Osmoregulation Flashcards
What is the apical membrane of a cell?
side that faces the environment
What is the basolateral membrane of a cell?
side that faces the lumen/blood
What is interstitial fluid?
fluid trapped between cells that does not exchange easily
- composition similar to plasma in most organisms (few exceptions)
- composition differs dramatically in freshwater vs. seawater environments
What is the endothelium?
separates skin layer from blood layer
What are the 3 homeostatic processes?
- osmotic regulation: osmotic pressure of body fluids
- ionic regulation: concentrations of specific ions
- nitrogen excretion: excretion of end-products of protein metabolism
What is the Fick equation?
calculates rate of diffusion (flux rate)
dQs/dt = Ds x A x dC/dX
- Ds: diffusion coefficient (Ds): includes size of molecule and hydration shell
- A: diffusion area
- dC/dX: size of concentration gradient, where x = distance
- direction of diffusion depends on concentration gradient
What is osmotic pressure?
pressure that draws water largely due to driving force of solute concentration
What counteracts osmotic pressure?
- hydrostatic pressure
- gravity
What are the 3 ways to classify/compare two solutions?
- solution with higher osmolarity is hyperosmotic
- solution with lower osmolarity is hyposmotic
- osmolarities that are the same are isosmotic
What is a semipermeable membrane permeable to? How does this affect responses in regulation?
- permeable to water
- impermeable to salt
- water moves quicker than ions (charged molecules)
- first response is water movement, second response is potential salt movement (depending on membrane permeability)
What is tonicity?
effect of a solution on cell volume
What are the 3 classes of tonicity?
- hypertonic solution: cells shrink – water leaves cell by osmosis
- hypotonic solution: cells swell – water enters cell by osmosis
- isotonic solution: cell neither shrinks nor swells – no net osmosis (but water is still always moving)
Why is water and solute regulation of the intracellular and extracellular space important? (2)
- increased intracellular osmolarity can directly interfere with cellular processes (ie. protein-protein interactions, cellular fluidity for diffusion)
- changes in osmolarity can result in movement of water across membrane, which changes cell volume
How does increased intracellular osmolarity affect proteins such as hemoglobin?
- crystallizes – cannot hold O2 anymore, therefore non-functional
- Hb is packed in RBC, on the verge of solubility and turning into crystals – dehydrated RBC can cause Hb to crystallize inside RBC
How does increased intracellular osmolarity affect cellular and membrane fluidity?
cellular fluidity and membrane fluidity is largely affected by volume
- stretches membranes
- changes barrier of membrane between inside and outside
Changes in osmolarity can result in movement of water across membrane, which changes cell volume. Why is this important?
cells are very susceptible to volume changes, but also well-designed to deal with changes
- moderate cell swelling → disruption of membrane
- excessive cell swelling → cell lysis
- excessive cell shrinkage → macromolecular crowding
How is cell volume regulated?
- cells transport solutes in and out of ECF (regulates composition) to control cell volume
- water follows solutes by osmosis
What is sodium regulated by?
- Na+/K+ ATPase: pumps 3 Na+ out for 2 K+ in
- Na+/H+ exchanger: driven by potential difference across cell membrane (does not require ATP) that is usually generated by NKA
What is potassium regulated by?
- Na+/K+ ATPase: pumps 3 Na+ out for 2 K+ in
What is chloride regulated by?
- generally distributed passively (Goldman equation)
What is calcium regulated by?
- Na+/Ca+ antiporter
- Ca2+ ATPase
What is regulatory volume increase (RVI)?
cells increase volume (swell) by actively importing ions, then water follows ions passively causing swelling and expansion
- different cells use different transporters
- usually achieved by activating NKCC
- alternatively by opening Na+ channels, Cl- channels, Na+/H+ exchangers
How do Na+/H+ exchangers work?
H+ comes from metabolism or CO2 and is pumped out while bicarbonate stays inside
What is regulatory volume decrease (RVD)?
cells actively decrease volume by exporting ions into lumen, then water follows passively
- different cells use different transporters
- usually achieved by opening K+ channels – K+ leaves cell (down electrochemical gradient) because RMP for K+ is -90 mV
- Cl- channels also open – Cl- leaves cell in response to hyperpolarizing effects of K+ movement
- Na+/K+ ATPase also pumps 3 Na+ out for 2 K+ in
How is water (and cell volume) regulated?
- passively regulated in response to changes in ions and osmolality
- different types of ion transporters exist that are species and tissue-specific in driving water transport
How does water cross cell membranes?
by moving through aquaporins (like ion channels, but for larger molecules) that work very specifically for polar water because they are charged
What are the ionic and osmotic challenges of marine environments?
most animals tend to gain salts and lose water
What are the ionic and osmotic challenges of freshwater environments?
animals tend to lose salts and gain water
What are the ionic and osmotic challenges of terrestrial environments?
animals tend to lose water
What are the two strategies to meet ionic challenges?
- ionoconformer
- ionoregulator
What are ionoconformers?
exert little control over ion profile within extracellular space
- exclusively found in marine animals – ie. many invertebrates and hagfish
- many invertebrates’ ECF is almost identical to seawater
What are ionoregulators?
control ion profile of extracellular space
- more popular – ie. most vertebrates
What are the two strategies to meet osmotic challenges?
- osmoconformer
- osmoregulator
What are osmoconformers?
internal and external osmolarity similar
- ie. marine invertebrates
What are osmoregulators?
osmolarity constant regardless of external environment
- ie. most vertebrates
What are the two classes describing the ability to cope with external salinities?
- stenohaline: can tolerate only narrow range
- euryhaline: can tolerate wide range
Classify cnidarians (ionoconformer/ionoregulator and osmoconformer/osmoregulator).
