1. Homeostasis and Endocrine Signaling32
Homeostasis and Endocrine Signaling

2. Why is it important in cells?
What is osmosis?
Why is it important in cells?

3. Concept 32.3: A shared system mediates osmoregulation and excretion in many animals
Osmoregulation is the general term for the processes by which animals control solute concentrations in the interstitial fluid and balance water gain and loss 3

4. Osmosis and Osmolarity
Cells require a balance between uptake and loss of water
Osmolarity, the solute concentration of a solution, determines the movement of water across a selectively permeable membrane
If two solutions are 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 4

5. (a) Osmoregulation in a marine fish Excretion of salt ions from gills
Figure 32.15
(a) Osmoregulation in a marine fish
Excretion of
salt ions
from gills
Gain of water and
salt ions from food
Osmotic water loss
through gills and
other parts of body
surface
SALT WATER
Gain of water
and salt ions from
drinking seawater
Excretion of salt ions and
small amounts of water in
scanty urine from kidneys
Key
Water
(b) Osmoregulation in a freshwater fish
Salt
Uptake of
salt ions
by gills
Gain of water and
some ions in food
Osmotic water gain
through gills and
other parts of body
surface
Figure Osmoregulation in marine and freshwater bony fishes: a comparison
Excretion of salt
ions and large
amounts of water
in dilute urine
from kidneys
FRESH WATER 5

6. Marine and freshwater organisms have opposite challenges
Marine fish drink large amounts of seawater to balance water loss and excrete salt through their gills and kidneys
Freshwater fish drink almost no water and replenish salts through eating; some also replenish salts by uptake across the gills 6

7. What do you think the difference between osmoregulators and osmoconformers might be?

8. Osmoregulatory Challenges and Mechanisms
Osmoconformers, consisting 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 8

9. What are the 3 different ways you can excrete nitrogenous wastes?

10. Proteins Nucleic acids Amino acids Nitrogenous bases Amino groups
Figure 32.16
Proteins
Nucleic acids
Amino
acids
Nitrogenous
bases
Amino groups
Most aquatic
animals, including
most bony fishes
Mammals, most
amphibians, sharks,
some bony fishes
Many reptiles
(including birds),
insects, land snails
Figure Forms of nitrogenous waste
Ammonia
Urea
Uric acid 10

11. Nitrogenous Wastes
The type and quantity of an animal’s waste products may greatly affect its water balance
Among the most significant wastes are nitrogenous breakdown products of proteins and nucleic acids
Some animals convert toxic ammonia (NH3) to less toxic compounds prior to excretion 11

12. Ammonia excretion is most common in aquatic organisms
Vertebrates excrete urea, a conversion product of ammonia, which is much less toxic
Insects, land snails, and many reptiles including birds excrete uric acid as a semisolid paste
It is less toxic than ammonia and generates very little water loss, but it is energetically more expensive to produce than urea 12

13. Nucleus of cap cell Cilia Flame bulb Interstitial fluid filters
Figure 32.18
Nucleus
of cap cell
Cilia
Flame
bulb
Interstitial
fluid filters
through
membrane.
Tubule
Opening
in body
wall
Figure Protonephridia: the flame bulb system of a planarian
Tubules of
protonephridia
Tubule
cell 13

14. Invertebrates
Flatworms have excretory systems called protonephridia, networks of dead-end tubules connected to external openings
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 14

15. Insects produce a relatively dry waste matter, mainly uric acid
In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph without a filtration step
Insects produce a relatively dry waste matter, mainly uric acid 15

16. Vertebrates
In vertebrates and some other chordates, the kidney functions in both osmoregulation and excretion
The kidney consists of tubules arranged in an organized array and in contact with a network of capillaries
The excretory system includes ducts and other structures that carry urine from the tubules out of the body
Animation: Nephron Introduction 16

17. In humans, how do we remove these wastes?

18. Excretory Organs Posterior vena cava Kidney Renal artery and vein
Figure 32.19a
Excretory Organs
Posterior vena cava
Kidney
Renal artery and vein
Aorta
Ureter
Figure 32.19a Exploring the mammalian excretory system (part 1: organs)
Urinary bladder
Urethra 18

