4. Responses to a Cold Environment

Success Criteria

Your learning has been successful if you can do the following:

I can explain how balance is re-established when the body gets too cold.
I can explain why life processes are affected if the negative feedback loop stops in a cold environment.
My explanations for the above includes specific biochemical or biophysical processes.

Vocabulary

Learn these so you can communicate this concept well.

Action potential: An electrical signal that is rapidly transmitted along the neuron.
Arteriole: Small blood vessels that can change diameter because of the smooth muscle around them.
Basal metabolic rate (BMR): The amount of energy (and therefore heat) the body produces to sustain basic life processes to keep you alive.
Core temperature: Actual temperature of the vital organs such as the heart, brain and liver.
Enzymes: Biological catalyst that speeds up a chemical reaction.
Hypothermia: Condition where the core temperature drops below 35°C.
Membrane permeability: How easily substances can pass through the cell membrane.
Neurotransmitter: A chemical signal that is released by neurons to communicate to another cell over very short distances.
Radiation: Loss of heat in infrared radiation emitted by the skin.
Shivering: Skeletal muscles rapidly contracting and relaxing to generate heat.
Smooth muscle: Muscles around your arterioles that contract and relax to cause vasoconstriction and vasodilation.
T4 (thyroxine): The most common thyroid hormone, but least potent.
Thermogenesis: Production of heat.
Thyroid gland: Effector organ located at the base of the neck that produces thyroxine (T4).
Vasoconstriction: The narrowing of arterioles due to smooth muscles contracting. Caused by the hormone, norepinephrine. reate different sounds when we speak.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

State which one is the odd one out: increased metabolism, sweating, vasoconstriction, shivering. Explain why in your books.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

1) Define what is considered to be normal body temperature in humans.

2) Explain why cells need a warm temperature to function.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Thyroid hormones:

What is the order?

Then, write an answer for it.

Exit Task

In your Learning Journal:

Re-write this interpreting question so it is asking about Death:

How did it happen?

Then, write an answer for it.

Reminder: In your internal report, you MUST link your report to specific scenarios provided to you.

How the Negative Feedback Loop NORMALLY Responds to a Cold Environment

The first normal response is always voluntary and behavioural - we may decide to put more clothes on, take shelter from the wind and rain, jump up and down or rub our hands together, or curl into a ball.

It is only when these voluntary responses are not enough, that the control centre stimulates involuntary responses.

1) Responses to increase the amount of heat produced

The THYROID GLAND (effector) produces THYROXINE hormone (T4), which is transported in the blood to all cells in the body to increase our BASAL METABOLIC RATE / aerobic respiration (response).
Skeletal muscles (effector) contract and relax, causing us to SHIVER (response).

These three responses generate heat through thermogenesis.

2) Responses to decrease heat loss (to get less cold).

The sympathetic nervous system (controller) triggers the adrenal gland (effector) to produce the hormone called norepinephrine.
SMOOTH MUSCLES in ARTERIOLES (effector) contract, causing VASOCONSTRICTION (response). This reduces blood flow to the extremities, so less heat is lost via RADIATION to the surroundings.
Sweat glands in the skin (effector) stop producing sweat (response). This stops heat being lost from evaporation.
(Vestigial - was once useful to our ancestors but little adaptive advantage to us) Erector pili muscles (effector) contract, erecting / raising the skin hairs (response). This traps a layer of warm air close to the skin, and stops this warm layer from being lost by convection.
- - Video: Arrecter pili muscle, General Histology
  - Video: Why do we get Goosebumps?

You MUST read the "Biophysical & Biochemical Processes of Sweating and Evaporative Cooling."

Biochemical process of producing thyroxine hormone.

You MUST read the reading called "Biochemical Processes for a Cold Environment"

Sky level - Fantastic and short explanation of how thyroid hormone is produced. No information on function.

Sun level - More than what you need to know, but it comprehensively outlines the production and function of thyroid hormone.

