Lecture notes from BIOL2018 adaptive physiology, osmoregulation. Covering: maintenance of homeostatic internal environment, water and ion exchange, osmo-conformers vs -regulators, excretory products, urine, kidneys, cartilaginous fish, marine fish, freshwater fish, freshwater to seawater exchanges ...
Osmoregulation
Maintenance of homeostatic internal environment
Osmoregulation – regulation of solute balance and the gain and loss of water
Excretion of nitrogen containing waste products of metabolism – impacts solute
balance
Maintaining solute (salt) concentrations in body fluids
Maintaining water balance in body
Gills, kidneys, and intestines are main osmoregulatory organs – some animals have
specialised organs for maintaining homeostasis
Water and Ion exchanges
Between:
External and internal fluids
Interstitial fluid
Extracellular fluid
Plasma
Intracellular fluid
Diffusion – the passive movement of molecules or particles along a concentration
gradient, or from regions of higher to regions of lower concentration
Osmosis – diffusion of water across a membrane that is semi-permeable i.e.
permeable to water
Osmotic pressure – the amount of pressure required to prevent any diffusion of
water across the semi-permeable membrane is called the osmotic pressure of the
solution with respect to the membrane
The size of the solute particles does not influence osmosis
If too much water enters red blood cells they can burst
If too much water is removed they can shrink to the extent that they are no long
functional
Osmoconformers Vs Osmoregulators
Osmoregulators – maintain an osmotic pressure themselves
Osmoconformers – osmotic pressure is dictated by surroundings
Some animals can be both – i.e. green crab is regulator at lower saline levels and
osmoconformers at higher saline concentrations
Euryhaline – Eurys = wide – halos=salt
Stenohaline – stenos = narrow
Stenohaline fish – tolerate narrow range of salinities – most marine fish and
freshwater fish
Euryhaline fish – tolerate a wide range of salinities – estuary, tidal zones, salt
marches (e.g. killifish)
Diadromous = migrating fish – anadromous = FW -> SW -> FW (e.g. salmon)
Catadromous = SW -> FW -> SW (e.g. eel)
Obligatory Osmotic changes
Some processes cannot be controlled by the animal
Transepithelial diffusion – respiratory epithelium – other moist epithelia
Ingestion – gain water by drinking, from diet variable water and salt content
Urination – excretion of waste
, Defecation – lose water to dilute and excrete waste
Excretory products and organs
Respiratory organs (gills and lungs) – CO2, NH4+, HCO3-, Na+, Cl-
Digestive systems (also liver) – undigested food, metabolic by-products such as
bilirubin, Na+ and water (diarrhoea)
Skin and glands – water and salts (sweat: Na+, Cl- and K+)
Renal systems – metabolites (urea/uric acid), hormone and drug by products
Mammalian kidney – a key excretory organ
Primary urine formed by filtration or by active solute secretion – similar to blood
plasma without proteins
Primary urine flows through kidney tubules, its volume and composition are
modified
Active/passive transport of solutes
Osmosis of water across epithelial cells
4 processes of urine production
1. Glomerular filtration – water, salts, glucose, urea etc. – non-selective except
excludes: RBC and large plasma protein
2. Tubular reabsorption - ~99% of water and most salts – glucose reabsorbed mostly in
proximal tubule by active transport (e.g. NaCl) or by passive diffusion
3. Tubular secretion - K+, HCO3-, H+, foreign substances in selective active transport –
tubular synthesis – NH4+ synthesised in tubular lumen from NH3 by deamination of
proteins in epithelial cells and H+
4. Excretion
The nephron and collecting duct
Regional functions of transport epithelium
Molarity Vs Osmolarity Vs Osmolality
Molarity – number of dissolved molecules per litre
Osmolarity – number dissolved particles per litre
Osmolality -
Vertebrate Nephrons
Size of nephron shows how much work is performed before urine is produced
Teleosts have short nephrons suggesting little water needs to be maintained
Elasmobranch have exceeding long nephrons as water and solutes need to be
maintained to combat evaporation
Osmoregulation in saltwater elasmobranchs
Medium 1000 mOsm
Hyperosmotic to seawater because they retain urea and TriMethylAmine Oxide as
osmolytes
Excess salt but no water loss
Cartilaginous fish
Maintain a lower osmolarity for salt than sea water
Salt diffuses through gills
Kidneys excrete some salt
Rectal glands (salt excretory glands) excrete NaCl
Animal fluids slightly hyperosmotic to seawater
Due to accumulation of urea and TriMethylAmine Oxide (TMAO) in fluids
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