HOMEOSTASIS

http://tle.westone.wa.gov.au/content/file/ea6e15c5-fe5e-78a3-fd79-83474fe5d808/1/hum_bio_Science_3a.zip/content/003_homeostasis/page_01.htm

Introduction

Homeostasis is the maintenance of a stable internal environment that enables the optimal functioning of body cells. Activities at the cellular level are controlled by feedback mechanisms for the maintenance of homeostasis of body temperature, body fluid composition, blood sugar, gas concentrations and blood pressure. In this topic these mechanisms will be explored and you will have the opportunity to plan and conduct a long-term investigation based on one of your own homeostatic mechanisms.

Homeostatic mechanisms

The human body is regulated by mechanisms that involve organs, glands, tissues and cells.
The constant monitoring and adjusting of these contribute to homeostasis,
which enables the body to function at an optimum steady state.
You will explore five major homeostatic mechanisms.

These homeostatic mechanisms control:

body temperature
body fluid composition
blood sugar
gas concentrations
blood pressure.



You will learn about:
: Homeostasis

the role of the cell membrane structure in active transport and as a receptor
DNA and how it controls the production of cellular materials
components of a stimulus-response feedback model for homeostasis
homeostatic mechanisms that control body temperature, body fluid composition, blood sugar, gas concentrations, blood pressure
physiological and behavioural mechanisms that influence the maintenance of homeostasis in the homeostatic mechanisms mentioned above.

Cellular activities - Getting past the gatekeeper!

If you have travelled by air interstate or overseas, you will be aware of the very strict security procedures that exist in airports. You must first pass through a number of check-in points to be permitted to board the plane. Your baggage passes through scanners that can identify potentially dangerous items by their shape.

Cell membranes also identify molecules by shape. They will only permit entry to certain molecules if their shape matches signal receptors within the membrane.

The role of the cell membrane

Many of the body's key chemical reactions and exchanges take place in the cells. In order for these reactions to proceed, the structure of the cell's outer membrane is highly specialised to allow for the efficient movement of substances into and out of the cell.

This movement occurs via the processes of simple and facilitated diffusion , osmosis and active transport .

When the steady state of the body is disrupted, signals are sent via the nervous and endocrine systems to the cells to instruct them to respond in such a way that will lead to the return of optimum conditions.

These signals are either:

chemical (hormones )
electrochemical (neurotransmitters or ions).
Cell membranes are structured to selectively control what goes into and out of the cell. Looking closer at this structure will assist in understanding the importance of the signalling process.
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Signal receptors
are Sites located on the cell membrane that link with signal molecules in order to transport them to the interior of the cell
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Simple diffusion
The movement of substances from areas of high concentration to low concentration (no energy required)
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Facilitated diffusion
Diffusion of substances across a semipermeable membrane with the assistance of carrier proteins
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Osmosis
The diffusion of water through a semipermeable membrane from an area of high water concentration to an area of low water concentration
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Active transport
Movement of substances across a semipermeable membrane from areas of low concentration to high concentration requiring the input of energy
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Hormones
Chemical messenger molecules secreted from endocrine glands that produce responses in their target cells or organs when present, eg thyroxine, prolactin and insulin
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Neurotransmitter
Chemical produced from an axon terminal that assists in passing an impulse from one neuron to another
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Revision

What is the role of ATP in active transport across the cell membrane?

Provides energy required to move substances against their concentration gradient (low to high concentration).


How does the structure of the protein channel allow for hormones to be moved across the cell membrane?

The shape of receptor sites along the channel will only bind with certain shaped molecules; it will only allow those through.

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The membrane of a cell is comprised of two layers of phospholipid molecules (orange in above diagram) with large protein molecules (blue in above diagram) embedded across its surface. These proteins are structured to act as receptors for signal molecules (hormones or neurotransmitters) sent to the cell. The proteins have channels through which the signal molecules pass. This only happens if the signal molecules match with the receptor sites in the protein channels with the assistance of energy released from ATP



This results in the movement of substances that may already be in high concentrations inside the cell.

