Osmoregulation & Excretion PDF

Module BL1004: Animal Physiology Prof. Rob McAllen Email: [email protected] Professor in Marine Biology| School of Biological, Earth & Office hours: by appointment Environme...

Module BL1004: Animal Physiology Prof. Rob McAllen Email: [email protected] Professor in Marine Biology| School of Biological, Earth & Office hours: by appointment Environmental Sciences, University College Cork, Ireland Osmoregulation and Excretion “Animal Excretory Systems” Chapter 44 pg 1029 Campbell  Overview: A Balancing Act Physiological systems of animals operate in a fluid environment. Relative concentrations of water and solutes must be maintained within fairly narrow limits. Osmoregulation regulates solute concentrations and balances the gain and loss of water.  Overview: A Balancing Act Freshwater animals show adaptations that reduce water uptake and conserve solutes. Desert and marine animals face desiccating environments that can quickly deplete body water.  Overview: A Balancing Act Excretion gets rid of nitrogenous metabolites and other waste products.  Osmoregulation balances the uptake and loss of water and solutes Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment. Cells require a balance between osmotic gain and loss of water. Osmosis = movement of water across a selective permeable membrane Osmolarity = the solute concentration of a solution, determines the movement of water across a selectively permeable membrane. If two solutions are isoosmotic, the movement of water is equal in both directions. If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic to the hyperosmotic solution.  Osmotic Challenges Osmoconformers, consisting only of some marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity. Most marine invertebrates are osmoconformers. Stenohaline (narrow) vs Euryhaline (wide) tolerances in Osmoconformers & regulators  Osmotic Challenges Osmoregulators expend energy to control water uptake in a hypoosmotic environment and loss in a hyperosmotic environment. Marine bony fishes are hypoosmotic to sea water. They lose water by osmosis and gain salt by diffusion and from food. They balance water loss by drinking seawater and excreting salts.  Osmotic Challenges Osmoregulators expend energy to control water uptake in a hypoosmotic environment and loss in a hyperosmotic environment. Freshwater animals constantly take in water by osmosis from their hypoosmotic environment. They lose salts by diffusion and maintain water balance by excreting large amounts of dilute urine. Salts lost by diffusion are replaced in foods and by uptake across the gills.  Animals That Live in Temporary Waters Some aquatic invertebrates in temporary ponds lose almost all their body water and survive in a dormant state. This adaptation is called anhydrobiosis.  Osmotic Challenges Land Animals Land animals manage water budgets by drinking and eating moist foods and using metabolic water. Desert animals get major water savings from simple anatomical features and behaviors such as a nocturnal life-style / underground existence.  Water balance in two terrestrial mammals Water Water balance in a balance in kangaroo rat a human (2 mL/day) (2,500 mL/day) Ingested Ingested in food (0.2) in food (750) Ingested in liquid Water (1,500) gain (mL) Derived from Derived from metabolism (1.8) metabolism (250) Feces (0.09) Feces (100) Water Urine Urine loss (0.45) (1,500) (mL) Evaporation (1.46) Evaporation (900)  Energetics of Osmoregulation Osmoregulators must expend energy to maintain osmotic gradients. Animals regulate the composition of body fluid that bathes their cells. Transport epithelia are specialized epithelial cells that regulate solute movement. They are essential components of osmotic regulation and metabolic waste disposal. They are arranged in complex tubular networks How do seabirds eliminate excess salt from their bodies? An example is in salt glands of marine birds, which remove excess sodium chloride from the blood.  An animal’s nitrogenous wastes reflect its phylogeny and habitat The type and quantity of an animal’s waste products may greatly affect its water balance. Among the most important wastes are nitrogenous breakdown products of proteins and nucleic acids. Nitrogenous wastes Proteins Nucleic acids Amino Nitrogenous acids bases Ammonia – toxic - needs lots of water. Common in aquatic Amino groups species. Urea - The liver of mammals and most adult amphibians converts ammonia to less toxic urea. The circulatory system Most aquatic Mammals, most Many reptiles animals, including amphibians, sharks, (including birds), carries urea to kidneys, where most bony fishes some bony fishes insects, land snails it is excreted. Conversion of ammonia to urea is energetically expensive; less water is required to excrete. Ammonia Very toxic Urea - less toxic Uric acid - not soluble Nitrogenous wastes Proteins Nucleic acids Amino Nitrogenous acids bases Uric Acid - Insects, land snails, and many reptiles, Amino groups including birds, mainly excrete uric acid. Uric acid is largely insoluble in water; can be secreted as a paste with little water loss. Uric acid is more Most aquatic animals, including Mammals, most amphibians, sharks, Many reptiles (including birds), energetically expensive to most bony fishes some bony fishes insects, land snails produce than urea. Ammonia Very toxic Urea - less toxic Uric acid - not soluble  Diverse excretory systems are variations on a tubular theme Excretory systems regulate solute movement between internal fluids and the external environment. Most excretory systems produce urine by refining a filtrate derived from body fluids. Key functions of most excretory systems: Filtration: pressure-filtering of body fluids Reabsorption: reclaiming valuable solutes Secretion: adding toxins and other solutes from the body fluids to the filtrate Excretion: removing the filtrate from the system.  