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Chapter 44

Chapter 44. Osmoregulation and Excretion. Overview: A Balancing Act. The physiological systems of animals from cells  tissues  organs  organ systems Operate in a fluid environment

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Chapter 44

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  1. Chapter 44 Osmoregulation and Excretion

  2. Overview: A Balancing Act • The physiological systems of animals from cells  tissues  organs  organ systems Operate in a fluid environment • The relative concentrations of water and solutes in this environment must be maintained within fairly narrow limits for these systems to function properly • Different types of animals face different types of challenges to maintaining osmolarity

  3. Figure 44.1 • Freshwater animals • Threat: to dilute the body • Show adaptations that reduce water uptake and conserve solutes • Desert and marine animals • Threat: face desiccating environments with the potential to quickly deplete the body water • Show adaptations to conserve water and eliminate excess salts

  4. 2 Key Homeostatic Processes • Osmoregulation • Regulates solute concentrations and balances the gain and loss of water • Excretion • How animals get rid of nitrogen containing metabolic wastes

  5. Concept 44.1: 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 • Osmosis • Cells require a balance between osmotic gain and loss of water • Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments

  6. Osmotic Challenges • 2 basic solutions to the problem of balancing water uptake with water loss • Osmoconformers are isoosmotic with their surroundings and do not regulate their osmolarity (Only available to marine animals) • Osmoregulators expend energy to control water uptake and loss in hyperosmotic or hypoosmotic environments • Energy cost depends on how different the animals osmolarity is from its environment

  7. Figure 44.2 • Most animals are said to be stenohaline • cannot tolerate substantial changes in external osmolarity • Euryhaline animals • Can survive large fluctuations in external osmolarity • Tilapia can live in any salt concentration from fresh to salt water

  8. Marine Animals • Most marine invertebrates are osmoconformers • Their total osmolarity is the same as that of sea water however, they differ considerably in the specific types of solutes and so must still regulate their internal environment • Most marine vertebrates and some invertebrates are osmoregulators

  9. 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 and small amounts of water in scanty urine from kidneys Excretion of salt ions from gills Marine Bony Fish • 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 • Disposing of NaCl via Specialized chloride transporter cells in their gills that actively pumps Cl- Out (and Na+ passively follows) (a) Osmoregulation in a saltwater fish

  10. Marine Chondrichthyans • Use different osmoregulatory strategy than bony fish • Rectal gland: specialized organ that unloads salt into the feces • Also, because sharks produce and maintain high amounts of Urea ( a byproduct of protein metabolism) a sharks total solute concentration is actually higher than the surrounding weater and so water diffuses into the shark when they eat (they don’t drink)

  11. Freshwater Animals • Have opposite problems those of the marine animals, water is constantly entering their bodies via osmosis • Many freshwater animals maintain body osmolarities significantly lower than their marine counterparts • Reduces energy needed for maintaining osmolarity in fresh water envieonments • Lose salts by diffusion

  12. 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 Excretion of large amounts of water in dilute urine from kidneys • Freshwater animals like Perch maintain water balance by excreting large amounts of dilute urine • Salts lost by diffusion • Are replaced by foods and uptake across the gills • Cl- ions are actively pumped into the gills, Na+ follows passively Figure 44.3b (b) Osmoregulation in a freshwater fish

  13. 100 µm 100 µm (a) Hydrated tardigrade (b) Dehydrated tardigrade 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 • Must have adaptations that keep their cells membranes intact Water Bear Figure 44.4a, b

  14. 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) Feces (100) Urine (0.45) Urine (1,500) Water loss Evaporation (900) Evaporation (1.46) Land Animals • Land animals manage their water budgets • By drinking and eating moist foods and by using metabolic water Figure 44.5

  15. EXPERIMENT 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. Removing the fur of a camel increased the rateof water loss through sweating by up to 50%. 4 RESULTS 3 CONCLUSION The fur of camels plays a critical role intheir conserving water in the hot desertenvironments where they live. Water lost per day (L/100 kg body mass) 2 1 0 Control group (Unclipped fur) Experimental group (Clipped fur) • Desert animalsget major water savings from simple anatomical features Figure 44.6

  16. Transport Epithelia • The ultimate function of osmoregulation is to maintain the composition of cellular cytoplasm • This is mainly accomplished by maintaining the composition of internal body fluid that bathes the cells • In animals with open circulatory systems this fluid is hemolymph • In animals with closed circulatory systems the fluid is interstitial fluid • Controlled indirectly by composition of the blood

  17. Transport Epithelia • Transport epithelia are specialized cells that regulate solute movement • Are essential components of osmotic regulation and metabolic waste disposal • Are arranged into complex tubular networks • Move specific solutes in controlled amounts in specific directions