Therefore, active transport is important for the maintenance of homeostasis , as often passive movement of substances will not be sufficient to restore a chemical balance that has been created by a change in the environment.

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Phospholipid

A double-layered arrangement of phosphate and lipid molecules that form cell membranes; the hydrophobic lipid ends face inwards and the hydrophilic phosphate ends face outwards. Sometimes known as a lipid bilayer

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Receptor

Specialised cells that receive stimuli from the environment

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Signal molecules

Neurotransmitters or hormones that either pass through or attach to receptors on the cell membrane to cause a response
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Adenosine triphosphate

Also known as ATP; high energy-carrying molecule found in cells
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Homeostasis

The maintenance of a steady state within the internal environment
----------------

What is the role of ATP in active transport across the cell membrane?

Provides energy required to move substances against their concentration gradient (low to high concentration).


How does the structure of the protein channel allow for hormones to be moved across the cell membrane?

The shape of receptor sites along the channel will only bind with certain shaped molecules; it will only allow those through.




يتبع

DNA - The cell production line


You may remember the DNA is a molecule mainly found in the nucleus of cells. It acts as a type of 'blue print for life'. Its information is translated into all the different types of tissues and structures that make up an organism.

Answering the following questions requires you to refresh your basic knowledge about DNA.

What are the names of the four bases that make up a DNA molecule and state the rule that applies to their pairing?

The names of the four bases are cytosine, guanine, adenine and thymine. These always bond together in DNA as 'cytosine and guanine' and 'adenine and thymine'.



How is DNA packaged when it is located in the nucleus?

As chromosomes in a double helix.



What is a gene?

A section of the DNA molecule that encodes for one protein.


Managing protein production

DNA controls cellular activity by managing the production of all proteins. This process is known as protein synthesis .

In the nucleus the DNA molecule is copied. This new copy molecule is called messenger RNA (mRNA) . This molecule is sent out into the cytoplasm of the cell where it attaches to specialised organelles called ribosomes. On these organelles, the mRNA is used to assemble the building blocks of proteins, called amino acids . These are joined to form polypeptide chains . The combination of many polypeptide chains creates a new functional protein.



This diagram shows the DNA molecule as a twisted ladder. It is splitting to allow each side of the ladder to act as a template for two new molecules to be formed. Each new molecule contains one strand of the original DNA and one new strand.


DNA is like a DVD

When we think about how DNA controls the cell's activities, it may be helpful to imagine our DNA molecule as our own personalised microscopic DVD, stored in our cell's nucleus.

Analogies are great ways to understand difficult concepts. DNA is a complex molecule but try to imagine that this molecule is very similar to a DVD. There are many similarities between DNA and a DVD. One similarity is that they both carry information in a linear manner. DNA is a long molecule of sequenced chemical bases and a DVD has information recorded in a thin spiralled band.

comparison between DNA and DVD
: both are similar in
1- function : they carry information
2- form of the information : the information is written in a special language or encoded
3- process to be taken before we get the information : the information has to be translated
4- Can we copy this information ? Yes cells make copies of the DNA and we can copy a DVD
Can we edit or manipulate the information on it ? Yes we can take sections of DNA and - 5
use them for medical purposes and also we can take sections of music or data

: DNA & DVD differ in the following

1- How is this information accessed? Cells translate the information on the DNA
but We use a DVD player or computer to translate the information

2- What sort of information is provided ? The information in DNA translates into proteins
but The information in DVD translates into music, data or movies

Homeostasis via feedback systems

A steady internal state makes it possible for our cells to operate at their optimum level. Changes that occur in the external environment affect the internal environment, which is being continually adjusted in response.

