Chapter 44:

Osmoregulation & Excretion

1. Osmoregulation

2. Nitrogenous Wastes

3. Excretory Processes

4. Hormonal Control ofOsmoregulation & Excretion

1. Osmoregulation

Balancing Uptake & Loss of Water, Solutes

Osmoregulation is the process of balancing the uptake and loss of water as well maintaining solute concentrations within acceptable levels:

• the key factor in this balance is osmosis, the diffusion of water from high to low concentration across a semi-permeable membrane

Osmosis & Osmolarity

Differences in osmolarity (the total solute concentration) across a membrane permeable only to the water will result in osmosis – the diffusion of water from higher to lower concentration: Solutes Water

Selectively permeablemembrane

Net water flow

Hypoosmotic side:• Lower solute

concentration• Higher free H2O

concentration

Hyperosmotic side:• Higher solute

concentration• Lower free H2O

concentration

The side with higher [solute] will gain

water and the side with lower [solute]

will lose water.

Osmolarity & Aquatic Animals

Some aquatic animals such as marine invertebrates are osmoconformers that are isoosmotic with their surroundings and thus don’t regulate their osmolarity. However most are osmoregulators that expend energy to regulate their osmolarity.

• most aquatic animals are stenohalineand cannot tolerate large fluctuations in the osmolarity of their environment

• some animals such as salmon and bull sharks are euryhaline and can survive large fluctuations in the osmolarity of their environment

Osmoregulation in Marine Fish

Marine bony fish are hypoosmotic to sea water and thus will lose water and take in excess salt. To counter this bony fish:

(a) Osmoregulation in a marine fish

Gain of waterand salt ionsfrom food

Excretionof saltionsfrom gills

Osmotic waterloss throughgills and otherparts ofbody surface

Excretion of salt ions andsmall amounts of water inscanty urine from kidneys

SALT WATER

Gain of waterand salt ionsfrom drinkingseawater

Water

Salt

Key

• ingest sea water

• reduce the amount of water excreted in the urine

• excrete excess salt from gills and kidneys

Osmoregulation in Freshwater Fish

(b) Osmoregulation in a freshwater fish

Gain of waterand some ionsin food

FRESH WATER

Uptakeof saltionsby gills

Osmotic watergain throughgills and otherparts ofbody surface

Excretion of salt ionsand large amounts ofwater in dilute urinefrom kidneys

Water

Salt

Key

Freshwater fish are hyperosmotic to sea water and thus will gain water and lose salt. To counter this freshwater fish:

• excrete large amounts of water from the kidneys

• uptake salt in the gills

• transport epithelia are cell layers specialized for moving solutes in specific directions typically arranged in tubular networks such as the nasal glands of marine birds to excrete excess salt

Transport Epithelia in Osmoregulation

Nasal salt gland

Capillary

Secretory tubule

Transportepithelium

Secretory cell of

transport epithelium

Vein Artery Lumen ofsecretorytubule

Saltions

Bloodflow

Saltsecretion

Nostril with saltsecretions

Central duct

2. Nitrogenous Wastes

uric acid

Forms of Nitrogenous Waste

All animals produce nitrogenous waste in the form of ammonia (NH3) due to the break down of proteins and nucleic acids. Ammonia is quite toxic and thus must be handled in 1 of 2 ways:

ammonia

1. rapid elimination from the body

2. conversion to the less toxic nitrogen compounds urea or uric acid

urea

Proteins Nucleic acids

Nitrogenousbases

Aminoacids

Amino groups

Most aquaticanimals, includingmost bony fishes

Mammals, mostamphibians,sharks, somebony fishes

Many reptiles(including birds),

insects, landsnails

Ammonia Urea Uric acid

• most aquatic animals can rapidly eliminate ammonia into the surrounding water

• mammals, amphibians and some aquatic animals expend energy to convert ammonia to urea which requires more water for excretion than uric acid

