Homeostasis
What are tolerance limits?
There are several variable factors in the internal environment which need to be kept within particular tolerance limits so that organisms can survive and operate most efficiently. When conditions are outside that range they either die, or have mechanisms for maintaining and internal environment suitable for life. Factors for which organisms have tolerance limits include, body temperature, water availability, blood glucose level and carbon dioxide concentration in the blood and tissue.
What are some areas in which organisms have tolerance limits?
Blood glucose concentration
Concentration of carbon dioxide in the blood an tissue
Water availability
Body temperature
Describe how temperature is a tolerance limit.
Temperature affects the growth and functioning of an organism. In particular within enzymes. Enzymes have a specific temperature for which they function optimally, when this deviates their effectiveness begins to decrease until a point of denaturation where they no longer function and thus the processes that are required to keep an organism alive stop
Describe how water availability is a tolerance limit.
The water/solute concentration balance in both the extracellular and/or intracellular environment is another critical factor that needs to be kept within specific tolerance limits. Shifts away from the normal levels can lead to swelling of shrivelling of cells as a result of osmosis. A solute concentration of about 0/9% NaCl us isotonic with blood plasma.
Describe how blood glucose is a tolerance limit.
Blood glucose levels are normally around 75-95mg/dL and sustained deviations from these levels cause serious health issues. Conditions such as hyperglycaemia, too high blood glucose and hypoglycaemia, too low blood glucose, if left untreated quickly lead to symptoms including loss of consciousness. Diabetes is a disease associated with a high level of blood glucose, usually as result of the lack of the hormone insulin or the tissues response to insulin.
Describe how CO2 concentration in the blood is a tolerance limit.
The carbon dioxide concentration in human blood is normally between 5-6% and again it is important that this is kept within these tolerance limits. Carbon dioxide is required to stimulate and control the rate and depth of breathing. At high levels, co2 dissolving in the blood can lead to the lowering of pH (acidosis). At levels greater than 10%, carbon dioxide is high toxic leading to unconsciousness and ultimately death.
Describe the stimulus response model.
Factors in the environment like, light, sound, touch, temperature and chemical concentration can cause either an increase or a decrease in the variable(s) and homeostatic responses work in the opposite way to reverse the changes. The control centre analyses information from the receptors and determines the appropriate response to the change. Feedback occurs when the response to a particular stimulus has an impact on the original stimulus.
What is a stimulus?
A stimulus us a variable factor in the internal or external environment that can be detected by the organism.
What is a receptor?
Receptors are the cells or tissues that can detects a change in the external or internal environment (a stimulus)
What is a transmission?
Transmission refers to the relay of the information via nerves and/or hormones to an effector
What is an effector?
An effector is usually a gland or muscle that brings about a repose after receiving the information such as movement or secretion. Effectors, in conjunction with receptors, help to maintain a constant internal environment by monitoring and resounding to changes in temperature, pH, oxygen concentration and solute concentration.
What is a response?
The responses is an action which occurs due to the initial stimulus
What is feedback?
The impact of the response on the initial stimulus. Feedback may be positive or negative.
Name the most common sensory receptors?
Photoreceptors (light), chemoreceptors (chemicals), mechanoreceptors (pressure), thermoreceptors (temperature)
What are sensory receptors?
Receptors respond to the intensity, duration and location of a stimulus. They send a nerve impulse o the central nervous system which interprets the signal and passes on a message to the effectors to elicit a response.
What would happen if we didn’t have sensory receptors?
If an organisms did not have sensory receptors it would not be able to detect and respond to changes in its environment. This would lead to changes in the organism’s internal environment and eventually death.
What are homeostatic control mechanisms?
Homeostatic control mechanisms are self regulating mechanisms that involve negative feedback. This is self regulating as it needs no outside action to occur, it is responding to a stimulus that it has itself caused.
What is negative feedback?
Negative feedback occurs when the reposes diminishes or reverses the original stimulus. Homeostasis is characterised by negative feedback as it is this mechanisms that returns the organ variable factor that was changed back to its steady state.
What is positive feedback?
Positive feedback occurs when the response reinforces or brings about an increase in the initial stimulus. This is less common.
What is an example of positive feedback?
When a newborn baby suckles on a mother’s breast nipple. This action causes a release of a hormone in the mother which further stimulates the release of milk which was the initial response.
3.2 - The Nervous System
What is the central nervous system?
It consists of the brain and the spinal cord. It detects internal changes int he brain, receives stimuli from the peripheral nerves, processes information and sends nerve impulses to relevant tissues and organs to elicit a response.
