Osmoregulation

1
Osmoregulation
2
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 ?

3
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

4
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

5
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 ?

6
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 ?

7
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
Overview
8/21
9
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

10
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.

11
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

12
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) ?

13
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
• VU CP VU that is 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

14
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 1920s 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

15
Tubular reabsorption II.
15/21

• descending part of Henles loop
• no microvilli, few mitochondria no active
  transport
• low permeability for NaCl and urea, high for
  water
• thin ascending part of Henles loop
• no microvilli, few mitochondria no active
  transport
• low permeability for water, high for NaCl
• thick ascending part of Henles 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) ?

16
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
  Henles 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

17
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 ------
  aCO2
• ? - 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

18
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, Henles loop Na/H exchanger,
  distal tubule, collecting duct HCO3- uptake
  through A-cells
• in distal tubule and collecting duct HCO3-
  secretion through B-cells ?
• in acidosis HCO3- level is low in the filtrate
  NH3 secretion binds to H, NH4 cannot go back,
  H secretion increases ?

19
Hyperosmotic urine
19/21

• birds and mammals can produce hyperosmotic urine
  - water reabsorption in the collecting duct due
  to osmotic pressure differences
• common characteristic Henles 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
  Henles 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 Henles loop , does
  not decrease the osmotic gradient

20
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

21
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

22
Extracellular ion concentrations
23
Osmoregulation in animals
24
Structure of mammalian kidney
25
Structure of a nephron
26
Glomerular filtration
27
Podocytes of the capsule
28
Podocytes of the capsule - EM
29
Juxtaglomerular apparatus
30
Na reabsorption
31
Processes of reabsorption
32
Mechanism of K secretion
33
pH regulation
34
Release of ammonia
35
Counter-current principle
36
Mechanism of urine concentration
37
Osmotic pressure in the kidney
38
Renin-angiotensin system
39
Actions of aldosterone
40
Regulation of ADH secretion
41
Methods of nitrogen release