1. Homeostasis and Endocrine Signaling32
Homeostasis and Endocrine Signaling

2. Multicellularity allows for cellular specialization with particular cells devoted to specific activities
Specialization requires organization and results in an internal environment that differs from the external environment

3. Organisms must maintain homeostasis
Organisms must maintain homeostasis. Why are homeostasis and regulation essential life functions for living things?

4. Homeostasis
Organisms use homeostasis to maintain a “steady state” or internal balance regardless of external environment
In humans, body temperature, blood pH, and glucose concentration are each maintained at a constant level
Regulation of room temperature by a thermostat is analogous to homeostasis

5. Sensor/ control center: Thermostat turns heater off. Response:
Figure 32.4
Sensor/
control center:
Thermostat
turns heater off.
Response:
Heating stops.
Room
temperature
decreases.
Stimulus:
Room
temperature
increases.
Set point:
Room temperature
at 20C
Figure 32.4 A nonliving example of temperature regulation: control of room temperature
Stimulus:
Room
temperature
decreases.
Room
temperature
increases.
Response:
Heating starts.
Sensor/
control center:
Thermostat
turns heater on. 5

6. Which body systems are responsible for maintaining homeostasis in animals? How are their modes of action different?

7. Coordination and Control Functions of the Endocrine and Nervous Systems
In the endocrine system, signaling molecules released into the bloodstream by endocrine cells reach all locations in the body
In the nervous system, neurons transmit signals along dedicated routes, connecting specific locations in the body 7

8. Endocrine cell Cell body of neuron
Figure 32.9
(a) Signaling by hormones
(b) Signaling by neurons
Stimulus
Stimulus
Endocrine
cell
Cell
body of
neuron
Nerve
impulse
Axon
Hormone
Signal
travels
everywhere.
Signal
travels to
a specific
location.
Blood
vessel
Nerve
impulse
Figure 32.9 Signaling in the endocrine and nervous systems
Axons
Response
Response 8

9. Regulating and Conforming
Faced with environmental fluctuations, animals manage their internal environment by either regulating or conforming
An animal that is a regulator uses internal mechanisms to control internal change despite external fluctuation
An animal that is a conformer allows its internal condition to change in accordance with external changes

10. Interpret the following figure.

11. (temperature conformer)
Figure 32.3 40
River otter (temperature regulator) 30
Body temperature (C) 20
Largemouth bass
(temperature conformer) 10
Figure 32.3 Regulating and conforming 10 20 30 40
Ambient (environmental) temperature (C) 11

12. An animal may regulate some internal conditions and not others
For example, a fish may conform to surrounding temperature in the water, but it regulates solute concentrations in its blood and interstitial fluid (the fluid surrounding body cells)

13. How does an organism know if there is a disruption of homeostasis?

14. Animals achieve homeostasis by maintaining a variable at or near a particular value, or set point
Fluctuations above or below the set point serve as a stimulus; these are detected by a sensor and trigger a response
The response returns the variable to the set point

15. Homeostasis in animals relies largely on negative feedback, a control mechanism that reduces the stimulus
Homeostasis moderates, but does not eliminate, changes in the internal environment
Set points and normal ranges for homeostasis are usually stable, but certain regulated changes in the internal environment are essential

16. We need to know a few examples of how homeostasis is moderated for the AP exam. Let’s take a closer look…

17. Thermoregulation: A Closer Look
Thermoregulation is the process by which animals maintain an internal temperature within a tolerable range

18. Describe the difference between endothermic and ectothermic organisms.

19. Endothermy and Ectothermy
Endothermic animals generate heat by metabolism; birds and mammals are endotherms = “warm-blooded”
Ectothermic animals gain heat from external sources; ectotherms include most invertebrates, fishes, amphibians, and nonavian reptiles = “cold-blooded”

20. Endotherms can maintain a stable body temperature in the face of large fluctuations in environmental temperature
Ectotherms may regulate temperature by behavioral means
Ectotherms generally need to consume less food than endotherms, because their heat source is largely environmental

21. How would you classify the following?

22. (a) A walrus, an endotherm
Figure 32.5a
Figure 32.5a Endothermy and ectothermy (part 1: endotherm)
(a) A walrus, an endotherm 22

23. (b) A lizard, an ectotherm
Figure 32.5b
Figure 32.5b Endothermy and ectothermy (part 2: ectotherm)
(b) A lizard, an ectotherm 23

24. Balancing Heat Loss and Gain
Organisms exchange heat by four physical processes
Radiation
Evaporation
Convection
Conduction
Heat is always transferred from an object of higher temperature to one of lower temperature

25. Generate definitions of each type of heat exchange from the figure.

26. Radiation Evaporation Convection Conduction Figure 32.6
Figure 32.6 Heat exchange between an organism and its environment
Convection
Conduction 26

