Osmoregulation Flashcards
Importance of animal body fluids
Fluid bathes organs, tissues, cells and organelles
Fluid composition often regulates how these function
Total volume of fluid in the average 70kg man
42L
Intracellular fluid volume
28 L
Extracellular fluid volume
14L
How is extracellular fluid divided
Interstitial fluid
Plasma
Transcellular fluid
Interstitial fluid volume
9.5 L
Plasma fluid volume
3.5 L
Transcellular fluid volume
1 L
Importance of water
Matrix in which ions are dissolved
Determines volume of cells and tissues
Key role in determining hydrostatic pressure
Importance of inorganic ions in body fluids
Ion concentration affects 3D conformation of enzymes
Produce and maintain electrical gradients across cell membranes
Crucial for nerve impulse transmission and muscle excitation
Body fluid relations
Freshwater fish have bodily fluid compositions that are more concentrated than the surrounding water
Need to expend considerable amounts of energy to maintain ionic imbalance as constantly absorb water from environment
Marine invertebrates have ionic concentrations little different to that of the surrounding (salty) water
Osmotic regulation
Maintenance of a constant osmotic pressure to the body fluid eg blood plasma
Ionic regulation
Maintenance of a constant concentration of an inorganic ion in a body fluid eg
Volume regulation
Regulation of the total amount of water in a body fluid eg
Osmosis
passive transport of water across a semi-permeable cell membrane or an epithelial layer (or artificial membrane) from a solution with lower osmotic pressure to a solution with higher osmotic pressure
Osmotic pressure
property of a solution that allows prediction of whether the solution, when in contact with another solution, will gain or lose water via osmosis
Measured in osmoles
Low osmotic pressure
High concentration of water (low solute)
High osmotic pressure
Low concentration of water (high solute)
Colligative property
Osmotic pressure is a colligative property of a solution
Also includes freezing point and water vapour pressure
Depends on the number of dissolved entities per unit volume of solution rather than the chemical nature of the dissolved entities
Colligative property
Osmotic pressure is a colligative property of a solution
Also includes freezing point and water vapour pressure
Depends on the number of dissolved entities per unit volume of solution rather than the chemical nature of the dissolved entities
Isosmotic
semi-permeable membrane are the same
Hyposmotic
internal solutions have a lower osmotic pressure than the surrounding fluids so lose water via osmosis
Hyperosmotic
internal solutions have a higher osmotic pressure than the surrounding fluids so gain water via osmosis
Osmoregulator
The blood osmotic pressure of an animal does not change with changing environmental osmotic pressure
Osmoconformer
Blood osmotic pressure is the same as the ambient osmotic pressure
Humans as osmoregulators
Maintain 300 mOsm irrespective of how much water is consumed
Freshwater fish
Body is hyperosmotic to water around it
Water moves into the body via osmosis
Salts lost to the water
Decreases osmotic pressure of the blood plasma (a challenge to osmotic regulation)
Dilutes ions in the blood plasma (a challenge to ionic regulation)
Increases volume of water in the blood plasma (a challenge to volume regulation)
Marine fish
Body is hyposmotic to water around it
Water moves from the body to the sea water via osmosis
Salt moves into body via gills and food
Increases osmotic pressure of the blood plasma (a challenge to osmotic regulation)
Concentrates ions in the blood plasma (a challenge to ionic regulation)
Decreases volume of water in the blood plasma (a challenge to volume regulation)
Marine shark
Body is hyperosmotic but hypoionic to water around it
Water moves from the seawater to the blood via osmosis via the gills
Salt also moves into body via gills and food
Decreases osmotic pressure of the blood plasma
Concentrates ions in the blood plasma (a challenge to ionic regulation)
Increases volume of water in the blood plasma (a challenge to volume regulation)
Sea water
34-36 g NaCl per kg H2O
Osmotic pressure = 1000 mOsm
Freshwater in lakes and rivers
< 0.5 g NaCl per kg H2O (often 0.1-0.2 g/kg)
Osmotic pressure = 0.5-15 mOsm
Calcium ion (Ca2+) concentration depends on geology and can have impact of water-salt physiology of freshwater animals
Brackish water
in estuaries varies in salinity both geographically and in time
0.5 - 30 g NaCl per kg H2O
Osmotic pressure = 15 – 850 mOsm
Evaporation and partial pressure of water vapour
Water as liquid has high partial pressure
Water vapour has a partial pressure = proportion of water vapour per unit volume of air
Evaporation takes place if partial pressure of water vapour in air is lower than that of liquid water
Elevated temperature increases partial pressures at saturation water vapour pressure
