•
Cell plasma membrane maintains ionic, but
not osmotic difference between intracellular
and extracellular fluids.
•
Epithelium surrounding the body often
maintains both ionic and osmotic difference
between animal and their environments.
•
Gills, salty gland and kidney are primary
organs of osmoregulation in vertebrates
•
Appropriate solute concentrations and water
are maintained by osomregulation
Exchange of water and salts depends on
•
The size of gradient
•
Surface area of the animal
•
Permeability of the animal’s surface
The surface-to-volume
ratio is greater for small
animals than large
animals. A small animal
will dehydrate or
hydrate more rapidly
than a larger animal
Various Strategies for Preserving Body Water
•Amphibians have moist, highly permeable skins
To avoid desiccation, stay in cool, damp microenvironment, stay
close to water
water and slats are stored in a large-volume lymphatic system and
an oversized urinary bladder.
•Insect’s waxy cuticle
•Burning fat to produce water in seal
Seals became fat when eating fish but get thin eating
marine invertebrate
The respiratory loss of water is minimized by temporal
countercurrent system
Water loss via respiration depends on
•
Difference between body temperature and air temperature
•
Humidity of inhaled air
Osomoregulatory in Various Classes of Animals
Euryhaline aquatic animal can tolerate a wide range of salinities
Stenohaline animals can tolerate only narrow osmotic range
Freshwater animals face two kinds of osmoregulatory problems:
•gain of water
•loss of salt
To prevent the net gain of water and net loss of salts, freshwater
animals
•Drink no water
•Produce a dilute urine
•Replace lost salts from ingested food
•Active transport salt from external environment
Marine invertebrates and hagfish (vertebrate) are isoosmotic to seawater, and have similar osmolarity and
ionic concentrations to seawater
Elasmobranch (e.g. shark, rays and skates, Latimeria) is
iso-osmotic to seawater by maintaining low concentration
of electrolytes and high concentration of urea and TMAO
(trimethylamine oxide)
Marine teleost, bird and mammals are hypo-osmotic to
seawater
Marine teleosts face two kinds of osmoregulatory problems:
•loss of water
•gain of salt
To prevent the net loss of water and net gain of salts, marine teleosts
•Drink water
•Active transport Na+, Cl- and K+ from gill to seawater.
•Secretion of divalent salts (Ca2+, Mg2+, SO42-) by kidney to urine
Marine reptiles and marine birds
•
Drink seawater
•
Kidney is unable to excrete
the salts
•
Salt gland (near eyes, nose
and in the tongue) secrete
concentrated salt solution
Most mammals lack salt
gland and will become
dehydrated if they drink
seawater
Desert animal faces double jeopardy
•Excess heat
•Absence of free freshwater
Camel strategies:
•Change body temperature
•Produce dry feces &
concentrated urine
•Store high levels of urea
Camel do not sweat and has
large body mass and thick
fur
Marine mammals:
Drink no water
Produce hypertonic urea
Absorb water from metabolic activity and ingested food
Terrestrial arthropods
Create high concentrated solutions in the rectum to absorb
water from the air
Salivary glands secrete highly concentrated KCl
Structure of the Kidney
•
•
2 distinct regions:
Outer cortex:
–
•
Many capillaries.
Medulla:
–
Renal pyramids separated
by renal columns.
Nephron is functional unit of
the kidney
Kidney Functions
•
•
•
•
•
•
Primarily on regulation of ECF through formation of urine.
Regulate volume of blood plasma and BP.
Regulate concentration of waste products in the blood.
Regulate concentration of electrolytes as Na+, K+, and HC03-.
Regulate pH.
Secrete erythropoietin.
Nephron
• Functional unit of the
kidney.
• Consists of:
• Blood vessels
– vasa recta
– peritubular capillaries
• Urinary tubules
–
–
–
–
Proximal tube
Loop of Henle
Distal tube
Collecting tube
Three main processes for urine
production
1. Filtration
2. Reabsorption
3. Secretion
Glomerular filteration
Ultrafiltration in glomerulus depends on
1. pressure difference
2. Membrane permeability
Fig. 12-9, p.534
Juxtaglomerular Apparatus
•
•
•
Region in each nephron where the
afferent arteriole comes in contact
the the thick ascending limb of the
loop. Two types of cells
Macula densa:
– Monitor the osmolarity and flow
– Inhibit renin secretion when
blood [Na+] in blood increases
Granular cells:
– Secrete renin.
– Converts angiotensinogen to
angiotensin I.
– Initiates the renin-angiotensinaldosterone system.
Arterial blood pressure
Driving pressure into glomerulus
Glomerular capillary pressure
GFR
Rate of fluid flow
through tubules
Stimulation of macula densa cells
to release vasoactive chemicals
Chemicals released that induce
afferent arteriolar vasoconstriction
Blood flow into glomerulus
Glomerular capillary pressure
to normal
GFR to normal
Fig. 12-13, p.538
Short-term
adjustment for
Arterial
blood pressure
Arterial blood pressure
Detection by aortic arch and
carotid sinus baroreceptors
Cardiac
output
Sympathetic activity
Total
peripheral
resistance
Generalized
arteriolar vasoconstriction
Afferent arteriolar
vasoconstriction
Glomerular capillary
blood pressure
GFR
Urine volume
Conservation of
fluid and salt
Fig. 12-14, p.539
Arterial blood pressure
Long-term
adjustment for
Tubular re-absorption
• Return of most of the filtered solutes and H20
from the urine filtrate back into the peritubular
capillaries.
• About 180 L/day of ultrafiltrate produced,
however only 1 – 2 L of urine excreted (>99%).
