AOS Unit 1Chapter 6

What is homeostasis?

• Homeostasis is the maintenance of a stable internal environment despite changes in the external environment.

• It ensures cells function optimally within specific limits.

• Cells like specific environments or limits

Why can we  stand outside in the heat for a little while before you  start to feel really uncomfortable

cells don’t instantly die when the external environment of your body changes because we maintain homeostasis

stimulus-response model

a model that describes how a system responds to react to a prompt

a stimulus positive feedback system

a stimulus–response process in which the response increases the stimulus 

negative feedback system

a stimulus–response process in which the response counters the stimulus 

stimulus (pl. stimuli)

an event or molecule that can initiate a response

receptor 

a structure that detects a signal or external change, usually a protein 

modulator/processing centre

location where information from receptors is sent to and compared to a set point, and where molecules altering  the functioning of an effector  are released.

effector

a molecule, cell, or organ that responds to a signal and produces a response 

hormone

a signalling molecule released from endocrine glands that regulates the growth or activity of target cells

response

 the action of a cell, organ, or organism caused by  a stimulus

What is controlled by  homeostatic mechanisms

External environment changes (heat, cold, dehydration, etc.)

Internal environment must stay constant

Stimulus–Response Model

Stimulus

• Change in internal/external environment

• e.g. increase in temperature

Receptor

• Receptor Detects the change (stimulis)

• Converts it into a chemical or electrical signal for modulator 

Modulator (processing centre)

• Usually brain or endocrine gland

• Compares to set point (ideal value)

• Sends signals to effectors

Effector

• hormone/Organ/cell that carries out response

• e.g. sweat glands, muscles

Response

• Action that occurs in response to stimulus 

• e.g. sweating to cool body

• any change in the function of a target cell, organ, or organism

Different types of receptors

Example of stimulus response model (NOT HOMEOSTASIS)-reflex arc

Seeing a hot object and pulling your hand away

1. Stimulus

• Touching a hot surface

2. Receptor

• Pain/temperature receptors in the skin detect heat

3. Modulator

• Spinal cord / brain processes the signal

4. Effector

• Arm muscles

5. Response

• Hand is pulled away quickly

Example of a Positive stimulus response model-Blood Clotting

Stimulus

• A blood vessel is damaged (cut or injury)

Receptor

• Platelets detect exposed damaged tissue

Modulator

• Platelets release chemical signals (clotting factors)

Effector

• More platelets are activated and attracted to the site

Response

• Platelets stick together until sealed and form a blood clot

The response (platelet activation) amplifies itself, leading to rapid clot formation

Why is blood clotting a postive feedback response

• Activated platelets release chemicals →

• This attracts more platelets →

• Which release even more chemicals

The response keeps increasing until the clot is fully formed

Negative feedback loop does what?

• Response counteracts the stimulus

• Returns system to set point

• Maintains homeostasis

the response attempts to revert the system back to the state it was in before the stimulus occurred.

Example of negative feedback loop  Body Temperature (Too Hot)

1. Stimulus

• Body temperature rises above 37°C

2. Receptor

• Thermoreceptors in the skin and brain detect the increase

3. Modulator

• Hypothalamus compares it to the set point (37°C)

4. Effector

• Sweat glands increase sweat production

• Blood vessels dilate (vasodilation)

5. Response

• Sweating → heat lost by evaporation

• Vasodilation → more heat lost from skin

Cell Signalling

3 Steps:

1. Reception → detect signalmechanical, electrical, or chemical signal 

2. Transduction → signal passed along

sending a signal between organisms, across the body, to a neighbouring cell, or back to the original receptor cell 

3. Response → action occurs

What are the four methods of heat transfer?