27. How might the circulatory system collaborate in this thermoregulation process?

28. Circulatory Adaptations for Thermoregulation
In response to changes in environmental temperature, animals can alter blood (and heat) flow between their body core and their skin
Vasodilation, the widening of the diameter of superficial blood vessels, promotes heat loss
How?
Vasoconstriction, the narrowing of the diameter of superficial blood vessels, reduces heat loss 28

29. In aquatic environments, we see adaptations for how thermoregulation takes place.

30. The arrangement of blood vessels in many marine mammals and birds allows for countercurrent exchange
Countercurrent heat exchangers transfer heat between fluids flowing in opposite directions and reduce heat loss 30

31. Canada goose 1 3 Artery Vein 35C 33 30 27 20 18 Key Warm blood
Figure 32.7
Canada goose 1 3
Artery
Vein
35C
33
30
27
Figure 32.7 A countercurrent heat exchanger
20
18
Key
Warm blood
10 9
Cool blood
Blood flow 2
Heat transfer 31

32. Other mechanisms for thermoregulation?

33. Acclimatization in Thermoregulation
Birds and mammals can vary their insulation to acclimatize to seasonal temperature changes
Acclimatization in ectotherms often includes adjustments at the cellular level
Some ectotherms that experience subzero temperatures can produce “antifreeze” compounds to prevent ice formation in their cells

34. In mammals (FYI: we are mammals, in case you didn’t know)… where is the physiological thermostat for thermoregulation?

35. Physiological Thermostats and Fever
Thermoregulation in mammals is controlled by a region of the brain called the hypothalamus
The hypothalamus triggers heat loss or heat-generating mechanisms
Fever is the result of a change to the set point for a biological thermostat
Animation: Negative Feedback
Animation: Positive Feedback

36. Sensor/control center: Thermostat in hypothalamus Response: Sweat
Figure 32.8a
Sensor/control center: Thermostat
in hypothalamus
Response: Sweat
Response:
Blood vessels
in skin dilate.
Stimulus:
Increased body
temperature
Body
temperature
decreases.
Figure 32.8a The thermostatic function of the hypothalamus in human thermoregulation (part 1: increase)
Homeostasis:
Internal body
temperature of
approximately
36–38C 36

37. Sensor/control center: Thermostat in hypothalamus Response: Shivering
Figure 32.8b
Homeostasis:
Internal body
temperature of
approximately
36–38C
Body
temperature
increases.
Stimulus:
Decreased body
temperature
Response:
Blood vessels
in skin constrict.
Figure 32.8b The thermostatic function of the hypothalamus in human thermoregulation (part 2: decrease)
Sensor/control center: Thermostat
in hypothalamus
Response: Shivering 37

38. Hormones, released in the endocrine system, are released to moderate a series of feedback mechanisms in varying organ systems.

39. Hormones may have effects in a single location or throughout the body
Only cells with receptors for a certain hormone can respond to it
The endocrine system is well adapted for coordinating gradual changes that affect the entire body 39

40. We need to know a few examples of endocrine pathways within the mammalian system. Let’s take a closer look…

41. Simple Endocrine Pathways
Digestive juices in the stomach are extremely acidic and must be neutralized before the remaining steps of digestion take place
Coordination of pH control in the duodenum relies on an endocrine pathway 41

42. Pathway Example  Stimulus Low pH in duodenum S cells of duodenum
Figure 32.10
Pathway
Example 
Stimulus
Low pH in
duodenum
S cells of duodenum
secrete the hormone
secretin ( ).
Endocrine
cell
Negative feedback
Hormone
Blood
vessel
Figure A simple endocrine pathway
Target
cells
Pancreas
Response
Bicarbonate release 42

43. The release of acidic stomach contents into the duodenum stimulates endocrine cells there to secrete the hormone secretin
This causes target cells in the pancreas to raise the pH in the duodenum
The pancreas can act as an exocrine gland, secreting substances through a duct,
Which substances???
or as an endocrine gland, secreting hormones directly into interstitial fluid
Which hormones??? 43

44. Neuroendocrine Pathways
Hormone pathways that respond to stimuli from the external environment rely on a sensor in the nervous system
In vertebrates, the hypothalamus integrates endocrine and nervous systems
Signals from the hypothalamus travel to a gland located at its base, called the pituitary gland 44

45. Major Endocrine Glands and Their Hormones Hypothalamus
Figure 32.11a
Major Endocrine Glands
and Their Hormones
Hypothalamus
Pituitary gland
Anterior pituitary
Pineal gland
Melatonin
Posterior pituitary
Oxytocin
Vasopressin
(antidiuretic
hormone, ADH)
Thyroid gland
Thyroid hormone
(T3 and T4)
Calcitonin
Adrenal glands
(atop kidneys)
Parathyroid glands
Parathyroid hormone (PTH)
Adrenal medulla
Epinephrine and
norepinephrine
Ovaries (in females)
Estrogens
Progestins
Figure 32.11a Exploring the human endocrine system (part 1: glands and hormones)
Adrenal cortex
Glucocorticoids
Mineralocorticoids
Testes
(in males)
Androgens
Pancreas
Insulin
Glucagon 45

