MSVU Animal Physiology U. Hoeger
Water and Salt Balance, Regulation
Renal Function and Excretion
Water, Salt, Excretion
Why are body fluids and their balancing so important?
- body fluids surround tissues, cells, organelles, molecules and provide the environment for their function
- water is the matrix (solvent), it determines ion concentrations in body and tissues,
and it controls volume (turgor) of body, tissue, cells
OSMOSIS
- salts (ions) built electrical and chemical gradients crucial for all physiological functions,
influence the 3D-structure of proteins
- intracellular and extracellular body fluids are never in equilibrium and
are always different from the environmental conditions
- differences can be minute (marine invertebrates)
or quite dramatic (freshwater animals, terrestrial animals)
- control and adjustment of body fluid composition is required to prevent equilibrium
- balancing water
- balancing salts and solutes
Physiological processes and thus life itself are based and depend on gradients and imbalances!
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
- bulk of body mass is water, in
animals up to 95 %
- daily turn-over of water in
animals is typically 7 - 25 %
- intracellular body fluids (cytosol)
- extracellular body fluids (interstitial fluids, blood plasma)
All body fluids are aqueous salt solutions, thus water and
salt balance are closely connected
Salt and water gain and losses must be balanced to avoid
severe problems by disturbing the water/salt balance
Note: Here BALANCE DOES NOT mean EQUILIBRIUM
Water, Salt, Excretion
Fluid Compartments
- intracellular fluid, interstitial fluid, and
blood plasma: only separated by thin
membrane layers or epithelial cells
- interaction between fluid compartments:
exchange of water, ions, and solutes
- osmosis (“diffusion of water”)
- diffusion (passive transport)
- active transport
Most animals are regulators! They are
not isotonic with their environment.
Compared with environment they are
- hyperosmotic (higher solvent content)
- hypoosmotic (lower solvent content)
Few animals are conformers, they are
isoosmotic with their environment
(restricted to brackish water environments)
Energetically expensive
Energetically cheap
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Shrimp is a regulator:
wide range of environmental osmotic pressure,
still able to keep internal osmotic pressure stable
Green Crab is regulator but over smaller range:
crab’s ability to regulate (maintain) internal osmotic
pressure is limited and it becomes conformer
Mussel is conformer:
internal osmotic pressure equals environmental
osmotic pressure
Being a conformer doesn’t mean there are no gradients! Conforming is restricted to osmotic pressure!
All animals regulate internal ionic conditions and total fluid volume,
Osmoregulation, ionic regulation, and volume regulation are closely connected
and can not be viewed as separated processes
Water, Salt, Excretion
Different environments, different problems
Open Ocean – animals are hypoosmotic to their environment, i.e. need to prevent osmotic water loss
to their environment to maintain water/salt balance
Brackish water -changing environmental conditions make animals at times hyper- or hypoosmotic
to their environment, i.e depending on the current state animals may have to prevent
water loss or void excess water
Freshwater- animals are always hyperosmotic, creating the need to void excess water to prevent
swelling and to control their salt balance
Terrestrial environments- water stress is common, animals have to deal with evaporative water loss.
Extent of the problem depends on given humidity and temperature.
Most environmental conditions demand water conservation
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Gain or loss of water
Gain or loss of salts (ions)
change of blood composition
require regulation
Change of osmotic pressure
Kidneys play mayor role in ion/water regulation
Gills and salt glands are of minor importance
Aim: control and if required selective removal of water, salts or solutes from blood/hemolymph plasma
Excess water, salts, solutes are voided as urine
Urine can be: isoosmotic, same osmotic pressure as body fluids
hyperosmotic, more concentrated than body fluid
hypoosmotic, more dilute tha body fluid
Kidneys control urine composition and can adjust it over wide range
- to save or void water
- to save or void salts and other solutes
All this is energetically expensive!
Water, Salt, Excretion
How do we loose water?
