Osmotic Regulation and Excretion
41.1 Osmoregulation and Osmotic Balance
Introduction to Osmoregulation
Osmoregulation balances concentrations of solutes and water across semi-permeable membranes, maintaining homeostasis.
Transport of Electrolytes across Cell Membranes
Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
Concept of Osmolality and Milliequivalent
Solution concentration is expressed by a solution's molality, while electrolyte concentration is expressed in terms of milliequivalents per liter.
Osmoregulators and Osmoconformers
Aquatic organisms with various salt tolerances adapt to their environments through osmoregulation and osmoconformation.
41.2 The Kidneys and Osmoregulatory Organs
Nephrons are the functional units of the kidneys, which are the primary osmoregulatory organs in mammals.
Kidney Function and Physiology
Urine is a byproduct of the osmoregulatory function of kidneys, which filter blood, reabsorb water and nutrients, and secrete wastes.
41.3 Excretion Systems
Contractile Vacuoles in Microorganisms
Contractile vacuoles absorb excess water and wastes from a microorganism's cell and excrete them into the environment by contracting.
Flame Cells of Planaria and Nephridia of Worms
Flame cells and nephridia remove the waste from bodies through filtration in a manner similar to a kidney.
Malpighian Tubules of Insects
Malpighian tubules remove wastes from insects by producing urine and solid nitrogenous waste, which are then excreted from the body.
41.4 Nitrogenous Wastes
Nitrogenous Waste in Terrestrial Animals: The Urea Cycle
Urea, a nitrogenous waste material, is the end product excreted in urine when ammonia is metabolized by animals, such as mammals.
Nitrogenous Waste in Birds and Reptiles: Uric Acid
Birds and reptiles have evolved the ability to convert toxic ammonia into uric acid or guanine rather than urea.
41.5 Hormonal Control of Osmoregulatory Functions
Epinephrine and Norepinephrine
Epinephrine and norepinephrine are released during the flight/fight response, causing vasoconstriction of blood vessels in the kidney.
Other Hormonal Controls
The renin-angiotensin-aldosterone system (RAAS) stabilizes blood pressure and volume via the kidneys, liver, and adrenal cortex.