Topic 3 Biology

Homeostasis

3.1 Maintaining the Internal Environment

Organisms function best within specific tolerance limits.
Factors with tolerance limits:
- Body temperature.
- Water availability.
- Blood glucose level.
- Carbon dioxide concentration.
Conditions outside tolerance limits impact organisms.
Organisms detect and respond to internal and external environment changes.
Homeostasis: Maintaining a relatively constant internal environment for optimum body function.
Homeostasis uses a stimulus-response and negative feedback model.
Sensory receptors: They detect changes in the internal and external environment.
Effectors: Muscles or glands that respond to the signal from the control center.
Stimulus-response model: A change triggers a response to maintain balance.
Negative feedback: The response inhibits the initial stimulus.
Homeostasis maintains a dynamic equilibrium, constantly adjusting to changes while keeping body functions within specific ranges.
Homeostasis maintains a steady state in response to external and internal environmental changes via:
- Structural adaptations: Physical features aiding tolerance.
- Physiological processes: Internal mechanisms detecting and responding to changes.
- Behavioral actions: Actions helping organisms survive.

Variable Factors & Tolerance Limits

Factors like pH, temperature, $CO_2$ levels, and ion concentrations must be kept at optimum levels.
Changes in internal or external environments can cause these variables to become elevated or lowered.
Homeostatic processes raise or lower variables to maintain optimum levels.
Several variable factors in the internal environment need to be kept within particular tolerance limits for organisms to operate efficiently.
Liebig’s Law of the Minimum: A limiting factor in short supply slows biological processes, like photosynthesis.
Variables like blood glucose, water, carbon dioxide, and temperature must stay within tolerance levels for proper organism function.
Body temperature in mammals, including humans, is usually maintained between $36°C$ and $38°C$ .
Enzymes work best at around $37°C$ , carrying out all of the cell’s metabolic activity.
Hyperthermia, a life-threatening situation, occurs when the internal temperature rises above $40°C$ . Hypothermia, also life-threatening, occurs when the temperature drops below $37°C$ .
Temperature tolerance for humans:
- Normal: $37°C$
- Hyperthermia: $38.5°C$ (of medical significance)
- Hyperpyrexia: $41.5°C$ (medical emergency)
- Death: $42.2°C$
- Hypothermia: $35°C$
- Severe hypothermia: $28°C$
- Unconsciousness: $27.8°C$
- Death: $25°C$
Water/solute concentration balance in extracellular and intracellular environments must be kept within specific tolerance levels.
Shifts away from normal levels can lead to swelling or shriveling of cells due to osmosis.
A solute concentration of about $0.9\% NaCl$ is isotonic with blood plasma.
Carbon dioxide concentration in human blood is normally between 5-6%, and it is important to keep it within these tolerance limits.
Carbon dioxide is a non-toxic gas at normal levels and is required to stimulate and control the rate and depth of breathing.
High levels of $CO_2$ dissolving in the blood can lead to lowering the pH (acidosis).
At levels greater than 10%, carbon dioxide is highly toxic, leading to unconsciousness and ultimately death.
Blood glucose levels are normally around 75-95 mg/dL (1dL = 100mL), and sustained deviations from these levels cause serious health issues.
Conditions outside the tolerance level are hyperglycemia (too high blood glucose) and hypoglycemia (too low blood glucose).
If left untreated, both conditions can quickly lead to symptoms including the loss of consciousness.
Diabetes is a disease associated with a high level of blood glucose, usually as a result of a lack of the hormone insulin or the tissue’s responses to insulin.
Normal values:
- Temperature: $36-38°C$
- Glucose: 75-95mg/dL
- $CO_2$ : 5-6%
Water balance: approx. $0.9\% NaCl$
Symptoms of diabetes:
- Eyes: blurred vision
Breath: smell of acetone
Brain: lethargy, stupor
Urinary: polyuria, glycosuria
Respiratory: breathing (hyperventilation)
Gastric: nausea, vomiting, abdominal pain

Homeostatic Process

The goal of homeostasis is maintaining equilibrium around a set point.
The body's systems attempt to return to this set point despite normal fluctuations.
A stimulus is a change in the internal or external environment, detected by a receptor.
The system responds by adjusting the deviation parameter toward the set point.

