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52 Cards in this Set
- Front
- Back
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What are the functions of the excretory systems? |
-Disposal of nitrogenous wastes & other metabolites -Maintenance of proper plasma water volume -Maintenance of osmotic balance -Maintenance of proper internal levels of inorganic solutes (including CO2 & HCO3-) |
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Explain the evolution of excretory function. |
-Simple, aquatic animals rely on diffusion & cellular transport -Next, aquatic animals evolved specialized excretory tissues with transport epithelia -Larger aquatic & all terrestrial animals evolved specialized tubule-containing organs lined with these transport epithelia (including animals that "returned" to aquatic life like dolphins) |
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Nitrogenous wastes are the result of what? |
The metabolism of amino & nucleic acids. |
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Nitrogen waste handling is related to what? |
Water availability |
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Describe nitrogen waste handling for most water-ventilating aquatic animals. |
Primary Form: Ammonia Advantage: Natural by-product; No energy used for synthesis Disadvantage: Highly toxic (Diluted to non-toxic concentrations in these aquatics) |
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Describe nitrogen waste handling for mammals, adult amphibians, sharks, and rays. |
Primary form: Urea Advantage: Least toxic; Semi-dry (less water loss) Disadvantage: More expensive metabolically |
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Describe nitrogen waste handling for insects, most reptiles, & birds. |
Primary form: Uric acid Advantage: Driest Disadvantage: Most expensive metabolically; Intermediate toxicity |
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What are the four processes in renal excretory organs? |
-Filtration: Small solutes & fluids pass through capillaries while cells & large molecules remain behind (Filtrate formed) -Secretion: Solutes are transported from renal tissues and/or blood across tubule epithelia into the tubule for excretion -Reabsorption: Solutes are transported from tubule fluid across epithelia back into the renal tissues and/or blood (Water may move with solutes by osmosis, if permeable) -Osmoconcentration: Water is transported from tubule fluid across epithelia back into the renal tissues and/or blood while solutes are left behind concentrating the tubular fluid |
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What is the first step of epithelia salt & water transport? |
-Sodium enters the apical cell side (side facing fluid being filtered) by diffusing through ENaC channels -Chlorine builds up outside and follows the charge gradient through an apical channel (ClC or CFTR channels) -Water follows through aquaporin channels (H2O follows Na) |
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Steps of epithelia salt & water transport? |
-Na+/K+ ATPase pumps on basolateral membranes -Lowers [Na+] inside the cell by pumping Na+ into the interstitial tissue -Maintains Na+ gradient for diffusion through ENaCs |
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What is the urine-forming organ for all vertebrates? |
The kidney which are: -Paired organs along the dorsal side of the abdominal cavity on each side of the vertebral column -Their blood is supplied by the renal artery, which exits through the renal vein -Urine drains into the renal pelvis & ureters to the bladder |
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What structures make up the kidneys? |
Small functional units called nephrons (~1 million per human kidney) |
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What is the outer portion of the kidney called and what is it's osmotic relation to blood? |
The renal cortex which is isosmotic to blood |
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What is the inner portion of the kidney called and describe it's concentration? |
The renal medulla which has a high solute concentration to promote reabsorption -In larger mammals, the medulla is divided into renal pyramids |
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Nephrons fill the inner & outer portions of the kidney which allows what in regards to orientation and arrangement? |
Creates a series of filtration apparatuses with attached filtrate-carrying tubules & associated small blood vessels |
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Describe the orientation & arrangement of the nephron in regards to vasculature. |
-The afferent arteriole supplies each nephron -The glomerulus is a ball-like tuft of filtering capillaries in the cortex -The efferent arteriole drains the glomerulus -The peritubular capillaries surround the nephron tubule and supply/exchange materials with the surrounding tissue (the tissue also exchanges with fluid in the nephron tubule) |
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Describe the orientation & arrangement of the nephron in regards to the nephron tubule and it's other discrete sections. |
-The Bowman's capsule surrounds the glomerulus & collects filtrates -The proximal tubule (longest part) promotes reabsorption of water, glucose, & ions (some secretion) -The loop of Henle reabsorbs water & solutes separately (The descending limb plunges into the medulla while the ascending limb returns to the cortex) -The distal tubule promotes secretion & reabsorption of ions as needed -The collecting duct promotes osmoconcentration of urine |
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What are osmoconforming Elasmobranchs and what are their adaptations in regards to osmotic challenges? |
-Cartilaginous fish that are isosmotic or hyperosmotic relative to seawater -They retain urea & trimethylamine oxide (TMAO) as osmolytes -The rectal gland excretes a hypertonic fluid high in NaCl |