Obligatory water losses are unavoidable
- evaporative loss during respiration (depends on breathing physiology and humidity of ambient air)
- urinary water loss (minimum required to void waste products from protein catabolism)
- fecal water loss (minimum required to void digestive waste products)
- transpiration over integument (can be minute or gigantic)
Minimizing obligatory water losses by producing highly concentrated urine, highly efficient
water reabsorbtion in hindgut, water impermeable integument, behavioral strategies……
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Obligatory water losses (which are unavoidable):
- daily water turnover within phylogentic groups shows allometric relationship with body size
- comparing phylogentic groups we find relationship with the respective metabolic rates
Daily water gain must
balance with water loss
Water, Salt, Excretion
Obligatory water losses (which are unavoidable):
- within a phylogentic group evaporative loss over integument is allometric function of body size
- smaller animals are at higher risk
of dehydration
WHY?
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Obligatory water losses (which are unavoidable):
- urinary water loss (minimum required to void waste products from protein catabolism,
and the ability of the renal system to produce concentrated urine)
Daily concern to meet water requirements
- allometric relationship between
mammalian body weight and the
ability to concentrate urine
- smaller animals are more prone to
water loss due to higher metabolism,
breathing frequency, and less favorable
body surface/volume relationship
- ability to produce hypoosmotic
urine helps to keep a neutral water
balance
Luxury of not having to worry about water
Note the differences within a body
size class between desert species
and semi-aquatic species
Water, Salt, Excretion
- isoosmotic urine doesn’t change osmotic pressure of plasma, no osmoregulation
- hypoosmotic urine voids excess water or conserves required salts
- hyperosmotic urine conserving water or voids excess salts
Ability to produce hyperosmotic urine is found in terrestrial species
facing the risk of dehydaration, i.e. way to conserve water
Water balance is influenced by availability of water
Salt balance is influenced by food/diet/water (source of salts)
- many plants, along with water, have high salt contents (problem for herbivores)
- marine predators have high salt uptake (marine prey is higher in salt than freshwater prey)
- some diets (e.g. nectar) are high water / low solute mixtures (void excess water)
Regulation of water / salt balance is required.
A job for the kidneys
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Sources of water
- fresh water (uptake through integument, by ingestion, or with food)
- saltwater can be used as water source if excess salt can be voided,
specialized mechanisms are required (salt glands, gills)
-without mechanisms to do void excess salt extrarenal (salt glands, gills)
consumption of saltwater leads to further water loss due to production
of urine to void excess salt (obligate water loss)
- consumption of salty food has the same dehydrating effect (animals specialized to such
food sources have the ability to produce urine with U/P ratios > 5 to void excess salt)
-metabolic water, product of aerobic catabolism
C6H12O6 + 6 O2 => 6 CO2 + 6 H2O
Amount of metabolic water depends on food source:
Carbohydrates ~ 0.56g H2O / g substrate
Lipids ~ 1.07g H2O / g substrate
Protein ~ 0.4 –0.5g H2O / g substrate
Water, Salt, Excretion
Metabolic water
Kangaroo rat – a desert survival specialist
- sustain itself from a diet of dry barley and metabolic water
Water balance:
+ 0.54g H2O / g food
(metabolic water)
- 0.33g H2O / g food due to respiration (obligatory loss)
- 0.14g H2O / g food due to urine (obligatory loss)
- 0.00g H2O / g food
in feces (obligatory loss)
+ 0.07g H2O / g food
gain used to balance
other water losses
Kangaroo rat survives due its capability to produce
concentrated urine and bone dry feces to minimize
water loss and maximize water conservation
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Salt Glands: Extrarenal Excretion
- animals in a marine environment benefit from extrarenal mechanisms to void excess Na-ions