• Minimum of 400 ml/day urine necessary to
excrete metabolic wastes (obligatory water loss).
Glucose re-absorption
• Filtered glucose and
amino acids are
normally reabsorbed by
the nephrons.
– Carrier mediated
transport:
• Saturation.
• Exhibit Tm. (320 mg
min-1, 3mgml-1)
Solute concentrations in
the interstitial fluid
increase from the cortex
to the depths of the
medulla.
Urea increases most in
the inner medulla. NaCl
increases most in outer
medulla
Proximal tube
70% Na+, Cl- and H20 reabsorbed across the PT into the
blood. 90% K+ reabsorbed.
• Fluid reduced to ¼ original volume but still iso-osmatic 300
mOsm/L
 Na+/K+ ATPase pump located in basal and lateral sides of
cell membrane creates gradient for diffusion of Na+ across
the apical membrane.
 Na+/K+ ATPase pump extrudes Na+.
 Cl follows electrical gradient into the interstitial fluid.
 H20 follows by osmosis.
 Reabsorption is constant, not subject to hormonal
regulation.

Fig. 12-16, p.541
Fig. 12-17, p.541
Descending Limb Loop of Henle
• Deeper regions of medulla
reach 1200 mOsm/L.
• Impermeable to passive
diffusion of NaCl & urea
• Permeable to H20.
• Hypertonic interstitial fluid
causes H20 movement out of
the descending limb via
osmosis.
• Fluid volume decreases in
tubule, causing higher [Na+]
in the ascending limb.
Ascending Limb Loop of Henle
• Na+ actively
transported across
the basolateral
membrane by Na+ /
K+ ATPase pump.
• Cl- passively
follows Na+ down
electrical gradient.
• K+ passively
diffuses back into
filtrate.
• Walls are
impermeable to
H20.
Distal tubule
• Transport K+, H+, and NH3
into the lumen
• Reabsorption of Na+, Cl-,
and HCO3• H20 follows passively
 subject to hormonal
regulation.
Collecting Duct
• Medullary area impermeable to high [NaCl] that surrounds
it.
• The walls of the CD are permeable to H20.
• H20 is drawn out of the CD by osmosis.
• Rate of osmotic movement is determined by the # of
aquaporins in the cell membrane.
• Permeable to H20 depends upon the presence of ADH.
• ADH binds to its membrane receptors on CD,
incorporating water channels into cell membrane.
Kidney Secretion
• Secretion of substances
from the blood to the
urine.
• Allows the kidneys to
rapidly eliminate certain
potential toxins.
• Substances (foreign &
normal metabolites)
conjugated with
glucuronic acid or its
sulfate, removed by
organic anionic and
cationic transport system
Renin-angiotensin system
Cells of macula densa senses
blood pressure decrease, they
stimulate releasing of renin
from the granular cells, leads to
an increase in angiotensin II
and aldosterone, promotes Na+
and water reabsorption
Role of Aldosterone
• 90% K+ reabsorbed in early part of the nephron.
• When aldosterone is absent, no K+ is excreted in the
urine.
• Final [K+] controlled in distal tube by aldosterone.
• High [K+] or low [Na+] stimulates the secretion of
aldosterone.
• Only means by which K+ is secreted.
• Control of plasma of K+ important in proper function of
cardiac and skeletal muscles
Na+ Reabsorption
• In the absence of aldosterone, 80% remaining Na+
is reabsorbed.
– 2% is excreted (30 g/day).
• Final [Na+] controlled in distal tube by aldosterone.
+
 Control of Na important in regulation of blood
volume and pressure.
(Antidiuretic
hormone) ADH
increase number
of water
channels
(aquaporins) and
thereby
promotes water
reabsorption
Atrial natriuretic peptide (ANP)
•
•
•
•
Produced by atria due to stretching of walls.
Antagonist to aldosterone.
Inhibits release ADH, renin and aldosterone
Increases [Na+] excretion and urine
production
Two factors control pH in mammals:
•
•
Excretion of CO2 via the lung (short term)
Excretion of acid via kidney (mainly)
Renal Acid-Base Regulation
• Kidneys help regulate blood pH by excreting H+
and reabsorbing HC03-.
• Most of the H+ secretion occurs across the walls of
the proximal tube in exchange for Na+.
– Antiport mechanism.
• Normal urine normally is slightly acidic because
the kidneys reabsorb almost all HC03- and excrete
H+.
• Returns blood pH back to normal range.
A type cell: acid
secreting cell
B type cell: base
secreting cell
Urinary Buffers
• Nephron cannot produce a urine pH < 4.5.
• IN order to excrete more H+, the acid must
be buffered.
• H+ secreted into the urine tubule and
combines with HPO4-2 or NH3.
• HPO4-2 + H+
H2PO4• NH3 + H+
NH4+
Buffering of the renal filtrate by H2PO4- and NH4- permits
greater secretion of protons.
Kidney in vertebrates
Hagfish: possess glomeruli, no tubules, excrete divalent ions
(Ca2+, Mg2+ , and SO42- ), carry out little osmoregulation,
Freshwater teleost: larger glomeruli, produce dilute urine
Marine teleost: produce little urine, excrete NH3 from gills
Amphibians and reptiles: lack of loop of Henle, can’t
produce concentrated urine
Mammals and birds: produce concentrated urine
Extrarenal osmoregulatory oragns
Salty gland: marine birds, reptiles, elasmobranch
Chloride cells in gills: marine teleost
Rectal gland: Elasmobranch
Uptake of salt in freshwater fish