1. Osmoregulation and Excretion
Chapter 44
Osmoregulation and Excretion

2. Overview: A Balancing Act
Osmoregulation regulates solute concentrations and balances the gain and loss of water
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3. Osmoregulation is based largely on controlled movement of solutes between internal fluids and the external environment
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4. Excretion gets rid of nitrogenous metabolites and other waste products
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5. Osmosis and Osmolarity
Osmolarity, the solute concentration of a solution, determines the movement of water across a selectively permeable membrane
Isoosmotic -the movement of water is equal in both directions
If two solutions differ in osmolarity, the net flow of water is from the hypoosmotic to the hyperosmotic solution
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6. Selectively permeable membrane
Figure 44.2
Selectively permeable membrane
Solutes
Water
Hyperosmotic side:
Hypoosmotic side:
Higher solute concentration •
Lower solute concentration •
Lower free H2O concentration •
Higher free H2O concentration •
Figure 44.2 Solute concentration and osmosis.
Net water flow

7. Osmotic Challenges
Osmoconformers, consisting only of some 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
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8. Marine Animals Most marine invertebrates are osmoconformers
Most marine vertebrates and some invertebrates are osmoregulators
Marine bony fishes are hypoosmotic to sea water
They lose water by osmosis and gain salt by diffusion and from food
They balance water loss by drinking seawater and excreting salts
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9. (a) Osmoregulation in a marine fish
Figure 44.3
(a) Osmoregulation in a marine fish
(b) Osmoregulation in a freshwater fish
Gain of water and salt ions from food
Excretion of salt ions from gills
Osmotic water loss through gills and other parts of body surface
Gain of water and some ions in food
Uptake of salt ions by gills
Osmotic water gain through gills and other parts of body surface
Figure 44.3 Osmoregulation in marine and freshwater bony fishes: a comparison.
Gain of water and salt ions from drinking seawater
Excretion of salt ions and small amounts of water in scanty urine from kidneys
Key
Excretion of salt ions and large amounts of water in dilute urine from kidneys
Water
Salt

10. Land Animals Body coverings
Desert animals get major water savings -nocturnal life style
Land animals maintain water balance by eating moist food and producing water metabolically through cellular respiration
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11. Water balance in a kangaroo rat (2 mL/day)
Figure 44.6
Water balance in a kangaroo rat (2 mL/day)
Water balance in a human (2,500 mL/day)
Ingested in food (0.2)
Ingested in food (750)
Ingested in liquid (1,500)
Water gain (mL)
Derived from metabolism (1.8)
Derived from metabolism (250)
Figure 44.6 Water balance in two terrestrial mammals.
Feces (0.09)
Feces (100)
Water loss (mL)
Urine (0.45)
Urine (1,500)
Evaporation (1.46)
Evaporation (900)

12. Energetics of Osmoregulation
Osmoregulators must expend energy to maintain osmotic gradients
The amount of energy differs based on
surroundings
How easily water and solutes move across the animal’s surface
The work required to pump solutes across the membrane
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13. Transport Epithelia in Osmoregulation
Transport epithelia are epithelial cells that are specialized for moving solutes in specific directions
arranged in complex tubular networks
An example is in nasal glands of marine birds, which remove excess sodium chloride from the blood
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14. Secretory cell of transport epithelium Lumen of secretory tubule Vein
Figure 44.7
Secretory cell of transport epithelium
Lumen of secretory tubule
Vein
Artery
Nasal salt gland
Ducts
Nasal gland
Salt ions
Capillary
Secretory tubule
Transport epithelium
Nostril with salt secretions
(a)
Location of nasal glands in a marine bird
(b)
Secretory tubules
Blood flow
Salt secretion
Figure 44.7 Countercurrent exchange in salt-excreting nasal glands.
Key
(c)
Countercurrent exchange
Salt movement
Blood flow
Central duct

15. waste products may greatly affect its water balance
significant wastes are nitrogenous breakdown products of proteins and nucleic acids
Some animals convert toxic ammonia (NH3) to less toxic compounds prior to excretion
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16. Most aquatic animals, including most bony fishes
Figure 44.8
Proteins
Nucleic acids
Amino acids
Nitrogenous bases
—NH2 Amino groups
Most aquatic animals, including most bony fishes
Mammals, most amphibians, sharks, some bony fishes
Many reptiles (including birds), insects, land snails
Figure 44.8 Forms of nitrogenous waste.
Ammonia
Urea
Uric acid

