5. Responses to a Hot 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 hot.
I can explain why life processes are affected if the negative feedback loop stops in a hot environment.
My explanations for the above includes specific biochemical or biophysical processes.

Vocabulary

Learn these so you can communicate this concept well.

Aerobic respiration: Enzyme controlled process which requires oxygen to produce 38 ATP from the breakdown of glucose.
Arteriole: Small blood vessels that can change diameter because of the smooth muscle around them.
Core temperature: Actual temperature of the vital organs such as the heart, brain and liver.
Enzymes: Biological catalyst that speeds up a chemical reaction.
Evaporative cooling: Loss of heat through sweat.
Membrane permeability: How easily substances can pass through the cell membrane.
Radiation: Loss of heat in infrared radiation emitted by the skin.
Set point: Temperature that the body is trying to maintain via thermoregulation (37°C)
Smooth muscle: Muscles around your arterioles that contract and relax to cause vasoconstriction and vasodilation.
Vasoconstriction: The narrowing of arterioles due to smooth muscles contracting. Caused by the hormone, norepinephrine.
Blood volume: Amount of blood in the circulatory system.
Blood pressure: Force with which blood pushes against the walls of blood vessels as it is pumped by the heart.
Heart rate: Number of times the heart beats per minute.
Hyperthermia: Condition when the core temperature rises to 39°C or higher.
Positive feedback loop: A mechanism where the body's response to a stimulus actually amplifies the stimulus instead of reducing it, making it worse.
Sweat gland: Effector located in the skin that produces sweat.
Vasodilation: The widening of arterioles due to smooth muscles relaxing.

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

1) List 2 causes of rising body temperature.

2) Think of 4 ways a rising body temperature can be countered.

3) What do you think "vasodilation" means?

Hei Mahi (Do Now)

Do Now in your OneNote/Notebook:

What's the difference between thyroid hormone and norepinephrine?

Exit Task

In your Learning Journal:

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

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 Hot Environment

The first normal response is always voluntary and behavioural - we may decide to take some clothes off, spread out or fan ourselves, or move to the shade.

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

1) Responses to increase heat loss (to cool down).

SWEAT GLANDS in the skin (effector) produces SWEAT (response). This sweat EVAPORATES from the skin, taking heat energy away with it.
SMOOTH MUSCLE in ARTERIOLES (effector) relax, causing VASODILATION (response). This allows more blood to flow to the extremities so more heat is RADIATED to the surroundings.
(Vestigial) Arrector pili muscles (effector) relax, lowering the skin hairs (response). This allows air to circulate over the skin so more heat is lost by convection.

2) Responses to decrease the amount of heat produced (to get less hot).

Skeletal muscles (effector) stop shivering (response).
THYROID GLANDS (effector) stop secreting THYROXINE to decrease the BASAL METABOLIC RATE, and therefore reduce THERMOGENESIS (response).

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

Biophysical & Biochemical Process of Sweating.

You MUST read the reading called "Biophysical & Biochemical Processes for a Hot Environment"

Biophysical process of evaporative cooling.

You MUST read the reading called "Biophysical & Biochemical Processes for a Hot Environment"

When the Negative Feedback Loop STOPS Working in a Hot Environment (Hyperthermia)

Hyperthermia

HYPERTHERMIA refers to those conditions that take place when core temperature climbs to 39°C or higher. When the body produces more heat than it can get rid of, the body can lose too much water from sweating, that the severe dehydration can cause the negative feedback loop to stop working.

Three Phases & Symptoms

Phase 1: Overexertion - a flushed red face and rapid short breaths.
Phase 2: Heat exhaustion - red skin, rapid breathing, profuse sweating, dry mouth, cramps, nausea, and vomiting
Phase 3: Heat stroke - core temperature reaches 41°C or higher, sweat is no longer produced, leading to a hot and dry skin, disorientation, collapse, and unconsciousness.

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

Switch to the Positive Feedback Loop = Uncontrollable Rise in Temperature

To try to bring back CORE TEMPERATURE to the SET POINT, sweating increases and dehydration occurs. Once the individual loses around 12% of their water, the situation becomes critical.

Severe dehydration causes flow-on effects that ultimately leads to a POSITIVE FEEDBACK LOOP, where the body's response to a stimulus actually amplifies the stimulus instead of reducing it. Positive feedback, therefore, results in the core temperature continues to rise uncontrollably - the body produces more heat and amplifies the stimulus rather than counteracts it.

Prolonged hyperthermia can be fatal, due to the effects of extreme hot temperatures on ENZYMES and MEMBRANE PERMEABILITY.

Dehydration leads to:

A decrease in BLOOD VOLUME and therefore BLOOD PRESSURE. The body tries to increase blood pressure by:
- - Increasing HEART RATE - however, this ends up increasing AEROBIC RESPIRATION and therefore the amount of heat produced
  - VASOCONSTRICTION, to direct blood to vital organs - however, this ends up damaging some other tissues (leading to vomiting and more dehydration) and reducing heat loss via radiation.
Sweating stops to conserve water (and blood volume).
- - Heat is no longer lost by EVAPORATIVE COOLING.

(Links to EXCELLENCE) Enzymes when core temperature is HOTTER than the set point.

At higher temperatures, enzymes can become denatured, meaning that their three-dimensional shape is altered and they lose their ability to function. The rate of denaturation increases with temperature, so as the body temperature increases, the activity of the enzymes also decreases.

The specific temperature at which this occurs can vary depending on the specific enzyme, but in general, a 1-2°C increase in core body temperature can cause denaturation of some enzymes. A core body temperature of around 41°C can be life-threatening, and at this temperature, many enzymes can become denatured and stop functioning properly.

In the case of hyperthermia, the elevated body temperature can cause denaturation of enzymes involved in cellular metabolism, leading to a decrease in metabolic rate. This can result in decreased energy production and decreased efficiency in cellular processes, leading to potential cellular damage and dysfunction.

Additionally, hyperthermia can affect the activity of enzymes involved in the regulation of body temperature itself. For example, hyperthermia can impair the activity of heat shock proteins, which help protect cells from heat stress, further exacerbating the effects of elevated body temperature.

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

When the body temperature decreases, the phospholipids and therefore the cell membrane becomes less fluid. This can lead to changes in the permeability of the membrane, making it more difficult for ions and other small molecules to cross the membrane causing disruptions in normal cell function.

For example, changes in ion gradients across the membrane can interfere with the normal function of ion channels and transporters, which play critical roles in maintaining cell membrane potential, transmitting nerve impulses, and transporting ions and other small molecules into and out of the cell.

This decreased permeability can also affect the function of membrane proteins and enzymes, which may become more rigid and less able to perform their normal functions. In extreme cases, low temperatures can cause the lipid bilayer to become so rigid that it can no longer maintain its integrity, leading to cell damage or death.

For example, hypothermia can also disrupt normal membrane-associated enzyme activity, which can interfere with a wide range of metabolic processes. For example, enzymes involved in the breakdown of glucose to produce ATP may become less efficient, reducing the cell's ability to produce energy.