 |language=en}}</ref>