19. Kidney Structure Nephron Organization Nephron Types Renal cortex
Figure 32.19b
Kidney Structure
Renal cortex
Renal medulla
Renal artery
Nephron Organization
Renal vein
Afferent arteriole
from renal artery
Glomerulus
Bowman’s
capsule
Ureter
Proximal
tubule
Renal pelvis
Peritubular
capillaries
Distal
tubule
Nephron Types
Cortical
nephron
Efferent
arteriole
from
glomerulus
Figure 32.19b Exploring the mammalian excretory system (part 2: kidney structure summary)
Branch of
renal vein
Descending
limb
Renal
cortex
Loop of
Henle
Vasa
recta
Collecting
duct
Renal
medulla
Ascending
limb
Juxtamedullary
nephron 19

20. What are the steps in urine formation?

21. Key functions of most excretory systems
Many animal species 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: Releasing processed filtrate containing nitrogenous wastes from the body 21

22. Filtration Capillary Filtrate Excretory tubule Reabsorption Secretion
Figure 32.17
Filtration
Capillary
Filtrate
Excretory
tubule
Reabsorption
Figure Key steps of excretory system function: an overview
Secretion
Urine
Excretion 22

23. Concept 32.4: Hormonal circuits link kidney function, water balance, and blood pressure
The capillaries and specialized cells of Bowman’s capsule are permeable to water and small solutes but not blood cells or large molecules
The filtrate produced there contains salts, glucose, amino acids, vitamins, nitrogenous wastes, and other small molecules 23

24. From Blood Filtrate to Urine: A Closer Look
Proximal tubule
Reabsorption of ions, water, and nutrients takes place in the proximal tubule
Molecules are transported actively and passively from the filtrate into the interstitial fluid and then capillaries
Some toxic materials are actively secreted into the filtrate 24

25. Descending limb of the loop of Henle
Reabsorption of water continues through channels formed by aquaporin proteins
Movement is driven by the high osmolarity of the interstitial fluid, which is hyperosmotic to the filtrate
The filtrate becomes increasingly concentrated all along its journey down the descending limb 25

26. Ascending limb of the loop of Henle
The ascending limb has a transport epithelium that lacks water channels
Here, salt but not water is able to move from the tubule into the interstitial fluid
The filtrate becomes increasingly dilute as it moves up to the cortex 26

27. The controlled movement of ions contributes to pH regulation
Distal tubule
The distal tubule regulates the K+ and NaCl concentrations of body fluids
The controlled movement of ions contributes to pH regulation 27

28. Collecting duct
The collecting duct carries filtrate through the medulla to the renal pelvis
Most of the water and nearly all sugars, amino acids, vitamins, and other nutrients are reabsorbed into the blood
Urine is hyperosmotic to body fluids
Animation: Bowman’s Capsule
Animation: Collecting Duct
Animation: Loop of Henle 28

29. Salts (NaCI and others) HCO3− OUTER MEDULLA H Urea Thin segment
Figure 32.20 1
Proximal tubule 4
Distal tubule
NaCI
Nutrients
H2O
HCO3−
H2O K
NaCI
HCO3− H
NH3 K H
Interstitial
fluid
CORTEX 3
Thick segment
of ascending
limb 2
Descending limb
of loop of
Henle
Filtrate
H2O
Salts (NaCI and others)
NaCI
H2O
HCO3−
OUTER
MEDULLA
NaCI H
Urea
Figure The nephron and collecting duct: regional functions of the transport epithelium 3
Thin segment
of ascending
limb 5
Collecting
duct
Glucose, amino acids
Some drugs
Urea
NaCI
H2O
Key
Active transport
INNER
MEDULLA
Passive transport 29

30. Proximal tubule Distal tubule Filtrate Interstitial fluid CORTEX
Figure 32.20a 1
Proximal tubule 4
Distal tubule
NaCI
Nutrients
H2O
HCO3−
H2O K
NaCI
HCO3− H
NH3 K H
Filtrate
Interstitial
fluid
CORTEX
Figure 32.20a The nephron and collecting duct: regional functions of the transport epithelium (part 1: cortex)
Active transport
Passive transport 30

