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Osmoregulation and Excretion
Fig. 44-1 Chapter 44 Osmoregulation and Excretion Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment Figure 44.1 How does an albatross drink saltwater without ill effect?
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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 Most animals are stenohaline; they cannot tolerate substantial changes in external osmolarity Euryhaline animals can survive large fluctuations in external osmolarity
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Fig. 44-4 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 Uptake of water and some ions in food Uptake of salt ions by gills Osmotic water gain 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 Excretion of large amounts of water in dilute urine from kidneys Figure 44.4 Osmoregulation in marine and freshwater bony fishes: a comparison (a) Osmoregulation in a saltwater fish (b) Osmoregulation in a freshwater fish
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anhydrobiosis 100 µm 100 µm (a) Hydrated tardigrade (b) Dehydrated
Fig. 44-5 anhydrobiosis 100 µm 100 µm Figure 44.5 Anhydrobiosis (a) Hydrated tardigrade (b) Dehydrated tardigrade
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Land Animals Water balance in a kangaroo rat (2 mL/day) Water
Fig. 44-6 Water balance in a kangaroo rat (2 mL/day) Water balance in a human (2,500 mL/day) Land Animals Ingested in food (0.2) Ingested in food (750) Ingested in liquid (1,500) Water gain (mL) Derived from metabolism (1.8) Derived from metabolism (250) Figure 44.6 Water balance in two terrestrial mammals Feces (0.09) Feces (100) Water loss (mL) Urine (0.45) Urine (1,500) Evaporation (1.46) Evaporation (900)
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Transport Epithelia in Osmoregulation
Animals regulate the composition of body fluid that bathes their cells Transport epithelia are specialized epithelial cells that regulate solute movement They are essential components of osmotic regulation and metabolic waste disposal They are arranged in complex tubular networks An example is in salt glands of marine birds, which remove excess sodium chloride from the blood
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EXPERIMENT Nasal salt gland Nostril with salt secretions Ducts
Fig. 44-7 EXPERIMENT Nasal salt gland Ducts Nostril with salt secretions Figure 44.7 How do seabirds eliminate excess salt from their bodies?
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Countercurrent exchange
Fig. 44-8 Vein Artery Secretory tubule Secretory cell Salt gland Capillary Secretory tubule Transport epithelium NaCl NaCl Direction of salt movement Figure 44.8 Countercurrent exchange in salt-excreting nasal glands Central duct Blood flow Salt secretion (a) (b) Countercurrent exchange
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The type and quantity of an animal’s waste products may greatly affect its water balance
Among the most important wastes are nitrogenous breakdown products of proteins and nucleic acids Some animals convert toxic ammonia (NH3) to less toxic compounds prior to excretion
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Concept 44.2: An animal’s Proteins Nucleic acids Amino acids
Fig. 44-9 Proteins Nucleic acids Concept 44.2: An animal’s nitrogenous wastes reflect its phylogeny and habitat Amino acids Nitrogenous bases Amino groups Most aquatic animals, including most bony fishes Mammals, most amphibians, sharks, some bony fishes Many reptiles (including birds), insects, land snails Figure 44.9 Nitrogenous wastes Ammonia Urea Uric acid
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Key functions of most excretory systems:
Capillary Filtration: pressure-filtering of body fluids Filtrate Excretory tubule reclaiming valuable solutes Reabsorption Figure Key functions of excretory systems: an overview Secretion adding toxins and other solutes from the body fluids to the filtrate Urine Fig Excretion removing the filtrate from the system
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Nucleus of cap cell Cilia Flame bulb Interstitial fluid flow
Fig Nucleus of cap cell Cilia Flame bulb Interstitial fluid flow Tubule Opening in body wall Figure Protonephridia: the flame bulb system of a planarian Tubules of protonephridia Tubule cell
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Coelom Capillary network Internal opening Components of
Fig Coelom Capillary network Components of a metanephridium: Internal opening Figure Metanephridia of an earthworm Collecting tubule Bladder External opening
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Malpighian tubules Salt, water, and nitrogenous wastes
Fig Digestive tract Rectum Hindgut Intestine Midgut (stomach) Malpighian tubules Salt, water, and nitrogenous wastes Feces and urine Figure Malpighian tubules of insects Rectum Reabsorption HEMOLYMPH
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Structure of the Mammalian Excretory System
The mammalian excretory system centers on paired kidneys, which are also the principal site of water balance and salt regulation Each kidney is supplied with blood by a renal artery and drained by a renal vein Urine exits each kidney through a duct called the ureter Both ureters drain into a common urinary bladder, and urine is expelled through a urethra
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Cortical nephron Juxtamedullary nephron Fig. 44-14 Renal medulla
Posterior vena cava Renal artery and vein Renal cortex Kidney Aorta Renal pelvis Ureter Urinary bladder Ureter Urethra (a) Excretory organs and major associated blood vessels (b) Kidney structure Section of kidney from a rat 4 mm Afferent arteriole from renal artery Cortical nephron Glomerulus Bowman’s capsule Juxtamedullary nephron 10 µm SEM Proximal tubule Peritubular capillaries Renal cortex Figure The mammalian excretory system Efferent arteriole from glomerulus Collecting duct Distal tubule Renal medulla Branch of renal vein Collecting duct To renal pelvis Descending limb Loop of Henle Ascending limb (c) Nephron types Vasa recta (d) Filtrate and blood flow
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Renal medulla Renal cortex Renal pelvis Ureter Section of kidney
