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OSMOREGULATION AND EXCRETION Key Concepts Osmoregulation balances the uptake and loss of water and solutes An animal’s nitrogenous wastes reflect its phylogeny and habitat Diverse excretory systems are variations on a tubular theme Nephrons and associated blood vessels are the functional units of the mammalian kidney The mammalian kidney’s ability to conserve water is a key terrestrial adaptation Diverse adaptations of the vertebrate kidney have evolved in different environments Key Words homeostasis ammonia excretion filtrate uric acid ureter filtration renal vein glomerulus urethra loop of Henle nephron cortical nephrons proximal tubule collecting duct peritubular capillaries efferent arteriole aldosterone antidiuretic hormone (ADH) osmoregulation urea renal cortex secretion (selective reabsorption) renal artery urinary bladder renal medulla Bowman's capsule distal tubule afferent arteriole vasa recta Types of metabolic waste produced by living systems 1. 2. 3. 4. Digestive waste Respiratory waste Excess water and salts (through osmoregulation) Nitrogenous waste (through excretion) Osmoregulation Balance of uptake and loss of water and solutes Controlled movement of solutes between internal fluids and environment Osmoconformer (marine animals isoosmotic with environment) Osmoregulator (freshwater, marine, and terrestrial animals that adjust internal osmolarity) Types of nitrogenous wastes Deamination – protein and nucleic acid metabolism Three main types differing in terms of: 1. 2. 3. Toxicity Amount of water needed for excretion Energy needed for synthesis 300 – 500 mL/gN 1 step rxn 50 mL/gN 4 step rxn 10 mL/gN 15 step rxn Type of Organism Structure Product of excretion Stomata, lenticels Insoluble crystals Crystals are kept inside plant cells Plants No excretory organ Osmoconformers, isoosmotic with environment Cnidarians and echinoderms Contractile vacuole Freshwater protists and sponges Other features Excretory Systems Dispose of metabolic wastes Regulate solute concentrations in the body Transport epithelia arranged in tubes 4 major processes 1. 2. 3. 4. Filtration, pressure-filtering of body fluids producing a filtrate (water, salts, sugars, amino acids, N-wastes) Reabsorption, reclaiming valuable solutes (glucose, salts, amino acids) from the filtrate Secretion, addition of larger molecules like toxins and other excess solutes from the body fluids to the filtrate Excretion, the filtrate leaves the system Type of Organism Structure Flatworms Flame cells Unsegmented roundworms Protonephridia, closed network of dead-end tubes lacking openings Annelids Metanephridia, open-ended network of tubes with internal openings that collect body fluids Product of excretion Other features Type of Organism Structure Molluscs Nephridia or metaphridia Crustaceans Antennal/green gland Insects Malpighian tubules and digestive tract Product of excretion Uric acid Other features Type of Organism Structure Product of excretion Marine fishes Gills Ammonia Other features Rectal glands – excrete excess NaCl Elasmobranchs (sharks, skates, rays) Kidneys Urea Freshwater fishes Gills Ammonia or urea Amphibians and mammals Kidneys Urea Liver converts ammonia to urea Reptiles and birds Kidneys Uric acid Salt glands From Blood Filtrate to Urine: A Closer Look 1 Proximal tubule – secretion and reabsorption NaCl HCO3 4 Distal tubule – secretion and reabsorption Nutrients H+ H2O K+ H2O NaCl K+ NH3 CORTEX Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs >> Same concentration of substances in blood plasma 2 Descending limb of loop of Henle – reabsorption -Permeable to water but not to salt 3 H+ Thick segment of ascending limb – reabsorption - Impermeable to water but permeable to salt NaCl H2O OUTER MEDULLA NaCl 3 Thin segment of ascending limb Key 5 Collecting Duct – permeable to water but not to salt, bottom portion is permeable to urea Urea NaCl Active transport Passive transport HCO3 INNER MEDULLA H2O Two solutes: NaCl and urea, contribute to the osmolarity of the interstitial fluid Cause the reabsorption of water in the kidney and concentrates the urine Osmolarity of interstitial fluid (mosm/L) 300 300 100 300 100 CORTEX NaCl H2O Active transport 300 400 400 600 600 H2O 200 400 300 H2O NaCl H2O H2O NaCl H2O Passive transport OUTER MEDULLA NaCl H2O H2O 400 600 NaCl H2O H2O Urea NaCl H2O 700 900 INNER MEDULLA H2O NaCl 900 H2O Urea H2O 1200 Urea 1200 1200 Nervous system and hormones regulate kidney functions Osmoreceptors in hypothalamus Antidiuretic hormone (ADH) Stimulated by a rise in the blood’s osmolarity (>300 mosm/L) Enhances fluid retention by making the kidneys reclaim more water Increases water reabsorption in the distal tubules and collecting ducts of the kidney Thirst Hypothalamus 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 H2O reabsorption helps prevent further osmolarity increase Collecting duct Homeostasis: Blood osmolarity The renin-angiotensinaldosterone system (RAAS) Responds to a loss of salt and water in the blood Stimulated by low blood volume or pressure Increases water and sodium ion reabsorption in the proximal and distal tubules Leads to an increase in blood volume and pressure Opposed by the hormone atrial natriuretic factor (ANF) Released by atria Inhibits release of renin 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 Renin Some medical aspects concerning the excretory system Urinary tract infection (UTI) bacterial infection cystitis/pyelonephritis treated by antibiotics and prevented through proper hygiene Kidney stones solidified crystals in kidneys or ureters Calcium oxalate Uric acid nephrolithiasis/urolithiasis prevention: Drinking adequate water Proper diet low in protein, N, and Na Avoid excess Vitamin C intake Dialysis Hemodialysis Peritoneal dialysis
