Osmoregulation
Ionic and osmotic balance
2/21
• in multicellular organisms the interstitial fluid is
the internal environment – composition resembles
that of the ancient sea: high Na+, low K+, low
Ca++ and Mg++
• osmoregulatory organs maintain this environment
isovolumic, isotonic, isoionic, isohydric, etc.
• further task: removal of poisonous end products
of metabolism (mostly NH3 from proteins)
• marine invertebrates: osmotic pressure and ionic
constitution in equilibrium with seawater
• marine vertebrates: ion concentration is one
third of seawater except in hagfish (Eptatretus
- Cyclostomata), MgSO4, Cl- much lower; in
sharks-rays ion concentration is lower, but
osmotic pressure is maintained by urea
• fresh water, terrestrial: ion concentration is one
third; hyperosmotic to freshwater, hyposmotic
to seawater
1
Osmotic exchange
3/21
• plasma membrane separates fluids with
different ionic composition, but equal osmotic
pressure
• epithelium separates fluids that are different
in both respects
• animals cannot isolate themselves from the
environment: exchange of gases, absorption of
nutritients; exception: Artemia salina
• obligate and regulated osmotic exchange
• obligate exchange depends on physical
factors, that animals cannot readily regulate
• regulated exchange compensates for changes
caused by the obligate
• only some parts of the epithelium participate
in osmoregulation: gill, kidney, salt gland,
enteric system
Obligate osmotic exchange
4/21
• occurs through the skin, respiratory epithelium
and other epithelia in contact with environment
• influencing factors:
– gradient: determines direction of exchange; a frog
sitting in a pond takes up water through the skin; a
marine fish is loosing water in the sea, but gains
NaCl; a freshwater fish is taking up water, but
loosing salt through the gill
– surface: small animal – relatively larger surface,
faster exchange, e.g. dehydration
– permeability:
• transcellular and paracellular exchange (but: tight junction);
• skin of amphibians, gill of fish have high permeability
• skin of reptiles, desert amphibians, birds, mammals are much
less permeable (leather containers), but mammals loose water
through sweating
– eating, metabolism, excretes: metabolic water is very
important for desert animals, but also for marine
mammals (seals put on weight eating fish, but loose
weight burning fat when is feeding on invertebrates)
– respiration: function of nose – condense water during
exhalation – dropping nose in winter
2
Osmotic regulation I.
5/21
• most vertebrates are strict osmoregulators exception: shark, ray and hagfish
• marine invertebrates are in equilibrium,
other invertebrates, similar to vertebrates,
are hyperosmotic in freshwater, hyposmotic
in seawater
• some of the invertebrates is conformer,
others are osmoregulators
• freshwater animals breathing from water
– they are hyperosmotic: 200-300 mOsm;
freshwater in general below 50 mOsm – water
inflow, salt outflow
– to compensate, they produce dilute urine, take up
salt with food and through active transport (fish,
frog), decrease the permeability of their skin
– they do not drink freshwater
Osmotic regulation II.
6/21
• marine animals breathing from water
– invertebrates are in equilibrium
– hagfish: Ca++, Mg++, SO42- regulation only
– shark-ray osmotic equilibrium due to urea
accumulation, excess salt removed by rectal glands
– fish are loosing water, drinking seawater; excess
salt removed through the gill (chloride cells)
• marine animals breathing air
– loosing water through respiratory epithelium and
other epithelia
– marine reptiles and birds are drinking seawater –
cannot produce strongly hyperosmotic urine (just as
fish) – excess salt removed through salt glands
– marine mammals do not drink seawater, water is
taken in with food and produced metabolically,
urine is hyperosmotic
– lion seal males spend 3 months on the beach
without eating or drinking; seal pups do similarly
for 8-10 weeks while mother is on the sea
3
Osmotic regulation III.
