A1.2 Homeostasis

🏠 Homeostasis

⚖ Homeostasis = regulation of the body’s internal environment so it remains relatively constant within physiological limits

🔄 It is a dynamic process, involving continuous adjustment and balance

🏃‍♂ The body adapts to stressors, including:

🔹 Internal stressors – e.g. low blood glucose levels

🔹 External stressors – e.g. heat, cold, humidity, lack of oxygen

🎯 A stable internal environment is essential for:

✔ Optimal performance

✔ Safe and effective body function

🧠 Role of the Hypothalamus in Homeostasis

📍 The hypothalamus is located in the diencephalon

🛠 It is the main control centre for homeostasis

🔧 Regulates key physiological processes, including:

❤ Blood pressure

💓 Heart rate

💪 Strength of cardiac muscle contraction

🌬 Respiration

🍽 Digestion

🌡 Body temperature

💧 Thirst and fluid balance

🔗 Interaction between the nervous and endocrine systems

🍎 Appetite and food intake

😴 Sleep–wake cycles

🏃‍♀ During exercise and extreme environments (e.g. heat, cold), the body must make rapid, coordinated adjustments

🔧 The body may need to:

⚡ Increase metabolism to meet energy demands

❤🌬 Adjust cardiovascular and respiratory responses to supply oxygen (O₂)

♻ Remove metabolic by-products (e.g. CO₂)

🌡 Regulate body temperature to avoid hyperthermia or hypothermia

🧠 Systems Involved in Regulation

⚡ Nervous system – sends nerve impulses to organs (fast responses)

🧪 Endocrine system – releases hormones into the blood (slower responses)

🔄 Feedback Mechanisms

📊 The body monitors variables such as body temperature and blood glucose level

🔗 A feedback mechanism has three components:

👁 Receptor – detects change (e.g. skin receptors sensing temperature)

🧠 Control centre – processes information (e.g. hypothalamus)

💪 Effector – produces a response (e.g. shivering when cold)

⚖ Types of Feedback

⬇ Negative feedback – reverses the effect of the stimulus

⬆ Positive feedback – amplifies the stimulus

✅ Homeostasis is mainly maintained by negative feedback, e.g. regulation of blood pressure

🧪 Regulation of Blood pH

⚖ Blood pH is a key component of homeostasis

Normal arterial blood pH: 7.35–7.45 (slightly alkaline)(0-14 scale)

🔬 Cellular metabolism produces carbon dioxide (CO₂)

CO₂ is transported in the blood to the lungs for removal

⚠ Increased CO₂ concentration → increased hydrogen ions (H⁺)

↓ pH (blood becomes more acidic)

Can harm tissues and organs

🧠 Monitoring systems:

👃 Chemoreceptors detect changes in blood pH and CO₂

🧠 Respiratory control centre in the brain monitors these changes

🌬 Response to low pH (high acidity):

⬆ Breathing rate and depth

⬇ CO₂ removed from the blood

🔄 Blood pH returns toward normal

🔁 This process is an example of negative feedback

❤ Regulation of the Heart

🔄 Cardiac cycle does not require nerve stimulation to contract

⚙ Heart regulation involves two control mechanisms:

💓 Intrinsic excitation

Electrical activity originates within the heart itself

🧠 Extrinsic excitation

Signals come from outside the heart

Includes input from:

🧠 Nervous system

🧪 Hormonal (endocrine) system

💓 Intrinsic Excitation of the Heart

🟢 Sinoatrial (SA) node

Heart’s natural pacemaker

Located in the right atrial wall

Generates electrical impulses that initiate heart contraction

🧪 SA node firing rate influenced by:

🫁 Blood O₂ and CO₂ levels

🧬 Hormones (e.g. epinephrine, norepinephrine)

➡ Impulse pathway

🔄 SA node → spreads through both atria

⚡ Atria contract → blood pushed into ventricles

🔘 Impulse reaches atrioventricular (AV) node

⏸ Brief delay allows ventricular filling

➿ Impulse travels via bundle of His

⚡ Conducted through Purkinje fibres

💪 Ventricles contract simultaneously → blood ejected into arteries

🚪 Valves

🔒 AV valves close during ventricular contraction

🔒 Semilunar valves close as ventricles relax

🔁 Cycle repeats

Atria refill with blood

Next contraction triggered by SA node firing

💓 Extrinsic Excitation of the Heart

🧠 Controlled by the autonomic nervous system

Adjusts the SA node firing rate

Maintains homeostasis

⚡ Sympathetic nervous system

⬆ Increases heart rate

⬆ Increases force of contraction

🧬 Releases epinephrine (acts as neurotransmitters)

