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Vocabulary:
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11,12,13,14,15,
16,17,18,19,20,
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The Kidney & It’s Function
I. Introduction
A. Vertebrate Physiology Reflects Their Origin in an
Aquatic Environment
B. Body Water Must Be Regulated to Maintain Proper Composition
of Cells
II. Osmoregulation
A. Most Marine Invertebrates Are Osmoconformers
1. Based on differential permeability
of plasma membranes
2. Osmolality is the measurement of
solute concentration in a solution
a) Moles of solute
per kilogram of water
b) Interchangeably
called osmotic concentration or osmotic pressure
c) Two solutions
with same osmotic concentration are isosmotic
3. Osmoconformer osmotic concentration
of body fluid equals that of environment
4. Are in osmotic equilibrium with
environment
B. The Problems Faced by Osmoregulators
1. Sharks are only vertebrate osmoconformers,
rest are osmoregulators
2. Maintain a constant osmotic concentration
independent of environment
a) Permits complex
patterns of internal metabolism
b) Requires constant
adjustment
3. Freshwater vertebrates
a) Maintain higher
salt concentration in body than in environment
b) Hyperosmotic
to the environment, water tends to enter body
c) Must prevent
water from entering and exclude excess that does enter
4. Marine vertebrates
a) Are hypoosmotic
to the environment
b) Body salt concentration
one-third of that of the environment
c) Must retain water
to prevent dehydration
5. Terrestrial vertebrates
a) Bodies have a
higher concentration of water than surrounding air
b) Tend to lose
water to evaporation from skin and lungs
C. How Osmoregulation Is Achieved
1. Simple protists and sponges
a) Removal of water
or salts coupled to removal of metabolic wastes
b) Possess contractile
vacuoles
2. Freshwater invertebrates
a) Utilize nephrid
organs
b) Membrane filters
water and wastes; proteins and sugars retained
3. Insects possess Malpighian tubules
a) Extension of
digestive tract branching off hindgut
b) Potassium ions
secreted into tubules by active transport
c) Osmotic gradient
pulls body fluids and organic wastes into tubules
d) Blood cells and
proteins too large to pass across membrane
e) Water and potassium
reabsorbed through epithelium in hindgut
4. Vertebrates utilize pressure-driven
filtration system
a) Insects secrete
solutes into excretory organ to pull water through filter
b) Higher blood
pressure in vertebrate closed circulatory systems
(1)
Blood pushed through filter, proteins and large molecules retained
(2)
Water reabsorbed as filtrate passes through a long tubule
(3)
Modified filtrate excreted as urine
c) Vertebrates selectively
reabsorb small molecules
d) Evolved various
membrane channels to reabsorb different molecules
e) Filtration and
reabsorption occurs in kidney
III. The Organization Of The Vertebrate Kidney
A. Introduction to the Renal System
1. Functional unit is the nephron
a) Human kidneys
are located in lower back
b) Each contains
one million nephrons
2. Kidney divided into outer renal
cortex and inner renal medulla
3. Series of converging tubules lead
from medulla to ureter
4. Ureter transports urine from kidney
to urinary bladder
5. Functions of vertebrate kidneys
a) Filtration:
blood passed through filter
b) Reabsorption:
desirable material recaptured from filtrate
c) Secretion: certain
ions, drugs and organic molecules transported from blood to filtrate
d) Excretion: materials
in nephron fluid eliminated
B. Filtration
1. Each nephron has tubular and vascular
component
a) Vascular component
is ball of capillaries called glomerulus
b) Embedded in first
part of tubular component, Bowman's capsule
c) Analogy of fist
in soft balloon
d) Capsule has slits
through which fluid can pass
2. Filtration process driven by hydrostatic
pressure of blood against capillary walls
a) Proteins and
large molecules cannot pass through walls or capsule slits
b) Water and small
solutes are pass through
c) Concentration
of small solutes in glomerular filtrate equal to that of blood plasma
d) Also equal to
that of extracellular fluid
e) Glomerular filtrate
destined to become urine
f) Filtered molecules
will be lost if not reabsorbed
C. Reabsorption and Secretion
1. Most of water and dissolved solutes
that enter glomerular filtrate returns to blood
a) In human 2,000
liters of blood passes through kidneys per day
b) 180 liters of
water leaves blood, enters filtrate
c) Most reabsorbed,
only 1-2 liters becomes urine
2. Reabsorption of water a consequence
of salt reabsorption
3. Reabsorption of glucose, amino
acids, other molecules driven by active transport carriers
a) Maximum rate
of transport reached when carriers are saturated
b) Renal glucose
saturation = 180 milligrams of glucose per 100 milliliters of blood
c) If blood glucose
is higher, extra will be lost in urine
(1)
Occurs in untreated diabetes mellitus
(2)
Glucose in urine is hallmark of disease
4. Secretion of foreign molecules
and waste products
a) Involves transport
