Osmosis and Osmolarity Cells require a balance between uptake and loss of water Osmolarity, the solute concentration of a solution, determines the movement of water across a selectively permeable membrane If two solutions are isoosmotic, the movement of water is equal in both directions If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic to the hyperosmotic solution © 2011 Pearson Education, Inc.

Figure 44.2 Selectively permeable membrane Solutes Water Net water flow Hyperosmotic side: Hypoosmotic side: Lower free H 2 O concentration Higher solute concentration Higher free H 2 O concentration Lower solute concentration

Osmotic Challenges Osmoconformers, consisting only of some marine animals, are isoosmotic with their surroundings and do not regulate their osmolarity Osmoregulators expend energy to control water uptake and loss in a hyperosmotic or hypoosmotic environment © 2011 Pearson Education, Inc.

Figure 44.3 (a) Osmoregulation in a marine fish (b) Osmoregulation in a freshwater fish Gain of water and salt ions from food Excretion of salt ions from gills Osmotic water loss through gills and other parts of body surface Gain of water and salt ions from drinking seawater Excretion of salt ions and small amounts of water in scanty urine from kidneys Gain of water and some ions in food Uptake of salt ions by gills Osmotic water gain through gills and other parts of body surface Excretion of salt ions and large amounts of water in dilute urine from kidneys Key Water Salt

Water balance in a kangaroo rat (2 mL/day) Water balance in a human (2,500 mL/day) Ingested in food (0.2) Ingested in food (750) Ingested in liquid (1,500) Water gain (mL) Water loss (mL) Derived from metabolism (1.8) Derived from metabolism (250) Feces (0.09) Urine (0.45) Feces (100) Urine (1,500) Evaporation (1.46) Evaporation (900) Figure 44.6

Figure 44.7 Nasal salt gland Ducts Nostril with salt secretions (a) Location of nasal glands in a marine bird (b) Secretory tubules (c) Countercurrent exchange Key Salt movement Blood flow Nasal gland Capillary Secretory tubule Transport epithelium Vein Artery Central duct Secretory cell of transport epithelium Lumen of secretory tubule Salt ions Blood flow Salt secretion

Figure 44.8 Proteins Nucleic acids Amino acids Nitrogenous bases —NH 2 Amino groups Most aquatic animals, including most bony fishes Mammals, most amphibians, sharks, some bony fishes Many reptiles (including birds), insects, land snails Ammonia Urea Uric acid

Concept 44.3: Diverse excretory systems are variations on a tubular theme Excretory systems regulate solute movement between internal fluids and the external environment © 2011 Pearson Education, Inc.

Excretory Processes Most excretory systems produce urine by refining a filtrate derived from body fluids Key functions of most excretory systems –Filtration: Filtering of body fluids –Reabsorption: Reclaiming valuable solutes –Secretion: Adding nonessential solutes and wastes from the body fluids to the filtrate –Excretion: Processed filtrate containing nitrogenous wastes, released from the body © 2011 Pearson Education, Inc.

Figure Capillary Filtration Excretory tubule Reabsorption Secretion Excretion Filtrate Urine

Figure Tubules of protonephridia Tubule Flame bulb Nucleus of cap cell Cilia Interstitial fluid flow Opening in body wall Tubule cell

Figure Components of a metanephridium: Collecting tubule Internal opening Bladder External opening CoelomCapillary network

Digestive tract Midgut (stomach) Malpighian tubules Rectum Intestine Hindgut Salt, water, and nitrogenous wastes Feces and urine Malpighian tubule To anus Rectum Reabsorption HEMOLYMPH Figure 44.13

Figure a Excretory Organs Kidney StructureNephron Types Posterior vena cava Renal artery and vein Aorta Ureter Urinary bladder Urethra Kidney Renal cortex Renal medulla Renal artery Renal vein Ureter Cortical nephron Juxtamedullary nephron Renal pelvis Renal cortex Renal medulla

Nephron Organization Afferent arteriole from renal artery Glomerulus Bowman’s capsule Proximal tubule Peritubular capillaries Distal tubule Efferent arteriole from glomerulus Collecting duct Branch of renal vein Vasa recta Descending limb Ascending limb Loop of Henle 200  m Blood vessels from a human kidney. Arterioles and peritubular capillaries appear pink; glomeruli appear yellow. Figure b

Concept 44.4: The nephron is organized for stepwise processing of blood filtrate The filtrate produced in Bowman’s capsule contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules © 2011 Pearson Education, Inc.

From Blood Filtrate to Urine: A Closer Look Proximal Tubule Reabsorption of ions, water, and nutrients takes place in the proximal tubule Molecules are transported actively and passively from the filtrate into the interstitial fluid and then capillaries Some toxic materials are actively secreted into the filtrate As the filtrate passes through the proximal tubule, materials to be excreted become concentrated © 2011 Pearson Education, Inc.

Animation: Bowman’s Capsule and Proximal Tubule Right-click slide / select “Play”

Proximal tubule Distal tubule Filtrate CORTEX Loop of Henle OUTER MEDULLA INNER MEDULLA Key Active transport Passive transport Collecting duct Nutrients NaCl NH 3 HCO 3  H2OH2O KK HH NaCl H2OH2O HCO 3  KK HH H2OH2O NaCl H2OH2O Urea Figure 44.15

Descending Limb of the Loop of Henle Reabsorption of water continues through channels formed by aquaporin proteins Movement is driven by the high osmolarity of the interstitial fluid, which is hyperosmotic to the filtrate The filtrate becomes increasingly concentrated © 2011 Pearson Education, Inc.

