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Chapter 21: Homeostasis of water and solutes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-1 Homeostasis • In animals, each cell has a membrane which separates the intracellular fluid from the surrounding extracellular fluid • Extracellular fluid provides a protective internal environment in which the cells live • Homeostasis is the term used to describe constancy of the extracellular environment in which the cells are located • Regulatory mechanisms can maintain consistency in some aspects of the extracellular environment Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-2 Homeostasis (cont.) • Animals can either conform or regulate in regards to certain aspects of their environment – Examples Osmoconform or Osmoregulate Thermoconform or Thermoregulate Ionoconform or Ionoregulate Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-3 Fig. 21.1: Osmoconformer and osmoregulator Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-4 Water and solutes • All organisms contain – high levels of water – ions Na+, K+, Cl–, Ca2+, SO42–, PO43– – organic solutes glucose, amino acids, proteins • These substances are essential for life – few animals can survive dehydration or freezing Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-5 Exchange with the environment • Water and solutes are exchanged continuously between an organism and its environment – intake of food and fluids – respiration – elimination of wastes • Aquatic animals also – gain or lose water by osmosis – gain or lose solutes by diffusion • Terrestrial animals also – lose water from body surface by evaporation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-6 Fig. 21.4: Exchange with the environment Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-7 Intracellular environment • Intracellular fluid must have the same osmotic concentration as extracellular fluid to prevent movement of water from one to the other – iso-osmotic • Solute composition differs from that of extracellular fluid – higher levels of K+ – lower levels of Na+ and Cl– Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-8 Tonicity • Cell volume is determined by solute content in intracellular fluid • Solutes move along diffusion gradients – movement of a solute across membranes depends on the membrane’s permeability to that solute – if solutes move across the membrane, water moves with them • Isotonic solutions maintain solute and water balance across a membrane – cells maintain cell volume by changing concentration of amino acids Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-9 Ionic and osmotic balance • Osmotic concentration of extracellular fluid of osmoconformers is identical to that of the external environment • Osmoregulators regulate the osmotic concentration • Ion concentration in ionoconformers is identical to that of the external environment • Ionoregulators regulate the ion concentration Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-10 Living in sea water • Many marine invertebrates are osmoconformers and ionoconformers – no regulation of osmotic or ionic concentrations – extracellular fluid varies with external environment • Almost all marine vertebrates – osmoconform and ionoregulate cartilaginous fish (sharks, rays, chimaeras), coelacanth – osmoregulate and ionoregulate most fish, amphibians, reptiles, birds, mammals Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-11 Fig. 21.5: Marine bony fish Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-12 Living in salt lakes • Hypersaline water has a greater salt concentration than sea water • Animals that live in salt lakes must osmoregulate and ionoregulate – fish excrete excess ions through salt pumps (chloride cells) on gills – brine shrimp (Artemia) excrete excess ions through salt pumps on appendages • Water lost by osmosis is increased by drinking salty water Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-13 Living in fresh water • Because fresh water has a low solute concentration, animals that live in it – gain water by osmosis – lose solutes by diffusion • Freshwater animals must osmoregulate and ionoregulate – water is excreted as dilute urine – ions are absorbed from the gut or actively taken up across the skin and gills Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-14 Fig. 21.7: Freshwater bony fishes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-15 Moving between sea and fresh water • Euryhaline animals move between salt and fresh waters – may change water balance strategies between different environments • Even those species that osmoregulate and ionoregulate in both environments must adjust pattern from fresh to salt – ‘freshwater’ hormone prolactin lowers water permeability of skin, gills and gut and increases ion retention and uptake – ‘seawater' growth hormone and cortisol increase ion excretion from gills Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-16 Living on land • Terrestrial animals face water loss through – evaporation from lungs and skin surface – excretion in urine and faeces • Water is usually replaced by drinking – in arid areas, food and metabolic water are important sources – some invertebrates can absorb water directly from air through skin of mouth or anus • Water loss is minimised by producing solid or concentrated wastes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-17 Nitrogenous wastes • Nitrogen produced by metabolism of protein must be excreted from the body • Toxic ammonia (NH3) is the first product of protein metabolism – excreted by aquatic animals • Terrestrial organisms convert NH3 to a less-toxic form – soluble urea (CON2H4) in invertebrates and mammals – insoluble uric acid (C5H4O3N4) in reptiles and birds – insoluble guanine (C5H5ON5) in spiders and scorpions Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-18 Nitrogenous wastes (cont.) • Advantages and disadvantages associated with different nitrogenous wastes • Ammonia (NH3) – toxic, requires water for excretion, no energy expenditure • Urea (CON2H4) – less toxic, less water required for excretion, some energy expenditure • Uric acid (C5H4O3N4) and guanine (C5H5ON5) – non-toxic, very little water required for excretion, substantial energy expenditure Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-19 Fig. 21.10: Nitrogenous waste Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-20 Question 1: The advantage of excreting wastes as urea rather than ammonia is that: a) Urea is less toxic than ammonia. b) Urea requires less water for excretion than ammonia. c) Urea does not affect the osmolar gradient. d) Urea can be exchanged for Na+. e) Both A and B are advantageous. Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-21 Excretion • Excretion regulates the internal environment by – controlling body water – maintaining solute composition – excreting metabolic waste products and unwanted substances • Excretion differs from elimination – excretion removes substances that have been metabolised – elimination expels unabsorbed food Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-22 Question 2: If you compared the maximum urine concentration of a desert mammal and a rainforest mammal, you would expect to find that: a) The maximum urine concentration would be higher for the rainforest mammal. b) The maximum urine concentration would be higher for the desert mammal. c) The maximum urine concentrations would be similar for the two mammals and similar to those in humans. Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-23 Excretory organs • Single-celled organisms and simple animals use contractile vacuoles to excrete material • More complex animals possess two mechanisms for excretion – Surface epithelial solute pumps regulate exchange of specific ions – Internal tubular excretory organs form liquid urine that contains a variety of materials Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-24 Epithelial excretory organs • Ion pumps on epithelia take up or excrete ions selectively • Epithelial salt glands of brine shrimp (Artemia) actively excrete Cl– and passively excrete Na+ • Ion pumps on gills of fish – marine species excrete Cl– and Na+ – freshwater species absorb Cl– and Na+ • Movement of ions may result in passive transport of water Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-25 Tubular excretory organs • Tubular excretory organs – – – – form urine reabsorb solutes and water secrete solutes change osmolarity • Excretory tubules form a filtrate of coelomic fluid or blood • As the fluid passes along the tubule, solutes and water are reabsorbed or solutes secreted to form urine Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-26 Fig. 21.13: Tubular excretory organs Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-27 Nephridia • Nephridia – ingrowths of body surface – coelomic fluid drawn into nephridia by cilia – excreted at nephridiopore • Protonephridia – blind-ending flame cells – fluid enters through perforations in walls • Metanephridia – ciliated funnel (nephridiostome) opens into coelomic cavity Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-28 Malpighian tubules • Malpighian tubules – – – – – open into digestive tract at junction of midgut and hindgut K+ is actively transported into lumen of tubule water and solutes follow ions selectively reabsorbed urine formed in tubules is emptied into hindgut for further modification • Malpighian tubules of insects and some spiders produce dry wastes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-29 Coelomoducts • Develop from coelomic lining • Coelomic fluid – is filtrate from blood vessels – cilia draw the fluid into funnels (coelomostomes) in coelom – excreted at coelomopore • Coelomoducts are present in many invertebrates, hagfishes and lampreys – blood vessels and coelomoducts combine into single structure in higher vertebrates Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-30 Kidneys • Kidneys are principal excretory organs of vertebrates • Each kidney is composed of nephrons (excretory tubules) • Nephrons of higher vertebrates are modified coelomoducts in which a cluster of capillaries (glomerulus) is associated with the tubule Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-31 Fig. 21.19: Four functions of nephron Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-32 Mammalian nephrons • At one end of a nephron tubule is an enlarged double-walled cup – outer wall forms renal or Bowman’s capsule – inner wall of podocytes encloses glomerulus • Proximal convoluted tubule extends from Bowman’s capsule • Loop of Henle • Distal convoluted tubule empties into collecting duct Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-33 Filtration • Fluid is filtered through walls of glomerular capillaries and podocytes into lumen of Bowman’s capsule • Erythrocytes and protein molecules are too large to pass out of capillaries • Otherwise filtrate has similar composition to blood Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-34 Reabsorption • Approximately 180 L of filtrate is produced each day in an adult human – 99 per cent of filtrate is reabsorbed • Proximal convoluted tubule reabsorbs most of the – – – – water NaCl glucose amino acids • Capillaries surrounding tubule return materials to circulation Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-35 Secretion • Peritubular capillaries secrete ions into renal tubules – – – – H+ K+ NH4+ specific organic molecules • H+ is secreted to regulate blood and urine pH – buffering H+ with NH3 to form NH4+ prevents excessively low pH Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-36 Osmoconcentration • All vertebrates produce iso-osmotic or hypoosmotic urine • Most mammals and some birds produce hyperosmotic urine • Hyperosmotic urine formed in juxtamedullary nephrons – long loops of Henle in medulla Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-37 Osmoconcentration (cont.) • Osmotic concentration gradient established between descending and ascending limbs of loop of Henle • Countercurrent multiplication of solute and water transport – ascending limb actively removes Cl– – Na+ follows passively – water cannot pass out of ascending limb because membrane is not permeable to it Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-38 Osmoconcentration (cont.) • Increased solute concentration draws water out of the descending limb • Urea from the collecting duct increases osmotic gradient • Medullary osmotic gradient draws water from fluid as it passes through collecting duct Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-39 Fig. 21.22: Loop of Henle Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-40 Control of kidney function • Hormones control kidney function – antidiuretic hormone (ADH) causes water retention – renin, released in response to low blood pressure, converts angiotensinogen into angiotensin II – angiotensin II decreases blood flow to capillary beds and stimulates reabsorption of water and NaCl from proximal tubules – angiotensin II causes release of aldosterone, which stimulates reabsorption of water and NaCl from distal tubules – atrial natriuretic factor (ANF) acts antagonistically to these other hormones, inhibiting renin, aldosterone and NaCl reabsorption Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-41 Salt glands • Many marine vertebrates possess salt glands that secrete NaCl or KCl – cartilaginous fish, coelacanth – marine reptiles – marine birds • Excreted salt solution more concentrated than sea water • Salt glands allow marine vertebrates to drink salt water Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-42 Summary • Homeostasis is consistency of the extracellular fluids that provides a stable environment for the cells • Intracellular fluid has the same osmotic concentration as extracellular fluid in animals, but solute concentration of intracellular and extracellular fluids invariably differ • Metabolic processes produce waste products which must be excreted • Environmental exchange occurs by passive and active processes Copyright 2010 McGraw-Hill Australia Pty Ltd PowerPoint slides to accompany Biology: An Australian focus 4e by Knox, Ladiges, Evans and Saint Slides prepared by Karen Burke da Silva, Flinders University 21-43