of molecules across membranes of capillaries and kidney tubules
b) Similar to reabsorption
but in opposite direction
c) Elimination may
be rapid
d) Example: penicillin,
must be administered in high doses
D. Excretion
1. Potentially harmful substances
eliminated via the kidney
2. Urine also contains nitrogenous
wastes
a) Example urea
and uric acid
b) Products of amino
acid and nucleic acid catabolism
3. May also contain excess K+, H+
and other ions
a) High H+ concentration
helps maintain blood pH in narrow range
b) Excretion of
water maintains blood volume and pressure
IV. The Evolution Of Kidneys Among Vertebrates
A. Freshwater Fishes
1. Vertebrate kidneys evolved in bony
freshwater fish
2. Fish body fluids have greater osmotic
concentration than surrounding water
a) Water tends to
enter body from environment
b) Solutes tend
to leave body and enter environment
3. Fish address problems by
a) Not drinking
water and excreting large volume of dilute urine
b) Reabsorbing ions
across nephron tubules from filtrate back into blood
c) Also actively
transport ions across gills from surrounding water into blood
B. Marine Bony Fishes
1. Bony marine fish probably evolved
from freshwater ancestors
2. Faced new problems, body fluids
hypoosmotic compared to surrounding seawater
3. Water tends to leave body by osmosis,
particularly across gills
4. Marine fish compensate by
a) Must drink large
amounts of seawater
b) Demands on kidneys
are opposite freshwater fish
c) Must retain water
and excrete ions from ingested seawater
5. Excretion of ions
a) Divalent calcium,
magnesium, sulfate remain in digestive tract, eliminated in anus
b) Some absorbed
into blood, as are monovalent ions K+, Na+ and Cl-
c) Most monovalent
ions actively transported out of blood across gills
d) Divalent ions
secreted into nephron tubules, excreted in urine
6. Excreted urine is isosmotic to
body fluids, more concentrated than freshwater fish, less concentrated
than birds and mammals
C. Cartilaginous Fishes
1. Virtually all except one species
are marine
2. Solve osmotic problem differently
than bony fishes
3. Kidneys reabsorb the metabolic
waste urea
a) Have a blood
urea concentration 100 times greater than mammals
b) Thus blood is
isotonic with surrounding sea
4. No net water movement, water loss
prevented
a) Do not drink
excessive amounts of sea water
b) Kidneys and gills
do not need to remove large quantities of ions
c) Enzymes and tissues
tolerant of high urea
D. Amphibians and Reptiles
1. Amphibian kidneys operate identically
to freshwater fish
2. Reptile kidneys are varied with
habitat
a) Freshwater varieties
have kidneys similar to fish and amphibians
b) Marine kidneys
similar to freshwater, excrete salt via salt glands on head
c) Terrestrial forms
reabsorb most of salt and water from filtrate
(1)
Conserves blood volume in dry environments
(2)
Urine cannot be more concentrated than blood plasma
E. Mammals and Birds
1. Produce urine with greater osmotic
concentration than body fluids
a) Can excrete wastes
in smaller volume of water, water retained in body
b) Kidneys of some
dessert animals very concentrated
c) Efficiency in
kangaroo rat so efficient it never has to drink water
2. Loop of Henle results in production
of hyperosmotic urine
a) Found only in
birds and mammals
b) Nephron with
long loop produces concentrated urine
c) Mammals have
some long and some short looped nephrons
d) Birds have only
short looped nephrons
(1)
Lose more body water in urine, drink more water
(2)
Marine birds drink sea water, excrete salt from salt glands near eyes
V. How The Mammilian Kidney Retains Salt And Water
A. Structure of Mammalian Tubule
1. Nephron begins in renal cortex
at Bowman's capsule, receives glomerular filtrate
2. Capsule connected to proximal convoluted
tubule in renal cortex
3. Fluid then enters loop of Henle
a) Moves down descending
limb into renal medulla
b) Goes back up
ascending limb into cortex
4. Empties into distal convoluted
tubule in cortex
5. Drains into collecting duct that
collects fluid from multiple nephrons
6. Collecting duct plunges deep into
medulla
7. Empties contents into funnel-shaped
renal pelvis
8. Drains urine into ureter
B. Movement of Ions and Water in the Nephron
1. Two-thirds of NaCl and water filtered
into capsule reabsorbed immediately across walls of proximal convoluted
tubule
a) Driven by active
transport of Na+ out of filtrate, into blood vessels
b) Cl- follows Na+
passively
c) Water follows
both because of osmosis
d) Filtrate is still
isosmotic to blood plasma
2. Most of remaining third of fluid
occurs in loop of Henle
a) Serves as countercurrent
multiplier
b) Countercurrent
flow exists between ascending and descending limbs of loop of Henle
3. Six stages of countercurrent flow
in region surrounding loop
a) Filtrate from
proximal convoluted tubule passes down descending limb onto medulla
(1)
Walls impermeable to salt or urea, freely permeable to water
(2)
Surrounding extracellular fluid is hyperosmotic to filtrate
(3)
Results from high concentration of salt and urea
(4)