Control of Homeostasis

When a change occurs in an animal’s environment, an adjustment must be made.
The receptor senses the change, sends a signal to the control center (usually the brain), which generates a response signaled to an effector.
The effector is a muscle (contracts or relaxes) or a gland that secretes.
Homeostasis is maintained by negative feedback loops.
Positive feedback loops push the organism further out of homeostasis but can be necessary for life.
Homeostasis is controlled by the nervous and endocrine systems.
If blood glucose levels decrease, the pancreas releases glucagon, causing glucose levels to increase.
Specialized cells in the parathyroid gland sense decreased calcium levels and release parathyroid hormone (PTH), increasing calcium absorption and possibly breaking down bone to liberate calcium.
Negative feedback loops are the predominant mechanism used in homeostasis.

Stimulus-Response Model

Factors in the environment cause increases or decreases in variables, and processes work in the opposite way to reverse the changes.
To understand the mechanisms involved in homeostasis, scientists use the stimulus-response model.
The elements of the model are:
1. Stimulus: Imbalance, change in a variable.
2. Receptor: Change is detected.
3. Input: Transmission of information along a pathway.
4. Control center: Analyzes information from the receptor and determines the appropriate response.
5. Output: Information sent to the effector.
6. Effector: Carries out the response.
7. Response: Feeds back to reverse the stimulus and returns the variable to homeostasis.

Negative Feedback Mechanisms

Any homeostatic process that changes the direction of the stimulus is a negative feedback loop.
The stimulus isn't allowed to continue as it did before the receptor sensed it.
If a level is too high, the body brings it down; if too low, the body raises it.
An example is animal maintenance of blood glucose levels. After eating, blood glucose levels rise, which is sensed by the nervous system.
Specialized cells in the pancreas release insulin, which lowers blood glucose levels.
Feedback occurs when the response to a particular stimulus impacts the original stimulus.
Negative feedback diminishes or reverses the original stimulus.
Positive feedback reinforces or increases the initial stimulus.
Homeostasis is characterized by negative feedback, which returns the changed variable to its steady state.
An increase in exercise raises metabolic activity, increasing blood temperature. Receptors detect this, and homeostatic responses by effectors lower the temperature.
Breastfeeding is an example of positive feedback. A newborn baby suckling causes the release of a hormone in the mother, stimulating milk release and promoting bonding.
Increased breastfeeding promotes increased oxytocin release, strengthening the bond.

Questions to test you runderstading

Outline reasons why homeostasis is such an important process in sustaining life
Organisms respond to external and internal stimuli in different ways e.g. structural, behavioural and physiological. Give one example of how an organism might respond in each of these three ways
The concept of tolerance levels can be applied to homeostasis.’ Explain the meaning of this statement and give an example to illustrate your understanding
State two reasons why it is important for humans to maintain the correct concentration of $CO_2$ in their blood
Compare and contrast the roles of receptors and effectors in the stimulus-response model
‘The maintenance of blood glucose concentration between 75-95 mg/dL is a result of negative feedback.’ Justify the above statement

Answers

Life’s processes can only occur within certain tolerance limits. Chemical reactions e.g. metabolism require optimum pH, temperature, osmotic balance etc. Homeostasis maintains these variables within the tolerance limits
Examples of responses include: a)Development of individual chemoreceptors to detect e.g. carbon dioxide levels. b)An animal seeking shade to avoid heat gain in hot weather. c)Increased levels of insulin in order to decrease blood glucose levels
Certain variables need to be maintained between upper and lower limits; the tolerance range for efficient functioning e.g. human temperature between approximately 36 to $38°C$ .
Reasons include: a) $CO_2$ is the stimulus for increased rate/depth of breathing and providing oxygen to tissues. b)Changing the carbon dioxide concentration alters the pH of the blood
Receptors are modified sensory neurons that respond to stimuli and can then initiate transmission of a message. An effector is a muscle or a gland that can bring about a response to a specific stimulus
This maintenance between the tolerance levels occurs as result of homeostatic processes. The response ‘feeds back’ to the initial stimulus and reverses it e.g. high blood glucose leads (via insulin) to a fall in glucose

3.2 The Nervous System

The nervous system is composed of the central nervous system and the peripheral nervous system.
Compare the structure and function of sensory neurons, interneurons, and motor neurons.
Describe the structure of a nerve pathway from receptor to effector.
Describe the role of synapses and neurotransmitters.
Describe the role and pathway of reflex responses.
Explain how the nervous… system works… to monitor pH in the brain to maintain a constant carbon dioxide level in the blood.