Fig b Renal medulla Renal cortex Renal pelvis Figure 44.14b The mammalian excretory system Ureter Section of kidney from a rat (b) Kidney structure 4 mm
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Figure 44.14cd The mammalian excretory system
Fig cd Afferent arteriole from renal artery Glomerulus Juxtamedullary nephron Cortical nephron 10 µm Bowman’s capsule SEM Proximal tubule Peritubular capillaries Renal cortex Efferent arteriole from glomerulus Collecting duct Distal tubule Renal medulla Branch of renal vein Collecting duct Descending limb To renal pelvis Figure 44.14cd The mammalian excretory system Loop of Henle Ascending limb Vasa recta (c) Nephron types (d) Filtrate and blood flow
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The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus Bowman’s capsule surrounds and receives filtrate from the glomerulus
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Juxtamedullary Cortical nephron nephron Renal cortex Collecting duct
Fig c Juxtamedullary nephron Cortical nephron Renal cortex Collecting duct Figure 44.14c The mammalian excretory system Renal medulla To renal pelvis (c) Nephron types
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10 µm Afferent arteriole from renal artery Glomerulus Bowman’s capsule
Fig d Afferent arteriole from renal artery Glomerulus 10 µm Bowman’s capsule SEM Proximal tubule Peritubular capillaries Efferent arteriole from glomerulus Distal tubule Branch of renal vein Collecting duct Descending limb Figure 44.14d The mammalian excretory system Loop of Henle Ascending limb Vasa recta (d) Filtrate and blood flow
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Filtration of the Blood
Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule Filtration of small molecules is nonselective The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules
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Pathway of the Filtrate
From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule, the loop of Henle, and the distal tubule Fluid from several nephrons flows into a collecting duct, all of which lead to the renal pelvis, which is drained by the ureter Cortical nephrons are confined to the renal cortex, while juxtamedullary nephrons have loops of Henle that descend into the renal medulla
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Blood Vessels Associated with the Nephrons
Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that divides into the capillaries The capillaries converge as they leave the glomerulus, forming an efferent arteriole The vessels divide again, forming the peritubular capillaries, which surround the proximal and distal tubules Vasa recta are capillaries that serve the loop of Henle The vasa recta and the loop of Henle function as a countercurrent system
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Concept 44.4: The nephron is organized for stepwise processing of blood filtrate
The mammalian kidney conserves water by producing urine that is much more concentrated than body fluids
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NaCl H2O NaCl H2O Salts HCO3- H+ Urea Glucose Amino acid Some drugs
Fig Proximal tubule Distal tubule H2O Salts HCO3- H+ Urea Glucose Amino acid Some drugs NaCl Nutrients H2O HCO3– H2O K+ NaCl HCO3– H+ NH3 K+ H+ Filtrate CORTEX Loop of Henle NaCl H2O OUTER MEDULLA NaCl NaCl Collecting duct Figure The nephron and collecting duct: regional functions of the transport epithelium Key Urea Active transport NaCl H2O Passive transport INNER MEDULLA
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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 secreted into the filtrate The filtrate volume decreases
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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 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 The thin segment: NaCl diffuses out to help maintain the osmolarity of the interstitial fluid The thick segment: actively transports NaCl into the interstitial fluid
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Distal Tubule Collecting Duct
The distal tubule regulates the K+ and NaCl concentrations of body fluids The controlled movement of ions contributes to pH regulation Collecting Duct The collecting duct carries filtrate through the medulla to the renal pelvis Water is lost as well as some salt and urea, and the filtrate becomes more concentrated Urine is hyperosmotic to body fluids
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Solute Gradients and Water Conservation
Urine is much more concentrated than blood The osmolarity of blood is about 300 mOsm/L Urine: 1,200mOsm/L, Australian hopping mice:9,300mOsm/L The cooperative action and precise arrangement of the loops of Henle and collecting ducts are largely responsible for the osmotic gradient that concentrates the urine NaCl (the loop of Henle) and urea (the collecting duct) contribute to the osmolarity of the interstitial fluid, which causes reabsorption of water in the kidney and concentrates the urine
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To maximize oxygen absorption by fish gills
The countercurrent mechanisms involve passive movement Fluid flow through gill filament Oxygen-poor blood Anatomy of gills Oxygen-rich blood Gill arch Lamella Gill arch Gill filament organization Blood vessels Water flow Operculum Water flow between lamellae Blood flow through capillaries in lamella To maximize oxygen absorption by fish gills Figure The structure and function of fish gills Countercurrent exchange PO2 (mm Hg) in water 150 120 90 60 30 Gill filaments Net diffu- sion of O2 from water to blood 140 110 80 50 20 PO2 (mm Hg) in blood Fig
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The countercurrent mechanisms involve passive movement
Canada goose Bottlenose dolphin Blood flow Artery Vein Vein Artery Figure Countercurrent heat exchangers 35ºC 33º 30º 27º 20º 18º 10º 9º To reduce heat loss in endotherms Fig
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countercurrent multiplier systems
The countercurrent system involvig the loop of Henle expends energy to actively transport NaCl from the filtrate in the upper part of the ascending limb of the loop. To expend energy to create concentration gradients, are called countercurrent multiplier systems.