• reptiles, birds & insects convert ammonia to uric acid which is more energetically expensive than urea but requires little water to be excreted

3. Excretory Processes

CapillaryFiltration

Excretorytubule

Reabsorption

Secretion

Excretion

Filtra

teU

rine

1

2

3

4

Excretory Systems in General

Excretory systems get rid of nitrogenous waste and osmoregulate through a series of general steps:

FILTRATION – removal of soluble portion of body fluid to form crude filtrate

REABSORPTION – reclaiming water and valuable solutes

SECRETION – transferring additional wastes to the filtrate

EXCRETION – elimination of urine from body

Protonephridia

Tubules ofprotonephridia

INTERSTITIALFLUID

Cilia

Tubule

Flamebulb

Capcell

Tubulecell

Opening inbody wall

Protonephridia are a simple type of excretory system consisting of networks of dead-end tubules that excrete a dilute waste fluid into excretory tubules:

• flame bulbs with cilia move fluid through perforated cap cells that filter out waste into excretory tubules

• found in flatworms

Metanephridia

Metanephridia are excretory organs in segmented worms (annelids), and some arthrophods and molluscs:

Coelom Capillarynetwork

Components of a

metanephridium:

Collecting tubule

Internal opening

Bladder

External opening

• blood filtrate collects in the coelom and is transferred via cilia into a collecting tubule

• reabsorption & secretion across the tubule results in a dilute urine that is released out of the body

Digestive tract

HindgutRectumIntestine

Malpighiantubules

Midgut(stomach)

Salt, water, andnitrogenous

wastes

Feces and urineTo anus

Rectum

Reabsorption

Malpighiantubule

HEMOLYMPH

Malpighian Tubules

In most insects & other terrestrial arthropods, excretion occurs via Malpighian tubules:

• nitrogenous & other wastes are transferred to Malpighian tubules connected to the gut

• waste then passes out of body through the anus

Kidneys

Posteriorvena cava

Excretory Organs

Renal arteryand vein

Aorta

Ureter

Urethra

Urinarybladder

Kidney

Renalcortex

Kidney Structure

Renalmedulla

Renalartery

Renalvein

Ureter

Renal pelvis

• kidneys are the main excretory organs in vertebrates

20

0 µ

m

Afferent arteriolefrom renal artery Glomerulus

Bowman’scapsule

Proximaltubule

Peritubularcapillaries

Distaltubule

Efferentarteriolefrom glomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

VasarectaCollecting

duct

Blood vessels from a human kidney.Arterioles and peritubular capillariesappear pink; glomeruli appear yellow.

Loop ofHenle

Renalmedulla

Renalcortex

Corticalnephron

Juxtamedullarynephron

Nephrons – the Function Unit of the Kidney

Nephron Structure

200 µ

m

Afferent arteriolefrom renal artery Glomerulus

Bowman’scapsule

Proximaltubule

PeritubularcapillariesDistal

tubule

Efferentarteriolefrom glomerulus

Branch ofrenal vein

Descendinglimb

Ascendinglimb

VasarectaCollecting

duct

Blood vessels from a human kidney.Arterioles and peritubular capillariesappear pink; glomeruli appear yellow.

Loop ofHenle

Glomerulus & Bowman’s Capsule

• the crude filtrate

produced

contains salts,

glucose, amino

acids, vitamins,

nitrogenous

wastes, & other

small molecules

Fluid from the blood leaks from the glomerulus into Bowman’s capsule:

The Proximal Tubule

proximal tubule

The following occurs in the proximal tubule:

• reabsorption of water, minerals & nutrients (e.g., glucose)

• some toxins are secreted into the tubule

• these activities result in a more concentrated filtrate

The Descending Loop of Henle

descending limb of the Loop of Henle

The following occurs in the descending limb of the Loop of Henle:

• reabsorption of water by diffusion through aquaporin channels

• interstitial fluid is hyperosmotic relative to filtrate

• further concentrates filtrate

The Ascending Loop of Henle

ascending limb of the Loop of Henle

The following occurs in the ascending limb of the Loop of Henle:

• reabsorption of salt ions

• interstitial fluid is hypoosmoticrelative to filtrate

• filtrate becomes more dilute

The Distal Tubule

distal tubule

The following occurs in the distal tubule:

• Na+, K+ & Cl- and other ions are transported in or out of the filtrate to maintain osmotic balance

• H+ and HCO3- ions are

transported in or out of filtrate to maintain pH balance

The Collecting Duct

collecting duct

The following occurs in the collecting duct:

• reabsorption of water and valuable solutes

• resulting urine is conducted to the renal medulla on its way to the renal pelvis and ureter

Proximal tubule1

2

5

4

3

Distal tubule

Descending limbof loop of Henle

Thick segmentof ascendinglimb

Thin segmentof ascendinglimb

Collectingduct

CORTEX

OUTERMEDULLA

INNERMEDULLA

Active transport

Passive transport

Interstitialfluid

Filtrate

Salts (NaCl and others)

H2O

HCO3

H+

Urea

Glucose, amino acids

Some drugs

H2O

NH3H+

K+H2OHCO3

NaCl Nutrients

NaCl

H2OHCO3

K+ H+

NaCl

NaCl

H2ONaCl

Urea

3

Summary of Nephron Function

300300

300

300

400400

600 600

900900

1,200

1,200

mOsm/L

H2O

H2O

CORTEX

OUTERMEDULLA

ActivetransportPassivetransport H2O

H2O

H2O

H2O

H2OINNERMEDULLA

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

NaCl

H2O

H2O

H2O

H2O

H2O

H2O

H2O

NaCl

Urea

Urea

Urea

NaCl

600

1,200

400

300

200

400

700

100

100

Osmolarity in the Cortex vs Medulla

The osmolarity of the interstitial fluids surrounding the nephron increases from cortex to medulla.

This gradient is responsible for the passive transport of water and solutes in various sections

of the nephron.

4. Hormonal Control of Osmoregulation & Excretion

Blood osmolarityincreases

(such as aftersweating profusely).

NORMALBLOOD OSMOLARITY

(300 mOsm/L)

Collectingduct

Distaltubule

Hypothalamusgenerates thirst.

Posteriorpituitary

ADH

Osmoreceptorsin hypothalamustrigger releaseof ADH.

Drinking ofwater

H2O reabsorption

Hypothalamus

Antidiuretic Hormone

• antidirutichormone or ADH (aka vasopressin) released by the posterior pituitary gland stimulates greater water reabsorption in the kidney blood when osmolarityincreases due to dehydration

…more on ADHADHreceptor

ADH

cAMPSecondmessenger

COLLECTINGDUCT CELL

LUMENCollectingduct

Proteinkinase A

Storagevesicle

Aquaporinwaterchannel

Exocytosis

H2O

H2O

• the effect of ADH is to increase the number of aquaporin water channels on the apical surface of collecting duct epithelial cells

• this results in more water reabsorption from the collecting duct to the interstitial fluid

Renin-Angiotensin-Aldosterone System (RAAS)NORMAL

BLOOD PRESSUREAND VOLUME

Blood pressure or

blood volume drops

(for example, due to

dehydration or blood loss).

Distaltubule

Sensors inJGA detectdecrease.

Juxtaglomerularapparatus (JGA)

JGAreleasesrenin.

Renin

Liver

Angiotensinogen

Angiotensin II

Angiotensin I

ACE

Adrenal glandArterioles constrict.

Aldosterone

Na+ and H2O are reabsorbed.• the RAAS is a

complex hormonal feedback system that increases water reabsorption in the kidney in response decreased blood pressure

1

2

3

4

5

Atrial natriuretic peptide (ANP) inhibits renin release when blood pressure rises.