A stimulus-response feedback model is a good way of showing how mechanisms are operating in the body in order to maintain homeostasis .
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Stimulus-response feedback model

A diagram used to explain how the changes to the body are responded to so as to maintain homeostasis
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Homeostasis

The maintenance of a steady state within the internal environment
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Things to remember about this model:

the loop represents a continuous process so there is no starting or finishing point
the response alters the original stimulus.
The following diagram reveals a stimulus-response feedback model. Use this diagram to answer the questions below



A negative feedback model can be represented with five boxes, all linked in a circular arrangement with arrows. The box at the top of the page represents the starting point of the feedback loop. It contains the stimulus or change in the environment. The second box which is linked by an arrow contains the receptor or the cells that detect the change. The third box is contains the modulator which is where the change is processed and information relayed on. The fourth box contains the effector which is organs, glands or tissues that are instructed to adjust output or secretions to cause an effect. This is linked by an arrow to the response in the last box. This is the change or outcome of the adjustments. The arrow that links the response box back to the stimulus box is a "dashed" or "broken line" arrow representing negative feedback that is where the response removes the initial stimulus.
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Negative feedback

A series of events where a response removes or reverses an initial stimulus or change; abbreviated to (-ve) feedback
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Receptor

Specialised cells that receive stimuli from the environment
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Qs

Why are four arrows represented by a solid line and one by a broken line?

The first four arrows represent information that is being passed from one place to another. The broken line arrow is representing negative feedback, that is, the stimulus is being reversed by the response.
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Name an organ which contains receptors.

Eye, ear, skin, brain.
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Give an example of a stimulus.

Increased temperature, decreased blood pressure.
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Suggest two examples of an effector.

Liver, kidneys, sweat glands.
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Regulation of body temperature

As humans, we maintain a relatively constant internal body temperature independent of the external temperature. The internal temperature of humans is approximately 37 °C. Any organism that can control its internal temperature is known as homeothermic.

Why is it so important to maintain a constant internal temperature?

Metabolic processes require an optimal temperature. At temperatures higher or lower than 37 °C, enzymes will not function optimally. Too high - they denature , too low - they will slow down the rate at which metabolic processes proceed. A rise of just 2 °C will cause disruption to the internal functioning of a human and should the temperature rise between 43 °C and 45 °C, death may occur. Our tolerance to lower temperatures is much greater. The temperature needs to fall below 23 °C to cause death.

How do we lose and gain heat?

The four methods of heat transfer in and out of the body are:

: conduction
Involves heat transferring from one object to another through direct contact

convection
Involves heat transfer via water or air currents

radiation
Involves heat transfer across space and doesn't require contact between two objects

evaporation.
Involves heat transferring out of a body as a result of liquid converting to a gas
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Denature
To cause a protein (eg enzyme) to lose some of its original properties; often this occurs due to temperature or pH changes
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Heat gain versus heat loss

In order to maintain a constant body temperature we must balance heat gain with heat loss.

How do we achieve this?

Humans can respond in two ways:

behavioural - where we consciously change our behaviour
physiological - where our body automatically alters its functioning without conscious control.

Raising body temperature

Think about the last time you felt cold. What sorts of things did you do that made you feel warmer? The body responds physiologically and behaviourally to a drop in the external temperature.

The following diagram is the feedback model. Study the diagram to discover the series of events that helps the body deal with a drop in temperature.



This is a negative feedback loop that illustrates the steps involved when the stimulus of a decrease in external temperature causes a decrease in body temperature. This information is detected by thermoreceptors located in the hypothalamus , skin, abdomen and spinal cord. The modulator, the hypothalamus, sends information via nerve and hormonal systems to effectors which are the skeletal muscles, blood vessels in the skin, cerebral cortex and body cells. The responses created include shivering which generates heat production in the muscles, vasoconstriction which reduces heat transfer via the skin surface, behavioural changes which involve deliberately increasing exercise, putting on more clothes and moving to a warmer place and lastly increasing metabolic rate to produce more heat. This response reverses the original stimulus and the internal body temperature begins to increase.
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Negative feedback

A series of events where a response removes or reverses an initial stimulus or change; abbreviated to (-ve) feedback
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Thermoreceptors

Receptor cells that detect changes in blood temperature and temperature of the external environment
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Hypothalamus

Region within the brain responsible for the regulation of many homeostatic mechanisms and endocrine activity
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Nerve

A bundle of nerve fibres held together by connective tissue
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Cerebral cortex

Specialised area of the cerebrum in humans that coordinates emotions, higher order thinking and problem solving, perception of senses and control of voluntary muscle movements
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access the Increase external temperature worksheet


Increase external temperature
Write the answers in the spaces below to complete the feedback loop that summarises how the body responds to an increase in external temperature.