The Central Nervous System and the Peripheral Nervous System

The nervous system detects and responds to stimuli from internal and external environments.
It comprises the central nervous system (CNS) and the peripheral nervous system (PNS).
The CNS (brain and spinal cord) primarily stores, arranges, and manages information, while the PNS mainly transmits information to and from the CNS.
Nerves in the PNS relay information from sensory receptors to the CNS and from the CNS to the effectors.
The nerves in the PNS can be further subdivided into the somatic (voluntary) and autonomic (involuntary) types.
Voluntary nerves are under our conscious control; for example, influencing the skeletal muscles, whereas the autonomic system is involuntary and consists of nerves involved in a range of unconscious responses from changes in the heart rate, gland activity and the activity of a range of body systems.
The autonomic system is divided into the sympathetic and parasympathetic systems
Nervous system
- Central nervous system
  - Brain: Controls the CNS
  - Spinal cord: Link between between brain and peripheral nerves
- Peripheral nervous system
  - Somatic nervous system
    - Controls skeletal muscles- voluntary
  - Autonomic nervous system
    - Regulates glands, blood vessels, internal organs - involuntary
      - Sympathetic nervous system
        Mobilises body for action, energy output
      - Parasympathetic nervous system
        Conserves energy, maintains quiet state
Parasympathetic vs Sympathetic
- Sympathetic:
  - Dilates pupil
  - Decreases salivation and increases sweating
  - Accelerates heart
  - Dilates lung bronchial tubes (airways)
  - Decreases digestive functions of stomach and pancreas
  - Decreases digestive functions of intestine
  - Inhibits bladder contraction
  - Adrenaline chain of secretion
- Parasympathetic:
  - Constricts pupil
  - Increases salivation
  - Slows heart
  - Constricts lung bronchial tubes (airways)
  - Increases digestive functions of stomach and pancreas
  - Increases digestive functions of intestine
  - Bladder contraction

Neurons

Receptors are modified cells or tissues that detect stimuli and send messages along nerve cells (neurons) to the CNS and muscles and glands to bring about a particular response.
There are three types of neurons in the nervous system:
- Sensory.
- Interneuron (relay).
- Motor.
Sensory neurons are nerve cells that transmit information from a receptor (e.g. cells that detect light, sound, temperature etc.) to the CNS.
Interneurons are located in the CNS and transmit information from sensory neurons to motor neurons.
Motor neurons transmit messages from the CNS to the effectors that are typically muscles or glands.
Components of a neuron:
- Dendrites: receive impulses from sensory receptors or other neurons and transmit information toward the cell body.
- Cell body: contains many cell organelles, including a nucleus, endoplasmic reticulum and mitochondria.
- Axons: long extensions of the nerve cell (neuron), necessary to transmit information to another cell or an effector.
- Axon branches: have tiny swellings at their ends that release neurotransmitter chemicals which transmit information between neurons or between a neuron and, for example, a muscle cell.
Synapse: the junction between neurons or between a neuron and a receptor/effector
Neurotransmitter: a chemical transmitter substance released at the synapse, that diffuses across the space and binds to receptors on the receiving membrane.