What is the peripheral nervous system?
It consists of all the nerves that aren’t part of the CNS. Cranial nerves connect to the brain, and spinal nerves connect to the spine. The PNS can be further divided into the somatic nervous system, and the autonomic nervous system.
What is the somatic nervous system?
This comprises the ‘voluntary’ nerves that control skeletal muscles. You use these when deciding to pick something up or kick a ball
What is the autonomic nervous system?
This controls all of the things that work automatically and do not require thought, e.g. gut movement, heart rate and breathing. This is further divided into the parasympathetic nervous system and the sympathetic nervous system.
What is the parasympathetic nervous system?
The parasympathetic nervous system controls homeostasis and the body at rest, “Rest and digest”
What is the sympathetic nervous system?
The parasympathetic nervous system is activated during great mental stress or when physical danger is encountered, “fight or flight”.
What are 3 parts all neurons have?
Cell body
Axon
Dendrites
In which way to nerve signals travel?
Nerves signals travel one way from the dendrite, though the axon, to the axon terminals, where they connect with the dendrites of the next nerve.
What are sensory neurons?
Sensory neurons are nerve cells that transmit information form a receptor to the CNS. Stimuli are sensed by specialised nerve endings called receptors which then triggers the nerve impulse which travels towards an interneuron in the CNS.
What are interneurons?
Interneurons are located in the CNS and transmit information from sensory neurons to motor neurons.
What are motor neurons?
Motor neurons transmit messages from the CNS to the effectors that are typically muscles or gland which results in the reposes of movement or secretion.
Are sensory neurons unipolar or multipolar?
They are unipolar as only on structure is directly attached to the cell body.
Are interneurons unipolar or multipolar?
They are multipolar because they have many structures extending from the cell body.
Are motor neurons unipolar or multipolar?
They are multipolar because they have many structures extending from the cell body.
Where are messages passed from?
Messages are passed from the axon of one neuron to the dendrite of another at synapse
What is the gap between the axon of one neuron to the dendrite of another called?
The synaptic cleft.
How do messages transmit from one neuron to another?
Step 1: Neurotransmitters are released into the synaptic cleft
Step 2: They diffuse across the space and bind to reactors on the receiving membrane
Step 3: this activates ion channels int he membrane leading to the nervous simple being transmitted further.
Why are nervous messages termed electrochemical?
They involve the electrical impulses along axons and the release of neurotransmitters across the synaptic cleft to transmit messages.
What are 4 examples of neurotransmitters?
Acetylcholine
Norepinephrine
Dopamine
Serotonin
What is acetylcholine?
Acetylcholine is a neurotransmitter that stimulates muscles and is also found in sensory neurons and the autonomic nervous system. The well-known poison botulin works by preventing vesicles in the axon from releasing acetylcholine, leading to paralysis. It also appears there is a strong link between acetylcholine and Alzheimer’s disease with about 90% loss of acetylcholine in the brain of those affected by this disease
What is norepinephrine?
Norepinephrine is a neurotransmitter that is linked with switching the nervous system into an ‘alert’ state. It increases heart rate and blood pressure. Amphetamine drugs (e.g. ‘Speed’) work by influencing the release of norepinephrine.
What is dopamine?
Dopamine is a neurotransmitter that acts as an inhibitor neurotransmitter. Several drugs, including cocaine, opium and alcohol increase levels of dopamine. The disease schizophrenia appears to be linked to increased levels of dopamine. Parkinson’s disease is though to be driven by a lack of dopamine in the brain. Interestingly, approximately 50% of dopamine is made by the gastrointestinal system and studies are showing links between out diet, gut microbiome and mental health.
What is serotonin?
Serotonin is 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. This further highlights the importance of a healthy diet and the link between poor diet and mental health conditions.
What is a reflex?
A reflex is an automatic response to a stimulus, like blinking when something is thrown at your face or quickly pulling your hand away from a hot fireplace.
Describe involuntary or autonomic responses.
Involuntary or autonomic reposes 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.
Describe the reflex arc.
A reflex arc is the pathway along which nerve impulses travel when a reflex action occurs.
A rector is stimulated and an impulse travels along a sensory neurone to an interneuron found in the spinal cord via a synapse
The interneuron stimulates a motor neuron via another synapse
Then 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.
Describe how carbon dioxide affects blood pH.