Steps:
1. Stimulus
2. Receptor
3. Modulator
4. Effectors
5. Response

Choose from the following:
a. Sweat glands, blood vessels in skin, cerebral cortex, body cells
b. An increase in external temperature causes an increase in body temperature
c. Thermoreceptors in skin, abdominal organs, spinal cord and hypothalamus detect increase in temperature
d. i) Increase in sweat gland secretion will increase evaporation.
ii) Vasodilation – skin blood vessels will release heat to environment.
iii) Behaviour changes to deliberately increase heat loss, seek cool places, remove clothing.
iv) Decrease in Metabolic rate will reduce heat production.
e. Hypothalamus sends information via nerve and hormonal systems to effectors

Blood glucose regulation

What is glucose?

Glucose is a simple sugar or monosaccharide - the building block of carbohydrates. When it combines with oxygen in cells, it produces energy. Without it none of our metabolic reactions could proceed.

Why do we need to regulate it?

Certain tissues like the brain and retina are very sensitive to changes in glucose levels. An excess or deficit of blood glucose for more than a few hours can result in the loss of consciousness and brain damage.

Two important organs that assist in the control of blood glucose regulation are the pancreas and the adrenal gland.
The pancreas contains specialised cells, the Islets of Langerhans, which respond to the ever changing levels of blood glucose.
The adrenal gland also plays a role in times of low blood glucose levels.
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Retina
Innermost layer of the eye; it lines the rear portion of the inside of the eye where light rays focus; houses photoreceptors (light receptors)
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The pancreas
The pancreas is a large mass of glandular tissue lying between the stomach and the small intestine. Much of the pancreas consists of exocrine tissue which secretes digestive enzymes into the small intestine. The pancreas also contains endocrine tissue that secretes hormones into the bloodstream.

Scattered throughout the pancreas are small masses of endocrine tissue called the Islets of Langerhans. These contain alpha and beta cells, which are responsible for secreting the hormones glucagon and insulin, respectively.



The Islets of Langerhans
The important cells within these clusters are the alpha and beta cells.

Alpha cells detect low blood glucose and respond by secreting the hormone glucagon. The target cells for glucagon are in the liver and the body's cells. Liver cells respond by breaking down glycogen to glucose (glycogenolysis ) and releasing it into the bloodstream where it is transported around the body to where it is required. A process called gluconeogenesis also takes place in the liver and this makes new glucose molecules from molecules other than carbohydrates, for use in the body. Lipolysis is a process that also contributes to increasing the blood glucose level. It involves the breakdown of fat (adipose tissue) from body stores into glucose.

Beta cells detect high blood glucose levels and in response, secrete the hormone insulin. Cell membranes respond to insulin by becoming more permeable to glucose and the liver converts glucose to glycogen (glycogenesis ). In addition, insulin stimulates the conversion of glucose to fat in adipose tissue (lipogenesis ). It also converts excess glucose into protein through protein synthesis

The adrenal glands
These glands also respond during times of low blood glucose. During exercise the adrenal medulla is stimulated to produce adrenalin. This fast acting hormone behaves in a similar way to glucagon, raising blood glucose levels quickly when required.



The adrenal cortex is stimulated by adrenocorticotrophic hormone (ACTH) during times of low blood sugar. It stimulates the release of cortisol, which promotes the mobilisation of fatty acids to provide energy for working muscles, rather than using glucose.
The adrenal gland is located above the kidney. The middle section of the adrenal gland is the medulla and the outer layer is the cortex.

