1. Osmoregulation and Excretion
Chapter 44
Osmoregulation and Excretion

2. 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

3. Desert and marine animals face desiccating environments
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

4. Osmoregulation Excretion
Regulates solute concentrations and balances the gain and loss of water
Excretion
Gets rid of metabolic wastes

5. Osmoregulation is based largely on controlled movement of solutes
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

6. Cells require a balance
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

7. Osmoconformers, which are only marine animals
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

8. 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

9. Marine Animals
Most marine invertebrates are osmoconformers
Most marine vertebrates and some invertebrates are osmoregulators

10. (a) Osmoregulation in a saltwater fish
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
Gain of water and
salt ions from food
and by drinking
seawater
Osmotic water loss
through gills and other parts
of body surface
Excretion of
salt ions
from gills
Excretion of salt ions
and small amounts
of water in scanty
urine from kidneys
Figure 44.3a
(a) Osmoregulation in a saltwater fish

11. Freshwater Animals Freshwater animals
Constantly take in water from their hypoosmotic environment
Lose salts by diffusion

12. (b) Osmoregulation in a freshwater fish
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
Uptake of
water and some
ions in food
Osmotic water gain
through gills and other parts
of body surface
salt ions
by gills
Excretion of
large amounts of
water in dilute
urine from kidneys
Figure 44.3b
(b) Osmoregulation in a freshwater fish

13. 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
(a) Hydrated tardigrade
(b) Dehydrated tardigrade
100 µm
Figure 44.4a, b

14. Land animals manage their water budgets
By drinking and eating moist foods and by using metabolic water
Water
balance in
a human
(2,500 mL/day
= 100%)
balance in a
kangaroo rat
(2 mL/day
Ingested
in food (0.2)
in food (750)
in liquid
(1,500)
Derived from
metabolism (250)
metabolism (1.8)
gain
Feces (0.9)
Urine
(0.45)
Evaporation (1.46)
Feces (100)
Evaporation (900)
loss
Figure 44.5

15. Desert animals Get major water savings from simple anatomical features
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
Control group
(Unclipped fur)
Experimental group
(Clipped fur) 4 3 2 1
(L/100 kg body mass)
Water lost per day
RESULTS
Removing the fur of a camel increased the rate of water loss through sweating by up to 50%.
The fur of camels plays a critical role in their conserving water in the hot desert environments where they live.
CONCLUSION
Figure 44.6

16. 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

17. An example of transport epithelia is found in the salt glands of marine birds
Which remove excess sodium chloride from the blood
Nasal salt gland
Nostril
with salt
secretions
Lumen of
secretory tubule
NaCl
Blood
flow
Secretory cell
of transport
epithelium
Central
duct
Direction
of salt
movement
Transport
Secretory
tubule
Capillary
Vein
Artery
(a) An albatross’s salt glands empty via a duct into the nostrils, 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 transport epithelium surrounded by capillaries, and drains into a central duct.
(c) The secretory cells actively transport salt from the blood into the tubules. Blood flows counter to the flow of salt secretion. By maintaining a concentration gradient of salt in the tubule (aqua), this countercurrent system enhances salt transfer from the blood to the lumen of the tubule.
Figure 44.7a, b

18. The type and quantity of an animal’s waste products
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

19. Among the most important wastes
Are the nitrogenous breakdown products of proteins and nucleic acids
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
Ammonia
Urea
Uric acid
NH3
NH2 O C N H HN
Figure 44.8

20. Forms of Nitrogenous Wastes
Different animals
Excrete nitrogenous wastes in different forms

21. Animals that excrete nitrogenous wastes as ammonia
Need access to lots of water
Release it across the whole body surface or through the gills

22. The liver of mammals and most adult amphibians
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

23. Insects, land snails, and many reptiles, including birds
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

24. 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

25. Concept 44.3: Diverse excretory systems are variations on a tubular theme
Regulate solute movement between internal fluids and the external environment

26. Most excretory systems
Excretory Processes
Most excretory systems
Produce urine by refining a filtrate derived from body fluids
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
Excretory
tubule
Filtrate
Urine 1 2 3 4
Figure 44.9

27. 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

28. 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

29. Protonephridia: Flame-Bulb Systems
A protonephridium
Is a network of dead-end tubules lacking internal openings
Nucleus
of cap cell
Cilia
Interstitial fluid
filters through
membrane where
cap cell and tubule
cell interdigitate
(interlock)
Tubule cell
Flame
bulb
Nephridiopore
in body wall
Tubule
Protonephridia
(tubules)
Figure 44.10

30. 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

31. Each segment of an earthworm
Metanephridia
Each segment of an earthworm
Has a pair of open-ended metanephridia
Nephrostome
Metanephridia
Nephridio-
pore
Collecting
tubule
Bladder
Capillary
network
Coelom
Figure 44.11