Blood pH is regulated by breathing out carbon dioxide when respiring aerobically
The CO2 produced is excreted by cells into the blood by diffusion
It dissolves int he blood forming carbonic acid (H2CO3)
Increased carbonic acid in the blood lowers blood pH, making it more acidic
Conversely, when too much CO2 is breathed out, there is a decrease in carbonic acid dissolved in the blood and the blood pH would increase, making it less acidic. This could occur when an individual hyperventilates.
How does monitoring blood pH control carbon dioxide levels in the blood?
Th blood pH is detected by chemoreceptors in the medulla oblongata (in the brain). A change in the blood pH would cause nervous message to be sent to the organs of the respiratory system (the diaphragm and intercostal muscles) to alter the rate of breathing, and therefore, the concentration of CO2 in the blood.
3.3 - The Endocrine System
What are the two types of hormones?
Protein and peptide hormones
Steroid hormones
What are protein and peptide hormones?
They are often polar and water soluble and do not travel into the cell; their action occurs as a result of binding to target cell membranes that possess complementary receptors to the hormone
What are steroid hormones?
They are non-polar and lipid soluble and bring about their effect by travelling into the cell and binding to internal receptors
What type of hormone is adrenaline, where is it secreted, what does it act on and what is its effect?
Adrenaline is an amino acid derivative, secreted from the adrenal medulla acts on most cells, mainly muscles, and it increases blood sugar levels, heart rate and blood pressure, blood flow to muscles, breathing rate and pupil dilation.
What type of hormone is noradrenaline, where is it secreted, what does it act on and what is its effect?
Noradrenaline is an amino acid derivative secreted by the adrenal medulla, it acts on cardiac muscles and smooth muscle, having a similar effect to adrenaline. It is also a neurotransmitter in the cardiac system.
What type of hormone is thyroxine, where is it secreted, what does it act on and what is its effect?
Thyroxine is an amino acid derivative, secreted by the thyroid gland, it acts on most cells, and increases oxidative metabolism.
What type of hormone is antidiuretic hormone (ADH), where is it secreted, what does it act on and what is its effect?
Antidiuretic hormone (ADH) is a peptide hormone, secreted by the hypothalamus, acts on renal collecting ducts, and increases reabsorption of water by kidneys.
What type of hormone is glucagon, where is it secreted, what does it act on and what is its effect?
Glucagon is a polypeptide hormone, secreted by the pancreas, acts on most cells, particularly liver and, stimulates the breakdown of glycogen to glucose increasing blood sugar level.
What type of hormone is insulin, where is it secreted, what does it act on and what is its effect?
Insulin is a protein hormone, secreted by the pancreas, acts on mainly liver and muscles cells and lowers blood sugar level, increases glycogen storage.
What type of hormone is thyroid stimulating hormone (TSH), where is it secreted, what does it act on and what is its effect?
Thyroid stimulating hormone (TSH) is a glycoprotein, secreted by the anterior pituitary, it acts on the thyroid and stimulates the production of thyroxine by the thyroid gland.
What type of hormone is aldosterone, where is it secreted, what does it act on and what is its effect?
Aldosterone is a steroid hormone secreted by the adrenal cortex, it acts on the kidneys and increases Na reabsorption and water reabsorption in kidneys and also increases blood pressure.
Describe hormones in the blood stream.
Endocrine glands secrete the hormones into capillaries that supply the glands. Hormones travel to target sites via the blood. The capillaries drain into veins which return the blood to the heart which pumps the blood to capillaries in all parts of the body via arteries. The hormones diffuse through the capillary walls into the tissue fluid where they are recognised by reactors on or in target cells.
Describe the control of metabolism.
Hormones can alter the metabolism of target cells, tissues or organs. Each hormone has a specific shape that is matched by a specific receptor on or in a target cell. Target cells make up target tissues and target organs. When a hormone binds to a rector it triggers a response from the cell. Hormones have no effect on cells that do not have the receptors. Hormones are active in the blood in very low concentrations and can have long-lasting effects.
Describe what happens after eating to blood glucose and insulin?
After eating, glucose enters the blood from the digestive system and causes blood sugar levels to rise
Glucose binds to reactors in the beta cells in the pancreas which triggers the beta cells to release insulin
Insulin causes fat and muscle cells to take up glucose from the blood and the liver to convert glucose to glycogen, a long chain carbohydrate that stores energy
Thus, blood sugar decreases
Describe what happens if blood sugar levels drop?
