EXCRETORY SYSTEM

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EXCRETORY SYSTEM
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Under the segmental somites therefore segmented
themselves into NEPHROTOMES In each nephrotome is
a NEPHRON ARCHINEPHRIC DUCT joins the nephrons
together
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NEPHRON consists of two layered cup of epithelium
RENAL CAPSULE (Bowmans capsule). Capsule
envelops a knot of capillaries GLOMERULUS with
an afferent arteriole and an efferent
arteriole. Whole thing RENAL CORPUSCLE. Rest
is simple epithelial tube which varies among
vertebrates - proximal tubule absorptive
variable length intermediate tubule distal
tubule collecting tubules excretory duct.
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• Filtration mechanism increased by
• 2 arterioles (rather than ateriole and veinule)
  and efferent smaller so pressure increases.
• Diffusion distance short, capillaries
  fenestrated, filtration slits on podocytes.
• Larger molecules held back (lt 60K m. wt)
  everything else goes through. Amino acids,
  water, sugars recovered, rest eliminated.

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Podocyte feet form outer diffusion barrier
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Proximal and distal convoluted tubules have
microvilli for absorption
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Recovery takes place in the tubules through
selective resorption, leaving urine to be
excreted. Resorption into PERITUBULAR
CAPILLARIES Blood supply varies A efferent
renal renal portal vein B renal portal vein
C efferent renal
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3 types of nephron arrangements
1. The first few nephrons at anterior end of
ammocoetes (larval lampreys) and some larval
amphibians external glomeruli. Filtrate goes
into coelom and then into duct by cilia
PRIMITIVE (present in many larvae and
invertebrates)
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2. Elasmobranchs, primitive actinopterygians,
many amphibians have connection to coelom
3. Most vertebrates have the nephron described
internal glomeruli
Suggests an evolutionary sequence
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Nephrons differentiate in rostral to caudal
sequence PRONEPHROS, MESONEPHROS,
METANEPHROS. Proposed that the primitive
condition is all pronephric and they all work
HOLONEPHROS (hypothetical ancestral kidney)
This resembles larval hagfish, but they dont all
work.
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PRONEPHROS forms at rostral end. Variable
number amniotes 1 3, non-functional
anamniotes 12, functional in larvae of hagfish,
lampreys, bony fish, amphibians, present in adult
hagfish. But when non-functional it initiates
formation of archinephric duct.
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MESONEPHROS forms after a gap, segmentation
gradually lost due to secondary tubules
developing. Functions in embryos and larvae of
all vertebrates. Forms a separate unit in
amniotes.
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OPISTHONEPHROS where mesonephros doesnt form a
separate unit and runs to caudal end e.g. adult
hagfish
Primitive opisthonephros
Advanced opisthonephros Fish, amphibians Lost
segmentation, caudal part enlarges, cranial part
used for sperm
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METANEPHROS amniote invention, mesonephros
functions in embryo only. Ureteric bud extends
from caudal archinephric duct, grows into caudal
end of nephric ridge, branches, induces renal
tubules. Ureteric bud becomes collecting tubules
and uterer.
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Ureteric bud separates from archinephric duct. In
males archinephric duct is involved with sperm
movement, in females it degenerates. In reptiles
and birds they enter the CLOACA
independently. Mammals have a separate hindgut
and u-g system and ureteric bud enters bladder
(from allantois)