- many marine vertebrates use salt glands to void
excess sodium they ingest with water and food
- nasal secretion (birds) or orbital secretions (reptiles)
- salt gland secretions are highly hypoosmotic with very
high sodium concentration
- birds and reptiles with salt glands can utilize saltwater
to meet water balance requirements
Marine mammals rely solely on ability of their kidneys
to produce highly concentrated urine, and saltwater is
not used as water source
Other marine vertebrates use forms of salt glands
(rectal glands in shark & Co.) and epithelial sodium
excretion (via gills) to equalize their salt balance
Water, Salt, Excretion
“Kidneys” (protonephridia, metanephriadia, malpighian tubules, antennal glands …)
- tubular structures in contact with the outside world
- produce and eliminate aqueous solution (urine) derived from blood
- regulate chemical composition, pH, volume and osmolarity of blood by
controlled excretion and reabsorption of solutes and water, and discharge
of potentially toxic metabolic waste products from the organism
Urine: complex solution of organic and inorganic solutes, nitrogenous waste,
ions (Na+, K+, Cl-, PO43-), creatine and water
Urine formation is always a two step process:
- primary urine: formed by ultrafiltration or secretion of plasma/hemolymp into the
renal tubule (product is very similar to blood/hemolymp)
- definitive urine: modification of primary urine during passage of the renal tubule,
product becomes very different from blood/hemolymp by highly selective
reabsorption and secretion of ions, organic and inorganic molecules and water
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MSVU Animal Physiology U. Hoeger
Glomerular Filtration and Primary Urine Formation
in the Vertebrate Kidney
Primary Urine vs Definite Urine
- Urine is produced by our KIDNEYS
- Produced by blood filtration
- We void 1 – 2 liters of DEFINITE URINE per day
- We produce 150 – 180 liters of PRIMARY URINE per day
- blood plasma is filtered every 30 minutes
- 60x our blood volume per day
- From 180 to 2 liters in 24 hours requires some heavy modification
- Solutes and most of the water are reabsorbed, other solutes are secreted
Today’s Topic: The Formation of PRIMARY URINE
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MSVU Animal Physiology U. Hoeger
Organ overview
Six organs = urinary system
Adrenal glands
Renal artery
Renal vein
Kidneys
Kidney (2)
Aorta
Inferior vena cava
Ureter (2)
Urinary bladder
Urethra
Kidney Gross Anatomy
Kidney Trivia:
Renal cortex
- ~ 150 g
- Approx. 11 cm x 6 cm x 3 cm
Renal medulla
“Kidney-shaped” Bar of Soap
- Enclosed and held in place by layers
of connective tissue
Renal pelvis
- Behind abdominal cavity (retroperitoneal),
below diaphragm
- Two major structures
- RENAL CORTEX
- RENAL MEDULLA
Ureter
- RENAL PELVIS collects urine
and drains it via the URETER
into URINARY BLADDER
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MSVU Animal Physiology U. Hoeger
Anatomy of the Kidney
Cortex
Pyramid
Column
Medulla
- Organized in “modules”
- RENAL PYRAMIDS
separated by
RENAL COLUMNS
Minor calyx
Major calyx
- Columns lead blood vessels into
Renal Cortex and Renal Medulla
Renal pelvis
- Pyramids contain the functional sub-units
of the kidney - NEPHRONS - (1.2 million / kidney)
The Nephron: Functional Sub-unit of the Kidney
Formation of PRIMARY URINE!
Proximal convoluted tubule
Distal convoluted tubule
Renal corpuscle, Malpighian body
Efferent arteriole
Collecting duct
Afferent arteriole
Renal vein
Loop of Henle
Calyx
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MSVU Animal Physiology U. Hoeger
Renal Corpuscle: Location of Primary Urine Formation
Afferent
arteriole
RENAL CORPUSCLE:
- Interaction between NEPHRON
and CIRCULATORY SYSTEM
Efferent
arteriole
Glomerular capsule
- GLOMERULAR CAPSULE (Bowman’s capsule)
proximal “Cul de Sac” of Nephron
- AFFERENT ARTERIOLE invades capsule
forming capillary tuft, the GLOMERULUS
- Increase of contact surface between
NEPHRON and CIRCULATORY SYSTEM
Glomerulus
- EFFERENT ARTERIOLE leaves capsule
What happens in the renal corpuscle?
FORMATION OF PRIMARY URINE BY PRESSURE FILTRATION OF BLOOD
Blood vs Primary Urine
BLOOD
(Glomerulus)
PRIMARY URINE
(Capsular Space)
Water
Blood cells
Electrolytes (Na+, K+, Ca2+ ….)