If blood sugar levels drop, it is detected by the alpha cells in the pancreas and they release glucagon
Glucagon binds to glucagon receptors on the liver cells and prompts them to convert glycogen back into glucose which is releases into the blood
Therefore, blood sugar levels rise
What is 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 which leads to high blood sugar levels (if untreated). There are two main types of diabetes, type 1 and type 2. In type 1 diabetes, the beta cells are killed by white blood cells as a result of an autoimmune disease, so the sufferer cannot produce insulin. In type 2 diabetes the cells become resistant to insulin and insulin does not work effectively.
Why do people with untreated diabetes have excessive urination?
Diabetics cannot reduce their blood glucose levels efficiently through the action of insulin so excess glucose is filtered by the kidneys and lost in the urine.
Why do people with untreated diabetes have thirst and dry mouth?
As excess fluid is lost through increases urination, diabetics can feel very thirsty and have a dry mouth, promoting the consumption of water.
Why do people with untreated diabetes have hunger and tiredness?
If a diabetic cannot produce enough insulin or their cells resist insulin, the cells cannot take up sufficient glucose to fuel their activity. This can result in a lack of energy and a feeling of hunger because the body’s cells haven’t been able to access enough energy.
Why do people with untreated diabetes have blurred vision?
Excess blood glucose levels cause the lens in the eye to swell, which increases refraction of light. As a result, the light does not hit the retina and a blurry image is seen.
Why do people with untreated diabetes have increased infections?
Increased glucose levels provide suitable conditions for microorganisms such as bacteria and fungi. This can result in increased infections, especially urinary tract infections.
What is thyroxine?
Thyroxine is a hormone produced by the thyroid gland and secreted into the circulator system. It acts on most cells in the body to regulate metabolism by increasing the basal metabolic rate (BMR). As a result of this, thyroxine promotes the generation heat.
What will a lack of thyroxine cause?
Slowing of thought and speech
Lethargy
Sleepiness
Decreased appetite
Cold intolerance
What will too much thyroxine cause?
Rapid heartbeat
Shortness of breath
Increases appetite
Intolerance to heat
Describe the hypothalamus’s role in the nervous and endocrine system.
They hypothalamus acts as a bridge between the nervous and endocrine system. It receives signal from incline nerves, sends impulses bia autonomic nerves, and secretes hormones and hormone-like substances that control the pituitary gland
Describe the process of thyroxine.
If thyroxine levels fall, this is detected by receptors in the hypothalamus region in the brain
The hypothalamus secretes thyrotropin releasing hormone (TRH) which stimulates the anterior pituitary gland to release thyroid stimulating hormone (TSH)
Thyroid stimulating hormone (TSH) travels though the bloodstream to the thyroid gland which then releases thyroxine
Increases levels of thyroxine increase metabolic rate and this stimulate energy release for ATP production and heat production
The hypothalamus will then detect increasing thyroxine levels and the stimulus is reversed.
What is osmoregulation?
The maintenance of constant osmotic pressure in the fluids of an organisms by the control of eater and salt concentrations.
What is the main organ involved in osmoregulation?
The kidneys. They continuously filter the blood and extract wastes for excretion while maintaining the correct balance of water, ions and other solutes the body needs. We have two, located on each side of the lower abdominal cavity. They form part of the excretory system. The adrenal gland sits on top of the kidney and secretes adrenaline.
Describe the process of osmoregulation and ADH.
The hypothalamus produces antidiuretic hormone (ADH) and it is stored in the posterior pituitary.
Osmoreceptors in the hypothalamus detect a rise in concentration of solutes in the blood and it signals the pituitary to release antidiuretic hormone (ADH)
The ADH travels to the kidneys in the blood where it binds to receptors in the cells that make up the wall of the collecting duct
The ADH signals the cell to produce aquaporins and send them to the side of the cell facing the collecting duct. This makes it more permeable to water and more ware is reabsorbed, reducing the concentration of solutes in the blood.
An increase in the reabsorption of water due to ADH increases blood volume, this, in turn increases blood pressure.
ADH is also secreted when stretch receptors in the circulatory system signal low blood pressure
There are 2 different stimuli that can trigger two different receptor types that will send a nerve message via the CNS to the effect that is the pituitary gland which secretes ADH
The ADH travels in the blood to two different target cell types (it can’t choose, it will act on both) causing vasoconstriction and increased water reabsorption
Thereby decreasing solute concentrations and increasing blood volume and pressure
What two things can signal hormonal responses?
