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Osmoregulation and Excretion A.P. Biology Ch. 44 Rick L. Knowles Liberty Senior High School Osmoregulation • Maintaining a balance of both water and ions across a membrane/organism. Solute and water homeostasis. • Osmolarity – moles of total solute per liter of water; usually in milliosmoles/L. • Mechanism of homeostasis varies with the environment in which they’ve adapted (freshwater, saltwater, terrestrial). Some Comparison Freshwater 0.5 -15 0 Distilled,deionized Water 300 Human Plasma 1,000 Seawater Milliosmoles/L (mosm/L) 5,000 Dead Sea • Most animals are said to be stenohaline: – And cannot tolerate substantial changes in external osmolarity; both osmoconformers and osmoregulators. • Euryhaline animals: – Can survive large fluctuations in external osmolarity. Tilapia, freshwater up to 2,000 mosm/L Figure 44.2 Osmoregulation and Nitrogenous Wastes • Other waste solutes must be removed from cells and organisms. • A waste product of metabolizing amino acids and nucleic acids (deamination)- ammonia. • Concept 44.2: An animal’s nitrogenous wastes reflect its phylogeny and habitat. • The type and quantity of an animal’s waste products: – May have a large impact on its water balance. Ammonia • Direct by-product of protein and nucleic acids (deamination). • Very toxic to cells. • Highly soluble in water. • Molecule of choice for freshwater organisms; eliminated easily through kidneys, gill epithelia, etc. • Downside: requires a lot of water. Urea • Saltwater and terrestrial mammals convert ammonia into urea. • Less toxic; accumulate more in tissue. • Less soluble in water than ammonia. • Allows conservation of water. Uric Acid • Birds and reptiles accumulate waste in an egg. • Convert ammonia into uric acid. • Insoluble in water; crystallizes. • Semisolid paste-guano. • Requires less water to eliminate. • Among the most important wastes – Are the nitrogenous breakdown products of proteins and nucleic acids Nucleic acids Proteins Nitrogenous bases Amino acids –NH2 Amino groups Most aquatic animals, including most bony fishes Many reptiles (including birds), insects, land snails Mammals, most amphibians, sharks, some bony fishes O H C NH3 Figure 44.8 Ammonia C O NH2 Urea C C C C NH2 O HN N N H N H Uric acid O Osmoconformers • Most marine protists and invertebrates. • Are isoosmotic with marine environment. • Open channels and carriers for most ion transport (Not all ions are in equilibrium). • Ex. Invertebrates like sea anemones, jellyfish, and only vertebrate, Class Agnatha- hagfish. Class Agnatha- Hagfish Show me a real hagfish! Video: Discovery- Blue Planet: Ocean World Osmoregulators • Maintain constant osmotic concentration in body fluids and cytoplasm despite external variations. • Continuous regulation since environment and intake (diet) changes. • Evolved special mechanisms for different environments. • Ex. Most Vertebrates The Problems • Freshwater Vertebrates- are hyperosmotic, water enters body, tend to lose ions. • Marine Vertebrates- are hypoosmotic, water leaves body, tend to gain ions. • Terrestrial Vertebrates- are hypoosmotic, water leaves body through respiration, perspiration, skin. Freshwater Protists • Problem: hyperosmotic; impossible to become isoosmotic with dilute fresh water; tend to gain water; lose ions; no excretory organ. • Solution: Contractile Vacuoles – active transport of water out of cell; less permeable to ions • Downside: Active transport requires energy. Freshwater Invertebrates • Water and wastes are passed into a collecting vessel or primitive excretory organ. • Membrane retains proteins and sugars and allows water and dissolved wastes to leave-selectively permeable. • Ex. Freshwater jellyfish, etc, • Concept 44.3: Diverse excretory systems are variations on a tubular theme. • Excretory systems: –Regulate solute movement between internal fluids and the external environment. Excretory Processes • Most excretory systems – Produce urine by refining a filtrate derived from body fluids Capillary Filtrate Excretory tubule 1 Filtration. The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule. 2 Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids. 3 Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule. Urine Figure 44.9 4 Excretion. The filtrate leaves the system and the body. • Protonephridia: Flame-Bulb Systems A protonephridium: – Is a network of dead-end tubules lacking internal Nucleus of cap cell openings. Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Figure 44.10 Tubule Nephridiopore in body wall • The tubules branch throughout the body: – And the smallest branches 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 Coelom Capillary network Bladder Collecting tubule Nephridiopore Figure 44.11 Nephrostome Metanephridia • Metanephridia consist of tubules: – That collect coelomic fluid and produce dilute urine for excretion. Terrestrial Insects • Problem: Must minimize water loss. • Solution: Use chitin as an exoskeleton. Malpighian Tubules • In insects and other terrestrial arthropods, malpighian tubules – Remove nitrogenous wastes from hemolymph and function in osmoregulation Digestive tract Rectum Intestine Midgut (stomach) Salt, water, and nitrogenous wastes Malpighian tubules Feces and urine Malpighian tubule Rectum Figure 44.12 HEMOLYMPH Hindgut Reabsorption of H2O, ions, and valuable organic molecules Anus Malpighian Tubules K+ Hemolymph Water and K+ K+ Water and waste K+ Na+/K+-ATPase Hindgut Conc. Waste Malpighian Tubules • Use Malpighian tubules- blind end tubules that extend into hemocoel (body cavity). • Cells waste and salts into hemolymphlumen of tubule by diffusion and active transport. • K+ are actively transported into lumen; set up a gradient. • Water and other ions leave the hemolymph and follow into the lumen by passive diffusion. • Empty into hindgut; water reabsorbed; urine is concentrated. • Na+/K+-ATPase moves ions from lumen of hindgut into hemolymph. Insects versus other Vertebrates • Insects use a gradient to pull water through a membrane; open circulatory system = low blood pressure. • Vertebrates- push water through a membrane; closed circulatory system = higher blood pressure. More Complex Organisms Need Another Solution Introducing the Vertebrate Kidney! Nephron (Tubule) Gill Epithelia is Permeable Hypotonic Env. Hypertonic Cells Water Freshwater Bony Fishes • Problems: Water enters cells from environment, solutes leave cells. • Solutions: Drink very little water; excrete large amounts of dilute (hypoosmotic) urine with large kidneys; reabsorb ions in kidney tubules (active transport) back into blood; use chloride cells in gill epithelium (active transport). • Freshwater animals maintain water balance: – By excreting large amounts of dilute urine. • Salts lost by diffusion: – Are replaced by foods and uptake across the gills. Osmotic water gain through gills and other parts of body surface Uptake of water and some ions in food Uptake of salt ions by gills Figure 44.3b (b) Osmoregulation in a freshwater fish Excretion of large amounts of water in dilute urine from kidneys Hypotonic Cells Water Hypertonic Env. Saltwater Bony Fishes • Problem: Tend to lose water, gain ions, mostly at gills. • Solutions: Drink large amount of water; kidney retains water and excretes ions (isoosmotic urine); use chloride cells in gills to actively transport some ions across gill epithelium. • Marine bony fishes are hypoosmotic to sea water: – Lose water by osmosis and gain salt by both diffusion and from food they eat. • These fishes balance water loss: – By drinking seawater. Gain of water and salt ions from food and by drinking seawater Osmotic water loss through gills and other parts of body surface Excretion of salt ions from gills Excretion of salt ions and small amounts of water in scanty urine from kidneys Figure 44.3a (a) Osmoregulation in a saltwater fish Cartilaginous Fishes • Problem: Same as marine bony fishes. • Solution: Reabsorb urea from nephron tubule back into the blood; 100X blood [urea] than mammals (special protective solute,TMAO to protect proteins)blood is slightly hyperosmotic kidneys and gills do not have to remove ions; do not have to drink large volume of water. Cartilaginous Fishes • Problem: Still must remove excess Na+ and Cl- that diffuse across gills, diet, etc. • Solution: Rectal Gland- uses Na+/K+-ATPase pumps to actively transport Na+ and Cl- out of blood by setting up a gradient. How the Rectal Gland Works Na + K+ Extracellular Fluid Na+ Cl- Na+/K+-ATPase Na+ Cotransporter Cl + Na Cl- Chloride Channel Lumen of Rectal Gland Cl- Na+ To Rectum How could a marine shark enter freshwater? By controlling the amount of solutes! Video: National Geographic Presents: Attacks of the Mystery Shark Rectal Gland • Very common mechanism for removing salt in marine animals. • Problem: Marine birds and reptiles have freshwater kidneys designed to reabsorb salt from urine into blood. • Use similar salt glands in nostrils to excrete salt. • An example of