- ionoconformers – ion composition very similar to seawater
- osmoconformers – osmolality is very similar to seawater, no net water flux between inside and outside
Classify hagfish (ionoconformer/ionoregulator and osmoconformer/osmoregulator).
- ionoconformers – ion composition very similar to seawater
- osmoconformers – osmolality is very similar to seawater, no net water flux between inside and outside
Classify sharks (ionoconformer/ionoregulator and osmoconformer/osmoregulator).
- ionoregulators – ions are regulated differently than seawater (Na+ and Cl- much lower than seawater, use urea and methylamines to make up the difference)
- osmoregulators
Classify marine bivalves (ionoconformer/ionoregulator and osmoconformer/osmoregulator).
- ionoregulators – use amino acids to make up the differences
- osmoconformers
Classify bony fish and all vertebrates (ionoconformer/ionoregulator and osmoconformer/osmoregulator).
- ionoregulators
- osmoregulator – osmolality is ⅓ seawater
Euryhaline Osmoconformer
- can tolerate wide range
- allows osmolarity to decrease in parallel with water until death
- internal osmolality of plasma or extracellular place decreases as external osmolarity decreases
Stenohaline Osmoconformer
- cannot tolerate much change – dies after very modest osmotic disruption
- internal osmolality changes as external osmolality changes
Euryhaline Osmoregulator
- defends nearly constant internal state for some time, but eventually succumbs
- internal osmolality does not change much as external osmolality decreases
Stenohaline Osmoregulator
- can defend its internal osmolarity over narrow range of external osmolarities
What are the 3 classes of solutes? What are they distinguished by?
distinguished by their effects on macromolecules
- perturbing solute
- compatible solute
- counteracting solute
What are perturbing solutes? How do they affect macromolecular function?
high concentration disrupts macromolecular function
- Na+, K+, Cl-, SO4+, charged amino acids
- charges induce 3D conformation change
What are compatible solutes? How do they affect macromolecular function?
changes in concentration have little effect on macromolecular function
- can increase in concentration if necessary (ie. to increase osmolality) without negative effects on proteins
- polyols (glycerol, glucose), uncharged amino acids
What are counteracting solutes? How do they affect macromolecular function?
disrupt function on their own, but counteract disruptive effects of other solutes when employed in combination
What is Km?
affinity of enzyme for substrate
How does Km change as perturbing solute concentration increases?
increasing perturbing solute concentration causes large increase in Km of enzyme
- enzyme requires more and more substrate to convert substrate to product
- reduces affinity of enzyme for substrate (need lots more substrate for same effect)
How does Km change as compatible solute concentration increases?
increasing compatible solute concentration does not affect Km of enzyme
How does Km change as counteracting solute concentration increases?
- increasing urea concentration alone increases Km of enzyme
- increasing TMAO concentration alone decreases Km of enzyme
- combining urea and TMAO (in 2:1 ratio) does not affect Km
ie. elasmobranchs are osmoconformers (plasma osmolarity is same as seawater), but their ion levels (Na+, Cl-) are lower
- balance this by increasing urea and TMAO
- urea is end product of protein breakdown, excreted in urine in humans – but sharks retain urea
What are the primary osmoregulatory epithelia of vertebrates?
- gills
- kidney
- digestive system
Which organisms can produce concentrated (hyperosmotic relative to blood) urine at the kidneys?
only birds and mammals
What are the osmoregulatory strategies of marine elasmobranchs?
- slightly hyperosmotic blood concentration relative to environment
- isosmotic urine concentration relative to blood
- does not drink seawater – hyperosmotic NaCl from rectal gland
What are the osmoregulatory strategies of marine teleosts?
- hyposmotic blood concentration relative to environment
- isosmotic urine concentration relative to blood
- drinks seawater – secretes salt from gills
What are the osmoregulatory strategies of freshwater teleosts?
- hyperosmotic blood concentration relative to environment
- hyposmotic urine concentration relative to blood
- drinks no water – absorbs salt with gills
What are the osmoregulatory strategies of amphibians?
- hyperosmotic blood concentration relative to environment
- hyposmotic urine concentration relative to blood
- absorbs salt through skin
What are the osmoregulatory strategies of marine reptiles?
- hyposmotic blood concentration relative to environment
- isosmotic urine concentration relative to blood
- drinks seawater – hyperosmotic salt gland secretion
What are the osmoregulatory strategies of desert mammals?
- hyperosmotic urine concentration relative to blood
- drinks no water – depends on metabolic water
What are the osmoregulatory strategies of marine mammals?
- hyposmotic blood concentration relative to environment
- hyperosmotic urine concentration relative to blood
- does not drink seawater – because cannot create urine that is more concentrated than seawater (requires specialized kidney)
What are the osmoregulatory strategies of marine birds?
- hyperosmotic urine concentration relative to blood
- drinks seawater – hyperosmotic salt-grand secretion
What are the osmoregulatory strategies of terrestrial birds?
- hyperosmotic urine concentration relative to blood
- drinks freshwater
What are the epithelial tissue properties for ion movement? (4)
- asymmetrical distribution of membrane transporters – transporters differ on apical and basolateral surfaces
- cells are interconnected (via tight junctions) to form impermeable sheet of tissue – prevents water loss
- high cell diversity within tissue
- abundant mitochondria to provide ATP for ion transport
What are the two main routes of solute transport that epithelial cells use?
- transcellular transport: movement through cell
- paracellular transport: movement between cells (“leaky epithelia”)
What are the types of transporters used for solute movement? (4)
- Na+/K+ ATPase (NKA) – creates electrochemical gradient
- various channels (Cl-, K+, Na+)
- electroneutral cotransporters
- electroneutral exchangers
How do freshwater fish osmoregulate?
- passively gains water and loses ions across gill and gut
- produces dilute urine in kidney to get rid of water
- actively absorbs ions at gill
How do seawater fish osmoregulate?