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Hormones
Chemical messenger molecules secreted from endocrine glands that produce responses in their target cells or organs when present, eg thyroxine, prolactin and insulin
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Glycogenolysis
The process of breaking down glycogen into glucose
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Gluconeogenesis
The process of manufacturing new glucose molecules from molecules other than carbohydrates, ie fats and amino acids
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Lipolysis
The process of breaking down fats to glucose
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Glycogenesis
The process of converting glucose to glycogen in the liver and muscles
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Lipogenesis
The process of storing glucose as fat in the tissues
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Protein synthesis
The process of building new proteins from long chains of amino acids; the cell's DNA is responsible for controlling this process
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Mobilisation
To activate or release into a system
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Fatty acids
The building blocks or basic units of lipids
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the steps in response to a low blood glucose level



The stimulus is a low blood glucose level due to exercising. This is detected by receptors in the pancreas (alpha cells) and the adrenal gland. The modulator, the alpha cells in the pancreas secretes glucagon, the adrenal medulla secretes adrenalin and the adrenal cortex secretes cortisol. These hormones target the liver and the body cells (effectors) to cause glucose to be released via the processes of glycogenolysis , gluconeogenesis and lipolysis . The response that occurs is an increase in blood glucose levels, which has a negative feedback effect to reverse the original stimulus.

Regulation of gas concentrations

Every cell in the body requires oxygen for respiration so that sufficient energy can be produced. Carbon dioxide, a waste product, is also produced and needs to be removed. Therefore, the levels of both gases must be regulated. This is achieved by a homeostatic mechanism that controls breathing. Unlike other homeostatic mechanisms, we have some voluntary control over this process.

Control of breathing

The intercostal muscles (between the ribs), the lungs and the diaphragm (a sheet of muscle) work together to move air into and out of the lungs. Nerve impulses control the muscles and these are sent from the respiratory centre, which is located in the medulla oblongata at the base of the brain.

The respiratory centre is comprised of two regions: the inspiratory centre that controls breathing in and the expiratory centre that controls breathing out. Messages are sent between these two centres to coordinate the breathing process.

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Nerve impulse
An electrical signal passed through a neuron or nerve
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Medulla oblongata
Structure located at the top of the spinal cord responsible for regulation of many homeostatic mechanisms including heart rate and breath rate
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Homeostasis and breathing rate

Oxygen, carbon dioxide and hydrogen ions are carried in the blood and their concentrations all have an effect on your breathing rate.

Two key structures involved in the homeostatic mechanism of breathing rate are the respiratory centre (located within the medulla located at the base of the brain) and the carbon dioxide receptors located in the aorta of the heart.



Hydrogen ions



When carbon dioxide dissolves in water it forms carbonic acid (H2CO3) which then breaks down to form hydrogen ions (H+) and bicarbonate ions (HCO3-). Increased H+ will cause a decrease in blood pH and this causes an increase in breathing rate.

Carbon dioxide

A slight increase in carbon dioxide concentration leads to a marked increase in breathing rate. As mentioned above, an increase in H+ leads to an increased rate of breathing. Together CO2 and H+ stimulate receptors located in the respiratory centre and in major arteries - the carotid and the aorta. Receptors that are stimulated by chemicals like CO2 or H+ are called chemoreceptors .

Oxygen

As oxygen is being continually used by cells, so its concentration in the blood decreases. In contrast to carbon dioxide, oxygen concentration needs to fall significantly before any increase in breathing rate results. Chemoreceptors that detect oxygen concentrations are located in the walls of two major arteries, the carotid and the aorta. These cells are known as the aortic and carotid bodies. When these cells detect a decrease in concentration, nerve impulses are sent to the respiratory centre. From here, impulses are sent to the respiratory muscles to stimulate the breathing process.

Remember the concentration of carbon dioxide is the main stimulus for breathing.