Plasma proteins
Glucose
Large anions
Amino acids
Protein-bound
- minerals
- hormones
Fatty acids
Molecules > 8 nm
Vitamins
Urea
Uric acid
Creatinine
Primary Urine = Blood Minus Large Solutes
Large Solutes held back by FILTER
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MSVU Animal Physiology U. Hoeger
The Glomerular Filter
Blood
Primary urine
ENDOTHELIAL CELL with FENESTRATIONS
PODOCYTES, specialized endothelial cells
of the inner capsular wall
Endothelial cells of Glomerulus und Glomerular Capsule form Filter
The Glomerular Filter
Glomerular blood capillary
Podocytes
1. Fenestrations (dia. 70 – 90 nm) hold back
blood cells and large macromolecules
Filtration slits
2. Proteoglycan gel of the basilar membrane
holds back molecules > 7 nm
3. Filtration slits (30 nm) holds back anions
with negatively charged protein diaphragm
Podocytes
Basilar membrane
Protein diaphragm
Fenestrated endothelium
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MSVU Animal Physiology U. Hoeger
Primary Urine is Formed by Pressure Driven Filtration
Pressure gradient between blood and
capsular fluid drives urine formation
Blood pressure in Glomerulus
Colloid osmotic pressure
Capsular fluid hydrostatic pressure
+ 6.7 kPa (50 mm Hg)
- 3.5 kPa (25 mm Hg)
- 1.9 kPa (14 mm Hg)
Net Filtration Pressure (NFP)
(positive = urine formation)
+ 1.3 kPa
(11 mm Hg)
NFP determines GFR
Blood Pressure
Colloid osmotic pressure
Hydrostatic pressure
Glomerular Filtration Rate GFR
Glomerular Filtration Rate (GFR) = Net Filtration Pressure (NFP) x KF
Filtration Coefficient KF varies with age and gender, ~ 11 ml/min
- GFR for average female = 105 ml/min = 150 liters primary urine a day
- GFR for average male = 125 ml/min = 180 liters primary urine a day
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MSVU Animal Physiology U. Hoeger
Regulation of Glomerular Filtration
Why Regulate Glomerular Filtration Rate?
GFR too high: urine flow in nephron tubule to fast to allow reabsorption
- dehydration and electrolyte depletion are possible consequences
GFR too low: urine flow in nephron tubule to slow, waste products are reabsorbed
- azotemia, accumulation of nitrogenous waste products in body fluids
How is Glomerular Filtration Rate Regulated?
- Renal autoregulation
- Sympathetic nervous system
- Endocrine system
Control of glomerular blood pressure
by vasoconstriction of afferent arteriole
Renal Autoregulation of Glomerular Filtration
Myogenic Mechanism:
SMOOTH MUSCLE CELLS of AFFERENT ARTERIOLE respond to
BLOOD PRESSURE INDUCED VESSEL STRETCH with contraction
- VASOCONSTRICTION lowers afferent blood flow,
glomerular blood pressure and GFR
Afferent arteriole
Juxtaglomerular smooth muscle cells
Efferent arteriole
BLOOD PRESSURE DROP REDUCES VESSEL STRETCH and
SMOOTH MUSCLE CELLS of AFFERENT ARTERIOLE relax
- VASODILATION increases afferent blood flow,
elevates glomerular blood pressure and GFR
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MSVU Animal Physiology U. Hoeger
Renal Autoregulation of Glomerular Filtration
Tubologlomerular Feedback:
MACULA DENSA CELLS in the NEPHRON TUBULE monitor urine flow and absorption
- PARACRINE CONTROL (prostaglandin) induces VASOCONSTRICTION
or VASODILATION of AFFERENT ARTERIOLE
Afferent arteriole
Macula densa
Juxtaglomerular smooth muscle cells
Efferent arteriole
Sympathetic Control of Glomerular Filtration
VASOCONSTRICTION of AFFERENT ARTERIOLES controlled
by SYMPATHETIC NERVE FIBERS releasing EPINEPHRINE
- response to strenuous exercise or circulatory shock
- limits renal blood flow in favor for increased blood flow to
heart, brain, or skeletal muscles
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MSVU Animal Physiology U. Hoeger
Endocrine Control of Glomerular Filtration
Renin – Angiotensin – Aldosterone Pathway
Response to Blood Pressure Drop
- Sympathetic stimulation of juxtaglomerular cells
- Release of RENIN from juxtaglomerular cells
- Renin cleaves ANGIOTENSIN I from precursor
- Angiotensin I transformed to ANGIOTENSIN II
ANGIOTENSIN II:
- vasoconstrictor in circulatory system
blood pressure increase
- promotes water reabsorption from nephron
blood volume and
blood pressure increase
- release of antidiuretic hormone (ADH)
from pituitary gland
blood volume and
blood pressure increase
- stimulation of thirst sensation
blood volume and
blood pressure increase
What happens next?