- passively lose water and gain ions across gill and gut
- cannot produce concentrated urine to conserve water
- drinks to obtain water
- actively secretes ions at gill
What transports ions in fish gills?
- ionocytes on gill arch filaments (site of active ion regulation)
- sometimes lamellae
What are ionocytes?
cells with lots of mitochondria, and high levels of Na+/K+ ATPase (NKA) activity to drive ion movement
What does the direction of ion transport depend on?
water salinity
What is the driving force for ion regulation in fish gills?
in both seawater and freshwater gill, Na+/K+ ATPase (NKA) on basolateral membrane is driving force for ion regulation – generates Na+ uptake or Cl- excretion
- then different transports direct entry and exit
What is the PNA- cell of freshwater gills?
site of sodium uptake
- NKA creates driving force for Na+ uptake
- Na+ taken up into cell by Na+ channel or Na+/H+ transporter
- H+ that gets pumped out of cell eliminates positive charge, which helps create driving gradient for Na+ uptake
- different species use different combinations for Na+ uptake
What is the PNA+ cell of freshwater gills?
site of calcium and chloride uptake
- NKA creates gradient
- CO2 diffuses into cell and dissociates into bicarbonate and protons in presence of CA
- H+ moves back into blood, and bicarbonate drives HCO3-/Cl- exchanger, which drives Cl- from water into cell
- Ca2+ ATPase pumps Ca2+ into cell, then Ca2+ is transported into blood through Ca2+ channel or Na+/Cl- exchanger
What occurs in seawater gills?
want to excrete salts
- NKA is also on basolateral membrane
- but different type of transporter (compared to freshwater cell) – cotransporter NKCC (non-ATPase-requiring transporter), then Cl- moves into environment
What is a diadromous fish?
fish that move between freshwater and seawater during their life cycle
What is a catadromous fish?
adults live in freshwater, breed in seawater
ie. eel
What is an anadromous fish?
adults live in seawater, breed in freshwater
ie. salmon
What is smoltification?
smolt must physiologically change/prepare for seawater while still residing in freshwater
- if it converts from freshwater to seawater form too early, it dies
- photoperiod is major driver of smoltification
Mechanisms of Salt Secretion (Hyperosmotic Environments)
see diagram
Mechanisms of Salt Secretion (Hyperosmotic Environments)
see diagram
What are salt glands?
glands in birds and reptiles located near eye that drain into ducts that empty near nostril
What do salt glands do?
excrete hyperosmotic solutions of Na+ and Cl- that are produced by ion pumps and countercurrent multiplier
- large amount of salt excreted in small volume of water (highly saline water droplet)
- pumps salt from blood into cell, and into lumen which excretes salt
- cells use and localize existing transporters found in every cell
Why can marine birds (ie. seagulls) drink seawater?
because they can create a solution that is hyperosmotic to seawater
Describe the countercurrent arrangement of the salt gland.
- solution in lumen and blood move in opposite direction
- solution moves into collecting duct and is eventually expelled
- arrangement maximizes efficiency of salt excretion
What is the rectal gland?
accessory excretory organ of elasmobranchs/sharks that empties into digestive tract
What does the rectal gland do?
actively transports Na+ and Cl- from blood into lumen of the gland
- ion transport similar to ionocytes and salt glands
- NKA generates potential (inside of cell = -60 mV)
- NKCC drives Cl- into cell
- once Cl- is in cell, it wants to leave
What is the rate of salt excretion from the rectal gland regulated by?
hormones – vasoactive intestinal peptide (VIP)
- elevated levels stimulate production of cAMP
- results in more Cl- channels being inserted into membrane
- increases capacity of Cl- excretion
Why don’t elasmobranchs use gills like marine fish for salt excretion?
- marine teleosts evolved much later, therefore gills may have been derived trait
- sharks needed additional structure for NaCl excretion (rectal gland), which later got lost as animals became more able to excrete NaCl by gills
Salt Gland vs. Rectal Gland
different organ systems for getting rid of Na+ and Cl-, but same basic principles
- often the same transporters
- often the same orientation of transporters – take advantage of potential energy generated by NKA, and put the respective channels where they need to be to have the respective movement of salts
How is water lost in terrestrial habitats?
across skin, respiratory surface, and in urine
How is water gained in terrestrial habitats?
metabolic water, drinking, food
How does SA:V ratio affect rate of water loss?
- humans have low SA:V ratio, which helps reduce water loss – lose 1% of their body water per hour
- fleas (parasites) lose 900% of their body water per hour – need to replace 9x their body water every hour, but they feed on blood so they have good access to water
How does the mammalian nasal countercurrent heat exchange system operate to greatly reduce water loss associated with breathing?
- inspiration: incoming air is warmed and humidified, cooling the nose
- expiration: outgoing air is cooled and loses water, wetting the nose
nasal countercurrent heat exchanger – operates to recycle and conserve water
- as we inhale cooler air, it moves past nasal mucosa and is warmed as air moves into lungs – warmed by drawing water osmotically from nasal mucosa (moist surface, high surface area)
- as we exhale air, warm humidified air moves out down nasal mucosa and comes in contact with progressively cooler nasal mucosa
- as air becomes cooled, water condenses out of solution from air and onto that surface, then it is available to rehumidify next breath of air
- the longer the system (ie. longer nose), the more potential for evaporative cooling and recovery
- breathing through mouth is not as efficient for water recovery
What are some mechanisms by which kangaroo rats conserve water?
