Excretory Systems and Osmoregulation

• Excretory systems and osmoregulation are key concepts in Biology
• This includes looking at homeostasis, osmoregulation, nitrogenous waste disposal, phylogeny, habitat, and regulation

Overview: A Balancing Act

• Physiological systems operate in a fluid environment
• Relative concentrations of water and solutes are maintained within narrow limits
• Osmoregulation regulates solute concentrations & water balance
• Freshwater animals possess adaptations to reduce water uptake & conserve solutes
• Desert and marine animals contend with desiccating environments that can quickly deplete body water
• Excretion eliminates nitrogenous metabolites and other waste products

The Origin of Life & Cellular Adaptation

• Life started as enzymes in the primordial sea
• Reaction rates were determined by the concentrations of substrates
• The first proto-organism enclosed its enzymes inside a membrane, becoming a cell
• Enclosing enzymes allows control of substrate concentration, preventing diffusion
• This is useful as it improves reactions and keeps out unwanted molecules, but can cause osmotic problems and retain hazardous byproducts

Homeostasis

• Metabolic processes require homeostasis
• A change in pH, temperature or potential of surrounding tissue can effect reactions
• Homeostasis is a physiological process maintaining internal systems at equilibrium
• Internal systems include blood pressure, body temperature, acid-base and solute concentration
• Can occur in extra- or intracellular spaces, with the latter being well-controlled
• Often involves a negative feedback system

Negative Feedback Loop

• The body constantly loses water through evaporation, perspiration, and urination
• Lost water is replaced through drinking and eating
• Urine concentration allows solutes to be excreted, in addition to conserving water

Regulators & Conformers

• An animal using homeostatic mechanisms to regulate its internal conditions is a regulator
• Endothermic animals and birds are examples of this
• Conformers allow some conditions to change in tune with the external environment
• Conforming is dependent on a relatively stable environment

The Internal Environment

• In most animals, cells are bathed by internal fluids rather than the environment
• This allows control of substrates needed for metabolism

Osmoregulation

• Osmoregulation - controls internal solutes such as water and salts/ions
• This avoids the build-up of toxic metabolic by-products, and deals with ammonia, as well as maintaining acid-base balance

Animals and Osmoregulation

• Osmoregulation is controlling water and internal solute balance
• This involves looking at freshwater animals, marine animals, those in temporary water, and land animals

Water Balance and Waste Disposal

• Osmoregulation
  • Balancing the uptake and loss of solutes and water with the external environment regulates the composition of body fluids
  • This leads to regulation of cytosol composition in cells because of osmosis
• Many animals expend energy on osmoregulation
• Regulating solute concentration also regulates water content by osmosis
• Most animals dispose of toxic by-products of metabolism as aqueous sollutions

Osmosis and Osmolarity

• All animals must regulate H₂O uptake and loss, otherwise too much H₂O and cells rupture or cells shrivel
• H₂O enters or leaves cells by osmosis - the movement of H₂O across a semi-permeable membrane
• This depends on the osmolarity of the solutions on either side of the membrane; they can be isoosmotic, hyperosmotic or hypoosmotic
• Osmolarity is defined as the total solute concentration expressed as moles per litre (mol/L)

Diffusion vs Osmosis

• Diffusion is where solute molecules move from high to low concentration
• Osmosis is where solvent molecules move from low to high solute concentration
• Osmolality drives osmotic pressure and movement

Osmoregulatory Challenges & Mechanisms

• The challenge to homeostasis depends on solute concentration of body fluids & tissues
  • Human blood measures at 300 mOsm/L
  • Fish blood measures at 300-350 mOsm/L
• It's also based on the concentration of environmental solutes
  • Marine (35ppt salinity) = 1000 mOsm/L
  • Freshwater (less than 3ppt salinity) = 1-10 mOsm/L
• Two strategies exist

Osmoregulators and Osmoconformers

• Osmoconformers are isosmotic with surroundings. They do not gain/lose H₂O
  • Many marine invertebrates. Low energetic cost.
• Osmoregulators control internal osmolarity, independent of environment
  • Body fluids/cells not iso-osmotic with external environment.
  • Must release or take up water depending on environmental conditions
  • Allows animals to live in saltwater, freshwater, and terrestrial habitats. -Energetic cost of active transport, e.g. bony fish can be 5% of metabolic rate