transport epithelia is found in the salt glands of marine birds. • Remove excess sodium chloride from the blood. Nasal salt gland (a) An albatross’s salt glands empty via a duct into the nostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist. Nostril with salt secretions Lumen of secretory tubule Vein Capillary Secretory tubule Artery NaCl Transport epithelium (b) One of several thousand secretory tubules in a saltexcreting gland. Each tubule Direction of salt is lined by a transport movement epithelium surrounded by capillaries, and drains into a central duct. Figure 44.7a, b Bloo d flow Central duct Secretory cell of transport epithelium (c) The secretory cells actively transport salt from the blood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentration gradient of salt in the tubule (aqua), this countercurrent system enhances salt transfer from the blood to the lumen of the tubule. Show me some marine reptiles! Salt glands in action! Video: Corwin Experience- Galapagos Animals That Live in Temporary Waters • Some aquatic invertebrates living in temporary ponds – Can lose almost all their body water and survive in a dormant state • This adaptation is called anhydrobiosis. 100 µm 100 µm Figure 44.4a, b (a) Hydrated tardigrade (b) Dehydrated tardigrade • The nephron, the functional unit of the vertebrate kidney – Consists of a single long tubule and a ball of capillaries called the glomerulus JuxtaCortical medullary nephron nephron Afferent arteriole from renal artery Glomerulus Renal cortex Bowman’s capsule Proximal tubule Peritubular capillaries Collecting duct To renal pelvis 20 µm Renal medulla SEM Efferent arteriole from glomerulus Loop of Henle (c) Nephron Figure 44.13c, d Distal tubule Collecting duct Branch of renal vein Descending limb Ascending limb (d) Filtrate and blood flow Vasa recta Vertebrate Kidneys • Four Functions: 1. Filtration 2. Reabsorption 3. Secretion 4. Excretion 1. Filtration • Glomerulus- tightly-woven ball of capillaries embedded in a cup-shaped tubule- Bowman’s capsule. • Slits/pores in capillaries and capsule allow liquid/solutes through but prevent cells and large proteins from entering the nephron. • Produces isoosmotic filtrate with blood Filtration of the Blood • Filtration occurs as blood pressure: – Forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule. Pathway of the Filtrate • From Bowman’s capsule, the filtrate passes through three regions of the nephron: –The proximal tubule, the loop of Henle, and the distal tubule • Fluid from several nephrons: –Flows into a collecting duct Blood Vessels Associated with the Nephrons • Each nephron is supplied with blood by an afferent arteriole: – A branch of the renal artery that subdivides into the capillaries • The capillaries converge as they leave the glomerulus – Forming an efferent arteriole. • The vessels subdivide again: – Forming the peritubular capillaries, which surround the proximal and distal tubules. From Blood Filtrate to Urine: A Closer Look • Filtrate becomes urine: – As it flows through the mammalian nephron and collecting duct. 1 Proximal tubule NaCl Nutrients HCO3 H2O K+ H+ NH3 4 Distal tubule H2O NaCl HCO3 K+ H+ CORTEX Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs Key Active transport Passive transport Figure 44.14 2 Descending limb of loop of Henle OUTER MEDULLA H2O 3 Thick segment of ascending limb NaCl NaCl 3 Thin segment of ascending limb NaCl INNER MEDULLA 5 Collecting duct Urea H2O Transport Epithelium 2. Reabsorption • Must return most of the water and solutes to the blood. (2000 l of blood 180 l water 1-2 l urine daily). • Reabsorb glucose, amino acids, divalent cations in proximal tubule by active transport carriers. • If not reabsorbed, lost in the urine. • Ex. Diabetes mellitus 3. Secretion • Foreign molecules and wastes (ammonia, urea) are secreted into lower portions of tubule. • Opposite direction as reabsorption (CapillaryTubule). • Ex. Antibiotics and other drugs, bacterial debris • Secretion and reabsorption in the proximal tubule: – Substantially alter the volume and composition of filtrate • Reabsorption of water continues: – As the filtrate moves into the descending limb of the loop of Henle 4. Excretion • Urine is a solution of: Harmful drugs, hormones, nitrogenous wastes, and excess K+, H+, water. • Homeostasis of: pH, electrolytes, blood volume and pressure. • As filtrate travels through the ascending limb of the loop of Henle: – Salt diffuses out of the permeable tubule into the interstitial fluid. • The distal tubule: – Plays a key role in regulating the K+ and NaCl concentration of body fluids. • The collecting duct: – Carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl. • Concept 44.5: The mammalian kidney’s ability to conserve water is a key terrestrial adaptation. • The mammalian kidney: – Can produce urine much more concentrated than body fluids, thus conserving water. Solute Gradients and Water Conservation • In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts: – Are largely responsible for the osmotic gradient that concentrates the urine. Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid. - Causes the reabsorption of water in the kidney and concentrates the urine. Osmolarity of interstitial fluid (mosm/L) 300 300 100 300 100 CORTEX Active transport Passive transport OUTER MEDULLA NaCl H2O H2O 400 200 H2O NaCl H2O H2O NaCl NaCl 600 300 400 400 H2O 400 NaCl H2O 300 H2O H2O 600 600 H2O Urea H2O INNER MEDULLA H2O NaCl 700 900 NaCl 900 H2O Urea H2O Urea 1200 1200 Figure 44.15 1200 • The countercurrent multiplier system involving the loop of Henle – Maintains a high salt concentration in the interior of the kidney, which enables the kidney to form concentrated urine. • The collecting duct, permeable to water but not salt: –Conducts the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis. • Urea diffuses out of the collecting duct: –As it traverses the inner medulla • Urea and NaCl: –Form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood. • Antidiuretic Hormone (ADH) – Increases water reabsorption in the distal tubules and collecting ducts of the kidney (a) Antidiuretic hormone (ADH) enhances fluid retention by making the kidneys reclaim more water. Osmoreceptors in hypothalamus Hypothalamus Thirst Drinking reduces blood osmolarity to set point ADH Increased permeability Pituitary gland Distal tubule STIMULUS: The release of ADH is triggered when osmoreceptor cells in the hypothalamus detect an increase in the osmolarity of the blood Collecting duct Homeostasis: Blood osmolarity Figure 44.16a H2O reabsorption helps prevent further osmolarity increase • The Renin-Angiotensin-Aldosterone System (RAAS) – Is part of a complex feedback circuit that functions in homeostasis Homeostasis: Blood pressure, volume Increased Na+ and H2O reabsorption in distal tubules STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (such as due to dehydration or loss of blood) Aldosterone Arteriole constriction Adrenal gland Angiotensin II Distal tubule Angiotensinogen JGA Renin production (b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase in blood volume and pressure. Renin Figure 44.16b • The South American vampire bat, which feeds on blood: – Has a unique excretory system in which its kidneys offload much of the water absorbed from a meal by excreting large amounts of dilute urine. Figure 44.17 • Concept 44.6: Diverse adaptations of the vertebrate kidney have evolved in different environments. • The form and function of nephrons in various vertebrate classes: – Are related primarily to the requirements for osmoregulation in the animal’s habitat. Terrestrial Animals • Land animals manage their water budgets – By drinking and eating moist foods and by using metabolic water. Water balance in a human (2,500 mL/day = 100%) Water balance in a kangaroo rat (2 mL/day = 100%) Ingested in food (750) Ingested in food (0.2) Ingested in liquid (1,500) Water gain Derived from metabolism (250) Derived from metabolism (1.8) Feces (0.9) Water loss Urine (0.45) Feces (100) Urine (1,500) Figure 44.5 Evaporation (1.46) Evaporation (900) • Desert animals: – Get major water savings from simple anatomical features Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of unclipped and clipped camels. EXPERIMENT RESULTS Removing the fur of a camel increased the rate of water loss through sweating by up to 50%. Water lost per day (L/100 kg body mass) 4 CONCLUSION Figure 44.6 The fur of camels plays a critical role in their conserving water in the hot desert environments where they live. 3 2 1 0 Control group (Unclipped fur) Experimental group (Clipped fur) • Exploring environmental adaptations of the vertebrate kidney MAMMALS Bannertail Kangaroo rat (Dipodomys spectabilis) Beaver (Castor canadensis) BIRDS AND OTHER REPTILES Roadrunner (Geococcyx californianus) Desert iguana (Dipsosaurus dorsalis) FRESHWATER FISHES AND AMPHIBIANS MARINE BONY FISHES Rainbow trout (Oncorrhynchus mykiss) Figure 44.18 Frog (Rana temporaria) Northern bluefin tuna (Thunnus thynnus)