31. Descending limb of loop of Henle OUTER MEDULLA INNER MEDULLA
Figure 32.20b 3
Thick segment
of ascending
limb 2
Descending limb
of loop of
Henle
NaCI
H2O
OUTER
MEDULLA
NaCI 3
Thin segment
of ascending
limb 5
Collecting
duct
Active transport
Passive transport
Urea
NaCI
H2O
Figure 32.20b The nephron and collecting duct: regional functions of the transport epithelium (part 2: medulla)
INNER
MEDULLA 31

32. Concentrating Urine in the Mammalian Kidney
The mammalian kidney’s ability to conserve water is a key terrestrial adaptation 32

33. 300 mOsm/L 300 300 300 H2O CORTEX 400 400 Active transport H2O Passive
Figure
300
mOsm/L
300
300
300
H2O
CORTEX
400
400
Active
transport
H2O
Passive
transport
H2O
H2O
OUTER
MEDULLA
600
600
Figure How the human kidney concentrates urine: the two-solute model (step 1)
H2O
H2O
900
900
H2O
INNER
MEDULLA
1,200
1,200 33

34. 300 mOsm/L 300 100 300 100 300 H2O NaCI CORTEX 400 200 400 Active
Figure
300
mOsm/L
300
100
300
100
300
H2O
NaCI
CORTEX
400
200
400
Active
transport
H2O
NaCI
Passive
transport
H2O
NaCI
H2O
NaCI
OUTER
MEDULLA
600
400
600
Figure How the human kidney concentrates urine: the two-solute model (step 2)
H2O
NaCI
H2O
NaCI
900
700
900
H2O
NaCI
INNER
MEDULLA
1,200
1,200 34

35. 300 mOsm/L 300 100 300 100 300 300 H2O NaCI H2O CORTEX 400 200 400 400
Figure
300
mOsm/L
300
100
300
100
300
300
H2O
NaCI
H2O
CORTEX
400
200
400
400
Active
transport
H2O
NaCI
H2O
NaCI
Passive
transport
H2O
NaCI
H2O
NaCI
H2O
NaCI
H2O
OUTER
MEDULLA
600
400
600
600
Figure How the human kidney concentrates urine: the two-solute model (step 3)
H2O
NaCI
H2O
Urea
H2O
NaCI
H2O
900
700
900
Urea
H2O
NaCI
H2O
INNER
MEDULLA
Urea
1,200
1,200
1,200 35

36. Water and salt are reabsorbed from the filtrate passing from Bowman’s capsule to the proximal tubule
In the proximal tubule, filtrate volume decreases, but its osmolarity remains the same
As filtrate flows down the descending limb of the loop of Henle, water leaves the tubule, increasing osmolarity of the filtrate
Salt diffusing from the ascending limb maintains a high osmolarity in the interstitial fluid of the renal medulla 36

37. Such a system is called a countercurrent multiplier system
The loop of Henle and surrounding capillaries act as a type of countercurrent system
This system involves active transport and thus an expenditure of energy
Such a system is called a countercurrent multiplier system 37

38. The filtrate in the ascending limb of the loop of Henle is hypoosmotic to the body fluids by the time it reaches the distal tubule
The filtrate descends to the collecting duct, which is permeable to water but not to salt
Osmosis extracts water from the filtrate to concentrate salts, urea, and other solutes in the filtrate 38

39. Adaptations of the Vertebrate Kidney to Diverse Environments
Variations in nephron structure and function equip the kidneys of different vertebrates for osmoregulation in their various habitats 39

40. Desert-dwelling mammals excrete the most hyperosmotic urine and have long loops of Henle
Birds have shorter loops of Henle but conserve water by excreting uric acid instead of urea 40

41. Mammals control the volume and osmolarity of urine
The kidneys of the South American vampire bat can produce either very dilute or very concentrated urine
This allows the bats to reduce their body weight rapidly or digest large amounts of protein while conserving water 41

42. Figure 32.22
Figure A vampire bat (Desmodus rotundas), a mammal with a unique excretory challenge 42

43. Homeostatic Regulation of the Kidney
A combination of nervous and hormonal inputs regulates the osmoregulatory function of the kidney
These inputs contribute to homeostasis for blood pressure and volume through their effect on amount and osmoregulatory of urine 43

