Homeostasis

Tolerance Range

Difference between maximum and minimum tolerance limits.

Limiting Factor

A variable that, when in low supply, will limit a process or the growth of an organism. Shown as plateau on graph.

Examples of Factors for which Organisms have Tolerance Limits

• Body temperature.

• Water availability.

• Blood glucose level.

• Carbon dioxide concentration in blood and tissues.

Homeostasis

The maintenance of a relatively constant internal environment, ensuring the body optimum conditions to function. Keeps conditions in a tolerance range, does not keep conditions static.

Reasons Homeostasis is Important for Humans

• Maintaining optimal conditions for maximum function.

• Maintaining optimal conditions for metabolic processes, such as aerobic respiration.

• Keeping toxic substances, such as carbon dioxide, at low concentrations to prevent impacts on metabolic processes.

Stimulus

A detectable change in the internal or external environment.

Sensory Receptors

Parts of cells or groups of cells in the body that detect stimuli.

Sensory Receptor Types

Thermoreceptor

• Sensory nerve sensitive to temperature changes.

• Found in skin, muscle cells, liver, Hypothalamus.

Mechanoreceptor

• Sensory nerve that responds to mechanical stimuli, such as touch or sound.

• Found in muscle tissues, skin, ear.

Photoreceptor

• Specialised neurons in retina that convert light into nerve impulses.

Chemoreceptor

• Sensory nerve that translates chemical substances into biological signals.

• Found in Hypothalamus, tongue, central nervous system.

Stimulus Response Model (Negative Feedback Loop)

1. Stimulus causes imbalance in homeostasis.

2. Receptors of sensory neuron detect stimulus.

3. Input sent through interneurons to control centre (brain).

4. Output sent via interneurons to motor neurons.

5. Effector of motor neuron creates response to change.

6. Imbalance in homeostasis is corrected.

*Loops back to beginning here.

Hypothalamus

Small region in brain that acts as a control centre for various bodily functions and behaviours. Regulates activities such as hunger, thirst, body temperature, and sleep. Also plays role in hormone production and emotion regulation.

Nervous System

Consists of Central Nervous System (brain and spinal cord), involved in receiving messages, processing and managing information, and Peripheral Nervous System, involved in the transmission of information to and from the CNS.

Types of Nerve Cells (Neurons)

Sensory Neurons

• In PNS.

• Send signals from stimulus towards CNS.

• Dendrites don’t extend directly from cell body.

• Axon branches from cell body.

• Long.

Interneurons

• In CNS.

• Relay signals from sensory neurons to motor neurons.

• Dendrites extend directly from cell body.

• Cell body is directly between dendrites and axon.

• Short.

Motor Neurons

• In PNS.

• Send signals from CNS to effector (muscles or glands).

• Dendrites extend directly from cell body.

• Long.

Nerve Impulses

Dendrites receive nerve impulses from other neurons and transmit information towards cell body. Axons transmit nerve impulse towards another cell.

Neurotransmission

Neurotransmitters are released by exocytosis and then diffuse across the synapse. Certain drugs can either increase or decrease a nerve impulse by:

• Preventing the production, or binding, of neurotransmitters.

• Increasing the production of, or impersonating, neurotransmitters.

Reflex Responses

Automatic response to stimulus without conscious thought, brain is not directly involved. Creates faster response.

Endocrine System

Involves endocrine glands that produce or secrete hormones, examples of use include growth, reproduction, blood-glucose concentration, and metabolic rate.

Hormones

Molecules that travel via the blood and bind to specific receptors in or on target cells. The four main types are:

• Peptides, are relatively short amino acid chains.

• Protein hormones, are long polypeptide chains.

• Steroids, are lipids.

• Amino acid derivatives, are amino acids that have been chemically altered.

Nervous and Endocrine System Comparison

Nervous:

• Direct pathway via neurons.

• Electro-chemical message.

• Highly specific target (cells in one area).

• Fast transmission of message.

• Shorter-term response.

Endocrine:

• Indirect pathway via blood.

• Chemical message.

• Targets one or more organs.

• Slow transmission of message.

• Longer-term response.

Blood Glucose Levels

Cells need a constant supply of glucose for energy (cellular respiration).

Low Blood Glucose (not eating and exercising):

• Pancreatic cells release hormone glucagon into blood.

• Glucagon binds to receptors on liver cells.

• Triggers liver cells to break down glycogen and release glucose into blood.

• Blood sugar levels are raised.

High Blood Glucose (eating):

• Pancreatic cells release hormone insulin into blood.

• Insulin binds to receptors on liver and muscle cells.

• Triggers the uptake of glucose into liver and muscle cells. Glucose molecules are then combined together to make glycogen for storage.

The site where glucagon binds to glucagon receptors is called the glucagon binding site, NOT the active site as it is not an enzyme.

Diabetes

Type 1: Damage to pancreas, leads to reduced insulin production, leads to reduction of uptake of glucose in liver and muscle cells, leads to high blood glucose levels.

