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 excretion uric acid filtration renal vein urethra nephron proximal tubule collecting duct efferent arteriole antidiuretic hormone (ADH) ammonia filtrate ureter renal cortex glomerulus loop of Henle cortical nephrons peritubular capillaries aldosterone osmoregulation urea 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 Digestive waste (elimination) 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: 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 Other features Plants Stomata, lenticels for gas exchange Insoluble crystals Crystals are kept inside plant cells Cnidarians and echinoderms No excretory organ - Osmoconformers, isoosmotic with environment Freshwater protists and sponges Contractile vacuole Porocytes and flagella

Excretory Systems Dispose of metabolic wastes Regulate solute concentrations in the body Transport epithelia arranged in tubes 4 major processes 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 Product of excretion Other features 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

Malpighian tubules and digestive tract Type of Organism Structure Product of excretion Other features Molluscs Metanephridia Crustaceans Antennal/green gland Insects Malpighian tubules and digestive tract Uric acid

Type of Organism Structure Product of excretion Other features Marine fishes Gills for osmoregulation, kidneys Ammonia Elasmobranchs (sharks, skates, rays) Kidneys Urea Rectal glands – excrete excess NaCl Freshwater fishes Ammonia or urea Amphibians and mammals Liver converts ammonia to urea Reptiles and birds Uric acid Salt glands

From Blood Filtrate to Urine: A Closer Look Proximal tubule – secretion and reabsorption Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs >> Same concentration of substances in blood plasma Key Active transport Passive transport CORTEX OUTER MEDULLA INNER Descending limb of loop of Henle – reabsorption Permeable to water but not to salt Thick segment of ascending limb – reabsorption - Impermeable to water but permeable to salt Thin segment limb Collecting Duct – permeable to water and salt, bottom portion is permeable to urea NaCl Distal tubule – secretion and reabsorption Nutrients HCO3 K+ NH3 1 4 3 2 5

Osmolarity of interstitial fluid (mosm/L) H2O NaCl 300 100 400 600 900 1200 700 200 Active transport Passive transport OUTER MEDULLA INNER MEDULLA CORTEX Urea Osmolarity of interstitial fluid (mosm/L) 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

Nervous system and hormones regulate kidney functions Osmoreceptors in hypothalamus Drinking reduces blood osmolarity to set point H2O reab- sorption helps prevent further osmolarity increase STIMULUS: The release of ADH is triggered when osmo- receptor cells in the hypothalamus detect an increase in the osmolarity of the blood Homeostasis: Blood osmolarity Hypothalamus ADH Pituitary gland Increased permeability Thirst Collecting duct Distal tubule 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

The renin-angiotensin- aldosterone system (RAAS) Increased Na+ and H2O reab- sorption in distal tubules Homeostasis: Blood pressure, volume STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (such as due to dehydration or loss of blood) Aldosterone Adrenal gland Angiotensin II Angiotensinogen Renin production Arteriole constriction Distal tubule JGA The renin-angiotensin- aldosterone 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

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