Excretion
Chapter 9
Functions of Excretory Organs
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Maintain solute concentrations
Maintain body fluid volume
Remove metabolic end products
Remove foreign substances
Nitrogen Excretion
• Nitrogen-based waste compounds derived from
proteins and nucleic acids
• Excretion classified by major waste produced
– Ammonotelic – ammonia is the principle waste
– Ureotelic – urea is the principle waste
– Uricotelic – uric acids and urate salts are the principle
wastes
Excretory Processes
• Ultrafiltration
– Movement of fluid (e.g. blood) through a
semipermeable membrane
– Membrane allows small particles to pass with the water,
large particles (proteins etc.) remain
• Active Transport
– Movement of solutes against their electrochemical
gradients (requires energy)
– Secretion – movement of solute into the lumen of the
excretory organ
– Reabsorption – movement of solute out of lumen
Generalized Excretory Organs
• Sponges, Coelentrates and Echinoderms –
none
• Playhelminths, Nematodes, Annelids –
nephridial organs
• Crustaceans – antenna glands
• Insects – Malpighian tubules
• Mollusks and Vertebrates – kidneys
Nephridial Organs
• Common in invertebrates
• System of tubes
– connected to the outside through nephridial pore
• Protonephridia
– Found in acoelous and pseudocoelous animals (platyhelminths,
nematodes, etc.)
– Blind-ended tubes with flame cells or solenocytes at closed end
• Create current
• Draw fluid in from surrounding tissues (filtration)
• Water then reabsorbed
Nephridial Organs
• Metanephridia
– Coelous Animals (e.g., annelids)
– Inner cells open into coelomic cavity
– Four components:
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Nephrostome – funnel-shaped filter
Coiled tubule – secretion and absorption
Bladder – storage
Nephridial pore
Antennal Gland
• Crustaceans
• Paired glands located in the head
• Consist of initial sac, long coiled excretory
tubule and terminal bladder
– Excretory pore at base of antenna
Malpighian Tubules
• Arachnids and insects
• Specialized region of digestive tract
– Located btw midgut and hindgut
• Blind-ended tubules
– Blind ends locates in hemocoel
– Some end near rectum
Malpighian Tubules and Rectum
• NO ultrafiltration
– Active secretion of K+ into lumen
– H2O follows passively along osmotic gradient
– Content altered by secretion and absorption
• Fluid secreted into hindgut
• Water and solutes reabsorbed in rectum
• Uric acid precipitates
Molluscan Kidneys
• Associated with pericardial cavities
– Ultrafiltration from heart
– Secretion/reabsorption by
renopericardial canal
– Stored in bladder (renal sac)
– Released into mantle and expelled
Vertebrate Kidneys
• Ultrafiltration followed by reabsorption
– Blood plasma is filtered, then important
solutes and water reabsorbed into the blood
– 99% of filtered material is reabsorbed
– Allows animals to filter out new substances
without developing new specialized secretory
mechanisms
Vertebrate Kidneys
• Consists of numerous tubular
units called nephrons
Kidney
• Blood delivered into the glomerulus
– Tuft of fenestrated capillaries
– Site of filtration (blood pressure forces
filtered plasma out)
– Filtrate collected by Bowman’s capsule
Kidney
• Enters tubular structures
• Proximal tubule
– Reabsorption of solutes and water
• Distal tubule
– Further reabsorption and secretion
• Collecting duct
– Join several distal tubules
Kidney
• Loop of Henle
(mammals and birds)
• Thin, single loop between
proximal and distal tubules
• Allows formation of
hyperosmotic urine
Glomerular Filtration
• Occurs through fenestrated capillaries
– Plasma with small particles filters out
– Blood cells and plasma proteins remain
• Blood pressure must exceed colloid
osmotic pressure
Tubular Secretion
• Removal of excess ions
(K+, Ca2+, Mg2+, H+)
• Removal of foreign
substances
• Active Transport
Tubular Reabsorption
• Active transport of
inorganic ions Na+
• Coupled transport of
glucose, amino acids, etc.
• Osmotic uptake of water
Hyperosmotic Urine
• Mammal kidneys can excrete a
hyperosmotic urine
– concentrating mechanism occurs in the
Loop of Henle
• Countercurrent Multiplication
– generates osmotic gradient that draws
H2O out of the tubules to be reabsorbed
– due to active reabsorption of Na+ and Cl-
Loop of Henle
• Mechanism
– descending limb
• permeable to water
– ascending limb
• impermeable to water
• lined w/ ion pumps (Na+ or Cl-)
Loop of Henle:
Ascending Limb
• Na and Cl actively transported out of lumen
• urea flows out of lumen in thin segment of
ascending limb
• Creates osmotic gradient
Loop of Henle:
Descending limb
• osmotic gradient generated btw interstitial
fluid and lumen
• H2O moves out of the lumen
• Filtrate concentrated to hyperosmotic levels
• Water leaving lumen diffuses into the vasa
recta (re-enters blood)
Loop of Henle:
Ascending limb
• osmotic concentration ’s
as solutes are moved out
of the filtrate by active
Na+ transport
Collecting Duct
• Water flows out as tubule
descends into medulla
• Water leaving lumen
diffuses into the vasa
recta (re-enters blood)
• Final urine produced is
hyperosmotic
Loop Length and Aridity
• Relative length of the loops is longer in
animals adapted to dry habitats than in
those from wetter habitats