Type 2: Mostly caused by obesity, insulin binds to receptor on muscle and liver cells but doesn’t result in uptake of glucose into cells, leads to damaged pancreas with reduction in insulin, results in high blood glucose levels.

Adrenaline

Also called epinephrine. Nerves and hormones work together, such as eyes sending neural signals to brain that result in the release of hormones, including adrenaline. Stress can result in fight-or-flight response which increases the availability of energy.

Physiological Responses Induced by Adrenaline

• Acts on iris to dilate pupils, this allows more light to enter eyes for better vision.

• Acts on cardiac muscle to increase heart rate, this increases blood flow to muscles.

• Acts on smooth muscle in arterioles to increase blood pressure, which increases amount of blood sent to muscles, increasing glucose and oxygen to muscle cells for respiration.

• Acts on smooth muscle in bronchi to increase air flow to lungs, which increases oxygen diffusing from lungs to blood.

• Acts on the liver to release glucose into the blood by converting glycogen to glucose, this increases the amount of glucose available for muscles for respiration.

High and Low Body Temperature

High body temperature can lead to denaturation of enzymes, therefore loss of important metabolic activities. Low body temperatures result in slowing of metabolic activities, often to dangerous levels. Too high or low temperatures cause negative feedback mechanisms involving nerves and hormones. The Hypothalamus detects temperature directly or receives messages from thermoreceptor neurons in the skin.

Thermoregulation for Body Temperature Below 37^oC

Hypothalamus sends nerve impulses for:

• Erection of hairs on the skin (pilo-erection), traps a layer of air close to the body and reduces heat loss.

• Vaso-constriction, restricts flow of blood to skin by constriction of blood vessels, diverting blood away from capillaries in skin, reducing heat loss.

• Involuntary shivering, contraction of muscles which produces heat.

• Release of thyroid stimulating hormone (TSH), causes the thyroid gland to release hormone thyroxine, stimulates metabolism which increases heat.

High thyroxine levels (hyperthyroidism):

• Increased metabolic rate.

• Higher body temperature.

• Rapid heartbeat and shortness of breath.

• Increased appetite.

Thermoregulation for Body Temperature Above 37^oC

Hypothalamus sends nerve impulses for:

• Flattened hair (pilo-relaxation), prevents trapping of air near skin, increases heat loss.

• Vaso-dilation, dilation of blood vessels near skin, increasing blood flow to skin, increasing heat loss.

• Sweating, increases water evaporation and increasing heat loss.

• Secretion of less thyroxine to reduce metabolic activity, produces less heat.

Low thyroxine levels (hypothyroidism):

• Reduced metabolic rate.

• Lower body temperature.

• Lethargy.

• Decreased appetite.

Osmoregulation

The process by which living organisms maintain the proper concentration of water and solutes in their body fluids. Occurs primarily in kidneys, which contain about a million nephrons, which filters blood so that waste products are removed and solute/water concentration is regulated.

Osmoregulation Stage 1 - Filtration

1. Blood is pumped through a ball of capillaries called the glomerulus.

2. The blood pressure causes the filtration (based on size) of water and smaller solutes (filtrate) between the cells in the Bowman’s Capsule.

3. The blood that remains in the blood vessels has a high concentration of solutes that were too large to filter in the glomerulus.

Osmoregulation Stage 2 - Reabsorption

1. Beneficial substances (glucose, amino acids, water, ions) are passively or actively reabsorbed from the nephron tubule back into the blood.

Dehydration

Hypothalamus detects high solute concentration in blood. Releases antidiuretic hormone (ADH) from pituitary gland in the blood. ADH binds to receptors on cells in the nephron wall resulting in addition of more aquaporins and an increase of reabsorption of water into the blood. This decreases volume of urine produced, and makes it more concentrated. An increase in reabsorption of water into blood results in:

• Lower solute concentration in blood.

• Higher blood volume.

• Higher blood pressure.

Effects of More or Less Water Diffusing Into Blood on Blood and Urine

More water diffusing into blood (from more ADH):

• Blood osmolarity decreases.

• Blood volume increases.

• Blood pressure increases.

• Urine volume decreases.

• Urine osmolarity increases.

Less water diffusing into blood (from less ADH):

• Blood osmolarity increases.

• Blood volume decreases.

• Blood pressure decreases.

• Urine volume increases.

• Urine osmolarity decreases.

Regulating pH Levels in the Blood

Some of the CO₂ from aerobic respiration reacts with water to form acid. This acid releases hydrogen ions (H⁺) which increases acidity of tissue fluid and blood. Receptors in respiratory centre of brain detect increases in H⁺ concentration. These neurons signal heart and lungs to increase heart and breathing rate to send more blood to lungs and increase CO₂ exhalation. H⁺ cannot pass from blood to brain, so CO₂ must first diffuse into brain tissue before reacting to form acid and releasing H⁺.