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Homeostatic Mechanisms

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Rebecca Dixon

on 25 September 2013

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Transcript of Homeostatic Mechanisms

Homeostatic mechanisms
Homeostasis is the technical word for the process of maintaining a constant internal environment despite external changes
The internal environment consists of blood, tissue cells, body cell contents and all the metabolic processes that are taking place within the body
Negative Feedback as a form of regulation
Negative feedback occurs when the important variable, sometimes known as the key variable, like the PH of the blood and tissue fluid, deviates from the acceptable range or limits. it triggers responses that return the variable back to within the normal ranges
The brain and nervous system play a vital role in controlling the homeostatic mechanisms and they also help to anticipate when the key variables might fall or rise beyond the acceptable range
Negative feedback systems require:
Receptors to detect change
A control center to receive the information and process the response
Effectors the reverse the change and re-establish the original state.
Regulation of heart rate
The heart is controlled by the autonomic nervous system which comprises of 2 branches, the sympathetic and parasympathetic nervous system. These 2 systems act like an accelerator and a brake for the heart.
The sympathetic nervous system is mainly active when the body is undergoing fear, stress and muscle work. It causes each heartbeat to increase in strength as well as causing an increased heartbeat.
The parasympathetic nervous system calms the heart and is active during resting and peace.
Every few second the S-A node sends out a cluster of impulses across the network of atrial muscle fibres to cause contraction. The impulses are caught by another group of cells forming the A-V node and relayed to a band of conductiong tissue of large, modified muscle cells called the Purkinje fibres
The transmission is delayed slightly in the A-V node to allow the atria to complete the contractions
Heart valves are located on fibrous figure of eight betweenthe atrial and the ventricular muscle and the first part of the conducting tissue enables the excitatory impulses to cross the ventricles.
Impulses now pass very rapidly so that the two ventricles contract together forcing the blood to the organs
Cardic centres
The Medulla of the brain is the lowest part and is located above the spinal cord often being known as the brain stem.
Two important centres for control of the heart rate are located in the medulla.
The cardio-inhibitory centre is responsible for the origins of the parasympsthetic fibres of the vagus nerve reaching the S-A node. Whilst the sympathetic fibres decend through the spinal cord from the vasomotor centre.
Role of internal receptors
Barorecptors detect changes in blood pressure and can be found in the walls of the aorta and part of the carotid arteries delivering blood to the heart and neck.
A small upward change in blood pressure in these arteries often indicates the extra blood has been pumped out by the ventricles due to the extre blood entering the heart on the right side
Barorecptors detect the change and rely information in nerve impulses to the cardiac centres.
Activity in the vagus nerve slows the heart rate down and decreases the blood pressure back to normal.
Effects of adrenaline on heart rate
Circulating adrenaline which is a hormone form the adrenal gland, it is released during fear, times of stress and exertion
It stimulates the S-A node to work faster making it boost the effect of the sympathetic nervous system
Effect of increased body temperature on heart rate
Thermo receptors indicate a rise in body temperature to the brain causing the hypothalamus to activate the sypmpathetic nervous system.
This causes the heart rate to increase.
Regulation of breathing rate
Only when taking deep breaths, speaking or holding a breath are we voluntarily controlling our breathing
When metabolism produces extra carbon dioxide, breathing rates will increase slightly.
A period of forced breathing, such as gasping, will lower carbon dioxide levels in the body
The homeostatic mechanisms will slow or stop breathing temporarily until levels returnn to normal.
Roles of internal receptors
Internal receptors can be stretch receptors in muscles and tissues that relay nervous impulses to the brain about the status of ventitlation form the degree of stretch muscles and tissues
Chemoreceptors detect changes in chemical stimuli and supply the brain with this information
There are central and peripheral chemoreceptors
The central chemoreceptors monitoring H+ ion concentration are located in the medulla of the brain, an increase in H+ ion concentration results in a increased breathing rate.
