Editing Common ostrich (section)

====System overview====
The common ostrich is well-adapted to hot, arid environments through specialization of [[excretory]] organs. The common ostrich has an extremely long and developed [[colon (anatomy)|colon]] {{endash}} a length of approximately {{cvt|11|-|13|m}} {{endash}} between the [http://medical-dictionary.thefreedictionary.com/coprodeum coprodeum] and the paired [[Pyloric caeca|caeca]], which are around {{cvt|80|cm}} long.<ref name=Skadhaugeetal /> A well-developed caeca is also found and, in combination with the [[rectum]], forms the [[microbial fermentation]] chambers used for [[carbohydrate]] breakdown.<ref name=Skadhaugeetal /> The [[catabolism]] of carbohydrates produces around {{cvt|0.56|g|gr}} of water that can be used internally.<ref name=zool241 /> The majority of their [[urine]] is stored in the coprodeum, and the [[feces]] are separately stored in the terminal colon.<ref name=Skadhaugeetal /> The coprodeum is located ventral to the terminal rectum and [[urodeum]] (where the [[ureters]] open).<ref name=Deeming /> Found between the terminal rectum and coprodeum is a strong sphincter.<ref name=Deeming /> The coprodeum and cloaca are the main osmoregulatory mechanisms used for the regulation and reabsorption of ions and water, or net water conservation.<ref name=Deeming /> As expected in a species inhabiting arid regions, dehydration causes a reduction in fecal water, or dry feces.<ref name=Deeming /> This reduction is believed to be caused by high levels of plasma [[aldosterone]], which leads to rectal absorption of sodium and water.<ref name=Deeming /> Also expected is the production of [[hyperosmotic]] urine; cloacal urine has been found to be 800 [[Osmole (unit)|mOsm]].<ref name=Deeming /> The U:P (urine:plasma) ratio of the common ostrich is therefore greater than one. Diffusion of water to the coprodeum (where urine is stored) from plasma across the [[epithelium]] is voided.<ref name=Deeming /> This void is believed to be caused by the thick [[mucosal]] layering of the coprodeum.<ref name=Deeming />

Common ostriches have two [[kidney]]s, which are chocolate brown in color, are granular in texture, and lie in a depression in the [[bird anatomy|pelvic cavity]] of the dorsal wall.<ref name=Shanawany /> They are covered by [[peritoneum]] and a layer of fat.<ref name=Deeming /> Each kidney is about {{cvt|300|mm}} long, {{cvt|70|mm}} wide, and divided into a [[Anatomical terms of location|cranial]], middle, and [[caudal (anatomical term)|caudal]] sections by large veins.<ref name=Deeming /> The caudal section is the largest, extending into the middle of the pelvis.<ref name=Deeming /> The [[ureters]] leave the ventral caudomedial surface and continue caudally, near the midline into the opening of the urodeum of the cloaca.<ref name=Deeming /> Although there is no bladder, a dilated pouch of ureter stores the urine until it is secreted continuously down from the [[ureter]]s to the urodeum until discharged.<ref name=Shanawany>{{cite book|last=Shanawany|first=M.M.|title=Ostrich Production Systems|year=1999|publisher=Food and Agriculture Organization of the United Nations|isbn=978-92-5-104300-4|page=32|url=https://books.google.com/books?id=BfjUW8ZVinkC&pg=PA253}}</ref>