  18. Nasal salt gland (a) An albatross’s salt glands empty via a duct into thenostrils, 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 (c) The secretory cells actively transport salt from theblood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentrationgradient of salt in the tubule (aqua), this countercurrentsystem enhances salt transfer from the blood to the lumen of the tubule. Secretory tubule Artery NaCl Transport epithelium (b) One of several thousand secretory tubules in a salt-excreting gland. Each tubule is lined by a transportepithelium surrounded by capillaries, and drains intoa central duct. Direction of salt movement Blood flow Secretory cell of transport epithelium Central duct Albatross • An example of transport epithelia is found in the salt glands of marine birds which remove excess sodium chloride from the blood Figure 44.7a, b

  19. 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

  20. Nucleic acids Proteins Nitrogenous bases Amino acids –NH2 Amino groups Many reptiles (including birds), insects, land snails Most aquatic animals, including most bony fishes Mammals, most amphibians, sharks, some bony fishes O H C N C HN C O NH2 C C C O N N O NH3 NH2 H H Ammonia Urea Uric acid • Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids Figure 44.8

  21. Forms of Nitrogenous Wastes • When these macromolecules are broken down for energy or converted to carbohydrates or fats enzymes remove nitrogen in the form of ammonia • Ammonia is a very toxic molecules • Many species must convert NH3 to less toxic forms before excreting it • Different animals excrete nitrogenous wastes in different forms

  22. Ammonia • Animals that excrete nitrogenous wastes as ammonia • Need access to lots of water • Ammonia secretion is most common in aquatic species who have access to lots of water • Release it across the whole body surface or through the gills

  23. Urea • The liver of mammals and most adult amphibians converts ammonia to less toxic urea • Main advantage of urea is low toxicity • Main disadvantage is that animals must expend energy producing it • Urea is carried to the kidneys, concentrated and excreted with a minimal loss of water

  24. Uric Acid • Insects, land snails, and many reptiles, including birds excrete uric acid as their major nitrogenous waste • Uric acid is • Nontoxic • largely insoluble in water • And can be secreted as a paste with little water loss

  25. The Influence of Evolution and Environment on Nitrogenous Wastes • The kinds of nitrogenous wastes excreted depend on an animal’s evolutionary history and habitat, especially availability of water, and on its reproductive strategies • The amount of nitrogenous waste produced is coupled to the animal’s energy budget

  26. Concept 44.3: Diverse excretory systems are variations on a tubular theme • Excretory systems regulate solute movement between internal fluids and the external environment

  27. Capillary 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. 1 Excretory tubule Filtrate 2 Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids. Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule. 3 4 Excretion. The filtrate leaves the system and the body. Urine Excretory Processes • Most excretory systems produce urine by refining a filtrate derived from body fluids Figure 44.9

  28. Key functions of most excretory systems are • Filtration, pressure-filtering of body fluids producing a filtrate • Reabsorption, reclaiming valuable solutes from the filtrate • Secretion, addition of toxins and other solutes from the body fluids to the filtrate • Excretion, the filtrate leaves the system

  29. Survey of Excretory Systems • The systems that perform basic excretory functions vary widely among animal groups • In general, though, they’re built on a complex network of tubules that provide a large surface area for the exchange of water and solutes

  30. Nucleus of cap cell Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Tubule Nephridiopore in body wall Protonephridia: Flame-Bulb Systems • A protonephridium is a network of dead-end tubules lacking internal openings found in flat worms • The tubules branch throughout the body with the ends being capped by a flame bulb • Each flamebulb has a tuft projecting into the tubule - Beating cilia draws water and solutes into the flame bulb for filtration • Most of the solutes are reabsorbed before excretion to maintain homeostasis in the hypotonic environment The Flamebulb functions mainly in osmoregulation with most nitrogenous wastes diffusing across the entire surface of the organism Figure 44.10

  31. Coelom Capillary network Bladder Collecting tubule Nephridio- pore Metanephridia Nephrostome Most Annelids Utilize Metanephridia • Have both excretory & osmoregulatory functions • Each segment of an earthworm • Has a pair of open-ended metanephridia • Metanephridium have internal openings that collect body fluid • Each is surrounded by a ciliated funnel called the nephrostome which fluid enters • Nephrostomes open to the outside via a nephridiopore Figure 44.11

  32. Digestive tract Rectum Hindgut Intestine Malpighian tubules Midgut (stomach) Salt, water, and nitrogenous wastes Feces and urine Anus Malpighian tubule Rectum Reabsorption of H2O, ions, and valuable organic molecules HEMOLYMPH Malpighian Tubules • In insects and other terrestrial arthropods, malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation • Malpighian tubules open into the digestive tract and dead-end at tips that are immersed in hemolymph • The transport epithelium secretes nitrogenous wastes from hemolymph into the tubules Figure 44.12

  33. Malpighian Tubules in Insects • Insects produce a relatively dry waste matter • Even though water follows the solutes into the tubules via osmosis, the fluid passes to the rectum where most of the solutes (and thus water) are pumped back into the hemolymph • Thus, it is mainly nitrogenous wastes that are excreted • This is an important adaptation to terrestrial life

  34. Vertebrate Kidneys • Kidneys, the excretory organs of vertebrates • Function in both excretion and osmoregulation • Built largely of tubules • Blood flow through the kidneys is known as renal circulation

