Vertebrate kidney variations and functions
Vertebrates have
two kidneys that are in the back of the abdominal cavity on either side of the
aorta. Each kidney has a coat of connective tissue called renal capsule. Inner
portion of the kidney is called medulla, the region between the capsule and the
medulla is cortex. There are three kinds of vertebrate kidneys:
(1) Pronephros:
Pronephros
appears only briefly in many vertebrate embryos and not at all in mammalian
embryos. In some vertebrates pronephros is first osmoregulatory and excretory
organ of the embryo (tadpoles and other amphibians larvae); in other hag fishes
it remains as the functioning kidney.
(2) Mesonephros:
During embryonic
development of amniotes or during metamorphosis in amphibians, mesonephros replaces
pronephros. Mesonephros is the functioning embryonic kidney of many vertebrates
and also adult fishes and amphibians.
(3) Metanephros:
Metanephros
gives way during embryonic development to metanephros in adult reptiles, birds
and mammals.
Physiological
differences between these kidney types are primarily related to the number of
blood filtering units they contain. Pronephric kidney contains fewer blood
filtering unit than either the mesonephric or metanephric kidneys. Larger
number of filtering units in the meta has allowed vertebrates to face the
rigorous osmoregulatory and excretory demands of freshwater and terrestrial
environments.
Metnephric kidney
work:
The filtration
device of metanephric kidney consists of over one million individual
filtration, secretion and absorption structures called nephrons. At the
beginning of nephron is filtration apparatus called glomerular capsule
(Bowman’s capsule). The capsules are in the cortical (outermost) region enters
and branches into fine network of capillaries called glomerulus. The walls of
these glomerular capillaries contain small perforations called filtration slits
that act as filters. Blood pressure forces fluid through these filters. Fluid
is now known as glomerular filtrate and contains small molecules such as
glucose, ions (Ca2+, PO43+), and urea or uric
acid.
Because the filtration slits are so small, large proteins and blood cells
remain in the blood and leave the glomerulus via efferent (outgoing) arteriole.
Efferent arteriole then divides into a set of capillaries called peritubular
capillaries that wind profusely around the tubular portions of the nephron.
Eventually they merge to form veins that carry blood out of the kidney.
Beyond the
glomerular capsule are proximal convolute tubule, the loop of the nephron (loop
of Henle) and the distal convoluted tubule. At various places along these
structures, glomerular filtrate is selectively reabsorbed, returning certain
ions (Na+, K+, Cl-) to the blood stream. Both
actively (ATP – requiring) and passive transport are involved potentially
harmful compounds such as Hydrogen (H+) and ammonium (NH4+) ions, drugs and
various other foreign material are secreted into the nephron lumen. In the last
portion of the nephron called collecting duct, final water re-absorption takes
place so that the urine contains an ion concentration well above that of the
blood. Thus the filtration secretion and re-absorption activities of nephron do
not simply remove wastes. They also maintain water and ion balance,
homeostasis.
Mammalian and to
a lesser extent avian and reptilian, kidneys can remove for more water from the
glomerular filtrate than can the kidneys of amphibians. For example human urine
is four times as concentrated as blood plasma, a camel’s urine is eight times
as concentrated, a gerbil’s is 14 times as concentrated and some desert rates
and mice have urine more than 20 times as concentrated as their plasma. This
concentrated waste enables then to live in dry or desert environments where
little water is available for them to drink. Most of their water is metabolically
produced from the oxidation of carbohydrates fats and proteins in the seeds
that they eat.
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