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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 Considerable energy is expended to maintain the osmotic gradient between the medulla and cortex This system allows the vasa recta to supply the kidney with nutrients, without interfering with the osmolarity gradient Descending vessel conveys blood toward the inner medulla: water lost and NaCl isgained by diffusion. Ascending vessel: these fluxess are reversed
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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
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Fig Osmolarity of interstitial fluid (mOsm/L) 300 300 300 300 H2O CORTEX 400 400 H2O H2O H2O OUTER MEDULLA 600 600 Figure How the human kidney concentrates urine: the two-solute model H2O H2O 900 900 Key H2O Active transport INNER MEDULLA 1,200 Passive transport 1,200
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Fig Osmolarity of interstitial fluid (mOsm/L) 300 300 300 100 100 300 H2O NaCl CORTEX 400 200 400 H2O NaCl H2O NaCl H2O NaCl OUTER MEDULLA 600 400 600 Figure How the human kidney concentrates urine: the two-solute model H2O NaCl H2O NaCl 900 700 900 Key H2O NaCl Active transport INNER MEDULLA 1,200 Passive transport 1,200
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Fig Osmolarity of interstitial fluid (mOsm/L) 300 300 300 100 100 300 300 H2O NaCl H2O CORTEX 400 200 400 400 H2O NaCl H2O NaCl H2O NaCl H2O NaCl H2O NaCl H2O OUTER MEDULLA 600 400 600 600 Figure How the human kidney concentrates urine: the two-solute model H2O NaCl H2O Urea H2O NaCl H2O 900 700 900 Key Urea H2O NaCl H2O Active transport INNER MEDULLA Urea 1,200 1,200 Passive transport 1,200
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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 Mammals The juxtamedullary nephron contributes 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
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Concept 44.5: Hormonal circuits link kidney function, water balance, and blood pressure
Mammals control the volume and osmolarity of urine The kidneys of the South American vampire bat can produce either very dilute or very concentrated urine This allows the bats to reduce their body weight rapidly or digest large amounts of protein while conserving water
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Antidiuretic Hormone (vasopressin)
The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney An increase in osmolarity ADHincrease in the number of aquaporin molecules in the membranes of collecting duct cells. Diabetes insipidus
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Fig COLLECTING DUCT LUMEN Osmoreceptors in hypothalamus trigger release of ADH. INTERSTITIAL FLUID Thirst Hypothalamus COLLECTING DUCT CELL ADH ADH receptor Drinking reduces blood osmolarity to set point. cAMP ADH Second messenger signaling molecule Pituitary gland Increased permeability Storage vesicle Distal tubule Exocytosis Aquaporin water channels H2O H2O reab- sorption helps prevent further osmolarity increase. STIMULUS: Increase in blood osmolarity H2O Figure Regulation of fluid retention by antidiuretic hormone (ADH) Collecting duct (b) Homeostasis: Blood osmolarity (300 mOsm/L) (a)
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Fig a-1 Osmoreceptors in hypothalamus trigger release of ADH. Thirst Hypothalamus ADH Pituitary gland STIMULUS: Increase in blood osmolarity Figure 44.19a Regulation of fluid retention by antidiuretic hormone (ADH) Homeostasis: Blood osmolarity (300 mOsm/L) (a)
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Fig a-2 Osmoreceptors in hypothalamus trigger release of ADH. Thirst Hypothalamus Drinking reduces blood osmolarity to set point. ADH Pituitary gland Increased permeability Distal tubule H2O reab- sorption helps prevent further osmolarity increase. STIMULUS: Increase in blood osmolarity Figure 44.19a Regulation of fluid retention by antidiuretic hormone (ADH) Collecting duct Homeostasis: Blood osmolarity (300 mOsm/L) (a)
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2.Alcohol is a diuretic as it inhibits the release of ADH