Excretory Systems Systems that perform basic excretory functions vary widely among animal groups. They usually involve a complex network of tubules. Protonephridia flame cells / planaria Metanephridia earthworm / similar to nephrons Malpighian Tubules insects Nephrons = the function unit of the kidneys / humans.  Protonephridia A protonephridium is a network of dead-end tubules connected to external openings. The smallest branches of the network are capped by a cellular unit called a flame bulb. These tubules excrete a dilute fluid and function in osmoregulation.  Metanephridia Each segment of an earthworm has a pair of open-ended metanephridia. Both excretory and osmoregulatory functions Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion.  Malpighian Tubules In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation. MT open into digestive tract. Insects produce a relatively dry waste matter, an important adaptation to terrestrial life. Highly efficient in water conservation  Kidneys Kidneys = excretory organs of vertebrates, function in both excretion and osmoregulation. Mammalian excretory systems center on paired kidneys, which are also the principal site of water balance and salt regulation. Each kidney is supplied with blood by a renal artery and drained by a renal vein. Urine exits each kidney through a duct called the ureter. Both ureters drain into a common urinary bladder, and urine is expelled through a urethra.  Kidneys: Nephrons = the Functional Unit The nephron = the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus. Bowman’s capsule surrounds and receives filtrate from the glomerulus capillaries.  Filtration : Glomerulus --> Bowman’s Capsule Filtration occurs as blood pressure = hydrostatic pressure forces fluid from the blood in the glomerulus to lumen of Bowman’s capsule. Filtration of small molecules is nonselective. The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules.  Pathway of the Filtrate From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule --> loop of Henle --> distal tubule… Fluid from several nephrons flows into a collecting duct ---> renal pelvis ---> ureter.  Pathway of the Filtrate Vasa recta are capillaries that serve the loop of Henle. The vasa recta and the loop of Henle function as a countercurrent system. The mammalian kidney conserves water by producing urine that is much more concentrated than body fluids.  The nephron is organized for stepwise processing of blood filtrate Proximal Tubule Reabsorption of ions, water, and nutrients takes place in the proximal tubule. Molecules are transported actively and passively from the filtrate into the interstitial fluid and then capillaries. Some toxic materials are secreted into the filtrate. The filtrate volume decreases.  The nephron is organized for stepwise processing of blood filtrate Descending Limb of the Loop of Henle Reabsorption of water continues through channels formed by aquaporin proteins. Movement is driven by the high osmolarity of the interstitial fluid, which is hyperosmotic to the filtrate. The filtrate becomes increasingly concentrated. Ascending Limb of the Loop of Henle In the ascending limb of the loop of Henle, salt but not water is able to diffuse from the tubule into the interstitial fluid. The filtrate becomes increasingly dilute.  The nephron is organized for stepwise processing of blood filtrate Distal Tubule The distal tubule regulates the K+ and NaCl concentrations of body fluids. Collecting Duct The collecting duct carries filtrate through the medulla to the renal pelvis. Water is lost as well as some salt and urea, and the filtrate becomes more concentrated. Urine is hyperosmotic to body fluids.  The nephron is organized for stepwise processing of blood filtrate Human kidneys process approximately 180 litters of filtrate per day 99% of water and nearly all sugars, amino acids, vitamins, are reabsorbed.  Adaptations of the Vertebrate Kidney to Diverse Environments The form and function of nephrons in various vertebrate classes are related to requirements for osmoregulation in the animal’s habitat. The juxtamedullary nephron contributes to water conservation in terrestrial animals. Mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops. Adaptations of the Vertebrate Kidney to Diverse Environments Birds and Other Reptiles Birds have shorter loops of Henle but conserve water by excreting uric acid instead of urea. Other reptiles have only cortical nephrons but also excrete nitrogenous waste as uric acid. Acknowledgements Majority of text and PowerPoint slides from Campbell’s Biology. Dr Ramiro Crego, School of BEES