- slightly longer nose
- remains in cool burrow during daytime
- respiratory moisture condensed in nasal passages
- free water in seeds
- metabolic water derived from dry seeds
- urine concentrated by countercurrent exchange in extra long loop of Henle
- feces dehydrates prior to defecation
Epithelia involved in ion and water balance (ie. gills, kidney) are typically also involved in what process?
excretion of nitrogenous wastes
What are the 3 forms in which ammonia nitrogen can be excreted?
type of nitrogenous waste produced has important implications for ion and water balance
- ammonia
- uric acid
- urea
What is ammonia produced from?
amino acid breakdown
What are ammonioteles?
organisms that excrete ammonia nitrogen as ammonia
most aquatic animals, including most bony fishes
- simple invertebrates (cnidarians, nematodes)
- aquatic molluscs
- agnathans, chondrichthians, bony fish, larval amphibians
What are uricoteles?
organisms that excrete ammonia nitrogen as uric acid
many reptiles (including birds), insects, land snails
- terrestrial molluscs (snails, slugs), terrestrial arthropods
- reptiles, birds
What are ureoteles?
organisms that excrete ammonia nitrogen as urea
mammals, most amphibians, sharks, some bony fishes
- some larval bony fish, estivating lungfish
- mammals
Is it good or bad to convert ammonia into different sources?
there are metabolic costs of converting ammonia into different sources, but huge benefits in terms of water balance
What is the type of nitrogen compound excreted by aquatic animals?
usually ammonia
What is the type of nitrogen compound excreted by terrestrial animals?
usually urea or uric acid
Can animals change their mode of nitrogen excretion?
yes – in response to water availability
- most fish lose ability to produce urea, but lungfish retained it
Humans produce urea. What happens if this is not managed well?
will get gout if not managed well (ie. high protein diet, or insufficient hydration)
- uric acid crystals form in joints and is painful
What are the advantages of ammonia excretion? (2)
ammonia released by deamination of amino acids
- first end product of breakdown
- some animals can store ammonia as glutamine (detoxify it), and later convert it back to glutamate – ammonia is produced when it can be handled
requires little energy to produce
What are the disadvantages of ammonia excretion? (2)
- highly toxic – dangerous if it cannot be managed
- requires large volumes of water to store and excrete (500 mL H2O per g NH4+)
Ammonia excretion is common in what animals?
common in aquatic animals where water is abundant
- seawater marine fish: can drink seawater
- freshwater marine fish: have lots of access to water
Ammonia Excretion
NH3 + H+ ⇄ NH4+
- NH3: (gas) moves rapidly across membranes down its partial pressure
- NH4+: more toxic form – can substitute for K+ in nervous tissue (interfering with normal nerve function), resulting in convulsions at high levels
- pK (relative proportion of NH3 or NH4+) = ~ 9
- if pH = 7, around 99% exists as NH4+
- if pH =9, around 50% exists as NH4+
Which form of ammonia moves easily across membranes?
under normal conditions, small amount of ammonia is NH3 (most is NH4+), which is the form that moves easily across membranes
Under normal conditions, what is blood pH and how does this affect total ammonia?
blood pH is such that the majority of total ammonia (NH3 and NH4+) is NH4+
- NH3 moves rapidly across membranes because it diffuses, but concentration is fairly low
- as NH3 moves across membrane into water, it combines with H+ to form NH4+ – advantage of this is that you get rid of NH3 and maintain partial pressure gradient from inside to outside
Ammonia Excretion in Fish Gills
- CO2 moves into cell in presence of CA, and dissociates into bicarbonate and protons
- protons get pumped out across gills by H+-ATPase, and help with ammonia excretion
- acid-trapping: acid being excreted is trapping ammonia as ammonium – partial pressure of ammonia is very low outside, allows for continued NH3 excretion
Is a low or high water pH more beneficial for ammonia excretion?
low (more acidic)
Why is a higher water pH (more alkaline) worse for ammonia excretion?
- any NH3 that moves out will remain as NH3, and NH4+ that is out there will probably be converted to NH3
- organism will die in few hours
- but some fish (ie. lungfish) can produce urea to live in alkaline environment
What are the advantages of uric acid excretion? (2)
- few toxic effects
- can be excreted in small volume of water (10 mL H2O per g uric acid)
What are the disadvantages of uric acid excretion? (1)
- expensive to produce
What is uric acid excreted as?
anhydrous white crystals
What organisms excrete ammonia nitrogen by uric acid excretion?
earliest adaptation for water conservation in terrestrial environments
What are the advantages of urea excretion? (3)
- only slightly toxic
- relatively inexpensive to produce
- requires (50 mL H2O per g urea produced)
What are the disadvantages of urea excretion? (1)
- urea is a perturbing solute
Where is urea synthesized and transported?
synthesized in liver and transported by blood to kidney
How do elasmobranchs use urea as an osmolyte?
use urea to increase plasma osmolarity
- helps prevent water loss in marine environment
- urea’s perturbing effects counteracted by methylamines (TMAO)
plasma of elasmobranchs have less Na+ and Cl-, but urea and methylamines (TMAO) are high
- 2-3x as much urea as TMAO, but if they occur together they are compatible and reduce problems
osmolality is similar to seawater even though ion concentrations are much lower because they retain urea
- metabolically convert ammonia to urea, and counteract urea negative effects with TMAO so that ⅓ to ½ of their total osmolarity is urea
Most animals maintain ion and water balance using some form of internal organ. What are the 4 organs?
- nasal gland
- rectal gland
- gills – are intimately in association with environment, and excrete or take salts into environment
- kidneys – air-breathers do not have access to unlimited water resource, therefore rely on kidney for water and ion balance
What are the 6 roles of vertebrate kidneys in homeostasis?
- ion balance – do not want to lose too many ions through urine
- osmotic balance – keep blood at appropriate osmolality
- blood pressure – reabsorb or dump water from urine
- pH balance – primarily done by ventilation, but kidney helps fine-tune acid-base status/levels
- excretion of metabolic wastes and toxins
- hormone production
What are the two layers of the mammalian kidney?
- outer cortex
- inner medulla
What is the nephron?
functional unit of the kidney – millions in mammalian kidney
- unique environment that optimizes water reabsorption and ion reabsorption from initial ultrafiltrate
What are the two main parts of the nephron?