Marine Animals: Osmoconformers

• Most marine invertebrates are osmoconformers
• Lobsters actively regulate specific solutes (e.g., Mg2+) in haemolymph
• No freshwater osmoconformers exist
• They would have body fluids too dilute to carry out life's processes

Freshwater Teleost: Osmoregulators

• Solute concentration is higher inside the cell in fresh water, while water concentration is higher outside

• This causes movement of water into the cell, potentially causing cytolysis

• Freshwater fish are hyperosmotic relative to their environment

• To balance this they do not drink freshwater, and have a small volume in food

• They actively acquire salts via gills and food

• They excrete dilute urine as chloride cells (Na-K Pump)

Marine Teleost: Osmoregulators

• Solute concentration is higher outside the cell in seawater, while the water concentration is higher inside

• Net movement of water is out of the cell, potentially causing Plasmolysis

• Marine fish are hypoosmotic relative to sea water

• To resolve this, they drink large amounts of sea water to get water, but also salts

• They secrete salt via the gills, in urine, small volume, and concentrate

• Their gills are comprised of chloride cells

Marine Sharks

• Marine sharks maintain internal salt concentration at 1/3 seawater
• Salts diffuse into body across gills
• They are not hypoosmotic to seawater
• Internal salts are made up by retaining high conc.s of Urea & Trimethylamine oxide (TMAO)
• Internal salt concentration "mimics" that of its environment, which minimizes passive water influx/efflux
• Excess inorganic electrolytes (NaCl) are excreted via kidneys using a special excretory organ known as Rectal Gland

Osmoregulation in Fish: Diadromy

• Diadromous fish spend part of their life in freshwater and part in seawater
• Catadromous fish born in seawater migrate upriver (e.g. eels)
• Anadromous fish born in freshwater migrate seaward (e.g. salmonids)

Animals That Live in Temporary Waters

• Extreme dehydration (desiccation) is fatal for most animals
• Temporary waters include temporary ponds, water droplets, and films around soil particles
• Some aquatic invertebrates in temporary ponds lose almost all their body water
• Some of these aquatic invertebrates survive in a dormant state known as anhydrobiosis
  • "life without water”
• Tardigrades, also known as water bears
  • When there is no water they undergo desiccation
  • They go from 85% water to 2% and can survive like this for years

Osmoregulation in Land Animals

• Land animals have water-impermeable layers to avoid water loss -wax in insects, shells in mollusks, skin in animals
• H₂O is lost via urine but obtained from foods, by drinking and by metabolism
• Animals living in dry environments have physiological mechanisms to avoid water loss

Terrestrial Regulation and Water Uptake

• There are only three possible avenues of water gain in a terrestrial animal -production of oxidative water via metabolic processes, uptake of liquid water, and uptake of water in the vapour phase
• All animals inevitably use the first mode, simply by virtue of being alive

Water Balance in Arid Environments

• The majority of water is obtained by drinking in desert environments
• Some desert animals can survive without drinking
• The kangaroo rat lives in the south-western USA and eats dry seeds without drinking water
• Almost all of its water is gained from metabolism of foodstuffs

Desert Mammals: Kangroo Rat

• Kangaroo Rats spend the day inactive in a humid burrow to reduce water loss
• They have no sweat glands; they Forage at night (no need to sweat), and have dry nasal passages for water condensation
• They have low volume urine & high water absorption in a large intestine
• They have a long Loop of Henle & high ADH level which means efficient water recovery

Osmoregulatory Mechanisms

• Osmoregulation regulates the composition of cytoplasm and interstitial fluids
• This occurs through the actions of transport epithelia, specialized cells that regulate solute movement in particular directions
  • Some face the outside environment directly, others are linked to tubes
  • The cells of these layers are joined by tight junctions, forming a boundary

Transport Epithelia

• In seabirds, excess salt is eliminated from their bodies via transport epithelia such as Nasal salt glands
• In sea birds such as the albatrosses, salt glands allow them to drink seawater
• Nasal glands contain transport epithelia transport and drain into a central duct that empties via the nostrils