Salt and Water Balance and Nitrogen Excretion

Transcription

1 Announcements Exam is in class on WEDNESDAY. Bring a #2 pencil and your UFID. You must come to your registered class section (except those with DRC accommodations). Office hours Mon 1-3 pm. Teaching evals: 40 Salt and Water Balance and Nitrogen Excretion 1

2 Excretory systems have four main functions: Regulate volume of fluid in the body Regulate overall solute concentrations, or osmolarity, in extracellular fluid Maintain specific solutes (Ca 2+, H +, Na +, glucose) at appropriate levels Eliminate nitrogenous wastes Result is osmotic equilibrium for optimal function of the organism and cells. Osmosis is movement of water across a selectively permeable membrane. Water will always move from an area of LESS solute (more water) to an area of MORE solute (less water). Selectively permeable membrane Solutes Net water flow Water Hyperosmotic side Hypoosmotic side 2

3 If the osmolarity on both sides of the membrane are equal, the cell and the solution are isosmotic (or isotonic). The inside of the cell is hyperosmotic to the solution. The solution is hypoosmotic to the cell. The inside of the cell is hypoosmotic to the solution. The solution is hyperosmotic to the cell. If the osmolarity of the extracellular fluid is different than the cytoplasm, water will move into or out of the cells via osmosis, and cells may be damaged. 3

4 9/20/13 Animals have different ways of meeting osmotic challenges. Osmoregulators actively regulate the osmolarity of their extracellular fluid. Osmoconformers equilibrate their osmolarity with seawater. Most animals are ionic regulators, regardless of their overall osmoregulation. Most marine invertebrates are osmoconformers isosmotic to seawater. Some of their internal solutes are different than for seawater, so they have to transport those to maintain homeostasis. 4

5 Sharks are essentially osmoconformers. Shark internal salt concentration slightly low but solute level is increased from urea in body tissues to be nearisosmotic with seawater. Trimethylamine oxide (TMAO) protects against damage from urea. Many marine vertebrates osmoregulate their body fluid to be hypoosmotic to seawater and therefore risk of dehydration of body tissues (must conserve fluids, excrete excess salt). Strategies: Drinking lots of seawater w/o absorbing all the ions. Ions eliminated by gills, kidneys, large intestine Little urine produced 5

6 Freshwater vertebrates are hyperosmotic relative to their environment and risk overdilution of body tissues (must conserve salt, excrete excess fluids). Strategies: Drink very little, produce lots of urine. Salts replenished by eating, reabsorption of ions, and active uptake across gills. Terrestrial animals must deal with lack of water and salt. Strategies: Body coverings shells, keratin, waxy exoskeletons Nocturnal behavior Ingesting moist foods Metabolic water via cellular respiration 6

7 Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes Animals must also eliminate nitrogenous waste from metabolizing protein and nucleic acids. Initially metabolized as ammonia, which is toxic if it builds up, so it s converted to urea or uric acid. More energetically costly to make Ammonia Urea More water needed to excrete (also more toxic) Uric Acid Concept 40.2 Excretory Systems Eliminate Nitrogenous Wastes Ammonotelic animals secrete NH 3 through their gills (aquatic only!). Ureotelic animals mostly excrete urea. It is water-soluble but results in large water loss. Uricotelic animals mostly excrete uric acid. It is insoluble in water and precipitates out of the urine with little water loss. Many animals can produce multiple types. 7

8 Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion In most systems urine is produced by filtering extracellular fluid. Pressure of blood against vessel walls forces fluids out across membrane (filtrate) while large solutes are retained. Materials in the filtrate are reabsorbed back to the blood/body fluids; other materials are secreted into filtrate. Solutes moving in and out affect osmotic movement (water). Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion In earthworms, blood pumped under pressure causes blood to filter across capillary walls into the coelom (body cavity). Each body segment has a pair of metanephridia that collect coelomic fluid, reabsorb most solutes, and excrete the nitrogenous wastes in dilute urine. 8

9 Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion The insect excretory system consists of Malpighian tubules blind-ended tubules that open into the gut. Tubule cells actively transport uric acid, K +, and Na + into the tubules. Water follows. Water and ions recovered in the hindgut; uric acid and other waste are excreted. Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Vertebrates are well-adapted to excrete excess water. Excretory system includes: Kidney the main excretory organ Ducts and structures that carry urine from tubules out of kidney and body (ureter bladder urethra). 9

10 Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Renal artery brings blood to kidneys (Kidneys receive 25% of blood exiting heart). Renal vein drains filtered blood away. Filtrate is excreted as urine. Aorta Posterior vena cava Renal artery and vein Ureter Urinary bladder Urethra Kidney Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion Nephron the main functional unit of the kidney, consisting of a renal tubule and the surrounding blood vessels. (millions per kidney) 80% of nephrons are in the cortex (cortical nephrons) 20% are juxtamedullary (extend into the medulla) 10

11 Concept 40.3 Excretory Systems Produce Urine by Filtration, Reabsorption, and Secretion A nephron begins with Bowman s capsule, which encloses the glomerulus. Filtration occurs when blood pressure drives water and solutes through fenestrations in glomerular capillaries and into filtration slits in Bowman s capsule. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The proximal convoluted tubule actively transports glucose, amino acids, and other valuable solutes, water follows. In mammalian kidneys, each nephron has a specialized feature which helps conserve water: the loop of Henle. 11

12 Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Tubule fluid flows in opposite directions in the two limbs of a loop of Henle countercurrent multiplier. The loop creates an osmolarity (concentration) gradient in the extracellular fluid in the medulla. This gradient allows urine to become concentrated in the collecting duct. Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Thin descending limb more permeable to than. Loses water to the neighboring interstitial fluid with high Na + and Cl concentration Key Active transport Passive transport CORTEX OUTER MEDULLA INNER MEDULLA Glucose, amino acids, and other valuable solutes Osmolarity of interstitial fluid (mosm/l) ,200 12

13 Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Thin ascending limb diffusion of Na + and Cl into the interstitial fluid Thick ascending limb actively transports Na + (Cl follows) to interstitial fluid Key Active transport Passive transport CORTEX OUTER MEDULLA INNER MEDULLA Glucose, amino acids, and other valuable solutes , Various ions & other compounds Osmolarity of interstitial fluid (mosm/l) ,200 Concept 40.4 The Mammalian Kidney Produces Concentrated Urine Fluid flows down collecting duct and loses water to interstitial fluid because of concentration gradient established by loops of Henle. Key Active transport Passive transport CORTEX OUTER MEDULLA INNER MEDULLA Glucose, amino acids, and other valuable solutes , Various ions & other compounds Urea Urea Urea 600 1,200 Osmolarity of interstitial fluid (mosm/l) ,200 13

14 9/20/13 Concept 40.4 The Mammalian Kidney Produces Concentrated Urine The juxtamedullary nephron is a key adaptation to terrestrial life, particularly for endotherms with higher metabolic demands. Mammals: longer loops in drier environments Birds: Also use loops, but loops not as long. Main adaptation to conserve water is uric acid paste. Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition Kidney function is under hormonal control. ADH (antidiuretic hormone) released by the posterior pituitary increases the permeability of membranes to water. Released in response to high osmolarity of blood or low blood pressure. 14

15 Concept 40.5 The Kidney Is Regulated to Maintain Blood Pressure, Blood Volume, and Blood Composition If the glomerular filtration rate begins to fall due to low blood pressure, the kidney releases renin which activates angiotensin, which increases blood pressure, in part by stimulating the release of aldosterone from the adrenal cortex. 15