- renal tubule – lined with transport epithelium, various segments with specific transport functions
- vasculature (blood supply) – glomerulus, capillary beds surrounding renal tubule
What is the glomerulus?
ball of capillaries, surrounded by Bowman’s capsule
Outer cortex and inner medulla have different types of nephrons depending on kidney. What are the 2 types of nephrons?
- cortical nephron: mostly in cortex
- juxtamedullary nephron: long loop of Henle descends into medulla
Describe the vasculature of the nephron and what it does.
- nephron is wrapped in vasculature system that is part of system that helps generate osmotic gradient in that renal pyramid, but also supplies blood to that nephron to reabsorb salt and water and excrete things
- arterial blood supply enters nephron, travels to glomerulus, and progressively gets more deoxygenated as it delivers O2 to metabolizing tissue
What are the 4 processes of urine production?
- filtration: blood perfuses glomerulus, creating initial ultrafiltrate
- reabsorption: specific molecules in filtrate removed
- secretion: specific molecules (secreted from blood to kidney for excretion) added to filtrate
- excretion: urine is excreted from body
Urine Production – Lecture 4
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What are the 4 parts of the tubule?
- proximal tubule: most of the solute and water reabsorption
- loop of Henle: descending limb, ascending limb
- distal tubule: reabsorption completed for most solutes
- collecting duct: drains multiple nephrons, carries urine to renal pelvis
Why do different tubule regions have differences in transport and permeability?
due to differences in epithelium along tubule
What occurs in the proximal tubule?
- most reabsorption of solutes and water takes place in proximal tubule – many solutes reabsorbed by Na+ co-transport, water follows by osmosis
- proximal tubule also carries out secretion
What occurs in the descending limb of the loop of Henle?
descending limb is permeable to water
- water is reabsorbed
- volume of primary urine decreases
- primary urine becomes more concentrated
What occurs in the ascending limb of the loop of Henle?
ascending limb is impermeable to water
- ions are reabsorbed
- primary urine becomes dilute
What happens to ions that are reabsorbed in the loop of Henle?
reabsorbed ions accumulate in interstitial fluid
- osmotic gradient created in medulla
Where is the nephron located?
embedded in renal pyramid
- deeper into pyramid → greater osmolality
What does the loop of Henle and collecting duct act as?
act as countercurrent multipliers to create osmotic gradients that facilitate transport processes
- basic physiological principle that takes advantage of gradients
- recall: nasal gland and salt gland of birds have countercurrent flow of luminal fluid and blood
- recall: swim bladder has countercurrent arrangement of arterial and venous vessels that allow for swim bladder inflation
What are osmotic gradients in the nephron maintained by?
maintained by vasa recta capillaries
What does the osmotic concentration of final urine depend on?
depends on permeability (number of aquaporins) of distal tubule and collecting duct, which can be regulated by vasopressin
- impermeable: produces dilute urine (formed in ascending limb) – not much water reabsorption from collecting duct, therefore water leaves body as urine
- permeable: produces concentrated urine (formed in collecting duct) – allows water reabsorption from collecting duct of nephron and ultrafiltrate (more water reabsorbed → more concentrated final urine)
What is the mechanism for concentrating urine?
- ascending limb of loop of Henle actively pumps Na+ out of tubule lumen and into interstitial fluid
- Cl- and K+ follow
- causes increased ion concentration in interstitial fluid of medulla
- causes water to move passively out of descending limb
- has lots of aquaporins, therefore water can move down its osmotic gradient
- deeper into medulla → greater osmolarity
- urine gets concentrated since water is drawn out
- moves up, and ions are pumped out – this is part of what generates osmotic gradient in medulla
What is the vasa recta and in what direction does it move relative to ultrafiltrate?
blood supply to nephron
- moves countercurrent to ultrafiltrate in nephron – crucial to how kidney works
Explain the mechanism of how the countercurrent arrangement of the loop of Henle and the vasa recta work to maintain osmolarity.
- ions (Na+, Cl-, and K+) are pumped out of ascending loop and into interstitial fluid, raising osmotic pressure outside and lowering it inside
- water flows out of descending tubule by osmosis (down its concentration gradient) into interstitial fluid, raising osmotic pressure in descending tubule (becomes more concentrated)
- deeper it goes → higher osmolarity
- at the point when vasa recta enters medulla, blood is isosmotic with cortex (300 mOsM)
- normal plasma osmolality
- as blood moves deeper into medulla, it loses water and picks up ions from interstitial fluid (osmolarity increases)
- water moves passively by osmosis
- ion uptake is actively driven
- when blood of vasa recta flows back towards cortex, high plasma osmolarity attracts water that is being lost from descending limb
- this decreases osmolarity of blood (300 mOsM)
Do desert animals typically have a short or long loop of Henle?
typically have long loop of Henle so they can reabsorb more water
- deeper loop of Henle and medulla → greater gradient can be generated, and greater amount of water reabsorption can occur
Do aquatic animals typically have a short or long loop of Henle?
typically have have short loop of Henle
- deeper loop of Henle and medulla → greater gradient can be generated, and greater amount of water reabsorption can occur
How must blood and ultrafiltrate volume be balanced?
volume increase of blood leaving relative to incoming has to be same as volume of ultrafiltrate that is lost as you move through system
- ultrafiltrate loses volume by the time it leaves
- vasa recta gains volume by the time it leaves
What 2 hormones affect kidney function and what kind of response do they each have?
- steroid hormones: slow response – ie. aldosterone
- peptide hormones: rapid response – ie. vasopressin
What 2 dietary factors affect urine output and what do they do?
