EXCRETION OF
WASTE
CAMPBELL & REECE
CHAPTER 44
The Urinary System
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HUMAN URINARY TRACT
Human urinary tract diagram
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REGULATION OF THE INTERNAL ENVIRONMENT
Regulation of the Chemical Environment
 Animals = avg. 70% water by weight; 2/3 within cells & 1/3 extracellular fluid (similar
to marine environment surrounding earliest cells)
Water Compartments
 Plasma- 7%
 Interstitial fluid/lymph- 28%
 Intracellular fluid- 65%
Blood Plasma Composition
 Must be regulated constantly (homeostatic environment); very selective process
 Monitoring/ analysis/ selection/ rejection
 Blood Composition is the responsibility of the EXCRETORY organs
 Nephridia: mollusks, annelids
 Malpighian tubules: insects
 Kidneys: vertebrates
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SUBSTANCES REGULATED BY THE KIDNEYS
Regulation needed:
 Excretion of metabolic wastes
 Regulation of the concentrations of ions and other chemicals
 Maintenance of water balance
Metabolic Wastes
 CO2
 Nitrogenous compounds: (NH3) ammonia= highly toxic, must convert into
nontoxic forms
 Mammals convert N-wastes to UREA (carried to kidneys to mix with water for
excretion)
 Uric acid salts (crystalline) birds, reptiles, insects waste = whitish paste
(dropping)
 Ion concentrations are regulated: Na+, Cl-, H+, Mg2+, Ca2+, HCO-3 Also excess
blood sugar (glucose)

Concentrations depend on amount of water in body (hydrated? dehydrated?)
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WATER BALANCE
Some examples of strategies:
 Freshwater- vs.- saltwater fish

Osmosis in /out? Hypertonic / hypotonic environment? "Which must gulp water
to maintain itself?" (salt glands for excretion of solutes)
Paramecium- contractile vacuoles = "bailing device"
 Sharks: Isotonicity with surrounding water = (high urea tolerance)
Sources of Water Loss and Gain in Terrestrial Animals
 Gain: drinking, eating water-containing foods,
 Water also gained as by-product of glucose-oxidation (0.6g water/g
glucose), protein oxidation (0.3 g water/g protein), fat oxidation (1.1
g water/g fat)
 Loss: sweat glands, urine, breathing (water vapor), feces
 Evolutionary adaptations: kangaroo rat = nocturnal, highly conc.
urine, no sweat glands

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FUNCTION OF THE KIDNEY- FILTRATION,
SECRETION, REABSORPTION, EXCRETION
The Kidneys (2) = 10 cm long, dark
red, bean- shaped
kidney animation
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NEPHRON- THE FUNCTIONAL UNIT OF THE KIDNEY.
EACH KIDNEY HAS ABOUT 1.3 MILLION OF THESE
LITTLE FILTRATION UNITS!
Renal tubule- composed of a
MAGNIFIED, THEY LOOK LIKE THIS: proximal (near) and a distal
(far) tubule(s), connected by a
loop of Henle
Nephron ends in Collecting
Duct
Bowman’s Capsule
Glomerulus
•Afferent arteriole- leads
in to the glomerulus
•Efferent arteriole- leads
out
"Peritubular" capillaries =
surround the nephron tubes
Venules, Capillaries
Loop of Henle
Renal pelvis
Collecting duct
Distal tubule
Proximal tubule
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THE KIDNEY’S JOB:
As blood flows though the nephrons, materials are
removed from the blood, enter the nephron.
Along the way, some materials are re-absorbed back into
the bloodstream (osmoregulation and selection happen
to maintain blood concentrations)
Filtration and removal of metabolic wastes from the blood
 Urea (nitrogen waste from proteins)
 Excess Salts
 Excess Water
 Excess Glucose (sugar)
 Excess Minerals (Na+, K+, Ca2+)
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FORMATION OF URINE:
Filtration: materials from the blood are forced out
of the glomerulus into the Bowman’s capsule
(“filtrate”).
 Reabsorption: some of the substances that left
the blood need to be returned, for homeostasis
purposes, and so are selectively diffused or
actively transported out of the proximal convoluted
tubule back into the capillaries
 Secretion: final “tweak”. Some more wastes and
H+ ions are secreted from the blood into the distal
convoluted tubule

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WHERE IT HAPPENS
diagram of the "what" and "where" HERE
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HUMAN URINARY TRACT
Human urinary tract diagram
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CONTROL OF KIDNEY FUNCTION: THE ROLE OF
HORMONES



kidney animation
Human urinary tract diagram
ADH (antidiuretic hormone)- formed by hypothalamus, released
by pituitary gland




Increase permeability of the Collecting ducts (of the nephrons) to water—
more water moves back into blood (decreases water excretion)
ADH release is triggered by BP sensors in walls of blood vessels
(especially dehydration, hemorrhage…)
Alcohol inhibits ADH secretion; symptoms
Aldosterone - produced by adrenal cortex; causes reabsorption
of NaCl into blood and secretion of K+ into nephron
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