=====Kidney function=====

Common ostrich kidneys are fairly large and so are able to hold significant amounts of [[solutes]]. Hence, common ostriches drink relatively large volumes of water daily and [[excretion|excrete]] generous quantities of highly concentrated [[urine osmolality|urine]]. It is when drinking water is unavailable or withdrawn that the urine becomes highly concentrated with [[uric acid|uric acid and urates]].<ref name=Deeming /> It seems that common ostriches who normally drink relatively large amounts of water tend to rely on [[renal function|renal conservation]] of water within the kidney system when drinking water is scarce. Though there have been no official detailed [[renal function|renal studies]] conducted<ref>{{cite web |last=Bennett |first=Darin C. |author2=Yutaka Karasawa |title=Effect of Protein Intake on Kidney Function in Adult Female Ostriches (''Struthio Camelus'') |year=2003 |pages=vii |url=http://www.publish.csiro.au/?act=view_file&file_id=EAv48n10posters.pdf}}</ref> on the [[Hagen-Poiseuille equation|flow rate]] ([[Hagen-Poiseuille equation|Poiseuille's Law]]) and composition of the ureteral urine in the ostrich, knowledge of [[renal function]] has been based on samples of [[urine|cloacal urine]], and samples or quantitative collections of [[urine|voided urine]].<ref name=Deeming /> Studies have shown that the amount of water intake and [[dehydration]] impacts the [[plasma osmolality]] and [[urine osmolality]] within various sized ostriches. During a normal hydration state of the kidneys, young ostriches tend to have a measured plasma osmolality of 284 [[mOsm]] and urine osmolality of 62 mOsm. Adults have higher rates with a plasma osmolality of 330 mOsm and urine osmolality of 163 mOsm. The [[osmolality]] of both plasma and urine can alter in regards to whether there is an excess or depleted amount of water present within the kidneys. An interesting fact of common ostriches is that when water is freely available, the urine osmolality can reduce to 60–70 [[mOsm]], not losing any necessary solutes from the kidneys when excess water is excreted.<ref name=Deeming /> Dehydrated or salt-loaded ostriches can reach a maximal urine osmolality of approximately 800 mOsm. When the plasma osmolality has been measured simultaneously with the maximal osmotic urine, it is seen that the urine:plasma ratio is 2.6:1, the highest encountered among avian species.<ref name=Deeming /> Along with dehydration, there is also a reduction in [[Hagen-Poiseuille equation|flow rate]] from 20 L·d<sup>−1</sup> to only 0.3–0.5 L·d<sup>−1</sup>.

In mammals and common ostriches, the increase of the [[renal function|glomerular filtration rate (GFR)]] and [[urine flow rate| urine flow rate (UFR)]] is due to a high protein diets. As seen in various studies, scientists have measured [[clearance (medicine)|clearance of]] [[creatinine]], a fairly reliable marker of glomerular filtration rate (GFR).<ref name=Deeming /> It has been seen that during normal hydration within the kidneys, the glomerular filtration rate is approximately 92&nbsp;ml/min. However, when an ostrich experiences [[dehydration]] for at least 48 hours (2 days), this value diminishes to only 25% of the hydrated GFR rate. Thus in response to the dehydration, ostrich kidneys [[secretion|secrete]] small amounts of very viscous glomerular filtrates that have not been broken down and return them to the [[circulatory system]] through [[blood vessel]]s. The reduction of GFR during dehydration is extremely high and so the fractional excretion of water (urine flow rate as a percentage of GFR) drops down from 15% at normal hydration to 1% during dehydration.<ref name=Deeming />

=====Water intake and turnover=====

Common ostriches employ adaptive features to manage the dry heat and [[solar radiation]] in their habitat.  Ostriches will drink available water; however, they are limited in accessing water by being flightless. They are also able to harvest water through dietary means, consuming plants such as the ''[[Euphorbia heterochroma]]'' that hold up to 87% water.<ref name=Deeming />

Water mass accounts for 68% of body mass in adult common ostriches; this is down from 84% water mass in 35-day-old chicks. The differing degrees of water retention are thought to be a result of varying body fat mass.<ref name=Deeming /> In comparison to smaller birds ostriches have a lower evaporative water loss resulting from their small body surface area per unit weight.<ref name=zool241 />

When heat stress is at its maximum, common ostriches are able to recover evaporative loss by using a [[metabolic water]] mechanism to counter the loss by urine, feces, and respiratory evaporation.  An experiment to determine the primary source of water intake in the ostrich indicated that while the ostrich does employ a [[metabolic water]] production mechanism as a source of hydration, the most important source of water is food. When ostriches were restricted to the no food or water condition, the metabolic water production was only 0.5&nbsp;L·d<sup>−1</sup>, while total water lost to urine, feces, and evaporation was 2.3&nbsp;L·d<sup>−1</sup>. When the birds were given both water and food, total water gain was 8.5&nbsp;L·d<sup>−1</sup>. In the food only condition total water gain was 10.1&nbsp;L·d<sup>−1</sup>. These results show that the [[metabolic water]] mechanism is not able to sustain water loss independently and that food intake, specifically of plants with a high water content such as ''Euphorbia heterochroma'', is necessary to overcome water loss challenges in the common ostrich's arid habitat.<ref name=Deeming />

In times of water deprivation, urine [[electrolyte]] and [[osmotic concentration]] increases while urination rate decreases. Under these conditions [[urine]] [[solute]]:plasma ratio is approximately 2.5, or [[hyperosmotic]]; that is to say that the ratio of solutes to water in the plasma is shifted down whereby reducing osmotic pressure in the plasma. Water is then able to be held back from [[excretion]], keeping the ostrich hydrated, while the passed urine contains higher concentrations of solute. This mechanism exemplifies how renal function facilitates water retention during periods of dehydration stress.<ref name=zool241 /><ref name="Withers" />

