Download Bowman`s capsule movie

Lecture 21, Excretory Systems:
Overview
• Excretory Systems:
– Essential to homeostasis because
they dispose of wastes and respond
to imbalances in ions
• Steps in excretion include:
– Filtration: blood exposed to
semipermeable membrane that
retains proteins and other large
molecules; water & small solutes
forced out
– Reabsorption: selective, active
transport of solutes: e.g., glucose,
AA, salts
– Secretion: solutes removed from
body fluid & forced out
Human Excretory System
Bowman’s capsule movie
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The Vertebrate
Nephron:
functional unit of
the kidney
The Nephron in detail
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Million nephrons: 80 km of
tubules
Bowman’s capsule: cup-like
swelling containing glomerulus
Glomerulus: capillary cluster
Proximal tubule
Loop of Henle: hairpin turn
– Descending limb
– Ascending limb
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Distal tubule
Collecting duct
Afferent arteriole: to glomerulus
Efferent arteriole: from glomerulus
Loop of Henle Movie
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Nephron: what happens, & where
• Proximal tubule: 2/3 H2O
& NaCl reabsorbed
• Descending limb:
permeable to H2 O but not
salt
• Ascending limb:
impermeable to H2 O
– Thin segment: permeable to
NaCl which diffuses
passively
– Thick segment: actively
transports NaCl out
• Distal tube:
– Reabsorbs HCO 3– Actively reabsorbs NaCl
– Passively absorbs Urea &
H2 O
Collecting Duct Movie
Counter-current Multiplier System
• Loop of Henle:
– Counter-current: Opposite direction of urine
flow in ascending and descending limbs
– Multiplier: Maintains concentration gradient
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How Urine is Concentrated
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The loop of Henle acts as a
countercurrent multiplier to
establish a concentration
gradient in renal medulla
Thick segment of ascending
limb pumps out NaCl out of
urine; H2 O cannot follow
because this region is
impermaeable to H2O; Sets up
conc. gradient
Increased conc. NaCl in tissue
fluid causes osmotic absorption
of H2O from descending limb,
thus concentrating the tubule
fluid that enters the ascending
limb
How Urine is Concentrated
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4. The urine entering the
collecting duct is less conc.
than tissue fluid, so as the urine
passes down the collecting duct
it loses H2O and becomes more
conc.
5. H2O resorbed from the
descending limb and the
collecting duct leaves the
medulla in the vas recta
6. Lower coll. duct is
permeable to urea as well as
H2 O. Urea is very conc. in
urine at this point, so it diffuses
into tissue fluid. Increased
osmolarity of tissue fluid
enhances contercurrent effect.
Urea enters ascending limb and
is recycled.
ADH Movie
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Hormonal Control of Kidney Function
ADH: antidiuretic hormone
• Reclaims more water by
increasing permeability of
distal tubules & collecting
ducts
The Renin-AngiotensinAldosterone System (RAAS):
Centers on juxtaglomular
apparatus (JGA)
• Response to decreases in
blood pressure: constricts
arterioles, decreasing blood
flow to capillaries
New Topic: Nervous Systems
• Nervous systems vary tremendously in
complexity
• Level of complexity varies w/ phylogeny
(evolutionary history) and mobility
• Simplest nervous systems found in
primitive, sessile species
Nervous System Types I
• Nerve net: simplest type
of metazoan NS
(Cnidaria &
Echinodermata)
• “Brain” plus
longitudinal and
transverse cords: found
in flatworms
• “Brain,” segmental
ganglia & nerves: found
in segmented worms
(e.g., earthworms)
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Nervous System Types II
• Brain and specialized
ganglia: found in some
inverts such as squid &
octopus
• Brain & spinal cord
(central & peripheral
nervous system):
vertebrates only
Vertebrate Brains
• Brains of 4 vertebrate
species of similar body
mass
• All contain:
– Hindbrain
– Midbrain
– Forebrain
• Most variation between
species exists in size &
complexity of forebrain
which is most highly
developed in primates
Neurons: Functional Units of NS
• Despite extensive variation in nervous
systems, nerve cells (called neurons)
function almost identically in animal
species as diverse as squid and humans
• Plasma membranes of neurons are capable
of generating electric signals called nerve
impulses.
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Structure of the Neuron
• Dendrites: bushlike
projections which bring in
information from other
neurons or sensory cells to
the neuron’s cell body
• Cell body: contains
nucleus and most of cell
organelles
• Axon: long projection
which carries information
away from cell body
• Axon terminal: At target
cell, axon divides into a
spray of fine nerve
endings; at tips of these are
swellings called axon
terminals
Glial Cells: important auxillary components
of nervous system
• Physically support
and orient neurons
• Insulate axons
• Schwann Cells
– A glial cell of the
peripheral nervous
system which
envelopes axons
with concentric
layers of insulating
plasma membrane
Other Types of Glial Cells
• Oligodendrocytes: glial cells of CNS which
performs function similar to Schwann cells
• Astrocytes: Star-shaped glial cells that create
blood-brain barrier
– Surround the smallest, most permeable blood vessels
in the brain and protect brain from damage by
chemical toxins; protection of brain is crucial because
brain cells do not regenerate (too well)
• Got hangover? Blame it on your astrocytes
– Protection of brain cells from toxins not perfect
because astrocytes are permeable to fat soluble
compounds such as alcohol and anesthetics.
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Generating & Conducting Nerve Impulses
• Membrane potential: difference in electrical charge across
the plasma membrane of a neuron
• Resting potential of a neuron:
– Membrane potential of a resting (unstimulated) neuron
– -60 to -70 millivolts
Resting Potential Video
More on Membrane Potentials
• Na+-K+ pump brings potassium ions into the nerve cells
– Expels 3 Na+ for every 2 K + ions it brings in
• Many negatively charged molecules are proteins which
are too large to pass out of the plasma membrane
• Membrane potential of resting neuron is polarized (has
non-zero charge) due
– Asymmetrical Na/K pump,
– Negatively-charged proteins
– Open K+ channels
• If a stimulus perturbs the resting potential and the inside
of the cell becomes more negative, the membrane is
hyperpolarized
• If inside becomes less negative, membrane is depolarized
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Basis of Membrane Potential
Membrane pumps:
• use energy (ATP) to move ions against a concentration gradient
• Na+-K+ pump moves K+ ions to the inside of the nerve cell
– Expels 3 Na+ for every 2 K+ ions it brings in
Ion Channel:
• Channels are selective
• Channels may be gated or un-gated
• Gated channels are either voltage-gated or chemically-gated
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