Homeostasis and Excretion
• Homeostasis - stable internal
• Done in three ways.
• 1Thermoregulation - maintenance of
specific body temperature.
• 2Excretion - get rid of wastes.
• 3Osmoregulation - maintenance of
water and solute balance.
• Warm-blooded animals
(endotherms) – regulators; regulate
internal body temperature
• Conformers - cold-blooded animals
(ectotherms); do not have constant
internal temperature (lizards) compensate for temperature
through behavior (sit in sun if cold).
• Temperature of hypothalamus
determines body temperature.
• Fevers - response to infection alters set-point to higher level than
normal to try and slow replication
of microorganism that infected
• Cells require water and specific
concentrations of salts.
• Most organisms have salt
concentration equal ocean.
• Cells must regulate salt
concentration in order for bodies
• Cells produce nitrogenous waste must be removed.
• Excretory system responsible for
balance of water and salt and
removal of nitrogenous wastes.
• Birds excrete uric acid - semiliquid
• Vertebrates - different system for
• Primary organ – kidney - forms
urine that passes to urinary bladder
• From bladder, urine passes to
exterior of body via urethra.
• Kidney filters blood - remove
harmful metabolic waste (urea)
while retaining cells, proteins, salts,
glucose, other essential factors in
• Also regulate volume, salt content
of extracellular fluids, like blood.
• Nitrogen removed from amino acid,
released to ammonia - toxic.
• Humans - liver converts ammonia to
urea - less toxic than ammonia kidneys remove urea from
• Basic unit of kidney that performs
functions - nephron
• Nephron is a small tubelike
structure - filters blood, modifies
filtrate to produce urine.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Nephron 4 jobs…
• 1- Filtration: filter blood
• 2- Secretion: secrete material into
• 3- Reabsorption: reabsorb material
• 4- Excretion: formation of urine
• Blood filtered enters nephron in
ball-shaped cluster of capillaries glomerulus.
• The glomerulus is inside a cuplike
structure called the Bowman’s
• Pressure of blood in glomerulus
squeezes liquid part of blood
through filtering structure.
• Blood cells - too large to pass
through; most proteins retained in
blood because of size and charge.
• Other smaller molecules (salts,
amino acids, glucose, water, urea)
pass easily into filtrate.
• Filtrate leaves blood, enters
structure around glomerulus Bowman’s capsule.
• Starting end of nephron.
• From Bowman’s capsule filtrate
moves down nephron tubule,
becoming increasingly modified.
• Passes through proximal convoluted
tubule, loop of Henle, distal tubule,
• Proximal convoluted tubule, active
transport pumps glucose, amino
acids, sodium, proteins out of
• Water follows through osmosis,
concentrating urine, reducing
volume of filtrate.
• Reabsorbed materials reenter blood
in capillaries that surround nephron.
• Conserves necessary materials that
may be wasted in urine
• From proximal tubule, filtrate
passes to loop of Henle.
• Glomerulus and Bowman’s capsule
both located in outer region of
kidney (cortex), loop of Henle dips
into inner kidney region (medulla).
• Medulla has high concentration of
• As filtrate passes down loop of
Henle, water drawn out of filtrate
due to osmosis, passing from low ion
concentration in filtrate to high ion
concentration of extracellular fluid
• When filtrate passes back up loop
of Henle, sodium is pumped out into
• Help further reduce volume of
urinary filtrate by drawing water
along with sodium; helps to
preserve high concentration of
sodium in medulla.
• After passing through distal tubule,
filtrate must pass through
collecting duct before passing out
to ureter and urinary bladder.
• Collecting duct passes back down
through high ion concentration in
• To make concentrated or dilute
urine, vasopressin (antidiuretic
hormone or ADH) regulates how
permeable walls of collecting duct
• Determines how much salt actually
secreted, how much is retained.
• Person needs water - secrete
vasopressin, excrete more
• Secreted by posterior pituitary
gland when stretch sensors in
arteries detect drop in blood
• Vasopressin acts on walls of
collecting ducts - make them more
permeable to water.
• Fluid of medulla very concentrated
with ions - water will flow out of
collecting ducts if walls of
collecting duct are water permeable
and allow osmosis.
• Saves water, creates concentrated
• No vasopressin present - walls of
collecting ducts do not permit
osmosis, urine will remain dilute.
• Regulates urine - aldosterone
• Secreted in response to low
extracellular sodium - distal tubule
increases resorption of sodium
from urinary filtrate.
• Water removed from filtrate by
osmosis, reducing urine volume,
increasing volume of extracellular
fluids - increases blood pressure.
• Secretion of aldosterone helps to
conserve water loss.