⬆ Increases cardiac output

🧘 Parasympathetic nervous system

⬇ Slows SA node firing rate (due to acetylcholine release from Vagus nerve)

⬇ Decreases force of contraction

🧬 Releases acetylcholine

⬇ Decreases heart rate and cardiac output

🔄 Involuntary control

Uses neurotransmitters to adjust heart activity

Responds to internal and external stimuli

🏃 During exercise

⬇ Parasympathetic stimulation

⬆ Sympathetic stimulation

⬆ Heart rate to meet increased demands

🩸 Regulation of Blood Glucose

🧠 Pancreas controls blood glucose

Releases insulin and glucagon

⬆ High blood glucose

🧬 Pancreas secretes insulin

➡ Increases glucose transport into cells (especially muscle)

⬇ Lowers blood glucose levels

⬇ Low blood glucose

🧬 Pancreas secretes glucagon

🔁 Stimulates glycogenolysis (glycogen → glucose)

🔁 Stimulates gluconeogenesis (protein/fat → glucose)

🏃 During prolonged exercise (≥30 min)

⬇ Insulin concentration

⬆ Insulin sensitivity of muscle cells

✔ Maintains plasma glucose levels

🔄 Regular exercise

⬆ Improves insulin sensitivity

⬇ Reduces insulin resistance

⬇ Lowers risk of high blood glucose

⚠ Insulin resistance

Cells become less responsive to insulin

Leads to higher blood glucose

Associated with sedentary lifestyle and poor diet

✅ Healthy regulation

🏃 Regular physical activity

🥗 Healthy diet

⬆ Better blood glucose control

🧠 Thermoregulation

Maintains core body temperature at 37 ± 1°C

Requires coordination of:

❤ Cardiovascular system

💪 Muscular system

🧠 Nervous system

🧍 Integumentary (skin) system

🔥 Production of heat in the body

☀ All energy originates from the Sun

🌱 Plants convert light → chemical energy

🍽 Humans obtain energy by eating plants or animals

🧬 Energy stored as carbohydrates, fats, proteins

⚡ Energy released in cells and stored as ATP

💪 Muscle contraction and heat

🔄 Muscle contraction is ~20% efficient

🌡 ~80% of energy released as heat

❗ Excess heat must be removed to prevent overheating

🚚 Heat loss

🔁 Heat transported from core → skin

🌬 Heat lost from skin to the environment

🧍 Core temperature (Tᶜ)

Remains reasonably constant throughout life

Maintained despite wide ambient temperature changes

🌡 Measuring core temperature (Tᶜ)

👄 Oral thermometer (mouth)

👂 Ear thermistor (tympanic membrane)

🍑 Rectal probe (very accurate)

📡 Gastrointestinal radio pill (swallowed; expensive)

🧠 Core vs shell temperature

🔥 Core temperature = deep body temperature

❄ Shell temperature = near skin surface

📉 Shell temperature is 1–6°C lower than core (environment dependent)

🧠 Temperature regulation

🌡 Temperature sensors located throughout the body (e.g. skin)

🎛 Hypothalamus acts as the body’s thermostat

⚡ Metabolic rate

Body heat production is proportional to metabolic rate

🏃 Exercise

Increases metabolic rate → more heat produced

🧪 Hormones

Thyroid hormones ↑ basal metabolic rate

🧠 Nervous system

Sympathetic activity (e.g. during exercise) ↑ metabolic rate

🌡 Body temperature

Higher body temperature ↑ metabolic rate

🍽 Food ingestion

Energy used for digestion, absorption and storage ↑ metabolic rate

👶➡🧑 Age

Children have a higher metabolic rate than adults

🚻 Sex differences

Metabolic rate is lower in biological females

⬆ Increases during pregnancy

🌡 Heat Transfer

⚖ Heat balance

Normal body temperature requires a balance between heat production and heat loss

🔄 Heat transfer methods

🔥 Conduction

🌬 Convection

☀ Radiation

💧 Evaporation

🏃 Exercise and heat production

Exercise ↑ heat production in skeletal muscles

Muscle temperature rises

⚠ Imbalance

Too much heat production + too little heat loss → hyperthermia

🌍 Environmental heat stress

Increases need for:

💦 Sweating

❤ Circulatory responses

🔁 Core to skin heat transfer

Heat from the core is transported by blood to the skin

Heat is then lost to the environment via the four mechanisms above

🔥 Conduction

🧠 Heat source

Heat is generated deep in the body

🔁 Heat transfer

Heat moves through body tissues to the skin surface

Then transfers to clothing, air, or water in direct contact with the skin

🌡 Rate of heat loss

Depends on the temperature gradient between the skin and surrounding surfaces

📉 Contribution to heat loss

In hot and humid conditions, conduction accounts for < 2% of total heat loss

🌬 Convection

🔄 Heat transfer by movement

Heat is moved by the motion of air or water

🩸 Internal convection

Blood transfers heat from deep body tissues to the skin

🌡 Low air movement

Air next to the skin warms up

➝ Heat loss slows down

💨 High air movement (wind)

Cooler air replaces warm air around the body

➝ Heat loss increases

☀ Radiation

📡 Heat transfer by energy waves

Energy is emitted from one object and absorbed by another

🌞 Environmental radiation

Solar energy from sunlight can be reflected (e.g. snow, sand, water) and warm the body

🌡 Heat gain condition

The body absorbs radiant heat when environmental temperature > skin temperature

💧 Evaporation

🌡 High environmental temperature

Effectiveness of conduction, convection and radiation decreases

💦 Primary heat loss method during exercise

Evaporative cooling becomes the main mechanism

🔄 How it works

Sweat changes from liquid to water vapour

This process removes heat from the body

🌡 Thermoregulatory Mechanisms

🧠 Non-shivering thermogenesis

Heat production without shivering

Driven by sympathetic nervous system

↑ Metabolic rate → ↑ heat production

💪 Shivering

Involuntary skeletal muscle contractions

Can increase heat production to ~4× basal rate

💨 Insensible water loss

Heat lost via water vapour from skin and respiratory passages

~580 kcal lost per litre of water vaporized

Accounts for:

~80% of heat loss during physical activity

~20% of heat loss at rest

🌡 Thermoregulatory Mechanisms

💦 Sweat glands

2–4 million sweat glands across the body

Sweat production depends on:

🧮 Density of sweat glands

💧 Amount of sweat per gland

Highest sweating rates:

Back and chest

Limbs only after large rise in core temperature

🔥 Exercise and sweating

Heat production increases rapidly at exercise onset

Core temperature rises → sweating increases

Sweating is not 100% efficient (some sweat drips off)

🌬 Evaporative cooling efficiency

Max heat loss occurs when sweat evaporates

Sweat that drips or is wiped off provides no cooling

Effectiveness depends on:

💧 Humidity

Dry air → high evaporation

Humid air → reduced evaporation

Relative humidity is the key factor affecting evaporative heat loss

👨‍🦱👩‍🦰 Sweat rate

Biological males typically have a higher sweat rate

Allows more efficient heat dissipation

⚖ Body size and shape

Biological females generally have:

Smaller body mass

Larger surface-area-to-mass ratio

This can be a thermoregulatory advantage in endurance events when aerobic ability is similar

🌡 Environmental influence

A higher surface-area-to-mass ratio is not always advantageous

In hot, dry conditions where air temperature > skin temperature:

Greater surface area can lead to faster heat gain

🔄 Hormonal fluctuations (menstrual cycle)

Progesterone peaks during the luteal phase

Associated with:

⏳ Delayed onset of sweating

🔻 Decreased skin blood flow

Results in a small increase in internal body temperature during exercise in the heat compared with the follicular phase

🏃‍♀ Performance impact

These temperature changes do not affect performance

Unless core temperature approaches ~40°C

🔬 Research note

Further research is needed to fully explain thermoregulatory differences related to sex hormone fluctuations

🔬 Study focus

Investigated whether menstrual cycle phase affects heat tolerance classification

🔄 Menstrual cycle phases

❌ No difference between follicular and luteal phases

Did not influence whether females were classed as heat-tolerant or heat-intolerant

🏃‍♀ Key predictor of heat tolerance

⭐ Maximal oxygen uptake (VO₂max)

Fitter individuals showed:

Improved sweating

Greater skin blood flow

Better heat dissipation

⚠ Heat illness risk

❌ No evidence females are at greater risk

✔ When standard heat risk management strategies are used

🧠 Key takeaway

Effects of sex and reproductive hormones on thermoregulation are complex

Depend on:

Exercise intensity

Environmental conditions