When the Negative Feedback Loop STOPS Working in a Cold Environment (Hypothermia)

Hypothermia

HYPOTHERMIA is a medical condition in which the core temperature of the body drops below 35°C. When the negative feedback loop is working, the body tries to generate more heat (THERMOGENESIS) through shivering and increasing the basal metabolic rate (through the action of thyroid hormone).

However, the negative feedback loop stops working when the body cannot generate enough heat to compensate for the rate at which heat is being lost. This can lead to range of neurological symptoms, and in severe cases, unconsciousness and death.

Three Phases & Symptoms

Phase 1: Mild hypothermia - blue extremities, loss of balance, slurred speech, lethargy, violent shivering, hypertension and cold diuresis.
Phase 2: Moderate hypothermia - irrational behaviour (paradoxical undressing), shivering stops, confusion, loss of muscle coordination.
Phase 3: Severe hypothermia - speech failure, loss of consciousness, unresponsive, cardiac arrest.

You must read the "Hypothermia Reading" for more information on each symptom.

Switch to the Positive Feedback Loop = Uncontrollable Fall in Temperature

To try to bring the core temperature back ot the set point, thermogenesis (by voluntary movements, involuntary shivering and increasing metabolism) increases to the point that accessible supplies of glucose run out. This has flow-on effects:

Lack of glucose and energy in the muscles causes SHIVERING and voluntary movements to stop - less heat is generated.
Lack of glucose and energy means that despite the increase in THYROID HORMONE, the body does not have enough energy left to generate more heat.
Lack of glucose and energy in the brain also causes neurological effects and reduced brain activity - irrational behaviour can worsen the condition, heart rate and breathing are slowed, loss of consciousness.

This results in the body no longer being able to generate more heat, and CORE TEMPERATURE drops uncontrollably.

Prolonged hypothermia can be fatal, due to the effects of extreme hot temperatures on enzymes and membrane permeability.

ENZYME activity slows down. This means that ATP synthase enzyme no longer produces enough energy for:

Muscle contraction
Brain activity (neurons sending signals to other parts of the body such as the heart and lungs)

MEMBRANE PERMEABILITY and fluidity decreases. This disrupts the distribution of ions:

Calcium ion stores are needed for muscle contraction
Calcium ion gradients are needed by neurons to send signals called NEUROTRANSMITTERS.
Sodium and potassium ion gradients are needed by neurons to send signals called ACTION POTENTIALS.

(Links to Excellence) Enzymes when the core temperature is COLDER than the set point.

At low temperatures, the movement of the enzyme molecules becomes sluggish, and their ability to bind to substrates and catalyze reactions decreases. This results in a decrease in the rate of chemical reactions, which can have a significant impact on the functioning of the body. For example, enzymes involved in energy production and metabolism become less efficient, leading to decreased energy production and an increase in fatigue.

Hypothermia occurs when the core body temperature drops below the normal range of 36-37°C. As the body temperature decreases, the activity of enzymes decreases as well. This can result in a slowing or cessation of many physiological processes, including cellular respiration, metabolism, and cellular function.

(Links to EXCELLENCE) Membrane permeability when the core temperature is COLDER than the set point.

An increase in temperature can cause the phospholipids to become more fluid, which can increase the fluidity of the cell membrane - more than normal. This can increase the permeability of the membrane, allowing ions and other small molecules to move more freely across the membrane - more than what's required to maintain a constant internal environment.

This increased permeability can lead to the loss of ions and small molecules, leading to cellular dehydration and altering the normal ion gradients across the membrane. This can disrupt normal cellular processes and contribute to the development of cellular damage.

(So, the cell could be spending energy on active transport of ions, but membrane that's too permeable could undo the work done by active transport).

This increased fluidity can also affect the function of membrane proteins, which are embedded in the lipid bilayer. In extreme cases, high temperatures can cause the lipid bilayer to become so fluid that it can break apart, leading to a loss of membrane integrity and cell death.