Rate of evaporation is increased as difference between liquid and air increases
What does water vapour pressure of an aqueous solution depend on
Solute concentration
Evaporation
The difference between water vapour pressure of the solution and air, divided by the distance between the solution and air
Ingestion of seawater or halophytes
Consumption of seawater by a hyposmotic whale will not gain water from the drink
Excretion of the excess chloride ions (Cl-) requires more water than is ingested because kidneys cannot urine that is iso-ionic to seawater
Bodily reserves of water are needed so drinking seawater leads to dehydration
Halophytes have high levels of salt within their tissues that can be ingested by herbivores
High sodium ion (Na+) concentrations mean that urine cannot concentrate enough ions to remove excess
Ingestion of high protein foods
Catabolism (breakdown) of sugars and lipids yields metabolically derived H2O (0.56 g g H2O per 1 g carbohydrate / 1.07 g H2O per 1 g lipid)
Metabolically derived water can be crucial in those animals with limited access to water
Protein diets generate more nitrogenous waste – ammonia, urea or uric acid – that needs to be excreted
High protein diets for a urea-excreting mammal require more water for excretion than for a low protein diet
Commonest amino acid catabolism pathway
α-amino acid + α-ketoglutamate → glutamate + α-keto acid
Glutamate taken up by mitochondria and oxidatively deaminated by glutamate dehydrogenase:
glutamate → NH3 + α-ketoglutamate
Ammonia (NH3) combines with water to form the weak base NH4OH, which disassociates into ammonium (NH4+) and hydroxide (OH-) ions
Gluconeogenesis and nitrogenous end-product formation
Glutamine synthetase reaction binds ammonium ions in an effective, non-toxic form for transport
glutamate + MgATP + NH4+→ glutamine + MgADP + Pi
This process consumes a lot of energy in the form of ATP. Protein catabolism also generates an excess of base as bicarbonate ions (HCO3-) relative to H+
Ammonia and ammonium
Ammonia (NH3) and ammonium (NH4+) are highly toxic
High levels lead to accumulation of glutamine and depletion of α-ketoglutamate and ATP
Loss of α-ketoglutamate is problematic for the tricarboxylic acid cycle
Glutamate is a neurotransmitter so disrupts the nervous system
High levels of NH3 alkalanises and NH4+ acidifies the cytoplasmic pH
NH3 diffuses into the mitochondria and binds with H+ to form NH4+ which reduces the pH gradient across the mitochondrial membrane and eliminates the electromotive force for ATP synthesis
Biological imperative is to prevent increases in ammonia concentrations
Excretion of ammonia at very low concentrations
Formation of glutamine
Formation of less toxic urea (CO(NH2)2) or uric acid (C5H4N4O3) [also liberates H+ that helps neutralise excess HCO3- released during protein catabolism]
Ways to prevent an increase in ammonia concentrations
Excretion of ammonia at very low concentrations
Formation of glutamine
Formation of less toxic urea (CO(NH2)2) or uric acid (C5H4N4O3) [also liberates H+ that helps neutralise excess HCO3- released during protein catabolism]
Urea formation
Urea is formed by the ornithine-urea cycle in the liver (and kidney)
Urea is a neutral molecule with low toxicity
Uric acid is a purine that metabolically produced from glutamine
It is effectively insoluble in water as the urate salt
3 methods of nitrogen excretion in amphibians
Ammonotelism - ammonia excretion with lots of water
Ureotelism - urea excretion with some water
Uricotelism - uric acid excretion with little water
Nitrogen excretion in reptile eggs
Many reptile eggs vary in eggshell structure but can gain water with the nest environment
Reptile embryos utilise protein for energy and generate little metabolic water
Produce more urea than uric acid
Nitrogen excretion in bird eggs
Bird eggs have hard shells and only lose water vapour during incubation
Bird embryos utilise lipid for energy and generate lots of metabolic water
Produce uric acid to conserve water
Invertebrate organs of blood regulation
Variety of kidney-like structures
Malpighian tubules in insects
Vertebrate organs of blood regulation
Kidney is a crucial organ
Regulates composition of blood by removing water, salts and other solutes in a controlled manner
Also gills in fish
Rectal and salt glands in marine reptiles and birds
Desert definition
aridity - < 25 cm of precipitation per year
Condensation
Sometimes conditions do allow for animals to be cooler than the ambient air which leads to condensation is water vapour pressure is high
Some mammals can condense water vapour from exhaled breathe on turbinate bones, e.g. camels
Which animals do not have a problem with osmoregulation
Marine invertebrates- have a permeable integument and are isosmotic and iso-ionic to sea water
Aquatic invertebrates in brackish or freshwater eg freshwater crayfish
Hyperosmotic and hyperionic to the ambient water
Impermeable cuticle