- diuretics: stimulate excretion of water
- antidiuretics: reduce excretion of water
What are the 3 extrinsic regulators of GFR?
hormones
- vasopressin (antidiuretic hormone – ADH)
- renin-angiotensin-aldosterone (RAA) pathway
- atrial natriuretic peptide (ANP)
Vasopressin (Antidiuretic Hormone – ADH)
What is vasopressin?
peptide hormone produced in hypothalamus, and released by posterior pituitary gland
Vasopressin (Antidiuretic Hormone – ADH)
What is the function of vasopressin?
increases water reabsorption from collecting duct by increasing number of aquaporins
Vasopressin (Antidiuretic Hormone – ADH)
What is the release of vasopressin stimulated by? What is the problem with this?
increasing plasma osmolarity detected by osmoreceptors in hypothalamus – stimulated when we are dehydrated
- increasing plasma osmolarity is a problem – need to make sure water is reabsorbed from kidney
Vasopressin (Antidiuretic Hormone – ADH)
What is the release of vasopressin inhibited by?
- increasing blood pressure detected by stretch receptors in atria and baroreceptors in carotid and aortic bodies
- alcohol – inhibits water reabsorption by aquaporins, which results in increased urine output
- caffeine – also increases blood pressure, which increases GFR (produce more ultrafiltrate), and decreases water reabsorption
Vasopressin (Antidiuretic Hormone – ADH)
How does vasopressin increase cell permeability?
- vasopressin binds G-protein-linked receptor
- receptor activates adenylate cyclase, increasing cAMP and activating protein kinase A
- phosphorylation of cytoskeletal and vesicle proteins occurs
- this triggers translocation of vesicle to cell membrane, with insertion of aquaporins
- aquaporins are in vesicles and readily available to be inserted into membrane (no need to synthesize them)
- more aquaporins inserted → more potential for water flux
Vasopressin (Antidiuretic Hormone – ADH)
Where does vasopressin stimulate H2O uptake?
in collecting duct
Describe the renin-angiotensin-aldosterone (RAA) pathway.
- renin secreted when blood pressure or GFR lower than normal
- renin converts angiotensinogen (inactive protein in plasma) to angiotensin I
- angiotensin converting enzyme (ACE) on blood vessel epithelia converts angiotensin I to angiotensin II
- angiotensin II causes synthesis and release of aldosterone from adrenal cortex
What does the renin-angiotensin-aldosterone (RAA) pathway regulate?
helps regulate blood pressure
What is angiotensin II?
vasoconstrictor – raises blood pressure by increasing resistance
What is aldosterone?
- mineralcorticoid in tetrapods – produced by adrenal cortex
- steroid hormone
What does aldosterone do?
increases Na+ (and water) retention – raises blood pressure by increasing blood volume
- targets cells in distal tubule and collecting duct
- stimulates Na+ reabsorption from urine
- enhances K+ excretion
- also stimulated by increases in circulating K+
How does aldosterone stimulate Na+ reabsorption from urine?
- aldosterone enters cell by diffusion
- binds to its receptor (transcription factor)
- activated transcription factor stimulates transcription of genes for transporters
- new transporter proteins are made in ER and exported in vesicles
- vesicles containing proteins are sent to plasma membrane
- slower pathway where new proteins need to be synthesized, then translocated in vesicles to membrane
- increasing number of transporters in specific region of membrane, therefore increasing potential for Na+ absorption
How does aldosterone affect Na+ reabsorption and K+ excretion?
distal tubule
- increased Na+ recovery
- increased K+ excretion
collecting duct
- increased Na+ recovery
When is atrial natriuretic peptide (ANP) produced?
produced in specialized cells within atria in response to stretch associated with increase in blood volume
What does atrial natriuretic peptide (ANP) do?
increases urine output and consequently lowers blood volume and pressure
- acts as antagonist with RAA pathway – increases excretion of Na+ in urine
- increases GFR by relaxing contractile cells that control size of filtration slits of glomerulus
- inhibits secretion of vasopressin, reducing water reabsorption
What is the pathway through which dehydration is compensated for by the kidney?
dehydration results in decreased blood volume, decreased blood pressure, and increased plasma osmolarity
blood volume and blood pressure is influenced by many cardiovascular components
- decreased hydrostatic pressure – results in ↑ blood volume (due to decreased capillary filtration, fluid shift from interstitial space to blood)
- decreased stretch of aortic or carotid baroreceptors (causes decreased frequency of APs, which affects cardiovascular control centres of medulla)
increase in angiotensin II normally leads to increase in aldosterone and increase in Na+ reabsorption
- BUT because of increase in osmolarity, it directly inhibits that response and results in decrease in aldosterone, and decrease in Na+ reabsorption by kidney, which lowers osmolarity
What is acid-base regulation regulated by?
- primary acid-base regulation is ventilatory – increase or decrease ventilation rate to alter CO2 levels
- regulation can be fine-tuned by kidney
How is the kidney involved in acid-base regulation?
- different regions of tubule secrete protons from blood into lumen – reduce acidity of blood
- bicarbonate from ultrafiltrate can be reabsorbed
- depending on metabolic state, acid is excreted into urine or bicarbonate (base) is recovered from urine, allowing us to fine-tune our blood system
Kidney is Involved in Acid-Base Regulation
What happens at high altitude?
acute mountain sickness occurs due to hyperventilation, which reduces CO2 and increases bicarbonate (become alkalotic)
- normally bicarbonate is recovered from urine, but carbonic anhydrase (CA) inhibitor prevents bicarbonate reabsorption into blood – bicarbonate is excreted through urine
- important in dealing with acute mountain sickness
Which 3 endocrine hormones affect kidney function and what do they do?
- diuretics: stimulate excretion of water
- antidiuretics: reduce excretion of water
- hormones: mediate diuresis and antidiuresis
What does the regulation of urine formation interact with?
interacts with cardiovascular function and blood pressure
Excretory Control of Homeostasis
What is the problem with increasing water intake and how is it solved?
