copyright © 2005 pearson education, inc. publishing as benjamin cummings osmoregulation and...
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Osmoregulation and excretion
• The physiological systems of animals operate in a fluid environment
• The relative concentrations of water and solutes in this environment must be maintained within fairly narrow limits
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• Freshwater animals show adaptations that reduce water uptake and conserve solutes
• Desert and marine animals face desiccating environments with the potential to quickly deplete the body water
Figure 44.1
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• Osmoregulation regulates solute concentrations and balances the gain and loss of water
• Excretion gets rid of metabolic wastes
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Osmoregulation
• Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment.
• Cells require a balance between osmotic gain and loss of water.
• Water uptake and loss are balanced by various mechanisms of osmoregulation in different environments.
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Osmotic Challenges
• Osmoconformers, which are only marine animals are isoosmotic with [at the same concentration as] their surroundings and do not regulate their osmolarity.
• Osmoregulators expend energy to control water uptake and loss in a hyperosmotic [more concentrated] or hypoosmotic [less concentrated] environment.
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Marine Animals
• Most marine invertebrates are osmoconformers
• Most marine vertebrates and some invertebrates are osmoregulators
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• Marine bony fishes are hypoosmotic to sea water and lose water by osmosis and gain salt by both diffusion and from food they eat
• These fishes balance water loss by drinking seawater and actively excrete salt through their gills. They produce little urine.
Figure 44.3a
Gain of water andsalt ions from foodand by drinkingseawater
Osmotic water lossthrough gills and other partsof body surface
Excretion ofsalt ionsfrom gills
Excretion of salt ionsand small amountsof water in scantyurine from kidneys
(a) Osmoregulation in a saltwater fish
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Freshwater Animals
• Freshwater animals constantly take in water from their hypoosmotic environment
• They lose salts by diffusion
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• Freshwater animals maintain water balance by excreting large amounts of dilute urine
• Salts lost by diffusion are replaced by foods and uptake across the gills
Figure 44.3b
Uptake ofwater and someions in food
Osmotic water gainthrough gills and other partsof body surface
Uptake ofsalt ions by gills
Excretion oflarge amounts ofwater in dilute urine from kidneys
(b) Osmoregulation in a freshwater fish
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Land animals
• Land animals manage their water budgets by drinking and eating moist foods and by using metabolic water.
• Animals adapted to deserts produce highly concentrated urine and use a wide range of behavioral adaptations to avoid the heat (burrowing, nocturnal activity)
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Figure 44.5
Waterbalance in a human
(2,500 mL/day= 100%)
Waterbalance in akangaroo rat
(2 mL/day= 100%)
Ingested in food (0.2)
Ingested in food (750)
Ingested in liquid(1,500)
Derived from metabolism (250)Derived from
metabolism (1.8)
Water gain
Feces (0.9)
Urine(0.45)
Evaporation (1.46)
Feces (100)
Urine(1,500)
Evaporation (900)
Water loss
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Benefits of insulation• Desert animals also get major water savings from simple
anatomical features
• Knut and Bodil Schmidt-Nielsen et al. observed that the fur of camels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the animals’ skin remained more than 30°C cooler.
• The Schmidt-Nielsens reasoned that insulation of the skin by fur may substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they compared the water loss rates of unclipped and clipped camels.
• Removing the fur of a camel increased the rateof water loss through sweating by up to 50%.
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Control group(Unclipped fur)
Experimental group(Clipped fur)
4
3
2
1
0
Wat
er lo
st p
er d
ay(L
/100
kg
body
ma
ss)
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Transport Epithelia
• Transport epithelia are specialized cells that regulate solute movement. The epithelia form a layer of cells across whose selectively permeable membranes solutes must flow.
• These epithelia are essential components of osmotic regulation and metabolic waste disposal and usually are arranged into complex tubular networks.
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• An example of transport epithelia is found in the salt glands of marine birds which remove excess sodium chloride from the blood.
• In birds such as petrels, gulls and albatrosses transport epithelia arranged in tubes in a salt gland actively secrete salt from the blood into collecting ducts [a countercurrent system is used to maximize salt concentration] that drain to the nostrils where the salt solution is shed.
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Nasal salt gland
Nostrilwith saltsecretions
Lumen ofsecretory tubule
NaCl
Bloodflow
Secretory cellof transportepithelium
Centralduct
Directionof saltmovement
Transportepithelium
Secretorytubule
Capillary
Vein
Artery
(a) An albatross’s salt glands empty via a duct into thenostrils, and the salty solution either drips off the tip of the beak or is exhaled in a fine mist.
(b) One of several thousand secretory tubules in a salt-excreting gland. Each tubule is lined by a transportepithelium surrounded by capillaries, and drains intoa central duct.
(c) The secretory cells actively transport salt from theblood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentrationgradient of salt in the tubule (aqua), this countercurrentsystem enhances salt transfer from the blood to the lumen of the tubule.