- Formation of DEFINITE URINE during passage
of nephron tubule and collecting duct
- Reabsorption and secretion of solutes
- Reabsorption of water
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MSVU Animal Physiology U. Hoeger
Water, Salt, Excretion
Vertebrate Kidney Function
- nephrons are arranged in a specific parallel order
- mammalian and avian nephrons are characterized by
specialized section between proximal and distal convoluted
tubules, the Loop of Henle
Loop of Henle and the parallel
arrangement of nephrons allow
production of very hyperosmotic,
highly concentrated urine!
- important adaptation to terrestrial life
and water stress
Only insects and few other arthropods
have mastered the problem of water
conservation, and are able to exist
even in arid conditions!
Water, Salt, Excretion
Mammalian Nephron
- primary urine produced by ultrafiltration,
in Bowman’s capsule
- proximal convoluted tubule, descending
from kidney’s cortex into the medulla
- thick segment of the descending limb of
Loop of Henle
- thin segment of Loop of Henle,
hair pin turn deep in the medulla
- thick segment of the ascending limb of
Loop of Henle
- distal convoluted tubule empties into the
collecting ducts
- collecting ducts join in the renal pelvis,
urine drains into the bladder via the ureter
During nephron passage definitive urine is
formed by secretion and reabsorption
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Urine Formation
Different sections of the nephron have different permeability properties to reabsorb or secret
- water (passive by osmosis, driven by osmotic pressure gradient)
- ions (some by active transport, some follow electro-chemical gradient)
- organic solutes (active transport)
Permeability is controlled by hormones and can be modulated to regulate urine production!
Proximal convoluted tubule:
- reabsorption of Na+, Cl-, and water (up to 20-40 % are reabsorbed)
- reabsorption of glucose and amino acids
Loop of Henle: allows mammals and birds production
of highly hyperosmotic urine
- reabsorption and secretion continues
- length of the Loop of Henle varies between nephrons
and between species
- length of loop of Henle is correlated with the ability
to concentrate urine, i.e. to conserve water
- longer loop = higher urine concentration
Mammalian Kidney Function
Review of transepithelial/transmembrane transport mechanisms
Passive Transport
Diffusion: passive, following “downhill” gradient (chemical, electrical, partial pressure),
membrane impermeable solutes require protein pores/channels/ carrier molecules.
Transport of water is always passive, following osmotic gradient.
Mechanisms creating the gradients may be active!
Active Transport
Moves solutes against gradient (“uphill”), cost energy
Requires transport proteins (carrier proteins, pumps, exchangers) or endo/exocytosis
Transport of larger molecules (sugars, amino acids etc.) requires transporters, often
highly specific
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Urine Formation
Production of hyperosmotic urine: How does it work?
- reabsorption of water concentrates solutes in tubule
- water is (re)moved by osmosis from descending tubule
i.e. medullary interstitial fluid has high solute concentration
is hyperosmotic = high osmotic pressure = water moves
- ascending urine can’t collect water from interstitial fluid
as walls of ascending Loop of Henle and distal
convoluted tubule are poorly permeable for water
- regulation of definite urine concentration is done
during passage of collecting duct, controlled by diuretic hormones
water permeability of collecting duct epithelia is adjusted
HOW IS OSMOTIC PRESSURE GRADIENT IN MEDULLA
CREATED, MAINTAINED, AND USED TO REGULATE URINE
CONCENTRATION, I.E. URINARY H2O AND SOLUTE DISCHARGE?
Mammalian Kidney Function
Mechanism of Urine Formation
Ultrafiltration and proximal convoluted tubule
- Primary Urine enters the proximal convoluted tubule
- proximal convoluted tubule starts in the renal cortex
and descends into the outer medulla
- epithelial cell of the proximal convoluted tubule
are covered with microvilli (surface area!)
