Chapter 9: Homeostasis & Circulation

Section 9.1:Homeostasis: Life in the Balance

pg. 300-303

In a room, I have a thermostat which has a

thermometer that records the room’s temperature.

The thermostat is connected to my furnace and is set at 20° Celsius.

Explain what happens on a cold day.

Like your thermostat,multicellular organismsmust maintain near-constantconditions within their internalenvironment so that all of theirindividual cells can continueworking together.(i.e. blood glucose = 100 mg/mL, blood pressure

= 120/80, body temperature = 37 degrees C)

Homeostasis- “steady state”

The body’s maintenance of a relatively stable internal environment despite a changing external environment.

e.g. blood glucose concentration, blood pH, blood pressure, body temperature

Dynamic Equilibrium-

State of balance achieved within an environment as the result of internal control mechanisms that continuously oppose outside forces that tend to change that environment.

Homeotherms:- (homeo = same, therm = temperature)

These organisms keep their body temperature relatively constant.E.g. birds & mammals

Poikilotherms – animals whose body temperature fluctuates depending on the temperature of their environment.

(i.e. cold-blooded animals like turtles, lizards, snakes)

What are some behavioural ways that humans control their body temperature?

Physiological Means of Maintaining Body Temperature

• The normal human body temperature is 37.5 degrees C. Your body generates heat as a by-product of metabolic processes.

• As warm blood from the interior of the body passes by the skin, heat is lost.

• The constriction of blood vessels is called vasoconstriction.

• The dilation of blood vessels is called vasodilation.

• The body maintains homeostasis through the use of feedback loops– Negative Feedback loops– Positive Feedback loops

Negative Feedback Loop• The body’s

reaction to increased or decreased temperature.

• A process that detects and reverses deviations from normal body temperature or other constants.

Negative Feedback Loop

• Involves three parts:– A sensor– An integrator– An effector

Negative Feedback Loop• Sensory receptors

– found in every organ and body tissue

– Its function is to send nerve impulses to the brain in response to environmental info

– Monitor pH, blood sugar, blood pressure and many other internal conditions on a continual basis

– i.e thermoreceptors in the skin

Negative Feedback Loop• Integrator

– The brain (hypothalmus)

– Takes info from sensor

– Compares this info to desired set point

– Sends a message to the effector

Negative Feedback Loop• Effector

– Any structure that takes action to return the value to the desired set point

– Causes the change in internal condtions

– i.e. blood vessels constrict or dilate

Negative Feedback Loop• Moves a system

toward balance and stability

• Essential to maintain homeostasis.

Positive Feedback Loop• Rare in healthy bodies• Acts to move a system

away from balance and stability

• Usually associated with disease or change – i.e. Addictions, child birth

Body systems play animportant role in maintaining homeostasis.

Equilibrium is maintainedas long as the systemis active.

Many Body Systems Interact to Maintain Homeostasis (Fig. 9.2, pg. 301)

1. Why do we need a circulatory system?

2. What does this system transport?

3. In humans, is this system open or closed? (Does blood ever leave the system)

Section 9.1Review Questions

• Do question #’s 1-6 on page 303

Section 9.1Answers

1. Explain how the brain helps to regulate body temperature.

The hypothalamus of the brain is the body’s thermostat. It regulates temperature by controlling blood flow to skin, sweating and shivering (muscle contraction).

2. Provide one example of dynamic equilibrium in the body.

Temperature regulation, control of blood sugar levels, etc.

3. Describe the role of the skin in maintaining homeostasis in the body.

The skin provides physical protection from environmental hazards (e.g. excess sunlight), acts as a barrier to infectious agents, and regulates body temperature.

4. Explain the difference between hypothermia and hyperthermia.

Hypothermia = core body temperature falls below the normal temperature range (37.5 degrees Celsius).

Hyperthermia = an elevation of core body temperature above the normal range.

5. Explain the graph showing the relationship between body temperature and ambient (environmental) temperature in a cat and a lizard. How is the cat able to maintain a more constant body temperature than the lizard?

In mammals, metabolic reactions in the body generate heat internally and various physiological mechanisms work to maintain this body temperature.

Question 5 (continued)

In reptiles, body heat is primarily absorbed from external sources, so body temperature is affected by changes in environmental temperature.

6. Explain the difference between a negative feedback loop and a positive feedback loop.

Negative feedback loops are systems that reverse the original stimulus.Positive feedback loops are systems that reinforce the original stimulus.