Fig b 1.Mutation in ADH production causes severe dehydration and results in diabetes insipidus 2.Alcohol is a diuretic as it inhibits the release of ADH COLLECTING DUCT LUMEN INTERSTITIAL FLUID COLLECTING DUCT CELL ADH ADH receptor cAMP Second messenger signaling molecule Storage vesicle Figure 44.19b Regulation of fluid retention by antidiuretic hormone (ADH) Exocytosis Aquaporin water channels H2O H2O (b)
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EXPERIMENT RESULTS Fig. 44-20
Prepare copies of human aqua- porin genes. Aquaporin gene Promoter Synthesize RNA transcripts. Mutant 1 Mutant 2 Wild type H2O (control) Inject RNA into frog oocytes. Transfer to 10 mOsm solution. Aquaporin protein Figure Can aquaporin mutations cause diabetes insipidus? RESULTS Injected RNA Permeability (µm/s) Wild-type aquaporin 196 None 20 Aquaporin mutant 1 17 Aquaporin mutant 2 18
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EXPERIMENT Prepare copies of human aqua- porin genes. Aquaporin gene
Fig a EXPERIMENT Prepare copies of human aqua- porin genes. Aquaporin gene Promoter Synthesize RNA transcripts. Mutant 1 Mutant 2 Wild type H2O (control) Inject RNA into frog oocytes. Figure Can aquaporin mutations cause diabetes insipidus? Transfer to 10 mOsm solution. Aquaporin protein
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RESULTS Injected RNA Permeability (µm/s) Wild-type aquaporin 196 None
Fig b RESULTS Injected RNA Permeability (µm/s) Wild-type aquaporin 196 None 20 Aquaporin mutant 1 17 Figure Can aquaporin mutations cause diabetes insipidus? Aquaporin mutant 2 18
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The Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus (JGA) to release the enzyme renin Renin triggers the formation of the peptide angiotensin II Angiotensin II Raises blood pressure and decreases blood flow to the kidneys Stimulates the release of the hormone aldosterone, which increases blood volume and pressure
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Fig Distal tubule Renin Juxtaglomerular apparatus (JGA) STIMULUS: Low blood volume or blood pressure Figure Regulation of blood volume and pressure by the renin-angiotensin-aldosterone system (RAAS) Homeostasis: Blood pressure, volume
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Fig Liver Distal tubule Angiotensinogen Renin Angiotensin I Juxtaglomerular apparatus (JGA) ACE Angiotensin II STIMULUS: Low blood volume or blood pressure Figure Regulation of blood volume and pressure by the renin-angiotensin-aldosterone system (RAAS) Homeostasis: Blood pressure, volume
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Low blood volume or blood pressure Angiotensin converting enzyme
Fig Liver Angiotensinogen Distal tubule Renin Angiotensin I Angiotensin converting enzyme Juxtaglomerular apparatus (JGA) ACE Angiotensin II STIMULUS: Low blood volume or blood pressure (due to dehydration or blood loss) Adrenal gland Figure Regulation of blood volume and pressure by the renin-angiotensin-aldosterone system (RAAS) Aldosterone Arteriole constriction Increased Na+ and H2O reabsorption in distal tubules Homeostasis: Blood pressure, volume
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Homeostatic Regulation of the Kidney
ADH and RAAS both increase water reabsorption, but only RAAS will respond to a decrease in blood volume A situation that causes an excessive loss of both salt and body fluids—a major wound, for example, or severe diarrhea will reduce blood volume without increasing osmolarity. This will not affect ADH release, but the RAAS will respond to the drop in blood volume and pressure by increasing water and Na+ reabsorption. Another hormone, atrial natriuretic peptide (ANP), opposes the RAAS
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ANP ANP is released in response to an increase in blood volume and pressure and the release of renin ANP( the wall of the atria of the heart)--> inhibits renin from the JGA Inhibits NaCl reabsorption by the collecting ducts and reduces aldosterone release from the adrenal glands lower blood volume and pressure Thus, ADH, the RASS, and ANP are provided to control the osmolarity, salt concentration, volume, and pressure of blood
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Conn’s syndrome? A tumor in the adrenal cortex high amounts of aldosterone The major symptom?
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