Active uptake of Na+ and Cl- via the gills
Production of copious dilute ammonotelic urine from antennas glands
End sac (coelomosac) is supplied with arterial blood via lacunae - filtration of plasma
Fluid then passes through labyrinth and nephridial tubule where molecules (eg glucose and NaCl) are reabsorbed from urine that is first stored in a bladder then exits the body via the nephropore
Water reabsorbed along the length of the antennal gland
Osmotic pressure is isosmotic in marine decapods
Hypotonic in freshwater species and length of nephridial canal correlates with osmolality of urine
Terrestrial invertebrates in air
Waterproofed cuticle to minimise loss of water in insects
Main excretory organ is Malpighian tubule which is engaged in active secretion of molecules across its wall (surrounded by haemolymph) to form primary urine in a blind-ended lumen that empties into the gut
Solutes are actively transported from the haemolymph between or through cells
Active recovery of water and ions in the hind gut
Most insects consuming solid food produce frass that is a mix of nitrogenous waste molecules and faeces
Insects feeding on liquid, blood or plant sap remove very little water from their urine and the excreta secreted is very liquid
Production of primary urine by secretion
Consider a tubule that has solution inside and the haemolymph outside, which are isotonic
(1). ATP energy is used to move solute X molecules into the lumen, which increases the osmolality of the tubular fluid
(2). Water then moves into the tubule from the haemolymph by osmosis which then dilutes solute Y
(3). Solute Y then passively diffuses into the tubule down its concentration gradient
(4). In this way the concentration of solutes X and Y are increased in the tubules.
Vertebrates in water
With the exception of hagfish – marine fish that are isosmotic to seawater (1000 mOsm) – all vertebrates are effectively regulating bodily fluids at ~300 mOsm
All modern fish (and vertebrates) arose from an ancestor adapted to freshwater
Need to regulate water loss or gain in aquatic habitats and water loss in terrestrial habitats
Need to regulate ion loss or gain in aquatic habitats
Need to excrete nitrogenous waste products efficiently
Osmolarity of body fluids in vertebrates in water
300 mOsm
What is the exception to vertebrates in water
Hagfish
A marine fish- hyposmotic to ambient water
gains water by ingestion of food and water
Loses water mainly via osmosis via gills and in urine and faeces
Gains solutes in food and diffusion across gills
Loses solutes by urine and faeces
Freshwater fish - Hyperosmotic to ambient water
Gains water by ingestion and via osmosis by the gills
Loses water in urine and faeces
Gains solutes in food and by active transport across gills
Loses solutes by diffusion from gills and in urine and faeces
Osmoregulation via the gills
Water is exchanged by osmosis across the semi-permeable epithelial membranes of the gills
Large surface area and perfumed with blood
Volume control is largely down to kidneys producing small (marine) or large (freshwater) amounts of urine
Gills are site of active ion exchange
Sodium and chloride ions are key components of bodily tissues
Freshwater fish lose ions in urine and faeces but also diffusion across the gills
Ionic regulation by cells in the gill epithelium requires active exchange of H+ (out) for Na+ (in) and active exchange of HCO3- (out) and Cl- (in)
Uses ATP and ions from the disassociation of carbonic acid produced by CO2 dissolving in water
Mitochondrial-rich (chloride) cells are often associated with gill arch
Pavement cells (pvc) thought to be site of oxygen exchange
Active transport of NaCl into blood
Site of oxygen cells in gills
Pavement cells
Active ion exchange cells in gills
Mitochondrial-rich chloride cells
Mitochondrial-rich chloride cells
Same cells are seen as being able to actively transport chloride ions
Na+, K+ and 2 Cl- move into cell via a cotransporter
ATPase removes the Na+
Cells are packed with mitochondria
Chloride ions move through a chloride channel to the sea water
Sodium ions move between cells out into seawater to balance the ionic gradient
Active loss of NaCl from blood
Marine shark - Hyperosmotic to ambient water
Body is hyperosmotic because retains urea within cells to increase osmolality of tissues
Water moves from the seawater to the blood via osmosis via the gills
Produces modest amounts of hyposmotic urine
Sharks venturing into brackish water lower their osmolality by reducing urea production
Hypoionic tissues relative to water around it
Salt also moves into body via gills and food
Excess salt ions are extracted from the blood by chloride cells in rectal glands and excreted with faeces
Salt glands
Marine turtles, lizards and birds are hyposmotic regulators
Kidney is unable to produce hyperosmotic urine that would remove the excess salts consumed with food and water