46. Hypothalamus Anterior pituitary Neurosecretory cells of the
Figure 32.11b
Hypothalamus
Neurosecretory
cells of the
hypothalamus
Portal vessels
Hypothalamic
hormones
HORMONE
Posterior
pituitary
Anterior pituitary
Endocrine cells
TARGET
Pituitary hormones
Figure 32.11b Exploring the human endocrine system (part 2: hypothalamus and pituitary)
FSH
TSH
ACTH
Prolactin
MSH GH
Mammary
glands
Testes or
ovaries
Thyroid
gland
Adrenal
cortex
Melanocytes
Liver, bones,
other tissues 46

47. Let’s consider another example…

48. Stimulus TSH circulation throughout body Sensory neuron Thyroid gland
Figure 32.11c
Stimulus
TSH circulation
throughout body
Sensory
neuron −
Hypothalamus
Thyroid
gland
Neurosecretory cell
TRH
Negative feedback
Thyroid
hormone
Thyroid hormone
circulation
throughout
body
Figure 32.11c Exploring the human endocrine system (part 3: hormone cascade) −
Anterior
pituitary
TSH
Response 48

49. Low level of iodine uptake High level of iodine uptake Thyroid scan
Figure 32.11d
Low level of
iodine uptake
High level of
iodine uptake
Figure 32.11d Exploring the human endocrine system (part 4: thyroid scan)
Thyroid scan 49

50. It also secretes antidiuretic hormone (ADH)
Hormonal signals from the hypothalamus trigger synthesis and release of hormones from the anterior pituitary
The posterior pituitary is an extension of the hypothalamus and secretes oxytocin, which regulates release of milk during nursing in mammals
It also secretes antidiuretic hormone (ADH) 50

51. Another example…Oxytocin

52. Pathway Example Stimulus Suckling Sensory neuron Hypothalamus/
Figure 32.12
Pathway
Example 
Stimulus
Suckling
Sensory
neuron
Hypothalamus/
posterior pituitary
Neuro-
secretory cell
Posterior
pituitary
secretes the
neurohormone
oxytocin ( ).
Positive feedback
Neuro-
hormone
Figure A neuroendocrine pathway
Blood
vessel
Smooth
muscle in
breasts
Target
cells
Response
Milk release 52

53. Feedback Regulation in Endocrine Pathways
A feedback loop links the response back to the original stimulus in an endocrine pathway
While negative feedback dampens a stimulus, positive feedback reinforces a stimulus to increase the response 53

54. There are different types of hormones : water soluble or lipid soluble Their receptors are located in different places at their target cells. They also have very different modes of action.

55. Pathways of Water-Soluble and Lipid-Soluble Hormones
The hormones discussed thus far are proteins that bind to cell-surface receptors and that trigger events leading to a cellular response
The intracellular response is called signal transduction
A signal transduction pathway typically has multiple steps 55

56. Lipid-soluble hormones have receptors inside cells
When bound by the hormone, the hormone-receptor complex moves into the nucleus
There, the receptor alters transcription of particular genes 56

57. Multiple Effects of Hormones
Many hormones elicit more than one type of response
For example, epinephrine is secreted by the adrenal glands and can raise blood glucose levels, increase blood flow to muscles, and decrease blood flow to the digestive system
Target cells vary in their response to a hormone because they differ in their receptor types or in the molecules that produce the response 57

58. Same receptors but different intracellular proteins (not shown)
Figure 32.13
Same receptors but different
intracellular proteins (not shown)
Different receptors
Different cellular responses
Different cellular responses
Epinephrine
Epinephrine
Epinephrine
 receptor
 receptor
 receptor
Glycogen
deposits
Figure One hormone, different effects
Vessel
dilates.
Vessel
constricts.
Glycogen
breaks down
and glucose
is released
from cell.
(a) Liver cell
(b) Skeletal muscle
blood vessel
(c) Intestinal blood
vessel 58

59. Evolution of Hormone Function
Over the course of evolution the function of a given hormone may diverge between species
For example, thyroid hormone plays a role in metabolism across many lineages, but in frogs it has taken on a unique function: stimulating the resorption of the tadpole tail during metamorphosis
Prolactin also has a broad range of activities in vertebrates 59

60. Tadpole Adult frog Figure 32.14
Figure Specialized role of a hormone in frog metamorphosis
Adult frog 60