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Nitrogenous wastes
• An animal’s nitrogenous wastes reflect its phylogeny and habitat
• The type and quantity of an animal’s waste products may have a large impact on its water balance
• Among the most important wastes are the nitrogenous breakdown products of proteins and nucleic acids.
• Different animals excrete nitrogenous wastes in different forms
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Proteins Nucleic acids
Amino acids Nitrogenous bases
–NH2
Amino groups
Most aquaticanimals, includingmost bony fishes
Mammals, mostamphibians, sharks,some bony fishes
Many reptiles(includingbirds), insects,land snails
Ammonia Urea Uric acid
NH3 NH2
NH2
O C
C
CN
CO N
H H
C O
NC
HN
O
H
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Ammonia
• Animals such as fish that excrete nitrogenous wastes as ammonia need access to lots of water.
• Ammonia is released across the whole body surface or through the gills.
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Urea
• The liver of mammals and most adult amphibians converts ammonia to less toxic urea
• Urea is carried to the kidneys, concentrated and excreted with a minimal loss of water
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Uric Acid
• Insects, land snails, and many reptiles, including birds excrete uric acid as their major nitrogenous waste
• Uric acid is largely insoluble in water and can be secreted as a paste with little water loss.
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Excretory Systems
• Diverse excretory systems are variations on a tubular theme
• Most excretory systems produce urine by refining a filtrate derived from body fluids
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• Key functions of most excretory systems are
– Filtration, pressure-filtering of body fluids producing a filtrate
– Reabsorption, reclaiming valuable solutes from the filtrate
– Secretion, addition of toxins and other solutes from the body fluids to the filtrate
– Excretion, the filtrate leaves the system
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Figure 44.9
Filtration. The excretory tubule collects a filtrate from the blood.Water and solutes are forced by blood pressure across the selectively permeable membranes of a cluster of capillaries and into the excretory tubule.
Reabsorption. The transport epithelium reclaims valuable substances from the filtrate and returns them to the body fluids.
Secretion. Other substances, such as toxins and excess ions, are extracted from body fluids and added to the contents of the excretory tubule.
Excretion. The filtrate leaves the system and the body.
Capillary
Excretorytubule
Filtrate
Urine
1
2
3
4
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Vertebrate Kidneys
• Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation.
• The mammalian excretory system centers on paired kidneys which are also the principal site of water balance and salt regulation.
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• Each kidney is supplied with blood by a renal artery and drained by a renal vein.
Posterior vena cava
Renal artery and vein
Aorta
Ureter
Urinary bladder
Urethra
(a) Excretory organs and major associated blood vessels
Kidney
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• Urine exits each kidney through a duct called the ureter.
• Both ureters drain into a common urinary bladder.
• Urine is voided from the bladder via a duct called the urethra.
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(b) Kidney structure
UreterSection of kidney from a rat
Renalmedulla
Renalcortex
Renalpelvis
Figure 44.13b
Structure and Function of the Nephron and Associated Structures
• The mammalian kidney has two distinct regions:an outer renal cortex and an inner renal medulla
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• The nephron is the functional unit of the vertebrate kidney.
• It consists of a single long tubule and a ball of capillaries called the glomerulus.
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Figure 44.13c, d
Juxta-medullarynephron
Corticalnephron
Collectingduct
To renalpelvis
Renalcortex
Renalmedulla
20 µm
Afferentarteriolefrom renalartery
Glomerulus
Bowman’s capsule
Proximal tubule
Peritubularcapillaries
SEM
Efferentarteriole fromglomerulus
Branch ofrenal vein
DescendinglimbAscendinglimb
Loopof
Henle
Distal tubule
Collectingduct
(c) Nephron
Vasarecta
(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 the Bowman’s capsule.
• Filtration of small molecules is nonselective and the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma.
• The filtrate contains water, urea, salts and other small molecules.
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Pathway of the Filtrate
• From the Bowman’s capsule, the filtrate passes through three regions of the nephron the proximal tubule, the loop of Henle, and the distal tubule. [proximal means closest to and distal furthest from in the case the Bowman’s capsule].
• Fluid from several nephrons flows into a collecting duct.
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Proximal tubule
FiltrateH2OSalts (NaCl and others)HCO3
–
H+
UreaGlucose; amino acidsSome drugs
KeyActive transport
Passive transport
CORTEX
OUTERMEDULLA
INNERMEDULLA
Descending limbof loop ofHenle
Thick segmentof ascendinglimb
Thin segmentof ascendinglimb
Collectingduct
NaCl
NaCl
NaCl
Distal tubuleNaCl Nutrients
Urea
H2O
NaClH2O
H2OHCO3 K+
H+ NH3
HCO3
K+ H+
H2O
1 4
32
3 5
Figure 44.14
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Proximal tubule
• Selective secretion and reabsorption in the proximal tubule substantially alter the volume and composition of filtrate from what is initially produced in the Bowman’s capsule.
