2. Overview
The balance of water in the body
depends on the rate of exchange of
solute between internal body fluids
and the external environment.
Excretory Systems are key to
homeostasis because the dispose of
metabolic wastes, and control body
fluid composition.
3. Nitrogenous Wastes
Animals excrete nitrogenous wastes
as ammonia, urea or uric acid
Animals that excrete ammonia need
access to lots of water because
ammonia can only be tolerated in low
concentrations
Urea combines ammonia with carbon
dioxide, and it has a very low toxicity
◦ Excreted by vertebrate mammals, turtles,
amphibians, sharks, etc.
4. Insects, birds, snails, and many
reptiles uric acid which is essentially
non toxic.
Its non toxicity makes it
hydrophobic, thus it can be excreted
without much water loss.
Requires more ATP to the synthesize
from ammonia.
5. Excretory Processes
Most animals produce fluid waste
known as Urine
◦ Body fluid (i.e. Blood) comes in contact
with a selectively permeable membrane of
the transport epithelium
◦ Filtration is driven by hydrostatic
pressure (blood pressure)
Water, small salts, sugars, nitrogenous
wastes, and amino acids are removed from
body fluids to create filtrate
6. Filtrate is converted into fluid waste
when reabsorption places useful
materials from the filtrate back into the
blood stream by active tranport.
Selective Secretion pumps
nonessential solutes and wastes into
the filtrate by active transport.
7. Excretory Systems
Excretory systems vary widely among
animal groups
◦ Flatworms that lack a coelom or body
cavity have protonephridia which form
network of dead end tubules connected to
external openings
◦ Annelids (ex: earthworms) have
metanephridia, excretory organs that
open internally to the coelom
8. Insects and terrestrial anthropods
have Malpighian tubules which
extend from dead end tips to the
digestive tract
Kidneys are present in vertebrates
and chordates
◦ Consist of highly organized tubules
associated with a network of capillaries
◦ Ducts and other structures carry urine
from the tubules and out of the body
9. Structure of Mammalian Excretory
System
The average kidney is about 10 cm
long
Blood is supplied to the kidney
through the renal artery and drained
by the renal vein
◦ take up less than 1% of the body’s mass
but receive ¼ of the blood that leaves the
heart
10. Urine exits the kidney through two
ducts called ureters
Ureters drain into the urinary
bladder
Urine is expelled through a tube called
the urethra
The sphincter muscles regulate
urination near urethra and bladder