Ascending Limb of the Loop of Henle In the ascending limb of the loop of Henle, salt but not water is able to diffuse from the tubule into the interstitial fluid The filtrate becomes increasingly dilute © 2011 Pearson Education, Inc.

Distal Tubule The distal tubule regulates the K + and NaCl concentrations of body fluids The controlled movement of ions contributes to pH regulation © 2011 Pearson Education, Inc.

Animation: Loop of Henle and Distal Tubule Right-click slide / select “Play”

Collecting Duct The collecting duct carries filtrate through the medulla to the renal pelvis One of the most important tasks is reabsorption of solutes and water Urine is hyperosmotic to body fluids © 2011 Pearson Education, Inc.

Animation: Collecting Duct Right-click slide / select “Play”

The Two-Solute Model In the proximal tubule, filtrate volume decreases, but its osmolarity remains the same The countercurrent multiplier system involving the loop of Henle maintains a high salt concentration in the kidney This system allows the vasa recta to supply the kidney with nutrients, without interfering with the osmolarity gradient Considerable energy is expended to maintain the osmotic gradient between the medulla and cortex © 2011 Pearson Education, Inc.

The collecting duct conducts filtrate through the osmolarity gradient, and more water exits the filtrate by osmosis Urea diffuses out of the collecting duct as it traverses the inner medulla Urea and NaCl form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood © 2011 Pearson Education, Inc.

Osmolarity of interstitial fluid (mOsm/L) 1, Key Active transport Passive transport INNER MEDULLA OUTER MEDULLA CORTEX H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O 1, Figure

Osmolarity of interstitial fluid (mOsm/L) 1, Key Active transport Passive transport INNER MEDULLA OUTER MEDULLA CORTEX NaCl H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O 1, Figure

Osmolarity of interstitial fluid (mOsm/L) Key Active transport Passive transport INNER MEDULLA OUTER MEDULLA CORTEX NaCl H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O H2OH2O Urea 1, Figure

Adaptations of the Vertebrate Kidney to Diverse Environments The form and function of nephrons in various vertebrate classes are related to requirements for osmoregulation in the animal’s habitat © 2011 Pearson Education, Inc.

Mammals The juxtamedullary nephron is key to water conservation in terrestrial animals Mammals that inhabit dry environments have long loops of Henle, while those in fresh water have relatively short loops © 2011 Pearson Education, Inc.

Antidiuretic Hormone The osmolarity of the urine is regulated by nervous and hormonal control Antidiuretic hormone (ADH) makes the collecting duct epithelium more permeable to water An increase in osmolarity triggers the release of ADH, which helps to conserve water © 2011 Pearson Education, Inc.

Figure Thirst Hypothalamus ADH Pituitary gland Osmoreceptors in hypothalamus trigger release of ADH. STIMULUS: Increase in blood osmolarity (for instance, after sweating profusely) Homeostasis: Blood osmolarity (300 mOsm/L) Drinking reduces blood osmolarity to set point. H 2 O reab- sorption helps prevent further osmolarity increase. Increased permeability Distal tubule Collecting duct

© 2011 Pearson Education, Inc. Animation: Effect of ADH Right-click slide / select “Play”

Collecting duct ADH receptor COLLECTING DUCT CELL LUMEN Second-messenger signaling molecule Storage vesicle Aquaporin water channel Exocytosis H2OH2O H2OH2O ADH cAMP Figure 44.20

Mutation in ADH production causes severe dehydration and results in diabetes insipidus Alcohol is a diuretic as it inhibits the release of ADH © 2011 Pearson Education, Inc.

Figure Prepare copies of human aquaporin genes: two mutants plus wild type. Synthesize mRNA. Inject mRNA into frog oocytes. Transfer to 10-mOsm solution and observe results EXPERIMENT RESULTS Aquaporin gene Promoter Mutant 1 Mutant 2Wild type Aquaporin proteins H 2 O (control) Injected RNA Permeability (  m/sec) Wild-type aquaporin None Aquaporin mutant 1 Aquaporin mutant 2

Angiotensinogen Liver JGA releases renin. Renin Distal tubule Juxtaglomerular apparatus (JGA) Angiotensin I ACE Angiotensin II Adrenal gland Aldosterone More Na  and H 2 O are reabsorbed in distal tubules, increasing blood volume. Arterioles constrict, increasing blood pressure. STIMULUS: Low blood volume or blood pressure (for example, due to dehydration or blood loss) Homeostasis: Blood pressure, volume Figure

Homeostatic Regulation of the Kidney ADH and RAAS both increase water reabsorption, but only RAAS will respond to a decrease in blood volume Another hormone, atrial natriuretic peptide (ANP), opposes the RAAS ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin © 2011 Pearson Education, Inc.

pH regulation by Kidney

Figure 44.UN01 Animal Inflow/OutflowUrine Freshwater fish. Lives in water less concentrated than body fluids; fish tends to gain water, lose salt Does not drink water Salt in (active trans- port by gills) H 2 O in Salt out Marine bony fish. Lives in water more concentrated than body fluids; fish tends to lose water, gain salt Terrestrial vertebrate. Terrestrial environment; tends to lose body water to air Drinks water Salt inH 2 O out Salt out (active transport by gills) Salt in (by mouth) H 2 O and salt out Large volume of urine Urine is less concentrated than body fluids Small volume of urine Urine is slightly less concentrated than body fluids Moderate volume of urine Urine is more concentrated than body fluids