=====Nasal glands=====
A number of avian species use [[Salt gland|nasal salt glands]], alongside their kidneys, to control [[Tonicity|hypertonicity]] in their [[blood plasma]].<ref name=saltglands>{{cite journal|title=Saline-Infusion-Induced Increase in Plasma osmolality Do Not Stimulate Nasal Gland Secretion in the Ostrich (''Struthio camelus'') |journal=Physiological Zoology|jstor=30163924|year=1995|volume=68|issue=1|pages=164–175|last1=Gray|first1=David A.|last2=Brown|first2=Christopher R.|doi=10.1086/physzool.68.1.30163924|s2cid=85890608}}</ref> However, the common ostrich shows no nasal glandular function in regard to this homeostatic process.<ref name=saltglands /> Even in a state of dehydration, which increases the [[Osmotic concentration|osmolality]] of the blood, nasal salt glands show no sizeable contribution of salt elimination.<ref name=saltglands /> Also, the overall mass of the glands was less than that of the duck's nasal gland.<ref name=saltglands /> The common ostrich, having a heavier body weight, should have larger, heavier nasal glands to more effectively excrete salt from a larger volume of blood, but this is not the case. These unequal proportions contribute to the assumption that the common ostrich's nasal glands do not play any role in salt excretion.

=====Biochemistry=====
The majority of the common ostrich's internal solutes are made up of [[sodium]] ions ({{chem2|Na(+)}}), [[potassium]] ions ({{chem2|K(+)}}), [[chloride]] ions ({{chem2|Cl(-)}}), total [[short-chain fatty acid]]s (SCFA), and [[acetate]].<ref name=Skadhaugeetal /> The caecum contains a high water concentration with reduced levels nearing the terminal colon and exhibits a rapid fall in {{chem2|Na(+)}} concentrations and small changes in {{chem2|K(+)}} and {{chem2|Cl(-)}}.<ref name=Skadhaugeetal /> 
The colon is divided into three sections and takes part in solute absorption. The upper colon largely absorbs {{chem2|Na(+)}} and SCFA and partially absorbs KCl.<ref name=Skadhaugeetal /> The middle colon absorbs {{chem2|Na(+)}} and SCFA, with little net transfer of K<sup>+</sup> and Cl<sup>−</sup>.<ref name=Skadhaugeetal /> The lower colon then slightly absorbs {{chem2|Na(+)}} and water and secretes {{chem2|K(+)}}. There is no net movements of {{chem2|Cl(-)}} and SCFA found in the lower colon.<ref name=Skadhaugeetal />

When the common ostrich is in a dehydrated state, plasma osmolality, {{chem2|Na(+)}}, {{chem2|K(+)}}, and {{chem2|Cl(-)}} ions all increase; however, {{chem2|K(+)}} ions return to controlled concentration.<ref name=hormones>{{cite journal |doi=10.1016/0300-9629(88)91088-2 |title=Plasma arginine vasotocin and angiotensin II in the water deprived common ostrich (''Struthio camelus'') |year=1988 |last1=Gray |first1=D.A. |last2=Naudé |first2=R.J. |last3=Erasmus |first3=T. |journal=Comparative Biochemistry and Physiology A |volume=89 |issue=2 |pages=251–256}}</ref> The common ostrich also experiences an increase in [[haematocrit]], resulting in a [[Hypovolemia|hypovolemic state]].<ref name=hormones /> Two antidiuretic hormones, [[Vasopressin|Arginine vasotocin (AVT)]] and [[angiotensin]] (AII), are increased in blood plasma as a response to [[hyperosmolality]] and [[hypovolemia]].<ref name=hormones /> AVT triggers [[Vasopressin|antidiuretic hormone]] (ADH) which targets the [[nephrons]] of the kidney.<ref name=zool241 /> ADH causes a reabsorption of water from the lumen of the [[nephron]] to the [[extracellular fluid]] osmotically.<ref name=zool241 /> These extracellular fluids then drain into blood vessels, causing a rehydrating effect.<ref name=zool241 /> This drainage prevents loss of water by both lowering volume and increasing concentration of the urine.<ref name=zool241 /> Angiotensin, on the other hand, causes [[vasoconstriction]] on the systemic arterioles and acts as a [[dipsogen]] for ostriches.<ref name=zool241 /> Both of these antidiuretic hormones work together to maintain water levels in the body that would normally be lost due to the osmotic stress of the arid environment.

Ostriches are [[uricotelic]], excreting nitrogen in the form of [[uric acid]] and related derivatives.<ref name=zool241 /> Uric acid's low solubility in water gives a semi-solid paste consistency to the ostrich's nitrogenous waste.<ref name=zool241 />