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Regulation (Homeostasis)

Urea portion of the abdominal cavity (Figure
Animals like sharks, adult amphibians, 7.5). The larger left kidney is located a bit
and mammals usually excrete urea as their higher than the right kidney. Unlike other
nitrogenous waste. Urea is a much less
toxic, and less soluble than ammonia. It can organs found in the abdomen, kidneys are
be excreted in a moderately concentrated located behind the lining (peritoneum)
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solution. This elimination strategy allows of the abdominal cavity. Thus, they are
body water to be conserved, an important considered retroperitoneal organs. These
advantage for terrestrial animals with bean-shaped organs are protected by the
limited access to water. The animals in
this group are termed as ureotelic. back muscles and the ribs as well as the fat
(adipose tissue) that surrounds them like
Uric acid a protective padding. Located above each
Uric acid is not toxic and it is insoluble kidney is an adrenal gland which secretes
in water. Poor solubility is an advantage
if water conservation is needed. Uric acid adrenaline hormone. For each kidney, the
can be concentrated even more readily renal artery supplies blood, which a renal
than urea can. Uric acid is usually excreted vein returns to the vena cava. The ureter
by insects, reptiles, and birds, these are removes urine produced by the kidney,
collectively termed as uricotelic animals. transfering it to the urinary bladder for
temporary storage, before being released
7.3.3 Kidney position, structure and through the urethra. The exit from the
functions in human body bladder is controlled by a sphincter (a ring
Kidneys are paired bean-shaped organ of muscles).
found on each side of the back of the lower

Right adrenal gland
Aorta

Right kidney Left renal artery
Left renal vein

Vena cava

Left ureter
Urinary bladder
Sphinter
Urethra

Figure 7.5: Human urinary system
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