Neuron Structure and Function

A sensory neuron has a cell body joined by two long processes: the dendron and the axon.
The extension of the dendrons, called dendrites, act as receptors and transmit information towards the cell body.
Interneurons have only short axons, enabling connections between a sensory neuron and a motor neuron.
Motor neurons usually have short dendrites and one long axon.
Neurons transmit messages to specific destinations in the form of electrochemical impulses; this process involves changing concentration gradients of sodium ( $Na^+$ ) and potassium ( $K^+$ ) ions, requiring movement across cell membranes through facilitated diffusion and active transport.
Structure
- Motor
  - motor:cell body, short dendrites, long axon
- Interneuron(relay)
  - cell body, short dendrites, short axon
- Sensory
  - Cell body, long dendrites, short axon
Location of cell body
- Motor
  - Cell body, short dendrites, long axon
  - Cell and dendrites in CNS
- Interneuron(relay)
  - cell body, short dendrites, short axon
  - Cell body etc inside CNC
- Sensory
  - Cell body, long dendrites, short axon
  - Cell body and dendrites outside CNS
Axons
- Motor
  - motor: End in effector
  - Axon outside CNS
- Interneuron(relay)
  - cell body, short dendrites, short axon
  - Axons synapse with other neurons
- Sensory
  - Cell body, long dendrites, short axon
  - Axon inside CNS
Dendrites
- Motor
  - Dendrites synapse with effectors
- Interneuron(relay)
  - Dendrites synapse with other neurons
- Sensory
  - Dendrites synapse with receptors
- Function
- Motor
  - Transmit impulses from CNS to effector
- Interneuron(relay)
  - Connect sensory and motor neurones to form nerve circuits
- Sensory
  - Transmit impulses from receptors to CNS

Myelin Sheath

Some axons have a fatty covering called a myelin sheath, formed by Schwann cells wrapping around the axons.
Nerve impulses travel faster along neurons with a myelin sheath, as impulses jump from node to node.

Synapses and Neurotransmitters

The electrical message cannot cross the synaptic cleft.
The arrival of a nerve impulse causes the release of a chemical transmitter substance into the synaptic cleft, called a neurotransmitter, that diffuses across the space and binds to receptors on the receiving membrane.
This activates ion channels in the membrane leading to the nervous impulse being transmitted further.
Nervous messages are termed electrochemical messages because they involve electrical impulses along axons and neurotransmitter release across the synaptic cleft.
The neurotransmitter can be broken down and deactivated by enzymes.
Neurotransmitter Transmission Overview:
- Neurotransmitters are packaged into vesicles in the presynaptic neuron.
- When an action potential excites the presynaptic neuron, the vesicles fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis.
- Neurotransmitters interact with and bind to receptors on the postsynaptic membrane, triggering an action in the postsynaptic cell.
- To terminate the signal, neurotransmitters are cleared from the synaptic cleft through diffusion, reuptake into the presynaptic neuron, or breakdown by enzymes.

Examples of Neurotransmitters

Acetylcholine

The first neurotransmitter discovered.
Stimulates muscles and is found in sensory neurons and the autonomic nervous system.
Botulinum toxin works by preventing vesicles in the axon from releasing acetylcholine, leading to paralysis.
A strong link exists between acetylcholine and Alzheimer’s disease, with about 90% loss of acetylcholine in the brains of those affected.

Norepinephrine

Discovered in 1946.
Linked with switching the nervous system into an ‘alert’ state.
Also called noradrenaline, it is both a neurotransmitter and a hormone.
As a neurotransmitter, it's a chemical messenger that helps transmit nerve signals across nerve endings to another nerve cell, muscle cell or gland cell.
As a hormone, it’s released by adrenal glands.
It’s part of the sympathetic nervous system, which is part of your body’s emergency response system to danger (the “fight-or-flight” response).
Increases heart rate and blood pressure.
Increases alertness, arousal and attention.
Constricts blood vessels, which helps maintain blood pressure in times of stress.
Affects your sleep-wake cycle, mood and memory.
Stress triggers the release of norepinephrine from adrenal glands.

Dopamine

Discovered in the 1950s.
Acts as an inhibitory neurotransmitter.
Several drugs increase levels of dopamine.
Schizophrenia is linked to increased levels of dopamine.
Parkinson’s disease is thought to be driven by a lack of dopamine in the brain.
Approximately 50% of dopamine is made by the gastrointestinal system.

Serotonin

An inhibitory neurotransmitter linked to emotion and mood.
Decreased levels have been associated with depression, anger management and obsessive-compulsive disorder (OCD).
Some drugs that are prescribed to assist with depression act on neurons by preventing them from removing excess serotonin thus leaving more in the synapses.
The gastrointestinal tract produces 95% of an individual’s serotonin, which acts on the brain.