• In the proximal tubule hydrogen ions (H+) and ammonia (NH3) are secreted into the tubule as are drugs and toxins.
• Nutrients (e.g. glucose and amino acids) are reabsorbed as are potassium (K+), NaCl and water.
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Descending limb of loop of Henle
• Reabsorption of water out of the filtrate continues as the filtrate moves into the descending limb of the loop of Henle, which is very permeable to water, but not very permeable to salts.
• NaCl concentration in the urine thus increases.
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Ascending Limb of Loop of Henle
• The ascending limb of the Loop of Henle in contrast to the descending loop is permeable to salt, but not to water.
• Thus, as filtrate travels through the ascending limb of the loop of Henle, salt diffuses out of the permeable tubule into the interstitial fluid.
• First the movement of salt is passive, but in the upper section of the ascending loop salt is actively pumped out into the interstitial fluid.
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Ascending Limb of Loop of Henle
• The movement of salt out of the ascending loop of Henle contributes to the development of an osmolarity gradient that is important to the kidney’s ability to produce a concentrated urine as we will see later.
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Distal tubule
• The distal tubule plays a key role in regulating the K+ and NaCl concentration of body fluids by regulating the K+ secreted and NaCl reabsorbed.
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Collecting Duct
• The collecting duct carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl
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Water conservation ability of the kidney
• The mammalian kidney’s ability to conserve water is a key terrestrial adaptation
• The mammalian kidney can produce urine much more concentrated than body fluids, thus conserving water
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Solute Gradients and Water Conservation
• In a mammalian kidney, the cooperative action and arrangement of the loops of Henle and the collecting ducts produce an osmotic gradient along the length of the loop of Henle in the kidney.
• Two solutes, NaCl and urea, contribute to the osmolarity of the interstitial fluid which causes the reabsorption of water in the kidney and concentrates the urine
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Figure 44.15
H2O
H2O
H2O
H2O
H2O
H2O
H2O
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
300
300 100
400
600
900
1200
700
400
200
100
Activetransport
Passivetransport
OUTERMEDULLA
INNERMEDULLA
CORTEX
H2O
Urea
H2OUrea
H2O
Urea
H2O
H2O
H2O
H2O
1200
1200
900
600
400
300
600
400
300
Osmolarity of interstitial
fluid(mosm/L)
300
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• Water passing down the descending loop of Henle meets more and more concentrated interstitial fluid and so water passively moves out of the tubule into the kidney.
• Because the ascending loop of Henle is not permeable to water, water does not flow back into the filtrate as it ascends. However, salts are actively pumped out to maintain the concentration gradient.
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Collecting duct
• The collecting duct, is also permeable to water but not salt.
• The collecting duct carries the filtrate through the kidney’s osmolarity gradient, and more water exits the filtrate by osmosis
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Collecting duct
• Urea also diffuses out of the collecting duct as it traverses the inner medulla
• Urea and NaCl together form the osmotic gradient that enables the kidney to produce urine that is hyperosmotic to the blood
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• Because of the countercurrent arrangement of the loop of Henle, the longer the loop the more urine can be concentrated.
• Thus, desert rodents have very long loops of Henle, which enable them to produce urine much more concentrated than humans can produce.
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• The South American vampire bat, which feeds on blood has a unique excretory system in which its kidneys offload much of the water absorbed from a meal by excreting large amounts of dilute urine up to 24% of bodyweight per hour.
Figure 44.17
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Regulation of Kidney Function
• 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
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Osmoreceptorsin hypothalamus
Drinking reducesblood osmolarity
to set point
H2O reab-sorption helpsprevent further
osmolarity increase
STIMULUS:The release of ADH istriggered when osmo-receptor cells in the
hypothalamus detect anincrease in the osmolarity
of the blood
Homeostasis:Blood osmolarity
Hypothalamus
ADH
Pituitarygland
Increasedpermeability
Thirst
Collecting duct
Distaltubule
(a) Antidiuretic hormone (ADH) enhances fluid retention by makingthe kidneys reclaim more water.
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RAAS system
• The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis.
• The juxtoglomerular apparatus located around arterioles supplying glomeruli detect drop in blood pressure. Releases enzyme renin.
• Renin stimulates production of angiotensin II which decreases blood flow to kidney and stimulates proximal tubules to reabsorb more water na dNaCl.
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Increased Na+
and H2O reab-sorption in
distal tubules
Homeostasis:Blood pressure,
volume
STIMULUS:The juxtaglomerular
apparatus (JGA) respondsto low blood volume or
blood pressure (such as dueto dehydration or loss of
blood)
Aldosterone
Adrenal gland
Angiotensin II
Angiotensinogen
Reninproduction
Renin
Arterioleconstriction
Distal tubule
JGA
(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increasein blood volume and pressure.