7/21
• terrestrial animals breathing air
– if freshwater is available then water lost
through breathing can be replenished by
drinking, salt loss (urine, faces, sweating)
compensated from food – sparing is important
– problem of shipwrecked sailors: kidney can
remove 6 g Na+/l urine, seawater contains 12 g
Na +/l – drinking of seawater leads to salt gain
– desert animals have two problems: heat and lack
of water
– kangaroo rat: remains in cool burrow during
daytime, active only during the night; gaining
water metabolically, water condensation in nose
– camel: cannot hide in burrow; when dehydrated
do not sweat, body temperature raises changing
between 35-41° C; hyperosmotic or no urine,
urea stored in tissues; faces dry
8/21
Overview
body fluids compared
to seawater
isosmotic
breathing
environment
group
problem, solution
seawater
seawater
invertebrates,
hagfish, shark,
ray
seawater
seawater
marine fish
water loss, drinking seawater, salt
excretion through gill
fresh
water
fresh water
freshwater fish
water inflow, active salt accumulation,
diluted urine, no drinking
marine reptile
water loss, drinking seawater, salt
gland
marine reptile
salt intake (food), hyperosmotic urine,
metabolic water, no drinking
amphibian
water inflow, active salt accumulation
through skin, diluted urine, no drinking
reptile, mammal
skin impermeable, water loss through
breathing, drinking freshwater as
needed
all groups
water loss through breathing and skin,
drinking freshwater as needed
desert animal
metabolic water, hyperosmotic urine,
dry faeces, behavior
marine bird
drinking seawater, salt gland
seawater
1/3 osmotic pressure
fresh water
air
air
no problem
4
Water compartments
9/21
• human body contains 60% water on average,
differences between male-female, young-old
• distributed in different compartments
• intracellularly 2/3, extracellularly 1/3
• of the extracellular water: 3/4 interstitially,
1/4 in blood plasma
• barriers and, transport rules are already known
• measurement of volumes applying dilution
principle: Evans-blue, inulin, tritiated water
• homeostasis is very important . cholera, diarrhea
- dehydration, working by a furnace in tropical
areas – water poisoning, severe burns –
dehydration due to loss of skin
• the most important regulator in humans is the
kidney, behavioral regulation is also important –
metabolic water is limited
Human kidney
10/21
• osmoregulatory organs always contain transport
epithelium (skin, gill, kidney, gut) : polarized apical (luminal, mucosal) and basal (serosal)
surfaces are different
• capacity of the transport epithelium is increased
by its special structure: tubular organization
• functioning of the mammalian kidney is well
known – though it does not represent all types of
vertebrate kidneys
• 0,5% of body weight, 20-25% of cardiac output
• cortex, medulla, renal pyramid, renal pelvis,
ureter, urinary bladder, urethra
• volume of urine is 1 l daily, slightly acidic (pH
6), composition, volume changes with the food
and the requirements of the water homeostasis
- beer, Amidazophen, etc.
5
The nephron
11/21
• functional unit of human kidney is the
nephron
• afferent and efferent arterioles, in between
glomerulus; Bowman capsule, proximal tubule,
loop of Henle, distal tubule, collecting duct
• most of the nephrons (85%) are cortical, the
rest juxtamedullary (15%) nephron
• steps in the formation of urine:
– ultrafiltration
– reabsorption
– secretion
• the kidney is very important in pH regulation
• the kidney removes ammonia formed during
the decomposition of proteins
Ultrafiltration
12/21
• in the kidney 15-25% of water and solutes is
filtrated, 180 l daily – proteins and blood cells
remain
• filtration depends on:
– the hydrostatic pressure between the capillaries and
the lumen of the Bowman capsule: 55-15 = 40 mmHg
– the colloid osmotic pressure of the blood: 30 mmHg –
effective filtration pressure 40-30 = 10 mmHg
– the permeability of the filter: fenestrated capillaries,
basal membrane (collagen + negative glycoproteins),
podocytes (filtration slits between pedicels)
• voluminous blood supply due to the relatively low
resistance – afferent arteriole is thick and short
– high pressure in the glomerulus
• regulation of the blood flow: basal miogenic
tone, paracrine effect of juxtaglomerular
apparatus, sympathetic effect (afferent
arteriole, glomerulus, podocyte)
6
Clearance
13/21
• clearance of a substance is the volume of plasma
that is completely cleaned from the given
substance in the kidney in every minute
CP = VU
that is
VU
C = -----P
C - clearance, P – concentration in plasma,
V – volume of urine in 1 minute, U – concentration in
urine
• clearance of a substance that is neither
reabsorbed nor secreted (e.g. inulin) equals the
glomerulus filtration rate : GFR
• clearance of a substance that is not only
filtrated, but completely secreted as well (e.g.