Salt glands are organs that allow for extra-renal salt excretion using chloride cells
Salt gland tubules are lined with chloride cells and lie in a counter-current system with perfusing blood
Salt is excreted down ducts on to the face as a hyperionic solution
Humidic animals
Occupy terrestrial habitats that are humid and water-rich environments
Annelids, gastropods, centipedes, terrestrial crabs and most amphibians
Terrestrial xeric invertebrates (insects and arachnids) and vertebrates
Adopted dry conditions
Evolved a waterproof integument that minimises cutaneous water loss
Relationship between body weight and weight-specific rate of evaporative water loss
Inverse relationship
Very small birds lose relatively greater amounts of water compared with large birds
Tree frog
secretes lipids (waxy esters) from skin glands and spreads them over its skin with its legs
Also is uricotelic rather than ureotelic - switching from urea to uric acid 70-fold saving of water
U/P ratio
Osmotic pressure of urine/ osmotic pressure of blood plasma
U/P = 1
Just like excreting blood plasma
U/P < 1 (hyposmotic)
Water preferentially excreted
Solutes preferentially retained
Osmotic pressure of plasma raised
U/P > 1 (Hyperosmotic)
Water preferentially retained
Solutes preferentially excreted
Osmotic pressure of plasma lowered
Kidney function
Primary urine produced by filtration or secretion from blood plasma is introduced into kidney tubules
Modification by selective reabsorption leads to definitive urine that is excreted
Urine crucial in removing nitrogenous waste whilst maintaining osmotic balance
Energetically expensive process
Production of primary urine by filtration
The vertebrate kidney is centered around the Bowman’s capsule and the loop of Henle, which form part of the nephron. Blood vessels come into the kidney and there are capillary ‘balls’ called glomeruli that sit within the Bowman’s capsule. The reduction in blood vessel lumen in the glomerulus forces plasma out of the blood and this cross the ‘leaky’ wall of the Bowman capsule’s wall so that this primary urine accumulates within the lumen of the nephron.
Diuresis
Formation of copious urine
Urine production is under hormonal control.
Volume of urine is controlled by changing the permeability of the kidney tubules to water
Posterior pituitary gland produces anti-diuretic hormone (ADH)
ADH renders the tubule wall more permeable to water and reduces glomerular filtration rate
Kidney function in amphibians
nephron is not structurally organized in the kidney and the Bowman’s capsule lies within the entangled convoluted tubule. The proximal tubule if thicker than the distal tubule, which opens up into a collecting duct that collects urine from many tubules and delivers it to the ureter.
the proximal part of the kidney tubule is freely permeable to water so the urine remains approximately isosmotic to the plasma but the walls of the distal tubule are poorly permeable to water so as Na+ and Cl- are selectively absorbed the water remains and the urine becomes progressively more dilute – a really useful trait if excreting ammonia.
Vasa recta
Capillary beds extending into the kidney medulla
Intraspecific variation in kidney morphology
Arid environments = long loops of henle (thickness of medulla)
Modification of primary urine
relies on the descending and ascending parts of the loop of Henle having different permeabilities to water and interstitial fluid composition
Walls of the ascending part of the tubule are impermeable to water
Single effect
Sodium pumps transport NaCl out of the ascending tubule – water cannot follow – into the interstitial fluid, which becomes hyperosmotic
Water is drawn out of the adjacent descending tubule and Na+ and Cl- can enter the descending tubule
Descending and ascending tubules contain fluid of different osmolality
Single effect
Sodium pumps transport NaCl out of the ascending tubule – water cannot follow – into the interstitial fluid, which becomes hyperosmotic
Water is drawn out of the adjacent descending tubule and Na+ and Cl- can enter the descending tubule
Descending and ascending tubules contain fluid of different osmolality
Countercurrent multiplication effect
Combination of single effect and very long loops of Henle mean that very concentrated urine can be produced
Countercurrent effect multiplies the osmotic differences between the descending and ascending tubules
Urine and interstitial fluid get more concentrated as you go down the descending tubule but become more dilute as it comes back up the ascending tubule
Urine then moves down collecting duct loses water through the wall (high ADH) and the urine is concentrated
Kidney function and urea
Urea enters primary urine in filtrate
Tubules and collecting ducts are relatively impermeable to urea
More urea is filtered than excreted so it accumulates in the medullary interstitial fluid
ADH controls amount of urea retained in tubules and collecting ducts