Sensory Receptors

Detecting changes in the external and internal environment is critical to organism survival.
The first two elements of the stimulus response model were (a) the stimulus and (b) the receptor.
Sensory receptors generally respond to the intensity, location and duration of the stimulus.
Stimuli are detected by specialized receptors, mostly dendrites of sensory neurons.
Some receptors respond to external stimuli, others to internal stimuli.
Receptors are classified by the stimulus they detect:
- Mechanoreceptor:
  - Stimuli detected: sound, touch, pressure, motion, stretch
  - Example locations in humans: ear, skin, muscle, tendons, ligaments
- Photoreceptor:
  - Stimuli detected: light
  - Example locations in humans: eye
- Thermoreceptor:
  - Stimuli detected: temperature changes
  - Example locations in humans: skin, brain (hypothalamus)
- Chemoreceptor:
  - Stimuli detected: detect changes in chemicals, such as solutes, glucose, $O_2$ , $H^+$ ions.
  - Example locations in humans: nose, tongue, carotid arteries, brain
- Nociceptor:
  - Stimuli detected: detect tissue damage, pain
  - Example locations in humans: skin, muscles, joints (brain interprets the pain message)

The Reflex Arc

There are two types of interactions in the human nervous system: involuntary and voluntary.
Involuntary or autonomic responses are not under the conscious control of the brain and are classified as automatic, rapid and instinctive responses that do not need to be learned.
They are known as reflexes or reflex actions and are among the simplest kind of behaviours in humans.
A reflex arc is the pathway along which nerve impulses travel when a reflex action occurs. In this action, a receptor is stimulated and an impulse travels along a sensory neuron to an interneuron found in the spinal cord via a synapse. The interneuron stimulates a motor neuron via another synapse and the impulse travels to an effector organ which brings about a response.
In a reflex response, the brain is not directly involved, to ensure a fast and automatic response to increase the likeliness of an individual being protected and/or surviving.
Reflex actions often have a protective function, for example, when you touch a hot object your hand jerks away rapidly.
Other reflexes include sneezing, secretion of saliva, coughing, blinking and the constriction or dilation of the pupil of the eye.

Suggested Exam Q

Refer to the following stimulus response diagram.
Which one of the following components of the nervous system are represented by the letters A, B, C and D?
Homeostasis is the maintenance of a relatively constant internal environment. This ensures:
- J. that internal conditions stay the same in response to external conditions.
- K. the optimum conditions for the body to function.
- L. the external and internal conditions stay constant.
- M. the internal and external conditions fluctuate consistently.

Blood pH Tolerance Limits

The normal blood pH is approximately 7.4. this will fluctuate very slightly for an individual throughout the day between the normal range of 7.35 to 7.45.
If it drops to 6.9 an individual is likely to be in a coma, and at 6.8, death is the likely result.
A rise in blood pH to 7.8 will also likely result in death.

Relationship Between Blood PH and Carbon Dioxide

Blood pH is regulated by breathing out carbon dioxide ( $CO_2$ ).
Cells produce $CO_2$ as a waste product when they respire aerobically (especially during intense exercise).
The $CO2$ produced is excreted by cells into the blood by diffusion. It dissolves in the blood forming carbonic acid ( $H2CO_3$ ), which is a weak acid.
Increased carbonic acid in the blood lowers blood pH, making it more acidic.
Conversely, when too much $CO_2$ is breathed out, there is a decrease in carbonic acid dissolved in the blood and the blood pH would increase, as it is less acidic. This could occur when an individual hyperventilates.