The nervous system
The nervous system stimulates the pancreas to secrete insulin
The nervous system stimulates the adrenal medulla rot secrete noradrenaline and adrenaline
Other hormonal messages
What effects does adrenaline have on the body?
Dilates - adrenaline dilates the smooth muscles around the blood vessels that supply blood to skeletal muscles
Constricts - adrenaline constricts the smooth muscle around the intestinal blood vessels so that blood can be redirected to where it is needed
Relaxes - adrenaline relaxes smooth muscles around the bronchi to increase airflow to the lungs so that more oxygen is available to be reabsorbed into the blood
Increases - adrenaline increases heart rate and cardiac output, raising blood pressure and flow
Stimulates - adrenaline stimulates the pancreas to increase glucagon secretion which causes the liver to release glucose
Contracts - adrenaline contracts the radial muscles in the iris to dilate the pupil and let more light in
3.4 - Comparing Nervous and Endocrine Systems
Describe the process of adrenaline.
Shock, danger, excitement or stress can cause the hypothalamus of the brain to activate nervous pathways (the sympathetic nervous system), which in turn causes the release of adrenaline from the adrenal medulla into the blood stream
Adrenaline overrides the normal homeostatic control. It inhibits the secretion of insulin and binds to specific protein reactors on the surface of liver cells
This activates a series of steps that ultimately converts glycogen to glucose, increasing blood glucose levels
This glucose can then be transported to cells to be used for energy required for the ‘fight or flight’ response
Compare the nervous and endocrine system.
The nervous system transmits electrochemical messages via nerves and neurotransmitters in synapses where as the endocrine system transits chemical messages via hormones. The nervous system transmits its messages through neurons or nerve cells whereas the endocrine system does it thorough the blood. The endocrine system is slower, typically second to hours or days whereas the nervous system is fast, typically within milliseconds. Likewise the endocrine system is long-lasting usually hours or longer in comparison to brief only lasting usually seconds in the nervous system. The nervous system usually targets specific tissues directly whereas the endocrine system is less specific - a range of target tissues is common.
What happens when there is a decrease in blood temperature?
The stimulus is a decrease in blood temperature
Blood temperature is monitored by the thermoregulatory centre in the hypothalamus of the brain. The thermoreceptors in the hypothalamus detect the decrease in blood temperature
A variety of nervous and hormonal transmissions are activated:
Nervous messages are sent to the skeletal muscles, which make them contract (shivering), burning energy and creating heat as a by-product
Increased nervous messages are sent to the smooth muscle cells in the arterioles and pre-capillary sphincters that supply blood to the skin surface resulting in vasoconstriction. This decreases flow to blood to the surface of the body, reducing loss of heat
Increased secretion of thyroxine and adrenaline occurs from the thyroid and adrenal glands respectively. This results in increased metabolic rate and increased heat production
Increased nervous messages are also sent to erector-pili muscles in the skin causing them to contract, to erect hairs. This may cause appearance of ‘goosebumps’
Collectively, these responses increase heat production and retain heat in the body resulting in an increase in blood temperature
The thermoreceptors in the hypothalamus detect when blood temperature is back to the normal temperature and the transmissions to increase temperature are terminated. The responses have inhibited the stimulus that caused the response therefore negative feedback has occurred
What happens when there is a decrease in blood temperature?
The stimulus is an increase in blood temperature
Blood temperature is monitored by the thermoregulatory centre in the hypothalamus of the brain. The thermoreceptors in the hypothalamus detect an increase in blood temperature
A variety of nervous and hormonal transmissions are activated
Nervous messages are sent to the sweat glands, causing them to release sweat onto the surface of the skin. The evaporation of the sweat off the skin, takes heat with it, resulting in a cooling sensation (evaporative cooling)
Decreased nervous messages are sent to smooth muscle cells in the arterioles and pre-capillary sphincters that supply blood to the skin surface resulting in vasodilation. This increases flow of blood to the surface of the body, increasing loss of heat
Decreased secretion of thyroxine and adrenaline occurs from the thyroid and adrenal glands respectively. This results in decreased metabolic rate and decreased heat production
Decreased nervous messages are sent to erector-pili muscles in skin causing hairs to lie flat increasing heat loss (as insulating layer of trapped air is not present)
Collectively, these responses decrease heat production and increase heat loss from the body, resulting in a decrease in blood temperature
The thermoreceptors in the hypothalamus detect when blood temperature is back to the normal temperature and the transmissions to decrease temperature and terminated. The responses have inhibited the stimulus that caused the response, therefore negative feedback has occurred