44 Excretion Text

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

PowerPoint Lectures for Biology, Seventh Edition

Neil Campbell and Jane Reece

Lectures by Chris Romero

Chapter 44Chapter 44

Osmoregulation and Excretion


• Overview: A balancing act

• 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


• 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


• Osmoregulation

– Regulates solute concentrations and balances the gain and loss of water

• Excretion

– Gets rid of metabolic wastes


• Concept 44.1: Osmoregulation balances the uptake and loss of water and solutes

• Osmoregulation is based largely on controlled movement of solutes

– Between internal fluids and the external environment


Osmosis

• 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


Osmotic Challenges

• Osmoconformers, which are only 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 said to be stenohaline

– And cannot tolerate substantial changes in external osmolarity

• Euryhaline animals

– Can survive large fluctuations in external osmolarity

Figure 44.2


Marine Animals

• Most marine invertebrates are osmoconformers

• Most marine vertebrates and some invertebrates are osmoregulators


• 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

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


Freshwater Animals

• Freshwater animals

– Constantly take in water from their hypoosmotic environment

– Lose salts by diffusion


• 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


Animals That Live in Temporary Waters

• Some aquatic invertebrates living in temporary ponds

– Can lose almost all their body water and survive in a dormant state

• This adaptation is called anhydrobiosis

Figure 44.4a, b (a) Hydrated tardigrade (b) Dehydrated tardigrade

100 µm

100 µm


Land Animals

• Land animals manage their water budgets

– By drinking and eating moist foods and by using metabolic water

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


• Desert animals

– Get major water savings from simple anatomical features

Figure 44.6

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)

Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University 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.

EXPERIMENT

RESULTSRemoving the fur of a camel increased the rateof water loss through sweating by up to 50%.

The fur of camels plays a critical role intheir conserving water in the hot desertenvironments where they live.

CONCLUSION


Transport Epithelia

• Transport epithelia

– Are specialized cells that regulate solute movement

– Are essential components of osmotic regulation and metabolic waste disposal

– Are arranged into complex tubular networks


• An example of transport epithelia is found in the salt glands of marine birds

– Which remove excess sodium chloride from the blood

Figure 44.7a, b

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.


• Concept 44.2: 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


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 ONC

HN

OH

• Among the most important wastes

– Are the nitrogenous breakdown products of proteins and nucleic acids

Figure 44.8


Forms of Nitrogenous Wastes

• Different animals

– Excrete nitrogenous wastes in different forms


Ammonia

• Animals that excrete nitrogenous wastes as ammonia

– Need access to lots of water

– Release it across the whole body surface or through the gills


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


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


The Influence of Evolution and Environment on Nitrogenous Wastes

• The kinds of nitrogenous wastes excreted

– Depend on an animal’s evolutionary history and habitat

• The amount of nitrogenous waste produced

– Is coupled to the animal’s energy budget


• Concept 44.3: Diverse excretory systems are variations on a tubular theme

• Excretory systems

– Regulate solute movement between internal fluids and the external environment


Excretory Processes

• Most excretory systems

– Produce urine by refining a filtrate derived from body fluids

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


• 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


Survey of Excretory Systems

• The systems that perform basic excretory functions

– Vary widely among animal groups

– Are generally built on a complex network of tubules


Nucleusof cap cell

Cilia

Interstitial fluidfilters throughmembrane wherecap cell and tubulecell interdigitate(interlock)

Tubule cell

Flamebulb

Nephridioporein body wall

Tubule

Protonephridia(tubules)

Protonephridia: Flame-Bulb Systems

• A protonephridium

– Is a network of dead-end tubules lacking internal openings

Figure 44.10


• The tubules branch throughout the body

– And the smallest branches are capped by a cellular unit called a flame bulb

• These tubules excrete a dilute fluid

– And function in osmoregulation


Metanephridia

• Each segment of an earthworm

– Has a pair of open-ended metanephridia

Figure 44.11 Nephrostome Metanephridia

Nephridio-pore

Collectingtubule

Bladder

Capillarynetwork

Coelom


• Metanephridia consist of tubules

– That collect coelomic fluid and produce dilute urine for excretion


Digestive tract

Midgut(stomach)