  35. Concept 44.4: Nephrons and associated blood vessels are the functional unit of the mammalian kidney • The mammalian excretory system centers on paired kidneys http://www.dnatube.com/video/2754/Nephron-Function • Kidneys are the principal site of water balance and salt regulation • Each mammal has 2 kidneys, each supplied with blood by the renal artery and drained by the renal vein • In humans, kidneys account for less than 1% of body mass but receive 20% of blood from the resting heart beat

  36. Posterior vena cava Renal artery and vein Kidney Aorta Ureter Urinary bladder Urethra (a) Excretory organs and major associated blood vessels • Urine exits each kidney through a duct called the ureter • Both ureters drain into a common urinary bladder • During urination urine is expelled via the urethra Figure 44.13a

  37. Renal medulla Renal cortex Renal pelvis Ureter Section of kidney from a rat Figure 44.13b (b) Kidney structure Structure and Function of the Nephron and Associated Structures • The mammalian kidney has two distinct regions • An outer renal cortex and an inner renal medulla

  38. Juxta- medullary nephron Cortical nephron Afferent arteriole from renal artery Glomerulus Bowman’s capsule Renal cortex Proximal tubule Peritubularcapillaries Collecting duct SEM 20 µm Efferent arteriole from glomerulus Distal tubule Renal medulla To renal pelvis Collecting duct Branch of renal vein Descending limb Loop of Henle Ascending limb Vasarecta (d) Filtrate and blood flow (c) Nephron The Nephron is the functional unit of the vertebrate kidney • Each nephron consists of a single long tubule and a ball of capillaries called the glomerulus • The glomerolous is surrounded by a bowmans capsule, which is a swelling at the blind end of the nephron Figure 44.13c, d

  39. Filtration of the Blood • Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule • Filtration of small molecules is nonselective • Capillaries are permeable to small molecules and water but not blood cells or macromolecules • And the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma • Salts Vitamins Glucose • Urea Amino Acids

  40. Pathway of the Filtrate • From Bowman’s capsule, the filtrate passes through three regions of the nephron • The proximal tubule • the loop of Henle • the distal tubule • Fluid from several nephrons flows into a collecting duct • Many collecting ducts enter into the renal pelvis which drains through a ureter

  41. Water Conservation & Evolutionary Adaptations • In the human Kidney about 80% of nephrons called cortical nephrons have reduced loops of Henle and are confined to the renal cortex • 20% of nephrons called juxtamedullary nephrons have well developed loops and • extend deeply into the adrenal medulla • Allow animals to produce urine that is hyperosmotic to to body fluids which is a helpful adaptation to water conservation • Only mammals and birds have juxtamedulary nephrons, the nephrons of other vertebrates lack loops of henle

  42. 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

  43. Exchange is Facilitated by Relative direction of Blood Flow to Filtrate Flow in Nephrons • Although excretory tubules and their surrounding capillaries are closely associated, they do not exchange materials directly • The tubules and capillaries are immersed in interstitial fluid through which various substances diffuse between the plasma within the capillaries and the filtrate within the nephron tubule

  44. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look • Filtrate becomes urine as it flows through the mammalian nephron and collecting duct Figure 44.14

  45. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look • The Proximal Tubule • Secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate • The nephron absorbs: H+ & NH3 • While the proximal tubule absorbs: bicarbonate, NaCl, water, nutrients, and K+ • *One of the most important functions of the proximal tubule is reabsorption of NaCl & water 1 Figure 44.14

  46. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look 2. The Descending Loop of Henle Reabsorption of water continues as filterate moves into the descending limb of the loop of henle Here, transport epithelium is free permeable to water but not salt or other solutes Interstitial fluid is maintained in a hyperosmotic state to encourage osmosis out of the nephron into the outer medulla 2 2 Figure 44.14

  47. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look 3 3. The Ascending Loop of Henle Here the transport epithelium is permeable to salt but not water NaCl diffuses out of the nephron into the interstitial fluid increasing its osmolarity This progressively dilutes the filtrate 3 Figure 44.14

  48. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look 4 3. The Distal Tubule Plays a key role in regulating K+ and NaCl concentration of body fluid by varying the amount of K+ secreted into the filtrate and the amount of NaCl reabsorbed from the filtrate Also contributes to pH regulation by secreting H+ or reabsorbing bicarbonate Figure 44.14

  49. Distal tubule Proximal tubule 4 1 NaCl Nutrients H2O HCO3 H2O K+ NaCl HCO3 H+ K+ H+ NH3 CORTEX Thick segment of ascending limb Descending limb of loop of Henle 2 3 Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct 3 5 Key Urea NaCl H2O Active transport Passive transport INNER MEDULLA From Blood Filtrate to Urine: A Closer Look 5. The Collecting Duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl 5 Figure 44.14

  50. 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 • 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 • Hyperosmotic urine is only possible because the body expends considerable amounts of energy actively transporting solutes against concentration gradients

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