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Osmoregulation of Marine Mammals, The kidneys superficially lobulate -…
Osmoregulation of Marine Mammals
URINE CONCENTRATION
Marine mammals can produce urine with an osmolality greater than of sea water. (Highest urine osmolality measured in any marine mammals was 2658 mosmol 1-^1
Increases in urine osmolality is associated with increased plasma osmolality.
Even mammals found in freshwater habitats can concentrate their urine in response to a hyperosmotic stimulus.
Marine mammals do not regularly concentrate Na+ and Cl to levels above that of sea water
Marine mammals they do not rely on the consumption of sea water to maintain their fluid balance.
KIDNEY STRUCTURE
Kidney are the principal organs and electrolyte regulation
In manatees (order Sirenia), the kidney is considered superficially lobulate since it lacks true reniculli and the cortex is continuous.
Kidney structures identified in cetacean kidneys that are lacking in those terrestrial mammalian kidneys studied are
1) specialized glycogen stores in the proximal convoluted tubule epithelial cells
2) highly concentrated bundles of medullary blood vessels.
3) presence of sporta perimedullaris musculosa, a layer of collagen, elastic fibers and smooth muscle separating the cortex from medulla
Kidney in marine mammals possess the anatomical prerequisites necessary to produce a highly concentrated urine, which is especially important for mammals in a hyperosmotic environment.
DOLPHINS AND WHALES
Hormonal Regulation
Renin in dolphin causes a vascular pressor effect as in other mammals
The pressor effect is dose-dependent
The respond is graded until a treshold is reached that results in a sustained elevation in blood pressure.
RAAS present and active and have same sensitivity of sea lion.
(Malvin et al., 1978) demonstrate a positive correlation between plasma renin activity and aldosterone content that suggesting the RAAS is present and active in bottlenose dolphins.
A correlation between urine flow rate and plasma AVP concentration in fasting dolphins was not observed, Malvin et al. ( Malvin et al., 1978) conclude that AVP do not show any significant on regulating urine volume and thus, water retention in these animals.
Kidney is considered superficially
lobulate since it lacks true reniculi and the cortex is continuous
made of
A single medullary pyramid inserted in a calyx
Contains discrete cortical tissue
Hundreds of individual lobes, or reniculi
SEALS AND SEA LIONS
Water immersion
Pinnipeds spend much of their time in the water, therefore the Henry-Gauer reflex, which raises urinary water loss when submerged, may work against them.
However, there was no increase in the output of urine when harbour seals were exposed to water immersion pressure effects via negative-pressure breathing, implying that seals do not have a Henry-Gauer reflex.
For seals, who are mammals that live largely in water, the lack of reflex is advantageous
When humans are immersed in water, they experience diuresis, which is connected to increased thoracic and mean arterial pressures.
This phenomenon is linked to the Henry-Gauer reflex, which was first discovered in dogs and describes the connection between the cardiovascular system and the kidney.
Osmolality
Primary hormones responsible for osmoregulation in mammals are angiotensin, atrial natriuretic peptide, aldosterone, and vasopressin.
Angiotensin I is converted to angiotensin II by renin, and angiotensin II stimulates the release of aldosterone from the adrenal gland, inducing the resorption of Na+ in the distal tubule of the nephron.
ANP opposes the actions of angiotensin II and aldosterone by inhibiting the synthesis and release of renin, resulting in an increase in excreted Na+.
Vasopressin stimulates the synthesis of water channels (aquaporins) in the collecting duct, and is the most potent antidiuretic agent.
Hyponatremia in seals is reconciled via the pituitary–adrenal axis.
Positive correlations between plasma renin activity and aldosterone levels suggest that electrolyte balance is regulated via the renin–angiotensin–aldosterone system (RAAS).
Tubular water resorption is mediated via AVP in seals.
Intravenous infusion of pitressin (synthetic AVP) in a water-loaded harbor seal resulted in an immediate decrease in urine flow rate together with concomitant increases in urinary electrolyte concentrations.
Under force-fasted conditions, Baikal and ringed seals exhibited an increase in excreted AVP associated with a concomitant decrease in urine flow rate and increase in urine osmolality.
Naturally fasting, postweaned elephant seal pups demonstrated an increase in urine osmolality despite a decrease in plasma AVP.
simulated diving/apnea
Adaptive advantage from the perspective of decreasing oxygen consumption by the kidneys and increasing the delivery of oxygenated blood to other tissues.
The regional shunting of blood flow to the kidneys appears to be the cause of this decrease in GFR,
MANATEES AND DUGONGS
The manatee's ability to concetrateits urine which led to a speculation that the manatee may consume sea water to maintain fluid homeostatis (Irvine et al., 1980)
Deuterium oxide were used in the water overturn studies that revealed that manatees do not consume sea water voluntarily and during the period of food deprivation, their water need are met from metabolic water production , as in pinnipeds and cetaceans ( Ortiz et al., 1999)
Diferences of manatees (fresh water) and dugong (salt water)
Dugong : kidney elongated internally
Manatees : kidney has several large lobes with a continuous cortex
Wild freshwater manatees have similar plasma osmolality and electrolyte concentrations to those in salt water, suggesting they have access to salts by temporarily living or eating aquatic vegetation
The sensitivity of the RAAS in
manatees is much greater than in pinnipeds and dolphins
commonly found in Na+-depleted environments
and do not drink sea water
renal blood flow and filtration rate are associated with dietary nitrogen levels
RPF and GFR may be relatively low because of
the low protein content of their diet
SEA OTTERS
Kidney
Has lobulate kidney, but larger with respect to body mass, compared to other marine mammals (Costa,1982.,Hoover & Tyler,1986).
Urine concentration
Has slightly greater urine concentrating-ability than freshwater otters (Ortiz,2001).
can excrete Na+ & Cl- in greater condition (Costa,1982).
They are able to actively consume sea water for free water (Costa,1982).
FUTURE DIRECTIONS
Seals in freshwater have not lost their osmoregulatory functions.
Comparative renal studies of representative species in each of the major taxonomic groups of marine mammals are needed to understand the evolution of osmoregulatory functions.
The underlying mechanisms that regulate water and solute excretion and resorption require further investigation, including a hormonal component.
More research is needed to compare osmoregulatory hormones between river and sea otters.
There are significant knowledge gaps that need to be addressed to better understand osmoregulation in marine mammals.
The kidneys superficially
lobulate
Lacks true reniculi and the cortex is continuous