Peripheral receptors, monitoring changes in oxygen concentration, increase breathing when oxygen levels decrease
Peripheral chemoreceptors are scattered around the aorta and carotid arteries in groups labelled the aortic and carotid sinuses
Autonomic nervous sytem - Parasympathetic and sympathetic nervous system
Most internal organs have a dual autonomic supply and the respiratory system is no different
The way that it acts is different
In the case of the bronsial muscle, the sympathetic causes it to relax and the parasympathetic causes contractions, resulting in the narrowing of the bronchi
Most of these fibres run in the vagus nerve in serving the heart
Sympathetic nerves emerge from a chain of ganglia to run to the bronchi.
Respiratory centre, diaphragm and intercoastal muscles
The cerebral cortex is the brain area responsable for voluntary conrtol of breathing
The involuntary centre, also known as the respiratory centre, is in the medulla and in the area above known as the pons.
The inspiratory centre is actively sending nerve impulses to the nerve to the diaphragm and the thoraic nerves are sending impulses to the intercaostal muscles to cause contraction
Inspiration ceases when the stretch receptors send bursts of impulses to the inspiratory centre, saying that the chest and lungs are fully expanded
Before exercise starts the body predicts the changes because the sympathetic nervous system is stimulated and adrenaline is released to increase the cardiac output and stroke volume
Regulation of body temperature
Humans are the only animal that can survive both tropical and polar regions of the earth
The fundamental precept is to keep the inner core of the body at normal temperatureswhile allowing the skin, limbs, etc. to adapt to chaning conditions of the external temperature
At very low temperatures such as -30c, the water component of the body would freeze and at high temperatures such as +50c, enzymes and body proteins would be permently altered or denatured
Life is not possible under these conditions so homeostatic regulation or thermo-regulation is vital
Sturcture and functions of the skin
The skin covers the outer surface of the skin and forms the largest organ. New cells are continually forming to replace those shed from the surface layers
The skin is a significant part of our built in immunity and forms not only a waterproof layer but also a microbe-proof covering.
It is an important part in the homeostatic regulation of body temperature and is condsidered to be part of the nervous system.
The skin varies in thickness throughout the body, it is thinner over the eyelids and lips but it is thicker on the soles of the feet.
It is divded into an outer and a deeper layer, the outer layer is called the Epidermis and the deeper layer is called the Dermis
Hair follicles are also extensions of the epidermis, which run down into the dermis and produce hairs made from Keratin
Attached to these are the sebaceous glands that coat the surface of the hairy parts, assisting in water proofing.
Sweat ducts penetrate the epidermis as they emerge from the sweat gland in the dermis. in the basal layer, there are collections of pigment cells known as the melanocytes that produce skin colour.
The pigment melanin protects against damage to deeper structures form ultra-violet light radiation.
The dermis is connective tissue, mainly areolar, in which blood vessels, nerves, sweat glands, elastic and collagen fibres intermingle.
Nerve endings form specialised receptors for temperature changes, touch, pain and pressure.
Hair erector muscles have their origins low down on the hair follicles and their attachments on the basal layer of the epidermis
When the hair erector muscles contract, usually from fear or the sensation of coldness, the hair becomes more erect making the skin surface lumpy.
Production of heat by the body
Heat is generated by the metabolic process taking place in the body.
Energy released during chemical reactions is used to drive processes like muscle contraction
The major functions of the skin:
Protect the underlying tissue agaisnt friction
Waterproof the body
Protect deeper structures from micro-organisms
Protect against ultra-violet radiation
For thermo-regulation
Relay nerve impulses generated form specialised skin sensory recpetors for heat, cold, touch, pain and pressure, informing the brain of changes in the environment
To synthesise vitamin D form sunlight acting on the adipose layers.
Loss of heat from the body
Skin capillaries form networks just below the outer layer or epidermis. when you are hot you need from the skin surface to cool yourslef down.
Conduction- warming up anything that you are in contact with.
Convection - when you warm up the layer of air next to your skin and it moves upwards, to be replaced by colder air form the ground
Radiation - heat will pass from your skin to warm up any colder objects arond you and conversely you will warm up buy radiation from any object than yourself, like a fire or the sun
Evaporation of sweat - when liquid water is converted into water vapour, it requires heat energy to do so. when you are hot, sweating will only cool the skin if it can take heat energy from the skin surface to convert to water vapour and evaporate.
