I. Introduction –Kidneys are the main organs controlling ion and water balance, but gills, skin and digestive mucosa also play a role
A. Osmotic Regulation– The control of tissue osmotic pressure requires the movement of solutes across membranes since animals cannot actively pump water.
B. Ionic Regulation– The control of ionic composition of body fluids – especially those that are important solutes.
C. Nitrogen Excretion –Ridding the body of nitrogen waste from protein catabolism in the form of ammonia, uric acid, and urea.
D. While some animals maintain osmotic/ionic balance like their surroundings, most need to regulate ions and osmotic composition.
1. Marine animals are challenged to expel ions (especially NaCl) against ionic gradients and obtain water against osmotic gradients.
2. Fresh water animals need to acquire ions from ion-poor water and dispose of excess water.
3. Terrestrial animals are challenged with a constant threat of dehydration.
II. Aquatic Environments
A. Ionoconformer– Internal conditions are similar to external conditions even if the external conditions change. ECF resembles seawater in terms of the major cations and anions.
1. Typically saltwater animals – typically simple animals such as sea anemones, coral, jellyfish, sea squirts.
B. Ionoregulator– Controls the internal conditions in ECF by using ion absorption and excretion strategies.
1. Examples include squid, shark, tuna, killer whale, flounder, sea turtle in saltwater
2. Examples include freshwater animals
3. Eases the burden of individual cells to regulate ions
C. Osmoconformer– Internal osmolarity is close to that of external environment even if external environment changes. (Osmolality is the solute concentration that will determine if water moves by osmosis.)
1. Examples include marine invertebrates such as clams and primitive vertebrates such as sharks.
D. Osmoregulator –Internal osmolarity is maintained within a narrow range regardless of external environment.
1. Marine vertebrates such as flounder, maintain internal osmolarity much below surrounding saltwater
2. Freshwater vertebrates such as goldfish, and freshwater invertebrates such as the mayfly and clam, maintain an internal osmotic pressure well above that of the surrounding water.
3. Many different osmoregulatory mechanisms have evolved in different animals.
III. Water sources – all living things need a source of water especially for terrestrial animals
A. Aquatic animals have a constant source of water
B. Diet is a mixture of water and solutes
C. Water is a metabolic by-product
IV.Inorganic and Organic Solutes – Potassium, sodium, chloride, sulfate ions, as well as amino acids, glucose, urea
A. Cell volumes are controlled by transporting solutes in and out of extracellular fluid. Note we transport solutes because water cannot be actively moved.
1. Regulatory Volume Increase (RVI) – importing ions causing an influx of water
a. Most common mechanism is the Na+/K+/2Cl– cotransporter which brings sodium, potassium and chloride into the cell and water follows
2. Regulatory Volume Decrease(RVD) – exporting ions, causing an efflux of water.
a. Activation of K+ channels cause K+ to exit cell resulting in hyperpolarization; Opening of Cl– channels cause Cl– to exit cell in response to hyperpolarization; As K+ and Cl–, both solutes exit cell, water follows
b. K+/Cl– Cotransporter activation results in both solutes to leave cell and water follows the solutes
c. Na+/Ca2+ exchanger expels Na+ and imports Ca2+; Calcium is then exported via the Ca2+ ATPase; water follows
d. Na+/K+ ATPase exports three sodium for every two potassium entering reducing intracellular osmolarity and water follows the majority of the solutes
V. Integument –epithelium is an osmotic barrier
A. Epithelium with aquaporins are more permeable to water than those without these proteins in the cell membrane
B. Solutes move by transcellular and paracellular transport (See unit 1)
C. Mucus (hydrophobic), keratin (protein forming dense envelope), insect cuticle (hydrophobic covering) reduce water loss
VI. Gills –transport ions into and out of water and the direction depends on the salinity of the water
VII. Digestive epithelium – digestion involves moving solutes and water across the gut wall
VIII. Salt glands – most birds and reptiles have salt gland that can excrete hyperosmotic solutions of Na+ and Cl– which allows the drinking of hypertonic water. Bird salt gland are found at base of beak and near the eye in reptiles
A. Salt gland is a series of secretory tubules that open to the surface and are surrounded by a network of capillaries
B. Salt can be concentrated in gland by countercurrent multiplier system –if blood moves left to right, it becomes more dilute as the salt escapes the bloodstream to the interstitial space; interstitial fluid is more concentrated with salt to the left and less concentrated to the right; fluid in the tubules moves right to left and are exposed to an increasing concentration of salt and therefore the fluid in the lumen of the tubules becomes increasingly concentrated.
IX. Rectal glands –salt excretion glands in sharks