PAH) equals the renal plasma flow : RPF
• knowing the hematocrit, renal blood flow (RBF)
can be calculated
Tubular reabsorption I.
14/21
• 180 l primary filtrate is produced every day,
but only 1 l is excreted, of 1800 g filtrated
NaCl only 10 g remains in the urine
• the process of reabsorption has been
successfully examined since the 1920’s using the
method of micropuncture
• role played by the subsequent sections of the
tubules:
– proximal tubule
• 70% of Na+ is reabsorbed by active transport, Cl- and water
follow passively, obligate reabsorption
• filtrate is isosmotic, but concentration of substances that
are not reabsorbed increases 4-fold
• on the apical membrane of epithelial cells microvilli
• virtually all filtrated glucose and amino acids are reabsorbed
using Na+ dependent symporter
• tubular maximum for glucose: below 1.8 mg/ml complete
reabsorption (normal value: 1.0 mg/ml), above 3.0 mg/ml
linear increase – sugar in urine in diabetes
• Ca++, phosphate and other electrolytes are reabsorbed as
needed – see later
7
Tubular reabsorption II.
15/21
– descending part of Henle’s loop
• no microvilli, few mitochondria – no active transport
• low permeability for NaCl and urea, high for water
– thin ascending part of Henle’s loop
• no microvilli, few mitochondria – no active transport
• low permeability for water, high for NaCl
– thick ascending part of Henle’s loop
• active reabsorption of Na+
• low water permeability
– distal tubule
• active reabsorption of Na+, and passive reabsorption
of water
• K+, H+ and NH3 transport as needed – see later (pH
regulation)
• transport is regulated by hormones – facultative
reabsorption
– collecting duct
• active reabsorption of Na+ at the cortical part, high
urea permeability in the internal medullary part
• regulated water permeability (ADH)
Tubular secretion
16/21
• several substances are secreted from the plasma
to the tubule in the nephron – best examined:
different electrolytes (K+, H+, NH3) organic
acids and bases
• active transport – recognizes substances
conjugated with glucuronic acid in the liver
• K+ is reabsorbed in the proximal tubule and
Henle’s loop (Na/2Cl/K transporter)
• if K+ concentration is too high, secretion in the
distal tubule depending on aldosterone and
coupled to Na+ reabsorption - K+ acts directly on
aldosterone, Na+ through renin-angiotensin
• conflicting demands: insulin secretion is induced
by high K+ - excess K+ is taken up by adipose
tissue
• secretion of H+ and NH3 serves pH regulation
8
pH regulation I.
•
•
•
•
17/21
normal pH 7.4 – 7.35 acidosis, 7.45 alkalosis
normal functioning is possible between 7.0-7.8
regulation: buffer systems, respiration, kidney
Henderson-Hasselbalch equation:
[A-]
pH = pK + log -----[HA]
• for CO2 two equations; following rearrangement:
•
[HCO3-]
pH = pK’ + log -----[αCO2]
• α - solubility of CO2
• pK’ = 6.08, i.e. not good buffer at normal pH –
as CO2 and HCO3- can be easily modified
(respiration, kidney)
• plasma proteins (14-15% Hgb, 6-8% other) pK is
same as blood pH – good buffers
• phosphate concentration low, negligible effect
pH regulation II.
