Download The Excretory System

The Excretory System
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Regulation of the osmotic and ionic
composition of intracellular fluids and
extracellular fluids is critical to maintain
homeostasis
– Osmoregulation – active regulation of
osmotic pressure of body fluids to keep
them from becoming too dilute or too
concentrated
– Excretion – process of ridding the body
of metabolic wastes and excess water
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Metabolic wastes include water, carbon dioxide, and
nitrogenous wastes
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Carbon dioxide is excreted mostly by respiratory
system
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Excretory organs (such as kidneys) remove most of
nitrogenous wastes and excess water
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Nitrogenous wastes include ammonia, uric acid, and
urea
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during the breakdown of amino acids, ammonia is
produced (deamination – process of removing
amino group – produces ammonia)
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ammonia is highly toxic – converted into less toxic
urea (urea is produced in liver in mammals and
amphibians) or uric acid (insects, many reptiles and
birds)
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Osmoregulation in marine invertebrates
– most release wastes across general surface
membranes
– water balance is not a problem since most are
isotonic with sea water (osmoconformers)
– Organisms living in coastal environments
must survive fluctuating conditions (fresh
water) – osmoregulators – maintain optimal
salt concentrations regardless of changes in
environment
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example: shore crab – body fluids are
hypertonic to brackish water (mixture of salt and
fresh water) – gills remove salts from water and
put into blood while excretory organs excrete
excess water that diffuses in
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Freshwater Animals – body fluids are
hypertonic to fresh water – must deal with
constant influx of water and loss of salts
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Aquatic mammals prevent this by having an
impermeable barrier (skin, fur) – possible
because breathe air
Fully aquatic animals must remain permeable
for gas exchange – excess water is
eliminated thru very dilute and copious urine
produced by kidneys and special cells in gills
absorb salts
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Marine Vertebrates – hypotonic to
seawater – results in excessive water
loss, excessive salt intake
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Drink water continuously, special cells in gills
remove excess salt
Nitrogenous wastes removed through gills
(ammonia) – very little urine is produced
Sharks – solve problem by retaining urea in
blood – keeps blood slightly higher conc.
than sea – salts excreted by special cells in
rectum
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Terrestrial Animals – biggest problem is
dessication
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replace water by drinking, food and products
of cellular respiration
metabolic wastes (ammonia) harder to get rid
of – ammonia is quickly converted to urea
(much less toxic)
Urea is very soluble and must be released in
a watery solution
Reptiles, birds, and insects excrete uric acid
instead (very insoluble) – conserves water
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Excretory mechanisms in Animals
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Contractile Vacuoles in protozoa – fill w/water
– contracts to eject water from cell – primarily
used for elimination of excess water and
some wastes
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Flame cell systems – flatworms
• beginning of tubular excretory system
• tubules run length of body – open to outside of body
thru tiny pores
• bulblike structures at end of tubules remove water
from tissue with help of cilia – travels down tubules
to pores
– Nephridia of earthworms – excretory organ
• closed circulatory system – blood vessels
have become associated with excretory
organs
• nephridium consists of nephrostome (open
ciliated funnel), a coiled tubule connecting
nephrostome to a bladder and a
nephridiopore (mat’ls pass to outside)
 blood capillaries surround tubule –
mat’ls move into nephrostome – also
picked up directly from blood by coiled
tubule
The Vertebrate Kidney
• Closely associated with the circulatory system
• Kidney structure:
– Cortex – outer part of kidney
– Medulla – inner portion
– Renal pelvis – center cavity
• Associated structures:
– Ureters – carry urine from each renal pelvis to bladder
– Urinary bladder – stores urine
– Urethra – tube that carries urine from the body
– Renal artery – carries blood to kidney from aorta
– Renal vein – carries blood away from kidney to inferior
vena cava
Kidneys are made up of microscopic nephrons –
functional units
• Bowman’s capsule – cup shaped top of nephron
• Glomerulus – network of blood capillaries tucked
into Bowman’s capsule
• Proximal convoluted tubule – 1st segment of the
renal tubule closest to Bowman’s capsule
• Loop of Henle – extension of tubule that reaches
down into medulla
• Distal convoluted tubule – part of tubule distal (far)
from Bowman’s capsule
• Collecting tubule (duct) – collects and sends urine
to renal pelvis
Formation of Urine
Occurs in three steps:
1. Filtration – blood pressure forces fluid
from blood in glomerulus into Bowman’s
capsule
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Water and dissolved substances enter
capsule
Filtrate – contains salt, glucose, vitamins,
nitrogenous wastes, and other small
molecules
Blood cells and plasma proteins are too big
and stay behind in blood
2. Reabsorption – substances move out of renal
tubule back into blood in capillaries surrounding
nephron
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Occurs in proximal conv. tubule, Loop of Henle, distal
conv. tubule, and collecting duct
100% of glucose is reabsorbed in proximal conv.
Tubule
Descending branch of Loop of Henle is permeable to
water but not very permeable to salts
Ascending branch of Loop of Henle is NOT permeable
to water - pumps actively transport salts into medullar
tissue
Tissue surrounding Loop of Henle becomes
concentrated causing more water to diffuse out by
osmosis at the descending portion – counter current
multiplier system
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Filtrate entering distal conv. tubule is actually
less concentrated (amount of salts depends on
salt intake)
Collecting tubule is permeable to water and
osmosis occurs into surrounding hypertonic
medullar tissues – urine becomes more
concentrated
Amount of water leaving with urine depends on
water levels in body
3. Secretion – last step of filtration
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Substances move from capillaries surrounding
nephron into urine (H+, K+, ammonia, certain
drugs)
Hormonal control of Kidneys
• Hypothalamus controls kidneys – monitors
amount of water in body fluids
• Makes a hormone called antidiuretic hormone
(ADH) – stored in posterior pituitary
• When water levels are too low, hypothalamus tells
pituitary to release stored ADH
• ADH causes collecting duct to become more
permeable – more water passes out of collecting
duct back into blood – conserves water, urine is
more concentrated
• When water levels are too high, hypothalamus
signals pituitary to release less ADH – collecting
ducts become less permeable to water – more
passes out with urine
Release of Urine
• Urine is carried by ureters to urinary bladder
– Elastic muscular bag – can greatly expand
– Special stretch receptors send messages to
brain
Urethra carries urine
away from bladder
and out of body –
closed off by
sphincter muscle