Conduction-

Convection -

Evaporation

Radiation -

Conduction-

The transfer of heat through physical contact with another object

When you touch something hot, heat from that object is transferred to your fingers via conduction

Convection -

The transfer of heat via the movement of a liquid or a gas between areas of a different temperature

The temperature is warmer in the second storey of your house because hot air rises, taking heat energy with it

Evaporation

-The loss of heat via the conversion of water from liquid to gas form

When you sweat, the water on your skin evaporates. Turning a liquid into a gas requires a lot of energy, and when sweat evaporates it takes away heat energy from your skin making you cool down

Radiation -

The transfer of heat via electromagnetic waves such as light (i.e. doesn’t require physical contact with another object)

The sun warms you via radiation. Conversely, when you stand in a cold room and you aren’t wearing much clothing, you lose heat to your environment via radiation

Thermoregulation occurs via

a negative feedback stimulus-response system.

Thermoregulation

 the homeostatic process of maintaining a constant internal body temperature 

Formula for total heat change

Total heat change= heat in + metabolic heat – heat out

metabolism

The set of chemical reactions within cells that help maintain the body’s normal functioning including converting food and drink to energy 

endotherm

an animal that produces the majority of its own heat via metabolic processes 

ectotherm an animal that obtains heat primarily from the environment, rather than its own metabolic heat

When Body is TOO HOT

Goal: lose heat + reduce heat production

increasing the amount of heat lost to the environment and decreasing the amount of heat produced by the body

• Sweat glands produce sweat which evaporates from the skin, taking heat energy with it

 • Small blood vessels in the skin vasodilate, increasing surface blood flow. Blood is warm, so by increasing the amount of blood at the surface of the body the heat lost to the environment via convection and conduction is increased 

• The cerebral cortex causes changes in behaviour, such as seeking shade 

• Arrector pili muscles in the skin relax, which flattens body hair against the skin.  Doing this increases the free flow of air against the skin, which increases the amount of heat lost due to convection

 • At a cellular level, signals are sent by the hypothalamus to slow metabolic processes which reduces the amount of heat made by the body.

vasodilation

The widening of blood vessels 

cerebral cortex

The outer layer of the brain that plays a key role in a number of processes including memory, attention, and perception

arrector pili muscles

arrector pili muscles small muscles attached to hair follicles

When Body is TOO COLD

Goal:decreasing the amount of heat lost to the environment, and increasing the amount of heat produced by the body

Skeletal muscles are stimulated to cause shivering, a process in which muscle cells are stimulated to move quickly which increases their metabolism and creates more  heat energy

 • Small blood vessels in the skin constrict through a process known as vasoconstriction, decreasing surface blood flow. This means that less body heat is lost to the environment vasodilation the widening of blood vessels s 

• The cerebral cortex causes changes in behaviour, such as putting on more clothing

 Arrector pili muscles in the skin contract, lifting hair follicles up, causing goosebumps, and trapping a layer of air. This layer of air serves as an insulating layer against the surrounding cold environment •

At a cellular level, signals are sent to increase metabolic processes such as cellular respiration which in turn results in more heat energy being produced • Brown fat cells are stimulated to produce heat via the burning of triglycerides.

skeletal muscle

 a type of muscle that is voluntarily controlled and that is usually attached to bones vasoconstriction the narrowing of blood vessels

What is glucose?

the main source of energy for all the cells in our body. 

Where do we get glucose from?

We get it from the food we eat in the form of carbohydrates

What happens when we eat carbohydrates?

-the digestive system breaks them down using enzymes into glucose.

-Glucose is then absorbed by the small intestine and released into the bloodstream via glucose transporters.

-glucose can travel around the body in the blood plasma where it gets taken up into cells. 

What happens to glucose in cells

 the process of respiration breaks glucose up into smaller parcels of energy called ATP that are used to power cell function

carbohydrates

a primary macronutrient and the body's main energy source, broken down into glucose to fuel cells, tissues, and the brain

glycogen

a highly branched chain of glucose molecules stored in the liver and muscles, acting as the primary, quick-access energy reserve in animals.

glucose transporter

a group  of membrane proteins that transport glucose across the plasma membrane

glycogenesis

the process of creating glycogen from glucose

glycogenolysis

the process  of breaking down glycogen into glucose 

Glucose from body stores

Glycogenesis is the process where glucose is joined together to form glycogen in the liver and skeletal muscle cells.