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What are osmoregulating marine teleosts and what are their adaptations in regards to osmotic challenges? |
-They are bony fish that are hypo-osmotic relative to seawater -They drink seawater to reverse water loss through the gills -The gills actively pump out NaCl & ammonia -The kidneys remove excess ions & excrete scant, concentrated urine |
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What are osmoregulating freshwater teleosts and what are their adaptations in regards to osmotic challenges? |
-They are bony fish that are hyperosmotic to freshwater -They take in water through the gills & the mouth -The gills take in NaCl while excreting ammonia -They excrete a large volume of highly dilute urine |
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What are the terrestrial developments of amphibian systems? |
-They demonstrate the transition to life on land -Their lungs cannot excrete waste/regulate salts like gills in early life -Nephrons resemble teleosts but can shut down filtration to near zero if dehydrated -The bladder is a water reservoir in case of dehydration (Salts actively transported back into the body; Arginine vasotocin, AVT, triggers water uptake through aquaporins in the bladder wall) |
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What are the terrestrial developments of avian urinary system (mix of mammalian & reptilian features)? |
-Uric acid is the primary nitrogenous waste (like reptiles) -Many bird nephrons resemble aquatic vertebrates & lack a loop of Henle for water conservation -A few mammalian-type nephrons with very long loops -Marine birds & reptiles also have nasal salt glands located near the eyes to excrete salt |
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What are the types of terrestrial nephrons and what is their major difference? |
-Cortical (short loop) -Juxtamedullary (long loop) |
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Describe cortical nephrons. |
-Have glomeruli in the outer cortex -Have short loops of Henle that dip only into the outer medulla |
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Describe juxtamedullary nephrons. |
-Have glomeruli in the inner cortex near the medulla -Have long loops that plunge deep into the inner medulla -Have peritubular capillaries that form hairpin vascular loops called vasa recta around the long loops |
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Describe filtration at the glomerulus. |
-The glomerulus is like a molecular sieve for movement of solutes & water -The glomerular capillary wall consists of a single layer of flattened endothelial cells (Perforated with fenestrations too small for proteins/cells; Materials leaving capillary are called "filtrate") |
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What are the forces regulating glomerular filtration? |
-The glomerular capillary blood pressure is higher than capillary hydrostatic pressure elsewhere (55mmHg vs 37mmHg); (The afferent arteriole is larger in diameter than efferent arteriole & most standard arterioles) -The standard plasma colloid osmotic pressure opposes filtration (30mmHg); (In healthy kidneys, proteins remain in the blood) -The capsular hydrostatic pressure is the pressure exerted by filtrate & is higher than standard tissue pressure (15mmHg) -NFP = 55 - (30 +15) = 10mmHg; (High glomerular capillary blood pressure is the key driving force) |
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Describe the glomerular filtration rate (GFR). |
-The GFR depends on the NFP, surface area, & permeability of the filtration membrane in the glomerulus -GFR = NFP x Filtration Co-efficient (Kf) -Kf is based on surface area & permeability & is usually close to 1 (So GFR ~ NFP in healthy individuals) -In disease states, including inflammation & fibrosis (scarring), the coefficient changes causing oliguria or polyuria (increased urine production) -20% of plasma that enters the glomerulus is filtered (GFR in an adult human is 115-125 mL/min or ~180 L/day) |
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Describe autonomic control of NFP. |
-Autoregulation by local factors prevents sporadic shifts in GFR with extrinsic controls by ANS -High resistance in the afferent arteriole will decrease blood flow, BP, & thus GFR (Vasoconstriction by high sympathetic tone; Vasoconstriction by high vasopressin and/or high angiotensin II; Compensates for drops in blood pressure by restoring blood volume) -Low resistance in afferent arteriole with increase blood flow, BP, & thus GFR (Vasodilation by decreased sympathetic tone; Compensates for high blood volume) |
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Describe reabsorption from the nephron tubules. |
-Tubular reabsorption is highly selective -Most mammals' kidneys reabsorb all energy nutrients & 99% of filtered salts & water while excreting wastes -Filtered substances must pass through (or occasionally between) tubular epithelial cells, interstitial fluid, & capillary walls |
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Reabsorption from nephron tubules may be... |
-Passive -Active -Some combo of the two |
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Where does reabsorption of most substances (75% of solutes) occur? |
The proximal convoluted tubule (PCT) |
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80% of kidney energy powers what? |
Na+ transport -Na+ reabsorption is active in most sections -Reabsorption of Na+ helps transport other molecules (e.g. passive Cl- flow, glucose & amino co-transport, H2O by osmosis, etc.) |
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~2/3rds of filtered Na+ is reabsorbed through what? |
The proximal convoluted tubule (PCT) |
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~1/4th of filtered Na+ is reabsorbed through what? |
The ascending limb of the loop of Henle (The descending limb is not Na+ permeable) |
