Osmoregulation and Excretion

1. Osmoregulation and Excretion

2. 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

3. Key Words • 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 • homeostasis • excretion • uric acid • filtration • renal vein • urethra • nephron • proximal tubule • collecting duct • efferent arteriole • antidiuretic hormone (ADH)

4. Types of metabolic waste produced by living systems • Digestive waste (elimination) • Respiratory waste • Excess water and salts (through osmoregulation) • Nitrogenous waste (through excretion)

5. 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)

6. 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

8. 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 Excretory Systems

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

14. 300 100 300 NaCl H2O Activetransport Osmolarity of interstitial fluid(mosm/L) 400 NaCl H2O Passivetransport 300 NaCl 100 H2O 300 300 CORTEX H2O OUTERMEDULLA NaCl H2O 200 400 400 600 H2O NaCl H2O H2O NaCl H2O H2O 400 600 600 900 H2O NaCl Urea H2O 900 H2O 700 Urea H2O INNERMEDULLA 1200 1200 Urea 1200 • 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

15. Osmoreceptors in hypothalamus Thirst Hypothalamus Drinking reduces blood osmolarity to set point ADH Increased permeability Pituitary gland Distal tubule 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 Collecting duct Homeostasis: Blood osmolarity Nervous system and hormones regulate kidney functions • 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

16. Homeostasis: Blood pressure, volume Increased Na+ and H2O reab- sorption 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 • 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 atrialnatriuretic factor (ANF) • Released by atria • Inhibits release of renin

17. 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