Water exits descending limb into surrounding tissue via osmosis
(5)
Filtrate becomes more concentrated
b) At bottom of
loop, walls become more permeable to salt, but less permeable to water
(1)
Salt diffuses out of tubule, filtrate more concentrated than extracellular
fluid
(2)
Salt concentration in filtrate and extracellular fluid become equal flowing
up ascending limb
c) Higher up in
thick segment of ascending arm, channels actively pump Na+ outward
(1)
Occurs against concentration gradient
(2)
Cl- passively follows, water cannot since walls are not permeable to water
(3)
Filtrate becomes more dilute toward distal convoluted tubule in cortex
(4)
Salt leaving filtrate is trapped in extracellular fluid of renal medulla
d) Some water reabsorbed
as filtrate passes through distal convoluted tubule
(1)
Filtrate now hypoosmotic to blood
(2)
Reabsorption concentrates fluid, becomes isosmotic to blood
(3)
Major solute in filtrate is urea. Most salt pumped out in ascending limb
e) Extracellular
fluid surrounding collecting duct is hyperosmotic to filtrate
(1)
Due to active extrusion of salt in ascending limb
(2)
Walls of collecting duct permeable to water
(3)
As filtrate descends into medulla through duct, water drawn out by osmosis
(4)
Filtrate becomes increasingly more concentrated through collecting duct
f) Deep in medulla
collecting duct walls become permeable to urea
(1)
Urea diffuses out of lower collecting duct into extracellular fluid
(2)
Produces high urea concentration in deep regions of medulla
(3)
Helps draw water out of descending limb of loop of Henle (stage one)
4. Operation successful because
a) Countercurrent
flow between descending and ascending limbs
b) Close proximity
of limbs to one another
c) Transport of
salt out of ascending limb creates hyperosmotic conditions to draw water
out of descending limb
d) Loss of water
increases concentration of fluid in ascending limb
(1)
Makes active transport process more effective
(2)
Helps concentrate medulla extracellular fluid
e) Interactions
create steep osmotic gradient
(1)
Gradient between renal extracellular fluid and filtrate in collecting
duct
(2)
Gradient drives reabsorption of water from remaining filtrate
VI. Excretion Of Nitrogenous Wastes
A. Animals Catabolize Nitrogen-Containing Amino Acids
and Nucleic Acids
1. Produce nitrogenous wastes that
must be eliminated
2. Metabolism of amino and nucleic
acids
a) Amino group removed,
combined with H+ to form ammonia in liver
b) Ammonia is toxic,
must be transported in very dilute solution
c) No problem for
freshwater fish with copious amounts of urine
B. Saltwater Fish and Terrestrial Vertebrates Adapt
to Transport Ammonia
1. Must conserve body water, can't
waste it to dilute ammonia
2. Evolved three solutions
a) Flushing
(1)
Fish breakdown protein in gills, not liver
(2)
Ammonia released there
b) Detoxification
(1)
Mammals convert ammonia to less toxic urea in liver
(2)
Transport urea in bloodstream to kidneys for excretion
c) Insolubilization
(1)
Birds and land reptiles convert ammonia to uric acid
(2)
Insoluble in water, crystallizes
(3)
Excreted as semi-solid paste
(4)
Mammals also excrete uric acid, but is a waste of purine nucleotide degradation
VII. The Kidney As A Regulatory Organ
A. Regulates Osmotic Concentration of Urine Via Water
Excretion
1. Blood volume, blood pressure maintained
by action of kidneys
a) Excrete hyperosmotic
urine when body needs to conserve water
b) Excretes hypoosmotic
urine when too much water has been ingested
2. Regulate plasma Na+ and K+ concentrations
3. Regulate blood pH
4. Coordinated primarily via actions
of two hormones: antidiuretic hormone, aldosterone
B. Regulation of Kidney Function by Antidiuretic Hormone
1. Hypothalamus produces antidiuretic
hormone, secreted by posterior pituitary
a) Increase in osmolality
of blood plasma stimulates its production
b) Osmoreceptors
in hypothalamus respond to elevated osmolality
c) Trigger sensation
of thirst, stimulate ADH secretion
2. Actions of ADH
a) Makes kidney
collecting ducts more permeable to urea
b) Extracellular
fluid in medulla is hyperosmotic to collecting duct filtrate
c) Water leaves
filtrate by osmosis, reabsorbed into blood
d) Greater ADH secretion,
ducts more permeable, more water reabsorbed
C. Regulation of Kidney Function by Aldosterone
1. Na+ ions are major solutes in blood
2. If Na+ decreased, blood osmolality
also decreased
a) Inhibits ADH
secretion
b) More water excreted
in urine as less is reabsorbed
c) Blood volume
decreases, lowers blood pressure
3. Kidneys compensate for Na+ decrease
via aldosterone secreted by adrenal cortex
a) Stimulates reabsorption
of Na+ at distal convoluted tubule
b) Thus decreases
amount of Na+ lost in the urine
c) Reabsorption
of Na+ followed by Cl- and then water
d) Net effect to
retain both salt and water
4. Also promotes secretion of K+ into
distal convoluted tubule
a) Lowers blood
K+ level
b) Helps maintain
K+ constant with changing amounts in diet
5. Lack of aldosterone is lethal
a) Excessive loss
of salt and water
b) Buildup of K+
in blood