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A max of 10% (varies with hormone levels) of filtered Na+ is reabsorbed through what? |
Distal convoluted tubule (DCT) |
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Describe glucose & amino acid reabsorption. |
-100% reabsorbed at normal blood concentrations (Secondary active co-transport with Na+ powered indirectly by basolateral Na+/K+ ATPase; Glucose/aminos transported into ECF by facilitated diffusion -All actively reabsorbed substances (except Na+) exhibit a transport maximum (Tm) aka a saturation point (Plasma membrane carriers exhibit saturation) -Diabetes mellitus (means "sweet through the urine"); Hyperglycemia leads to glycosuria; Filtered load exceeds Tm and excess spills over into urine |
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Water reabsorption occurs exclusively through what? |
Passive osmosis -Obligatory reabsorption in PCT & ascending limb of the loop of Henle -Reabsorption amounts from DCT & collecting duct (CD) subject to hormonal control |
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In water reabsorption, H2O passes through what? |
AQPs in the apical & basolateral membranes -AQP-1 in PCT are always expressed -AQP-2 in DCT & CD are regulated by AVP/ADH |
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What is the only reabsorbed waste product? |
Urea -40% of filtered urea is passively reabsorbed -60% excretion is sufficient to outpace urea production & maintain homeostasis -Trace nitrogenous wastes (in mammals), uric acid, & ammonia, are toxic & thus not reabsorbed |
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Describe urea handling in relation to osmotic water reabsorption. |
Urea reabsorption promotes osmotic water reabsorption along the length of the nephron tubules in both cortical & medullary regions -Most urea reabsorption occurs in the distal CD running through the inner medulla |
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Describe tubular secretion. |
Active transfer of selected substances from the peritubular capillaries into the tubular lumen -Supplemental mechansim for elimination of selected compounds from the body H+ secretion is important in regulation of acid-base balance -Takes place in PCT, DCT, & CD using proton pumps (H+ ATPases) & H+/Na+ co-transporters (antiport) -Extent of H+ secretions depends on compensations required to maintain systemic blood pH |
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Describe potassium secretion. |
Plasma K+ concentrations are tightly controlled -Elevated plasma K+ may increase or decrease excitability of nerve & muscle tissue Obligatory active transport reabsorption in PCT Active transport secretion in DCT & CD -Most K+ in urine is derived from controlled K+ secretion in distal parts of the nephron -K+ secretion is coupled to Na+ reabsorption by basolateral Na+/K+ ATPase |
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Describe hydration changes in regards to osmoconcentration. |
-When a mammal is euhydrated, an isotonic urine is produced at a moderate rate (1mL/min in humans) -When overhydrated, kidneys produce a large amount of dilute, hypotonic urine (diuresis & polyuria) -When dehydrated, kidneys produce a small amount of concentrated, hypertonic urine (antidiuresis & oliguria) |
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Describe osmoconcentration changes anatomically. |
Concentration of the urine is modified by the vertical osmotic gradient maintained in the interstitial fluid surrounding the nephrons -Isotonic in cortex -Hypertonic in medulla |
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Describe how the loop of Henle forms a gradient and countercurrent multiplication. |
-Fluid from the PCT is isotonic with blood plasma -Descending limb of the loop is highly permeable to water but not salt (water diffuses out; salt concentrates) -Hairpin turn has low water permeability & is permeable to sodium due to leak channels (salt & a little water diffuse out) -Thick ascending limb actively transports NaCl out of the tubular lumen & is impermeable to water (Pumped salts form hypertonic gradient) |
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Describe countercurrent multiplication. |
Close proximity & countercurrent flow between descending & ascending limbs allow important interactions between them -NaCl is reasbsorbed from the ascending limb by active transport, increasing osmolarity of the interstitial fluid in the upper medullar -Water in the descending limb is driven by osmotic pressure to the concentrated solutes in the upper medulla |
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Describe hormonal regulation of urine output. |
Vasopressin (ADH) controls the usage of the medullary gradient (set up by loop) to regulate urine concentration -Fluid entering the DCT & CD is hypotonic (100 mOsm) -Progressively increasing concentrations (osmoconcentration) of interstitial fluid drives water reabsorption if permeable -Water permeability depends on AVP-dependent expression of AQP-2 channels in the apical membrane (more AVP = more channels = more water reabsorption) |
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How is urine output affected by an underhydration stimulus? |
-High AVP/ADH secretion -High apical AQP-2 -High water reabsorption -High urine concentration -Low urine volume |
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How is urine output affected by an overhydration inhibition? |
-Low AVP/ADH secretion -Low apical AQP-2 -Low water reabsorption -Low urine concentration -High urine volume |
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Weak osmoconcentrators (e.g. beavers) don't have what? |
Long nephron loops |
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Desert-dwelling mammals (e.g. kangaroo rat) have what? |
Almost all justamedullary nephrons with very long loops of Henle increasing osmoconcentration capabilities -Elongated medulla has an exaggerated vertical osmotic gradient |