Blood glucose level

The amount of glucose in the blood at any time

Normal range: ~4.0 – 7.8 mmol/L

(≈ 1 teaspoon of glucose in the body)

hyperglycaemia

the state of having blood glucose levels above the normal range (>7.8 mmol/L)

hypoglycaemia

The state of having blood glucose levels below the normal range (<4.0 mmol/L)

How does homeostasis maintain constant blood glucose levels? negative feedback loop

by releasing insulin to lower blood glucose levels and glucagon to increase blood glucose levels.

Islets of Langerhans

regions of the pancreas that contain cells  that secrete hormones

alpha cells

cells that occupy  the islets of Langerhans and  secrete glucagon 

beta cells

cells that occupy  the islets of Langerhans and  secrete insulin 

insulin

a hormone secreted by beta cells of the pancreas when blood glucose levels are elevated 

glucagon

a hormone secreted by alpha cells of the pancreas when blood glucose levels are low

when blookd glucose levels are high

• Stimulus:
  Blood glucose rises above ~5 mmol/L (e.g. after eating)

• Receptor:
  Beta (β) cells in the islets of Langerhans (pancreas) detect the increase

• Modulator:
  Pancreas (islets of Langerhans)
  → decides to release insulin

• Effector(s):

  1. Skeletal muscle & fat cells

  2. Liver cells

• Response:
  Blood glucose levels decrease back to normal (~5 mmol/L) 

Effect number 1 of Insulin (When Blood Glucose is High)

 Effector 1: Skeletal Muscle & Fat Cells

• Insulin binds to receptors on these cells

• Causes insertion of glucose transporters into the cell membrane

• → increases glucose uptake via facilitated diffusion

Why needed:

• Glucose is hydrophilic → cannot pass through lipid membrane easily

Once inside the cell:

• Used in cellular respiration → ATP (energy)

• Stored as:

  • Glycogen in skeletal muscle

  • Fatty acids in fat cells (long-term storage)

Insulin activates enzymes for glycogenesis

→ converts glucose into glycogen for storage

Note:

• Liver already absorbs glucose

Insulin mainly increases conversion to glycogen, not uptake

Effect number 2 of Glucagon (When Blood Glucose is low)

Liver Cells

• Glucagon Stimulates liver cells to  to break down glycogen into glucose and release it into the bloodstream via glycogenolysis. 

• Glycogen → glucose

• Glucose is then released into the bloodstream

🔁 Negative Feedback Link

• When blood glucose returns to ~5 mmol/L:

  • Alpha cells stop releasing glucagon

  • Liver stops breaking down glycogen

• → System switches off

a) Insulin binds with receptors on skeletal muscle cells, causing (b) vesicles embedded with glucose transporters to (c) fuse with the cell membrane, (d) allowing more glucose into the cell.

osmolality

the total concentration of solute in a given weight of water

• keeps stable water balance in the body Keeps osmolality

Water moves by osmosis:

• From low solute → high solute

What Happens if Balance is Wrong?

Hypertonic environment (extracellular fluid -high solute concentration)

• Water moves out of cells via osmosis

• Cells shrink (crenate)

• → cannot function properly

Hypotonic environment (the extracellular fluid-low solute concentration,)

• Water moves into cells

• Cells swell / may burst

Isotonic (ideal)

• Equal solute concentration

• → no net water movement

  

How the Body the imbalance of water

• Easier to change water amount than solutes

• So body:

  • Adds water → lowers concentration

  • Removes water → increases concentration

Goal:

• Keep extracellular fluid isotonic to cells

• Water balance controls cell size + function

• Controlled by adjusting water in body fluids

• Prevents:

  • Cell shrinking

  • Cell bursting

Water balance = keep cells stable (not too swollen, not too shrunk)”

Other functions of water

1. Production of Urine

helps:

-Dissolve urea and other wastes

-Remove waste via the excretory system

2. Removal of Heat (Thermoregulation)

Loss of sweat, heat energy lost

3. Maintaining Blood Volume

Blood plasma is about 92% water

Maintains:

• Blood pressure

• Efficient transport of substances (e.g. glucose, hormones)

4. Protection of the Brain & CNS

• Water forms cerebrospinal fluid (CSF)

• CSF:

  • Surrounds brain and spinal cord

  • Acts as a shock absorber

Formula for total water change

Total water change = water in + metabolic water – water out

What is the stimulus response model when water levels decrease?

Stimulus:

• Decrease in body water
  → causes:

• Increased  blood osmolality

• decreased blood volume & pressure

Receptors:

Osmoreceptors (hypothalamus)  detects the increased osmolality

Baroreceptors (heart, arteries, kidneys)  detects the decreased  pressure/volume

Modulators:

Hypothalamus + posterior pituitary gland → ADH pathway

Kidneys → renin pathway

What is the ADH hormone?

Antidiuretic Hormone

ADH pathway

ADH Pathway (Osmoreceptors)

• Posterior pituitary releases ADH

• ADH acts on:

  • Distal convoluted tubule

  • Collecting duct

👉 Causes:

• Insertion of aquaporins

• increased water reabsorption into blood

👉 Also:

• Stimulates thirst centre → drink more water

 The body retains more water, the osmolality of blood decreases.

Renin–Aldosterone Pathway (Baroreceptors)

• Kidneys release renin

• → leads to aldosterone release

👉 Causes:

• ↑ sodium reabsorption in kidney

• Water follows via osmosis

Aldosterone activates sodium potassium pumps in the cells lining the distal convoluted tubule and collecting duct,

increased reabsorption of water from kidney filtrate

When water levels increase (Negative feedback loop)

🔹 Full stimulus–response modelstimulus

• Increase in body water

•  decreased blood osmolality (more diluted)

• Increased  blood volume & pressure 

Receptors:

• Osmoreceptors (hypothalamus) → detects decreased osmolality

• Baroreceptors (blood vessels & heart) detects  increased pressure/volume 

Modulator:

• Hypothalamus + posterior pituitary gland

• ↓ ADH (antidiuretic hormone) release

Effectors:

• Kidneys (distal convoluted tubule + collecting duct)

• Thirst centre (hypothalamus)

Response:

1. Less ADH → fewer aquaporins inserted

   • decreased water reabsorption from kidney filtrate

   •  more water stays in urine

2. Urine becomes dilute (light colour, high volume)

3. Thirst centre suppressed

   • ↓ fluid intake

🔁 Final outcome (negative feedback)

• Water levels ↓ back to normal

• Blood osmolality ↑ back to normal

• Blood volume & pressure ↓ to set point

Crenate

when a cell shrinks due to water leaving the cell by osmosis in a hypertonic environment.

osmoreceptor

a type of receptor found primarily in the hypothalamus that detects changes in osmolality

 baroreceptor

a type of receptor found throughout the body that detects changes in blood pressure

Antidiuretic hormone (ADH)

Is a hormone released from the posterior pituitary gland that increases water reabsorption in the kidneys, helping the body conserve water and maintain blood osmolality.

aquaporin

a family of transmembrane proteins facilitating the transport of water into and out of a cell

Renin

is an enzyme released by the kidneys when blood pressure or blood volume is low. It starts a process that helps the body reabsorb more water and increase blood pressure.

Aldosterone

 hormone from the adrenal glands that helps the kidneys reabsorb sodium and water, and excrete potassium, raising blood volume and pressure.

type 1 diabetes

an autoimmune disease in which beta cells of the pancreas are destroyed, resulting in an inability to regulate blood glucose levels

What causes type 1 diabetes?