Although conduction and convection to take place, they cannot be changed significantly to alter body temperature. the main methods of regulating temperature are by changing radiation and sweat-evaporation processes.
Role of the hypothalamus
The receptors for temperature, both heat and cold, are located in the peripheral skin and around the internal organs.
These are sepcially adapted cells with nerve fibres that run up the spinal cord to the temperature control centre in the hypothalamus of the brain.
The hypothalamus sends nerve imuplses to the muscles , sweat glands and skin blood vessels to cause changes that counteract the external changes.
Roles of the parasypathetic and sympathetic nerves
The parasypathetic nervous system has no role in thermo-regulation but the sympathetic nervous system controls both the sweat glands and the calibre of the arterioles
Role of the arterioles and sweat glands
As thermoreceptors tell the hypothalamus in the brain that the temperature is rising, sweat glands are activated by the sympathetic nerves and the arterioles are dilated to let more heat reach the surface of the skin, incereasing heat loss by radiationand evaporationof sweat.
If the core temperature is cooling, the sympathetic is active in causing the constriction of the arterioles but sweating is 'turned off'. This reduces heat loss, makes the skin older to the touch and preserves the core temperature.
The reason is that core temperature overrides the peripherial skin thermoreceptors when conflicting information is recieved.
Effects of shivering
Muscular activity generates heat so in a cold environment we may swing our arsm, stomp our feet, rub our face, hands and feet and also shiver.
This is a very effective way to generate heat, as it is all available to warm the body up.
Implications of surface area to volume ratio in the care of babies.
Babies have a larger surface area to volume ratio than adults and cannot effect changes to gain or lose heat for themselves, this meansd they are more at risk of developing Hyperthermia or Hypothermia.
Babies do not sweat much and newborns do not shiver. Therefore, it is important in cold weatherto wrap babies warmly, including the extremities and the head, and to gurad against over-heating in hot weather
Fever
It is one type of Hyperthermia and is most usually caused by infection; other types are heat stroke and heat exhuastion - all can be life threatening
Factors released as a result of disease act on thermoreceptors in the hypothalamus, raising the upper set point.
Consequently the sufferer feels cold, curls up, pulls on covers, looks pale due to vasoconstriction and even experience experiences intense shivering known as riggers.
It is not until the new sety point has been reached that sweating and other heat loss mechanisms begin. When the infection has subsided the set point is reset at a lower level.
Regulation of blood glucose levels
Role of the pancreas, liver, insulin and glucagon
After a meal rich in carbohydrates, blood glucose levels will start to rise. This increased level of glusoce stimulates the production of the hormone insulin from the beta cells in the islets of Langerhorns in the pancreas.
Insulin has two main functions:
To regulate the concentration of glucose in the blood
To increase the passage of glucose into th actively respiring blood cells by active aborsption
In the absence of insulin, very little glucose is able to pass through cell membranes and so the plasma level of glucose rises. Individuals with untreated Diabetes Mellitus (caused by a lack of insulin secretion) have high plasma glusoce levels and this leads to other biochemical disterbances.
In healthy people, the plasma glucose hardly varies at all because of liver cells, under control insulin, convert glucose into liver glycagon for storage.
When blood gluscose starts to fall as a result of fasting or being used up by respiring cells, another hormone, glucagon, from the aplha cells in the islets of Langerhans, is secreted and this converts liver glucagon back into gluscose for release into the bloodstream
These two hormones regulate the amount of glucose in the blood plasma by negative feedback mechanisms. Both have receptors attached to their islet cells to identify rising and falling plasma glucose levels.
Insulin also promotes the conversion of glucose into fat and delays the conversion of amino acids into energy.
It is also necessary to identify the role of another hormone, adrenaline, in the homeostatsis of glucose. Adrenaline, released by the adrenal glands when the sympathetic nervous system is active under stressful conditions, acts antagonistically to insulin and overrides it, to convert glucagon in the liver to glucose.
This outpouring of glucose provides energy for muscles to become active under emergency conditions. In addition, adrenaline converts fats to fatty acids for muscle contraction. When the emergency is over, insulin will once more become active and store any surplus as before.
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