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52 Cards in this Set

  • Front
  • Back

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-)

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)

Nitrogenous wastes are the result of what?

The metabolism of amino & nucleic acids.

Nitrogen waste handling is related to what?

Water availability

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)

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

Describe nitrogen waste handling for insects, most reptiles, & birds.

Primary form: Uric acid

Advantage: Driest

Disadvantage: Most expensive metabolically; Intermediate toxicity

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

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)

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

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

What structures make up the kidneys?

Small functional units called nephrons (~1 million per human kidney)

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

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

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

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)

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

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

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

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

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)

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

What are the types of terrestrial nephrons and what is their major difference?

-Cortical (short loop)

-Juxtamedullary (long loop)

Describe cortical nephrons.

-Have glomeruli in the outer cortex

-Have short loops of Henle that dip only into the outer medulla

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

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")

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)

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)

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)

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

Reabsorption from nephron tubules may be...



-Some combo of the two

Where does reabsorption of most substances (75% of solutes) occur?

The proximal convoluted tubule (PCT)

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.)

~2/3rds of filtered Na+ is reabsorbed through what?

The proximal convoluted tubule (PCT)

~1/4th of filtered Na+ is reabsorbed through what?

The ascending limb of the loop of Henle

(The descending limb is not Na+ permeable)

A max of 10% (varies with hormone levels) of filtered Na+ is reabsorbed through what?

Distal convoluted tubule (DCT)

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

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

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

What is the only reabsorbed waste product?


-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

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

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

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

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)

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

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)

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

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)

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

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

Weak osmoconcentrators (e.g. beavers) don't have what?

Long nephron loops

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