Malpighiantubules

RectumIntestine

Hindgut

Salt, water, and nitrogenous

wastes

Feces and urineAnus

Malpighiantubule

Rectum

Reabsorption of H2O,ions, and valuableorganic molecules

HEMOLYMPH

Malpighian Tubules

• In insects and other terrestrial arthropods, malpighian tubules

– Remove nitrogenous wastes from hemolymph and function in osmoregulation

Figure 44.12


• Insects produce a relatively dry waste matter

– An important adaptation to terrestrial life


Vertebrate Kidneys

• Kidneys, the excretory organs of vertebrates

– Function in both excretion and osmoregulation


• Concept 44.4: Nephrons and associated blood vessels are the functional unit of the mammalian kidney

• 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

Figure 44.13a

Posterior vena cava

Renal artery and vein

Aorta

Ureter

Urinary bladder

Urethra

(a) Excretory organs and major associated blood vessels

Kidney


• Urine exits each kidney

– Through a duct called the ureter

• Both ureters

– Drain into a common urinary bladder


(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


• The nephron, the functional unit of the vertebrate kidney

– Consists of a single long tubule and a ball of capillaries called the glomerulus

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


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

– And the filtrate in Bowman’s capsule is a mixture that mirrors the concentration of various solutes in the blood plasma


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


Blood Vessels Associated with the Nephrons

• Each nephron is supplied with blood by an afferent arteriole

– A branch of the renal artery that subdivides into the capillaries

• The capillaries converge as they leave the glomerulus

– Forming an efferent arteriole

• The vessels subdivide again

– Forming the peritubular capillaries, which surround the proximal and distal tubules


Proximal tubule

Filtrate

H2OSalts (NaCl and others)HCO3

–

H+

UreaGlucose; amino acidsSome drugs

Key

Active transport

Passive transport

CORTEX

OUTERMEDULLA

INNERMEDULLA

Descending limbof loop ofHenle

Thick segmentof ascendinglimb

Thin segmentof ascendinglimb

Collectingduct

NaCl

NaCl

NaCl

Distal tubule

NaCl Nutrients

Urea

H2O

NaClH2O

H2OHCO3 K+

H+ NH3

HCO3

K+ H+

H2O

1 4

32

3 5

From Blood Filtrate to Urine: A Closer Look

• Filtrate becomes urine

– As it flows through the mammalian nephron and collecting duct

Figure 44.14


• Secretion and reabsorption in the proximal tubule

– Substantially alter the volume and composition of filtrate

• Reabsorption of water continues

– As the filtrate moves into the descending limb of the loop of Henle


• As filtrate travels through the ascending limb of the loop of Henle

– Salt diffuses out of the permeable tubule into the interstitial fluid

• The distal tubule

– Plays a key role in regulating the K+ and NaCl concentration of body fluids

• The collecting duct

– Carries the filtrate through the medulla to the renal pelvis and reabsorbs NaCl


• Concept 44.5: 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


Solute Gradients and Water Conservation

• In a mammalian kidney, the cooperative action and precise arrangement of the loops of Henle and the collecting ducts

– Are largely responsible for the osmotic gradient that concentrates the urine


• 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

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


• The countercurrent multiplier system involving the loop of Henle

– Maintains a high salt concentration in the interior of the kidney, which enables the kidney to form concentrated urine


• The collecting duct, permeable to water but not salt

– Conducts the filtrate through the kidney’s 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


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

Figure 44.16a

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.


• The renin-angiotensin-aldosterone system (RAAS)

– Is part of a complex feedback circuit that functions in homeostasis

Figure 44.16b

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.


• Another hormone, atrial natriuretic factor (ANF)

– Opposes the RAAS


• 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

Figure 44.17


• Concept 44.6: Diverse adaptations of the vertebrate kidney have evolved in different environments

• The form and function of nephrons in various vertebrate classes

– Are related primarily to the requirements for osmoregulation in the animal’s habitat


• Exploring environmental adaptations of the vertebrate kidney

Figure 44.18

MAMMALS

Bannertail Kangaroo rat(Dipodomys spectabilis)

Beaver (Castor canadensis)

FRESHWATER FISHES AND AMPHIBIANS

Rainbow trout(Oncorrhynchus mykiss)

Frog (Rana temporaria)

BIRDS AND OTHER REPTILES

Roadrunner(Geococcyx californianus)

Desert iguana(Dipsosaurus dorsalis)

MARINE BONY FISHES

Northern bluefin tuna (Thunnus thynnus)

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44 Excretion Text