1. Homeostasis & Feedback
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2. What is homeostasis? Process that occurs in all living things
All organ systems work together to achieve homeostasis
Ability of an organism to maintain its internal environment, despite changes to its internal or external environment
Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.
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3. How does homeostasis work?
Feedback pathways
A cellular relay race!
Specific organs and structures must communicate with each other in response to changes in the body
Keeps levels of certain processes within a normal range
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4. What things in your body need to be kept within a range?
Body Temperature
Blood pressure
Blood pH
O2 and CO2 concentration
Osmoregulation-Water balance
Blood glucose
Ion levels
Hormone levels
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5. Cellular Relay Race Stimulus Receptor Integrating center Effector
Response
Reverses the stimulus
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6. Feedback Loops: Types Negative feedback loop Positive feedback loop
original stimulus reversed
most feedback systems in the body are negative
used for conditions that need frequent adjustment
Positive feedback loop
original stimulus intensified/amplified
seen during normal childbirth

7. Negative Feedback Loop

8. Homeostasis – Negative Feedback Loop
Blood glucose concentrations rise after a sugary meal (the stimulus), the hormone insulin is released and it speeds up the transport of glucose out of the blood and into selected tissues (the response), so blood glucose concentrations decrease (thus decreasing the original stimulus).

9. Homeostasis of Blood Pressure
Baroreceptors in walls of blood vessels detect an increase in BP
Brain receives input and signals blood vessels and heart
Blood vessels dilate, HR decreases
BP decreases

10. Thermoregulation (Regulation of Body Temperature)
Ectothermy (cold-blooded animals)
body temperatures vary with that of the environment
e.g. reptiles, fish, amphibians
Endothermy(warm-blooded animals)
keep body temperature constant even in winter by increasing metabolic rate
e.g. birds, mammals

11. Ectotherm vs Endotherm

12. Advantages Disadvantages Endothermy Maintains stable body temp
Cooling & heating the body
cooling and heating the body
high levels of aerobic metabolism
sustains vigorous activity for much longer than ectotherms
Long distance running
Flight
Disadvantages
Greater food consumption to meet metabolic needs
Human metabolic mate at 200C & at rest
1,300 to 1,800 kcal per day.
American alligator metabolic rate at 200C & at rest 60 kcal per day at 200C.

13. Mechanisms for thermoregulation
Insulation
Fur
Hair
Feathers
Fat
Blubber
Evaporative cooling
sweating, panting, bathing
Shivering
Nonshivering thermogenesis & brown fat
Circulation adaptations
Countercurrent exchange
Vasodilatation (cooling)
Vasoconstriction (heat conservation)
Behavioral responses

14. Regulating Body Temp in Humans

15. Some ectotherms that experience subzero body temperatures protect themselves by producing “antifreeze” compounds (cryoprotectants) that prevent ice formation in the cells.
In cold climates, cryoprotectants in the body fluids let overwintering ectotherms, such as some frogs and many arthropods and their eggs, withstand body temperatures considerably below zero.
Cyroprotectants are also found in some Arctic and Antarctic fishes, where temperatures can drop below the freezing point of unprotected body fluids (about -0.7oC).

16. Cells can often make rapid adjustments to temperature changes.
For example, marked increases in temperature or other sources of stress induce cells grown in culture to produce stress-induced proteins, including heat-shock proteins, within minutes.
These molecules help maintain the integrity of other proteins that would be denatured by severe heat.
These proteins are also produced in bacteria, yeast, and plants cells, as well as other animals.
These help prevent cell death when an organism is challenged by severe changes in the cellular environment.

17. Positive Feedback during Childbirth
Stretch receptors in walls of uterus send signals to the brain
Brain induces release of hormone (oxytocin) into bloodstream
Uterine smooth muscle contracts more forcefully
More stretch, more hormone, more contraction etc.
Cycle ends with birth of the baby & decrease in stretch

18. Lactation in Mammals
Suckling by an infant stimulates sensory neurons in the nipples, generating signals in the nervous system that trigger the release of the neurohormone oxytocin
In response to the oxytocin, the mammary gland secrete milk.
Milk release leads to more suckling and more stimulation.

19. Role of Body Systems in Homeostasis

20. Control of Homeostasis
Homeostasis is continually being disrupted by
External stimuli
heat, cold, lack of oxygen, pathogens, toxins
Internal stimuli
Body temperature
Blood pressure
Concentration of water, glucose, salts, oxygen, etc.
Physical and psychological distresses
Disruptions can be mild to severe
If homeostasis is not maintained, death may result

21. Alteration in the mechanisms of feedback often results in deleterious consequences
Diabetes mellitus
Response to decreased insulin
Cannot absorb glucose out of the blood
Dehydration
Response to decreased antidiuretic hormone (ADH)
Grave’s disease (hyperthyroidism)
imbalance of metabolism caused by overproduction of thyroid hormone which control the way that every cell in the body uses energy (metabolism).
Blood clotting

22. Life Sciences-HHMI Outreach
Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.

23. Plant responses to water limitations
When plants under water stress, they close their stomata and decrease their transpiration rates (water loss that occurs when stomata open to get carbon dioxide for photosynthesis)
Life Sciences-HHMI Outreach. Copyright 2009 President and Fellows of Harvard College.

24. Operons in gene regulation
An operon is a cluster of coordinately regulated genes.  It includes structural genes(generally encoding enzymes), regulatory genes(encoding, e.g. activators or repressors) and regulatory sites(such as promoters and operators).

25. Ripening of Fruit
First fruit that starts to ripen releases ethylene gas that stimulates other fruits to ripen and create more ethylene gas