17. Forms of Nitrogenous Wastes
Animals excrete nitrogenous wastes in different forms: ammonia, urea, or uric acid
These differ in toxicity and the energy costs of producing them
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18. They release ammonia across the whole body surface or through gills
Animals that excrete nitrogenous wastes as ammonia need access to lots of water
They release ammonia across the whole body surface or through gills
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19. Urea
The liver of mammals and most adult amphibians converts ammonia to the less toxic urea
The circulatory system carries urea to the kidneys, where it is excreted
Conversion of ammonia to urea is energetically expensive; excretion of urea requires less water than ammonia
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20. Uric Acid
Insects, land snails, and many reptiles, including birds, mainly excrete uric acid
Uric acid is relatively nontoxic and does not dissolve readily in water
It can be secreted as a paste with little water loss
Uric acid is more energetically expensive to produce than urea
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21. Excretory Processes
Most excretory systems produce urine by refining a filtrate derived from body fluids
Key functions of most excretory systems
Filtration: Filtering of body fluids
Reabsorption: Reclaiming valuable solutes
Secretion: Adding nonessential solutes and wastes from the body fluids to the filtrate
Excretion: Processed filtrate containing nitrogenous wastes, released from the body
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22. 1 Filtration Capillary Filtrate Excretory tubule 2 Reabsorption 3
Figure 44.10 1
Filtration
Capillary
Filtrate
Excretory tubule 2
Reabsorption
Figure Key steps of excretory system function: an overview. 3
Secretion
Urine 4
Excretion

23. Protonephridia
A protonephridium is a network of dead-end tubules connected to external openings - flatworms
The smallest branches of the network are capped by a cellular unit called a flame bulb
These tubules excrete a dilute fluid and function in osmoregulation
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24. Each segment of an earthworm has a pair of open-ended metanephridia
Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion
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25. Malpighian Tubules
In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation
Insects produce a relatively dry waste matter, mainly uric acid, an important adaptation to terrestrial life
Some terrestrial insects can also take up water from the air
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26. Salt, water, and nitrogenous wastes
Figure 44.13
Digestive tract
Rectum
Hindgut
Intestine
Midgut (stomach)
Malpighian tubules
Salt, water, and nitrogenous wastes
Feces and urine
To anus
Malpighian tubule
Figure Malpighian tubules of insects.
Rectum
Reabsorption
HEMOLYMPH

27. Kidneys
Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation
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28. Excretory Organs Kidney Structure Nephron Types Cortical nephron
Figure a
Excretory Organs
Kidney Structure
Nephron Types
Cortical nephron
Juxtamedullary
nephron
Renal cortex
Posterior vena cava
Renal medulla
Renal artery
Renal artery and vein
Kidney
Renal vein
Renal cortex
Aorta
Ureter
Figure Exploring: the Mammalian Excretory System
Ureter
Renal medulla
Urinary bladder
Urethra
Renal pelvis

29. Proximal tubule Distal tubule Filtrate CORTEX Loop of Henle
Figure 44.15
Proximal tubule
Distal tubule
NaCl
Nutrients
H2O
HCO3
H2O K
NaCl
HCO3 H
NH3 K H
Filtrate
CORTEX
Loop of Henle
NaCl
H2O
OUTER MEDULLA
NaCl
Collecting duct
Figure The nephron and collecting duct: regional functions of the transport epithelium.
Key
Urea
Active transport
NaCl
H2O
Passive transport
INNER MEDULLA

30. Osmoreceptors in hypothalamus trigger release of ADH.
Figure
Osmoreceptors in hypothalamus trigger release of ADH.
Thirst
Hypothalamus
Drinking reduces blood osmolarity to set point.
ADH
Increased permeability
Pituitary gland
Distal tubule
STIMULUS: Increase in blood osmolarity (for instance, after sweating profusely)
Figure Regulation of fluid retention in the kidney by antidiuretic hormone (ADH).
H2O reab- sorption helps prevent further osmolarity increase.
Collecting duct
Homeostasis: Blood osmolarity (300 mOsm/L)

31. The Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis
A drop in blood pressure near the glomerulus
juxtaglomerular apparatus (JGA) to release the enzyme renin
3. Renin triggers the formation of the peptide angiotensin II
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32. Angiotensin II
1. Raises blood pressure and decreases blood flow to the kidneys
2. Stimulates the release of the hormone aldosterone, which increases blood volume and pressure
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33. Juxtaglomerular apparatus (JGA)
Figure
Liver
Angiotensinogen
JGA releases renin.
Distal tubule
Renin
Angiotensin I
ACE
Angiotensin II
Juxtaglomerular apparatus (JGA)
STIMULUS: Low blood volume or blood pressure (for example, due to dehydration or blood loss)
Adrenal gland
Figure Regulation of blood volume and blood pressure by the renin-angiotensin-aldosterone system (RAAS).
Aldosterone
Arterioles constrict, increasing blood pressure.
More Na and H2O are reabsorbed in distal tubules, increasing blood volume.
Homeostasis: Blood pressure, volume

34. Homeostatic Regulation of the Kidney
ADH and RAAS both increase water reabsorption, but only RAAS will respond to a decrease in blood volume
Another hormone, atrial natriuretic peptide (ANP), opposes the RAAS
ANP is released in response to an increase in blood volume and pressure and inhibits the release of renin
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