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pH
The scale used to measure the acidity or alkalinity of a substance; the scale is numbered 1-14 with 1 being highly acidic and 14 highly alkaline
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Chemoreceptors
Receptor cells that detect changes in chemical concentrations, ie oxygen, carbon dioxide and hydrogen ion concentrations
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Nerve impulse
An electrical signal passed through a neuron or nerve
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Control of breathing

Write the answers in the spaces below to complete the stimulus-response feedback loop for control of breathing.

http://www.4shared.com/file/SKbF8z50/control_of_breathing.html?

Steps:

1. Stimulus
2. Receptor
3. Modulator
4. Effectors
5. Response

Choose from the following:
a. Respiratory centre in medulla
b. Increased CO2, increased H+
c. Increased stimulation of respiratory muscles causes increased breathing rate and depth to reduce CO2 and H+ levels
d. Chemoreceptors in respiratory centre and in carotid and aorta
e. Diaphragm intercostal muscles

Regulation of blood pressure

The circulatory system plays a major role in the regulation of temperature, body fluids, glucose and gases.

The blood transports hormones , gases and nutrients and allows for heat gain or loss.

The pressure at which the blood is pumped and distributed is also controlled by a homeostatic mechanism.

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Research ,Questions

report and reflect
Part 1: Research and report

a. Define blood pressure.
b. The following headings should be used to complete short summaries that will help your understanding of this mechanism.

How is heart rate and blood pressure controlled?
• Cardiac output
• Heart rate
• Stroke volume
• Venous return
• Blood pressure
• Diameter of blood vessels
• SA Node
• AV Node
• Cardiovascular regulating centre
• Baroreceptors.
Part 2: Reflect
a. In a table format, list the body organs, structures, tissues and cells that take part in the mechanism for regulating blood pressure.
Organs Structures Tissues/cells

b. Write the answers in the spaces below to complete the stimulus–response feedback loop that demonstrates the regulation of blood pressure. The first feedback loop should show what occurs if a person experiences an increase in blood pressure. The second feedback loop should show what occurs when a person experiences a decrease in blood pressure.

Increased blood pressure response


Steps:
1. Stimulus


2. Receptor


3. Modulator


4. Effectors


5. Response


Decreased blood pressure response


Steps:
1. Stimulus


2. Receptor


3. Modulator


4. Effectors


5. Response


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Research, report and reflect answeres – Solution

Part 2: Reflect

a. Organs
heart, brain

Structures
arteries, veins

Tissues/cells
SA node, AV node, baroreceptors

b. Your answers should be similar to the following:
Increased blood pressure response
Steps:
1. Stimulus
Increase blood pressure (increased cardiac output)
2. Receptor
Baroreceptors in carotid/aortic bodies
3. Modulator
Cardiovascular regulating centre (medulla) via parasympathetic fibres
4. Effectors
Heart – SA Node and AV Nodes
5. Response
Decrease in heart rate/stroke volume (cardiac output) due to reduced firing of SA/AV nodes
Decreased blood pressure response
Steps:
1. Stimulus
Decrease blood pressure (decreased cardiac output)
2. Receptor
Baroreceptors in carotid / aortic bodies
3. Modulator
Cardiovascular regulating centre (medulla) via sympathetic fibres
4. Effectors
Heart – SA/AV nodes, blood vessel walls
5. Response
Increase in Heart rate/stroke volume (cardiac output) due to increased firing of SA/AV nodes. Vasoconstriction, increase venous return

HOMEOSTASIS VIDEOS
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USMLE Step 1 Review 52 02 Physiology Homeostasis And Control Systems
https://www.youtube.com/watch?v=8AGOvE4Cafg&list=PLE371A278DFF39023&index=1


USMLE Step 1 Review 52 09 Physiology Membrane Transport 1of 2
https://www.youtube.com/watch?v=SmxslZk3AJM&list=PLE371A278DFF39023&index=7

USMLE Step 1 Review 52 10 Physiology Membrane Transport 2 of 2
https://www.youtube.com/watch?v=YG8F7ozquog&list=PLE371A278DFF39023&index=8