18/21
• respiratory alkalosis and acidosis: caused by
hyper-, or hypoventilation
• metabolic alkalosis – e.g. Cl- loss because of
vomiting
• metabolic acidosis – anaerobic energy production,
ketosis in diabetes mellitus
• in the first case: kidney compensates, in the
second: breathing (short-term) and the kidney
(long-term)
• proximal tubule, Henle’s loop: Na+/H+ exchanger,
distal tubule, collecting duct: HCO3- uptake
through A-cells
• in distal tubule and collecting duct: HCO3secretion through B-cells
• in acidosis HCO3- level is low in the filtrate:
NH3 secretion – binds to H+, NH4+ cannot go
back, H+ secretion increases
9
Hyperosmotic urine
19/21
• birds and mammals can produce hyperosmotic
urine - water reabsorption in the collecting
duct due to osmotic pressure differences
• common characteristic: Henle’s loop, the longer
the loop, the more concentrated the urine –
very long in kangaroo rat
• pressure difference is achieved through the
counter-current principle
• Na+ transport in the ascending part of the
Henle’s loop – do not enter the descending
part, but attracts water leading to the same
result
• in addition, urea present in high concentration
because of the reabsorption of water, can only
leave the tubule in the internal medulla
• osmotic pressure increases from the cortex to
the medulla
• blood supply to the tubules (vasa recta) is
running in parallel to the Henle’s loop , does not
decrease the osmotic gradient
Regulation of the kidney
20/21
• granular cells in the juxtaglomerular apparatus
produce renin in response to a decrease in blood
pressure or NaCl delivery to the distal tubule
• renin cuts off angiotensin I (10 amino acids)
from angiotensinogen (glycoprotein)
• converting enzyme (mostly in the lung) cuts off 2
amino acids from angiotensin I – angiotensin II
• angiotensin II enhances aldosterone secretion in
the adrenal gland, increases blood pressure
through vasoconstriction and increases ADH
production
• aldosterone increase Na+ reabsorption through 3
different ways: facilitation of the pump, ATP
production, increased apical Na+ permeability
• ADH producing cells detect blood pressure and
osmolality and are sensitive to alcohol
• atrial natriuretic peptide (ANP) – released in the
atria when venous pressure increases - inhibits
renin, aldosterone, ADH production
10
Nitrogen removal
21/21
• part of the digested amino acids are reused,
amino groups from the others have to be
removed as NH3 and NH4+ are poisonous
• three forms: ammonia, urea, uric acid
• ammonia
– poisonous – huge volume is needed to provide low
concentration in the cell and high outward gradient
– 0.5 l water/1 g nitrogen
– fish, aquatic invertebrates, mammals in low amount
– transport in the form of glutamine from the liver to
the kidney
• urea
– less poisonous, 0.05 l water/1 g nitrogen
– synthesis requires ATP
– vertebrates, except fish, synthesize urea in the
ornithine-urea cycle, fish and invertebrates from uric
acid
– hominoids cannot metabolize uric acid (from nucleic
acids) – can accumulate and lead to gout
• uric acid
– low solubility: 0.001 l water/1 g nitrogen
– white precipitate - guano in birds (uric acid, guanine)
– fish, reptiles, terrestrial arthropods
Extracellular ion concentrations
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Tab. 14-1.
11
Osmoregulation in animals
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-8.
Structure of mammalian kidney
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-13.
12
Structure of a nephron
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-14.
Glomerular filtration
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-18
13
Podocytes of the capsule
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-19.
Podocytes of the capsule - EM
Berne and Levy, Mosby Year Book Inc, 1993, Fig. 41-7
14
Juxtaglomerular apparatus
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-20.
Na+ reabsorption
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-25.
15
Processes of reabsorption
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-24.
Mechanism of K+ secretion
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-28.
16
pH regulation
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-29.
Release of ammonia
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-30.
17
Counter-current principle
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, SL. 14-2.
Mechanism of urine concentration
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-32.
18
Osmotic pressure in the kidney
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-32.
Renin-angiotensin system
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-26.
19
Actions of aldosterone
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-27.
Regulation of ADH secretion
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-35.
20
Methods of nitrogen release
Eckert: Animal Physiology, W.H.Freeman and Co., N.Y.,2000, Fig. 14-49, 50.
21