44. Antidiuretic Hormone
Antidiuretic hormone (ADH) makes the collecting duct epithelium temporarily more permeable to water
An increase in blood osmolarity above a set point triggers the release of ADH, which helps to conserve water
Decreased osmolarity causes a drop in ADH secretion and a corresponding decrease in permeability of collecting ducts
Animation: Effect of ADH 44

45. Osmoreceptors trigger release of ADH. ADH STIMULUS: Increase in blood
Figure
Osmoreceptors
trigger release
of ADH.
ADH
STIMULUS:
Increase
in blood
osmolarity
Figure Regulation of fluid retention in the kidney by antidiuretic hormone (ADH) (step 1) 45

46. Osmoreceptors trigger release of ADH. Thirst ADH Increased
Figure
Osmoreceptors
trigger release
of ADH.
Thirst
ADH
Increased
permeability
Distal
tubule
STIMULUS:
Increase
in blood
osmolarity
Figure Regulation of fluid retention in the kidney by antidiuretic hormone (ADH) (step 2)
Collecting duct 46

47. Osmoreceptors trigger release of ADH. Thirst Drinking ADH of fluids
Figure
Osmoreceptors
trigger release
of ADH.
Thirst
Drinking
of fluids
ADH
Increased
permeability
Distal
tubule
STIMULUS:
Increase
in blood
osmolarity
Figure Regulation of fluid retention in the kidney by antidiuretic hormone (ADH) (step 3)
H2O
reabsorption
Collecting duct
Homeostasis 47

48. The Renin-Angiotensin-Aldosterone System
The renin-angiotensin-aldosterone system (RAAS) also regulates kidney function
A drop in blood pressure near the glomerulus causes the juxtaglomerular apparatus (JGA) to release the enzyme renin
Renin triggers the formation of the peptide angiotensin II 48

49. STIMULUS: Low blood pressure
Figure
Distal
tubule
JGA
releases
renin.
Renin
Juxtaglomerular
apparatus (JGA)
STIMULUS:
Low blood
pressure
Figure Regulation of blood volume and blood pressure by the renin-angiotensin-aldosterone system (RAAS) (step 1) 49

50. STIMULUS: Low blood pressure
Figure
Liver
Distal
tubule
Angiotensinogen
JGA
releases
renin.
Renin
Juxtaglomerular
apparatus (JGA)
Angiotensin I
ACE
Angiotensin II
STIMULUS:
Low blood
pressure
Figure Regulation of blood volume and blood pressure by the renin-angiotensin-aldosterone system (RAAS) (step 2) 50

51. STIMULUS: Low blood pressure Arterioles constrict.
Figure
Liver
Distal
tubule
Angiotensinogen
JGA
releases
renin.
Renin
Juxtaglomerular
apparatus (JGA)
Angiotensin I
ACE
Angiotensin II
STIMULUS:
Low blood
pressure
Figure Regulation of blood volume and blood pressure by the renin-angiotensin-aldosterone system (RAAS) (step 3)
Arterioles
constrict. 51

52. Liver Distal tubule Angiotensinogen JGA releases renin. Renin
Figure
Liver
Distal
tubule
Angiotensinogen
JGA
releases
renin.
Renin
Juxtaglomerular
apparatus (JGA)
Angiotensin I
ACE
Angiotensin II
STIMULUS:
Low blood
pressure
Adrenal gland
Figure Regulation of blood volume and blood pressure by the renin-angiotensin-aldosterone system (RAAS) (step 4)
Aldosterone
Arterioles
constrict.
Na and H2O
reabsorbed.
Homeostasis 52

53. Angiotensin II
Raises blood pressure and decreases blood flow to capillaries in the kidney
Stimulates the release of the hormone aldosterone, which increases blood volume and pressure 53

54. Coordination of ADH and RAAS Activity
ADH and RAAS both increase water reabsorption
ADH responds to changes in blood osmolarity
RAAS responds to changes in blood volume and pressure
Thus, ADH and RAAS are partners in homeostasis 54