-body’s immune system recognises beta cells in the pancreas as non-self and attacks them using autoantibodies 

leads to

- no insulin

-blood glucose are left un regulated

-alpha cells are also impaired and can no longer function properly.

autoantibodies

proteins created by the immune system that destroy an organism’s own tissues

type 2 diabetes

a disease  in which the body becomes resistant to the effects of insulin and/or doesn’t produce enough insulin to maintain normal blood glucose levels

What can lead to hypoglycaemia and hyperglycaemia

As a result of beta cell attack, people with type 1 diabetes are insulin deficient

hyperglycaemia

the state of having blood glucose levels above the normal range (>7.8 mmol/L)

Why is insulin important for the regulation of blood glucose levels?

 it facilitates the entrance of glucose into cells as well as the production of glycogen. Because glucose can’t be absorbed by people with type 1 diabetes, they are at risk of hyperglycaemia. 

Short term effects of type 1 diabetes

increased urination and excessive thirst

– if levels of glucose in the blood are high, glucose can push through the walls of the glomerulus and end up in the kidney filtrate.

-The presence of glucose in the filtrate increases its osmolality, meaning that the normal osmotic processes that reabsorb water in the nephron don’t function properly.

-As a result, water remains in the filtrate and a larger amount of dilute urine is produced .

-This can cause a person with type 1 diabetes to lose large amounts of water, become dehydrated, and feel thirsty

• excessive hunger and lethargy

- because their cells aren’t receiving the glucose they need to function properly, people with type 1 diabetes often feel tired and lethargic. 

-In an attempt to correct the lack of glucose entering the system, the body stimulates  the sensation of hunger • 

weight loss – the loss of large amounts of water and the inability of cells to grow  and function normally due to a lack of glucose can lead to weight loss in people with type 1 diabetes.

Long term effects of type 1 diabetes

vision loss – the tiny vessels carrying blood to the eye become damaged and leaky, leading to swelling and reduced blood flow

) • heart disease and stroke – the blood vessels supplying the heart and brain can become damaged and blocked, causing cells to be deprived of oxygen and die 

• tingling or numbness in the feet and/or hands – nerves are sensitive to elevated blood glucose levels and can become damaged

 • prolonged wound healing – the damaged blood vessels in people with long term wounds take longer to heal or won’t at all

• kidney damage – the blood vessels that carry blood to the kidney are very small and sensitive, and can easily be damaged by increased blood glucose levels.

hypoglycaemia

the state of having blood glucose levels below the normal range (<4.0 mmol/L)

How can someone become hypoglycaemic

• Too much insulin injected

• Not enough food

• Too much exercise

• ↓ glucagon (alpha cells also impaired)

Effects of hypoglycaemia

• Weakness, dizziness

• Can lead to loss of consciousness or death

Management of Type 1 Diabetes

• Insulin injections (short + long acting)

• Insulin pump (automatic control)

• Regular blood glucose monitoring

Hyperthyroidism

condition where the thyroid gland produces too much T3 and T4, increasing metabolic rate.

triodothyronine

a hormone produced and secreted by the thyroid gland. Also known as T3

-regulating metabolism, growth, body temperature, and heart rate

thyroxine

-a hormone produced and secreted by the thyroid gland. Also known as T4

-essential for regulating metabolism, heart function, muscle control, and brain development.

Graves diseaae (autoimmune)

-The immune system produces an autoantibody called thyroid-stimulating immunoglobulin (TSI)

-antibody recognises and binds to the TSH receptors on the thyroid, stimulating the thyroid to release T3 and T4

Leads to

-elevated levels of thyroid hormones

-constant stimulation to release hormones

The components of the body when thyroid levels are elevated

Treatment

• Medication (reduce hormones)

• Radioactive iodine

• Surgery (remove thyroid)