- epithelial cells are mitochondria-rich, to meet
energy requirements of active reabsorption
- 60% of water, salts, and all organic solutes (glucose!)
are reabsorbed into peritubular blood capillaries and
transported away
- reabsorption by active Na+-transport and active
transport of glucose, amino acids, phosphate and
other solutes from tubule lumen into blood
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Urine Formation
Loop of Henle
Role of the Loop of Henle: concentrate salts (Na+) in the interstitial fluid of the medulla
to built osmotic pressure gradient
Descending limb of the Loop of Henle
- permeable for water, impermeable for salt,
water flows out of the tubule into the interstitial fluid
Ascending limb of Loop of Henle
- impermeable for water
- Na+ is pumped out into interstitial fluid, creating
osmotic gradient between tubule lumen and
interstitial fluid (high osmotic pressure in the medulla)
- loss of Na+ results in hypoosmotic urine as it ascends
in the Loop of Henle from the inner medulla to the outer
medulla (Paradox?! We don’t want hypoosmotic urine?!)
Mammalian Kidney Function
Mechanism of Urine Formation
Loop of Henle
Role of the Loop of Henle: concentrate salts (Na+) in the interstitial fluid of the medulla
to built osmotic pressure gradient
Descending limb of the Loop of Henle
- permeable for water, impermeable for salt,
water flows out of the tubule into the interstitial fluid
Ascending limb of Loop of Henle
- impermeable for water
- Na+ is pumped out into interstitial fluid, creating
osmotic gradient between tubule lumen and
interstitial fluid (high osmotic pressure in the medulla)
- loss of Na+ results in hypoosmotic urine as it ascends
in the Loop of Henle from the inner medulla to the outer
medulla (Paradox?! We don’t want hypoosmotic urine?!)
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Osmotic Gradient Formation in the Medulla
1) The Single Effect
Creates an extreme Na+/Cl- gradient between
tubule lumen and medullary interstitial fluid
- active transport of Na+ (Cl- follows) from
ascending segment of the Loop of Henle
into the medullary interstitial space
- water can’t follow through impermeable wall
Result: increased interstital ion concentration,
and increasingly hypoosmotic urine in the ascending
segment due to loss of salt.
Water is osmotically drawn from descending
segment, resulting in a more hyperosmotic
urine as descends down into the medulla.
Important: At this point we don’t care about urine
osmolarity. We built osmotic gradients!
Mammalian Kidney Function
Mechanism of Osmotic Gradient Formation in the Medulla
II. Countercurrent Multiplier Mechanism increases Osmotic Pressure Gradient
urine is not stationary,
it flows
Single Effect
-the single effect is multiplied
by the movement of urine
Result: steep hyperosmotic interstitial gradient is created, and
hypoosmotic, salt reduced urine leaves the loop.
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Osmotic Gradient Formation in the Medulla
II. Countercurrent Multiplier Mechanism increases Osmotic Pressure Gradient
urine is not stationary,
it flows
we want/need?
Single Effect HYPOOSMOTIC! Is that what -the
single effect is multiplied
by the movement of urine
Countercurrent Multiplier
only works when urine flow
is maintained!
Result: steep hyperosmotic interstitial gradient is created, and
hypoosmotic, salt reduced urine leaves the loop.
Mammalian Kidney Function
Urine Formation: Counter Current Multiplier to drive Osmotic Gradient to the Max.
Flow of urine through the nephron creates and enables countercurrent multiplier mechanism
between Loop of Henle and renal medulla
Key to understand Function and Mechanism of
Counter Current Multiplier is to realize that
there is constant flow of urine in those segments!
-the single effect is multiplied
by the movement of urine
Flow allows to build gradient that by far exceeds what
Single Effect counter current exchanger would be able to do!
The following Flash animation may be helpful to
better understand this mechanism!
http://www.cellphys.ubc.ca/undergrad_files/urine.swf
Result: urine enters the loop, a highly hyperosmotic interstitial gradient is created, and
hypoosmotic, salt reduced urine leaves the loop. HYPOOSMOTIC! That is not what we want/need?