- PROBLEM: increases plasma volume because water is taken up from stomach into cardiovascular system
- PROBLEM: decreases plasma osmolarity which starts to sets up fluid shifts so water can move from plasma to cells (causing swelling)
- SOLUTION: increase urine volume/formation
- salt excretion not affected because do not want to lose salt
- RESOLUTION: decrease plasma volume
Excretory Control of Homeostasis
What is the problem with increasing salt intake and how is it solved?
- PROBLEM: increases plasma osmolarity
- may not change plasma volume
- SOLUTION: excrete salt – reduce NaCl reabsorption from nephron, downregulate number of transporters that drive salt from nephron
- do not want to alter urine volume, but want to conserve water – cannot excrete salt without water loss, but it can be done efficiently
Water and Gatorade
What happens if you only drink water?
will partially replenish water being lost
- water will move osmotically into blood volume – but salt is lost, therefore there is a certain total amount of water that can be taken up and to maintain normal osmolality
- but if you continue to drink more water (thirsty, blood volume still low), plasma osmolarity decreases
- salts leave by one of three pathways: (a) urinated out – blood pressure decreases (b) enters lymphatic system (c) enters body cells
Water and Gatorade
What happens if you only drink gatorade?
(gatorade: salts + water)
salt and water is taken up so that blood volume can be increased and restored – do not need to worry about water being excreted via urine, lymphatic system, or cells
Water and Gatorade
During a marathon, how does body temperature differ in people only drinking water and people only drinking gatorade?
body temperature increases in people only drinking water, and is constant in people only drinking gatorade
- people drinking water more likely not to finish race due to osmoregulatory collapse
water drinkers:
- satisfying thirst by drinking water, but blood volume is not being expanded back to normal because there are no salts (salts leave through one of three pathways above)
- their blood pressure is not as high
- total blood volume is reduced, therefore their ability to cool themselves down is impaired because they still have to make sure that they are perfusing necessary tissues (like diving)
- brain/heart/muscles are primary, thermoregulation is secondary
gatorade drinkers:
- gatorade salt moves into plasma compartment, takes water, restores blood volume
What is the malpighian tubule?
blind ending sac in insects that extends into coelomic cavity/fluid
- similar to lymphatic system
- wrapped in stellate cells and principal cells
- equivalent to vertebrate kidney – empties into hindgut
Describe how the malpighian tubule produces urine.
no ultrafiltrate created
primary urine formed by secretion – not filtration
- reabsorption and secretion in hindgut further modifies primary urine
- secretion from coelomic fluid into Malpighian tubules is driven mainly by H+-ATPase (most cells we already talked about use Na+/K+ ATPase to generate potential across membranes)
What are many insecticides based on?
based on how they affect Malpighian tubule and hindgut function
How do hormones affect the malpighian tubule?
- diuretic hormones increase urine formation
- less is known about antidiuretic hormones
What are the ionoregulatory organs of sponges?
use simple contractile vacuoles to expel cellular waste (including water)
What are the ionoregulatory organs of worms (and other simple animals)?
protonephridia – flame cell + flagella
- similar to vertebrate kidney tubule
How does the protonephridia of worms work?
fluids are taken from interstitial space into lumen with reabsorption
- flagella beats to create current, which draws fluid into flame cell and tubule cell
In what organisms are protonephridia most developed?
(worms and other simple animals)
- most developed in freshwater organisms
- seawater is dehydrating environment if trying to regulate ion levels at seawater
- organism is more concentrated than freshwater – freshwater moves in, has way of removing freshwater that is continuously coming in
What are the ionoregulatory organs of molluscs and annelids?
metanephridia – more complex
- fluid taken from blood or coelom into lumen with some reabsorption
Describe mammalian nephrons.
- glomerulus, proximal tubule, loop of Henle, distal tubule, collecting duct
- major innovation of birds and mammals was loop of Henle, allowing production of concentrated urine
- mammals producing more concentrated urine have longer loop of Henle and relatively thicker medulla
- birds and reptiles without loop of Henle conserve water by excreting uric acid
Describe the evolution of mechanisms to tolerate thermal stress in mammals.
- urine-concentrating abilities repeatedly evolved in multiple desert-dwelling mammalian lineages
- lower mean annual aridity index → greater degree of mammal’s urine-concentrating abilities
What do all fish (including sharks) nephrons lack?
loop of Henle
- this is exclusive to birds and mammals
Describe chondrichthian (shark) nephrons.
- glomerulus, ciliated neck, proximal tubule, distal tubule, collecting duct
- shark extracellular fluid is slightly hyperosmotic to seawater due to high urea concentrations
- countercurrent arrangement recovers up to 90% of urea from primary urine, possibly using Na+-urea transporter
- final urine slightly hyposmotic relative to shark plasma, and isosmotic to seawater
What is the role of the kidney in freshwater bony fish?
- ions reabsorbed from primary urine
- excretion of lots of very dilute urine
- most ion, water, and nitrogen excretion responsibilities met by gills and skin
- huge glomeruli – battling water influx, need to get rid of lots of water, can create ultrafiltrate
What is the role of the kidney in seawater bony fish?