Monitoring Blood pH and Controlling Carbon Dioxide Levels in the Blood

The blood pH is detected by chemoreceptors in the medulla oblongata.
A change in the blood pH would cause nervous messages to be sent to the organs of the respiratory system to alter the rate of breathing, and therefore, the concentration of $CO_2$ in the blood.
If the blood pH decreases, the rate of breathing would be increased to breathe out more $CO_2$ and increase pH back to 7.4.
Conversely, if the blood pH increases, the rate of breathing would be decreased to breathe out less $CO2$ and decrease pH back to 7.4. Therefore, our breathing rate is controlled by our blood pH (levels of $CO2$ ) and not the amount of oxygen available to cells.
Negative feedback
- Stimulus
  - Increased $CO_2$ levels, decreased blood pH
- Receptors
  - Chemoreceptors stimulated
    - Medulla oblongata(Brain)
    - Carotid bodies
    - Aortic bodies
- Effectors
  - Intercostal muscles
  - Diaphragm
  - Heart muscle
  - Blood vessels
- Response
  - Increased rate and depth of breathing
  - Increased heart rate
  - Decreased $CO_2$ levels
Increased $CO_2$ levels leads to decreased blood pH.
Raised $CO_2$ and lowered pH are detected by chemoreceptors in the brainstem (medulla oblongata) and tissues in the walls of the aorta and carotid arteries.
Nerve impulses are transmitted to the effectors.
Increasing the rate and depth of breathing and increasing the heart rate lower $CO2$ levels. This reversing the stimulus, and restoring the normal level of blood $CO2$ .

3.3 The Endocrine System

The endocrine system releases hormones that are amino acid derivatives, peptides, proteins, or steroids.
Hormones travel to target sites via the blood.
Hormones can alter the metabolism of target cells, tissues, or organs.
Compare the action of insulin and glucagon in blood sugar regulation.
Describe how diabetes mellitus can result from a hormonal imbalance.
Describe the action of thyroid stimulating hormone and thyroxine in metabolism.
Describe the effect of antidiuretic hormone (ADH) on the nephron in osmoregulation..
Discuss links between osmoregulation, blood volume, and blood pressure.
Describe the role of thyroid-stimulating hormone in the production of thyroxine.
Explain how the… endocrine system works … to:
- enable osmoregulation
- maintain blood sugar level

Introduction to the Endocrine System

The endocrine system is the body system based on hormones.
Hormones are described as chemical messengers.
They are synthesised and released by endocrine glands, often called ductless glands, because they release hormones directly into the blood without the hormone passing through a duct or tube.

Hormones

Chemical messages that travel through the blood
Produced by endocrine glands
Control homeostasis in:
- Growth, reproduction, solute concentration, glucose concentration, blood temperature
List of glands hormones and target organs
- Pineal gland
  - Melatonin
  - Many
  - Pituitary gland
  - FSH/LH
  - Ovaries
  - ADH
  - Kidneys
  - Growth hormone
  - Many others
  - Oxytocin
  - Uterus
  - Prolactin
  - Breast tissue
  - Thyroid gland
  - Thyroxin
  - Liver
  - Adrenal glands
  - Adrenaline
  - Many
  - Cortisol
  - Many
  - Pancreas
  - Insulin
  - Liver
  - Glucagon
  - Liver
  - Ovaries
  - Estrogen
  - Uterus
  - Progesterone
  - Uterus
  - Testis
  - Testosterone
  - Many
The pituitary gland in the base of the brain is often referred to as the master endocrine gland because it controls the action of other endocrine glands.
Hormones travel in the blood to all parts of the body, but act on specific cells or organs with appropriate receptors (binding sites) for that hormone. These cells or organs are called target cells/organs.
Hormones are found in low concentration in the blood as they are act in tiny amounts.
Hormones are 3 types:
- Peptides - consist of a chain of amino acids
- Proteins - larger 3D structures
- Steroids - Lipids and insoluble in water
Peptides and proteins: water soluble - bind to receptors on cell membranes and transmit messages - faster
Steroids: Lipid soluble - move through membranes and into cell - slower but longer lasting
Property
- Peptide/protein hormones
  - Synthesis
    - generally synthesised in the RER and often requires cleavage to activate
  - Storage
    - stored in vesicles
  - Solubility
    - most are polar (water soluble) and travel in the blood
  - Receptors
    - bind to receptors in the cell membrane which activates secondary messengers inside the cell
  - Examples
    - insulin, glucagon, anti-diuretic hormone (ADH)
- Amino acid derived hormones
  - Synthesis
    - synthesised from one amino acid (tyrosine or tryptophan)
  - Storage
    - stored before release
  - Solubility
    - some are lipid soluble (thyroxine) and non-polar (lipid soluble)
    - others are water soluble (adrenaline)
  - Receptors
    - some act on cell membrane receptors, others bind to receptors inside the cell
  - Examples
    - T3, T4 (thyroxine) (which act similar to progesterone, steroids)
    - adrenaline, melatonin (which act similar to proteins)
- Steroid hormones
  - Synthesis
    - synthesised from cholesterol
  - Storage
    - released into bloodstream immediately
  - Solubility
    - non-polar (lipid soluble)
  - Receptors
    - are primary messengers that bind to intracellular receptors
  - Examples
    - oestrogen, testosterone
The protein and peptide hormones (polar/water-soluble) don't enter the cell; their action results from binding to target cell membranes with complementary receptors.
Steroid hormones (non-polar/lipid-soluble) travel into the cell and bind to internal receptors.