32. Metanephridia consist of tubules
That collect coelomic fluid and produce dilute urine for excretion

33. In insects and other terrestrial arthropods, malpighian tubules
Remove nitrogenous wastes from hemolymph and function in osmoregulation
Digestive tract
Midgut
(stomach)
Malpighian
tubules
Rectum
Intestine
Hindgut
Salt, water, and
nitrogenous
wastes
Feces and urine
Anus
tubule
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH
Figure 44.12

34. Insects produce a relatively dry waste matter
An important adaptation to terrestrial life

35. Kidneys, the excretory organs of vertebrates
Vertebrate Kidneys
Kidneys, the excretory organs of vertebrates
Function in both excretion and osmoregulation

36. The mammalian excretory system centers on paired kidneys
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

37. 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
Figure 44.13a

38. Urine exits each kidney
Through a duct called the ureter
Both ureters
Drain into a common urinary bladder

39. 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
(b) Kidney structure
Ureter
Section of kidney from a rat
Renal
medulla
cortex
pelvis
Figure 44.13b

40. The nephron, the functional unit of the vertebrate kidney
Consists of a single long tubule and a ball of capillaries called the glomerulus
Juxta-
medullary
nephron
Cortical
Collecting
duct To
renal
pelvis
Renal
cortex
medulla
20 µm
Afferent
arteriole
from renal
artery
Glomerulus
Bowman’s capsule
Proximal tubule
Peritubular capillaries
SEM
Efferent
arteriole from
glomerulus
Branch of
renal vein
Descending
limb
Ascending
Loop of
Henle
Distal
tubule
(c) Nephron
Vasa recta
(d) Filtrate and
blood flow
Figure 44.13c, d

41. Filtration of the Blood
Filtration occurs as blood pressure
Forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule

42. 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

43. 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

44. 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

45. From Blood Filtrate to Urine: A Closer Look
Filtrate becomes urine
As it flows through the mammalian nephron and collecting duct
Proximal tubule
Filtrate
H2O
Salts (NaCl and others)
HCO3– H+
Urea
Glucose; amino acids
Some drugs
Key
Active transport
Passive transport
CORTEX
OUTER
MEDULLA
INNER
Descending limb
of loop of
Henle
Thick segment
of ascending
limb
Thin segment
Collecting
duct
NaCl
Distal tubule
Nutrients
HCO3 K+
NH3 1 4 3 2 5
Figure 44.14

46. 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

47. 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

48. Concept 44.5: The mammalian kidney’s ability to conserve water is a key terrestrial adaptation
Can produce urine much more concentrated than body fluids, thus conserving water

49. 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

50. Osmolarity of interstitial fluid (mosm/L)
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
H2O
NaCl
300
100
400
600
900
1200
700
200
Active transport
Passive transport
OUTER MEDULLA
INNER MEDULLA
CORTEX
Urea
Osmolarity of interstitial fluid (mosm/L)
Figure 44.15

51. 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

52. 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

53. 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

54. Regulation of Kidney Function
The osmolarity of the urine
Is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys

55. Antidiuretic hormone (ADH)
Increases water reabsorption in the distal tubules and collecting ducts of the kidney
Osmoreceptors
in hypothalamus
Drinking reduces
blood osmolarity
to set point
H2O reab-
sorption helps
prevent further
osmolarity
increase
STIMULUS:
The release of ADH is
triggered when osmo-
receptor cells in the
hypothalamus detect an
increase in the osmolarity
of the blood
Homeostasis:
Blood osmolarity
Hypothalamus
ADH
Pituitary
gland
Increased
permeability
Thirst
Collecting duct
Distal
tubule
Figure 44.16a
(a) Antidiuretic hormone (ADH) enhances fluid retention by making the kidneys reclaim more water.

56. The renin-angiotensin-aldosterone system (RAAS)
Is part of a complex feedback circuit that functions in homeostasis
Increased Na+
and H2O reab-
sorption in
distal tubules
Homeostasis:
Blood pressure,
volume
STIMULUS:
The juxtaglomerular
apparatus (JGA) responds
to low blood volume or
blood pressure (such as due
to dehydration or loss of
blood)
Aldosterone
Adrenal gland
Angiotensin II
Angiotensinogen
Renin
production
Arteriole
constriction
Distal
tubule
JGA
Figure 44.16b
(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase in blood volume and pressure.

57. Another hormone, atrial natriuretic factor (ANF)
Opposes the RAAS

58. 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

59. The form and function of nephrons in various vertebrate classes
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

60. Exploring environmental adaptations of the vertebrate kidney
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)
Figure 44.18