- produce only small amounts of urine
- most ion, water, and nitrogen excretion responsibilities met by gills and skin
- some marine fish lack glomeruli (aglomerular kidney), therefore lack ability to create ultrafiltrate and need to rely on secretion
What is the advantage of marine fish having aglomerular kidneys?
minimizes water loss
- glomerular kidney would create ultrafiltrate resulting in enormous water loss that needs to be reabsorbed or else it will be lost
What is the disadvantage of marine fish having aglomerular kidneys?
can only secrete things that there are specific transporters for (whereas humans can create ultrafiltrate and dump everything we do not want to take back)
Are amphibians freshwater or seawater organisms?
almost exclusively freshwater
How does kidney function change in amphibians with development and why is that important for their survival?
larval: pronephros – tubule opens into coelom
- little need for water retention in aquatic life – excretion of dilute urine
adult: more mammal-like nephron
- conserve water on land – reduce GFR, reabsorb water from bladder
What is comparative physiology?
integrative scientific discipline applying physiological concepts, tools, and knowledge to characterize biological diversity and its ecological implications
- understanding and predicting how organisms, populations, and ecosystems respond to environmental change and stressors
- using sub-lethal physiological indicators to really understand how changes in environment influence animals
How does the internal osmolality of fish in freshwater and seawater compare?
they are similar
Describe the osmolarity of freshwater compared to the osmolality of fish and explain what happens to ions and water.
osmolality of freshwater is lower than osmolality of fish
- actively take up Na+ and Cl- from water across gills
- passively lose ions
- passively gain water
- high urine output – because freshwater wants to move in
Describe the osmolarity of seawater compared to the osmolality of fish and explain what happens to ions and water.
osmolality of seawater is greater than osmolality of fish
- passively gain ions
- passively lose water
- drink seawater to get water in
- actively absorb Na+ and Cl- across gut, which draws water osmotically
- excrete divalent ions (Ca2+ and Mg2+) by kidney
- excrete Na+ and Cl- at gills
What happens when fish are transferred from freshwater to seawater (ie. when they smolt)?
increase in osmolality up to some point, then plasma osmolality may decrease as they acclimatize and reach new stable level that may be slightly higher than in freshwater
- euryhaline: can move from freshwater to seawater
- stenohaline: plasma osmolality increases, and animal dies at some point
24 Hour Seawater Challenge Tests
How can you assess smolt condition and whether fish are ready to be transferred to seawater?
hatcheries transfer “smolts” from FW to SW for 24 h and then measure physiologically relevant parameters to assess smolt status
- relevant physiological parameters: regulation of hormones, Na+/K+ ATPase activity and amount, NKCC, ion levels, urine flow rate (excreting or taking up ions with pumps – different isoforms of pumps in sweater and freshwater), gill surface area, water flux across membrane, urine ion composition, plasma osmolarity, drinking rate of fish (smolt would drink more)
- relevant behavioural parameters: visible stress, level of equilibrium, overall activity, startle response, exercise capacity
- fish that can tolerate higher salinity exhibit recovery of osmoregulatory status within around 5 days – if not within 5 days, they likely will not recover
24 Hour Seawater Challenge Tests
Results – 24 hours of exposure
20 g/L resulted in death of all fish
- large increases in plasma osmolality above 9 g/L h at 24 h
- lethal if fish osmolarity increases or decreases by 30% – not much tolerance for large changes
- large increases in both plasma [Na+] and [Cl-] above 9 g/L h at 24 h
- no significant changes in gill Na+/K+ ATPase within 24 h
24 Hour Seawater Challenge Tests
Results – 48 hours of exposure
100% lose equilibrium and die at 16 g/L
24 Hour Seawater Challenge Tests
Results – 72 hours of exposure
100% lose equilibrium and die at 13 g/L
- in 11 and 13 g/L no recovery in plasma osmolarity by 72 h
- in 11 and 13 g/L no recovery in plasma [Cl-] by 72 h
- no significant changes in gill Na+/K+ ATPase within 72 h
What is the salinity tolerance of the Lake Qinghai scaleless carp, and how might predicted salinity increases within the lake affect this species?
- lake salinity is currently 9 g/L (compared to human and fish plasma also at 9, and seawater at 30)
- water level of Lake Qinghai is dropping 10-13 cm/year due to agricultural water use, and therefore salinity of lake will increase
- Lake Qinghai scaleless carp appear to live on the edge of their salinity tolerance – changes from 9 g/L to 11 g/L results in struggling, and small changes in salinity above 9 g/L result in large perturbations that do not appear to be compensated
- at current water diversion rates, lake water salinity will increase to 11 g/L within 50 years
- are these short term studies predictive of long term salinity tolerance?
What appears to be the metabolic advantages of living in lake water (which is isosmotic with plasma) relative to rivers?
tested how stressful current salinity fish live in is
- fish are isosmotic to their plasma
- ion regulation is driven by Na+/K+ ATPase, ion transporters – could be metabolically expensive
- but results showed that metabolic rate is lower at lake salinity than in river salinity – maybe because ion composition is similar in lake and plasma
but what about higher salinities – when water is hyperosmotic to blood plasma?
Filtration
- podocytes
- mesangial cells
- GFR
- podocytes: cells with foot processes that form filtration structure supporting/holding the capillary
- mesangial cells: control blood pressure and filtration within glomerulus by contracting and restricting flow (like smooth muscle)
- glomerular filtration rate: affected by (a) glomerular capillary hydrostatic pressure AKA blood pressure (b) Bowman’s capsule hydrostatic pressure AKA lumen hydrostatic pressure (c) oncotic pressure
- net movement of water from blood to glomerular filtrate because hydrostatic pressure exceeds combination of Bowman’s capsule hydrostatic pressure and oncotic pressure
Reabsorption
- sites of reabsorption
- modification of kidney filtrate
- glucose reabsorption
- renal threshold
- passive reabsorption
- sites of reabsorption: proximal tubule, loop of Henle, distal tubule, collecting duct
- modification of kidney filtrate: (a) 80% of blood that comes in leaves (b) 20% of blood that comes in generates ultrafiltrate, 19%/most is reabsorbed (c) overall, > 99% of plasma entering kidney returns to systemic circulation
- glucose reabsorption: (a) secondary active transport driven by Na+
- renal threshold: glucose reabsorption transports fully saturated
What is primary ultrafiltrate?
initial filtrate filtered in Bowman’s capsule that is isosmotic to blood – no protein
Secretion
- sites of secretion
- sites of secretion: proximal tubule, distal tubule, collecting duct