Example of Stimulus Hormonal Model That Helps Maintain Homeostasis

Control of blood glucose levels
If blood glucose levels fall outside tolerance limits then negative feedback will act to pull them back in line.

Blood Glucose Regulation

Glucose is an essential substrate for energy supply.
It is broken down in both aerobic and anaerobic respiration to release energy for ATP production.
It is important for the body to maintain blood glucose levels within the tolerance range of 75 to 95 mg/dL.
High blood glucose levels can cause complications, such as increased blood pressure, cardiovascular disease, nerve damage, blindness and increased infections.
Chemoreceptors in the Islets of Langerhans of the pancreas detects changes in blood glucose levels in the blood.
In response to an elevated blood glucose level, insulin is released from the beta (β) cells of the Islet of Langerhans into the blood, which binds to specific receptors on cells, especially liver, muscle and adipose (fat) cells, stimulating them to take up glucose from the blood.
Under normal circumstances, the blood glucose levels will decrease to the normal tolerated range as a result.
This is detected by the chemoreceptors, resulting in the termination of insulin release.
This is an example of negative feedback, where the response reverses the stimulus.
The liver is the major store of carbohydrates, converts excess glucose absorbed from the blood into a complex carbohydrate called glycogen, and used for respiration.
In response to decreased blood glucose levels, glucagon is released from the alpha (α) cells of the Islets of Langerhans into the blood.
Glucagon binds to specific receptors on liver cells, stimulating them to break down their glycogen stores into glucose and release the glucose into the blood.
This increases blood glucose levels back into the normal tolerated range.
This is detected by the chemoreceptors, resulting in the termination of glucagon release. This is also an example of negative feedback
The pancreas produces and secretes the hormones insulin and glucagon,
Homeostasis: normal blood glucose level
- Imbalance
  - Receptor
    - Pancreas
  - Communication
    - Insulin
  - Stimulus
    - Increased glucose levels
  - Muscle cells
    - Stimulates glucose uptake
  - Liver
    - Glucose =glycogen
  - Blood glucose falls to normal range
- Imbalance
  - decreasing blood glucose level
  - Receptor
    - pancreas
  - Communication
    - Glucagon
  - Liver
    - Glycogen = glucose
  - Stimulates glycogen breakdown
  - Blood glucose rises to normal range
- Homeostasis: normal blood glucose level

Diabetes

What is diabetes?
Caused by the bodies inability to control insulin levels
This results in inbalanced glucose levels
Can cause headaches, nausea or seizures

Diabetes Mellitus

Diabetes mellitus (diabetes) is a metabolic disorder where a person does not produce enough insulin, and/or the body does not respond properly to insulin (insulin resistance).
This means that individuals with diabetes cannot easily regulate their blood glucose levels.
Decreased blood glucose levels, known as hypoglycaemia, causes symptoms related to lack of energy due to decreased glucose availability to cells. These can include feeling weak, headaches, sleepy, dizzy, etc.
As the brain requires a constant supply of energy, generally in the form of glucose, an extended lack of supply can result in unconsciousness and even death.
Elevated blood glucose levels, known as hyperglycaemia, can cause a range of symptoms in diabetics.
As both type 1 and 2 diabetes can result in elevated blood glucose levels, the symptoms outlined in the table below can be applied to both conditions
If diabetes is not suitably managed and the individual continues to experience chronic high blood glucose levels, it can lead to further complications such as cardiovascular disease, nerve damage, eye damage