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MSVU Animal Physiology U. Hoeger
Mammalian Kidney Function
Mechanism of Urine Formation
Distal Convoluted Tubule
Distal convoluted tubule receives hypoosmotic urine from the ascending limb of the Loop of Henle
- rich in mitochondria to provide ATP required for active transport mechanisms
(both reabsorption and secretion)
- active transport of ions in the distal convoluted tubule controlled by the endocrine system
- parathyroid hormone promotes Ca2+-reabsorption and PO43—secretion
-aldosterone promotes Na+-reabsorption and K+-secretion
-natriuretic peptide promotes Na+-secretion
-secretion of H+ or NH4+ (ammonium) regulates pH
Distal convoluted tubule plays major role in voiding undesired substances
Urine is now reduced to ~3% of the volume of the primary urine
STILL RELATIVELY HYPOOSMOTIC AND DILUTED !
Mammalian Kidney Function
Mechanism of Urine Formation
Collecting Duct
Collecting Duct descends from Cortex through Medulla in Renal Pelvis
- descending urine faces the high osmotic pressure of medullary
interstitial fluid, generated by the counter-current multiplier system
Water permeability of the collecting duct
is regulated by the endocrine system!
Antidiuretic hormone ADH regulates final water reabsorption from urine,
i.e. concentration and volume of the definitive urine!
- presence of ADH and vasopressin (another hormone) increases
water permeability of collecting duct wall
- water is osmotically removed from urine (up to 75 % of the
urine volume entering collecting duct)
- lack of ADH and vasopressin renders collecting duct wall
impermeable for water
- water can not be removed and urine stays diluted
Definite urine leaves the collecting duct through the renal papilla into the
renal pelvis, and finally reaches the bladder via the ureter
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MSVU Animal Physiology U. Hoeger
Urine Formation in Non-Mammals
Reptiles and amphibians: simpler than mammalian
nephrons. Loop of Henle not present!
Limited ability to produce hyperosmotic urine
Freshwater fish: nephrons similar to amphibian
type. No need to produce hyperosmotic urine
Marine fish: reduced nephrons, poor glomerular
filtration, secretion is important for formation of
primary urine. Some species have no glomerular
structure (aglomerular)
Urine Formation in Invertebrates
All nephridial organs follow the same principals:
- tubular structures on one end in contact with body fluid,
on other end in contact with environment
- formation of primary urine by filtration or secretion
- modification of primary urine by reabsorption and secretion
Protonephridia:
- blind tubules, ending in ciliated flame cells and opening through
nephridiophore to the exterior
- fluid uptake into tubule by filtration and secretion
- active reabsorption
Metanephridia:
- ciliated funnel opens into the body cavity (coelom) and takes up fluid
- cilia pump water carrying ions and metabolic waste through tubule
- fluid is modified in glandularized duct by reabsorption and secretion
- resulting urine is finally discharged through nephridiophore to the exterior
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MSVU Animal Physiology U. Hoeger
Urine Formation in Invertebrates
Crustaceans: antennal or green glands are the renal organs.
Each gland is basically a giant nephron
Exit
Urine Formation in Invertebrates
Insects:
Malpighian Tubules and Hindgut are the “renal” organs.
-
Malpighian tubules form primary urine by secretion,
and discharge into hindgut
-
modification (H2O reabsorption etc.) is
done in the hindgut.
Insects can produce hyperosmotic urine
by secretion of solutes into the rectum!
Insects produce Uric Acid, another way to
conserve water and minimize obligatory water
loss with excretions.
Exit
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MSVU Animal Physiology U. Hoeger
Nitrogen Disposition and Excretion
Toxic Waste Disposal
Catabolism of proteins produces CO2, H2O and nitrogenous end products
- nitrogenous end products are tricky, they are either cheap to make and relatively toxic,
or require some energy investment and are of low toxicity
- mammals, birds, and reptiles excrete nitrogenous end products entirely in their urine
- fish and other aquatic animals void nitrogenous waste over gills or integument
Ammonothelic animals (fish, aquatic invertebrates) produce NH3 (ammonia) or
NH4+ (ammonium), both are quite toxic but cheap to synthesize!
- instantaneous voiding and dilution in the environment required to prevent problems
Ureotelic animals (amphibians, mammals, some invertebrates) produce urea, highly soluble
in water, diffuses easy over membranes, non-toxic but costly to synthesize.
- water is required as solvent, obligatory water loss with urine is unavoidable!
Uricotelic animals (insects, reptiles, birds) produce uric acid. Non-toxic, most expensive
product to synthesize.
- insoluble in water (precipitates as crystals), minimizes obligatory water loss with urine
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