Human Physiology and Practical Skills
1. Homeostasis
• The maintenance of a stable internal environment while adjusting to changing
external conditions. It works within a negative feedback loop and if it’s
successful, life continues.
Internal environment External environment
Core body temperature External temperature
Blood pressure Exercise
Arterial oxygen content Feeding
Blood glucose Fasting
Blood volume Hydration status
• All regulated variables must be kept within stable limits. They don’t have a definitive
limit, rather a definitive range which enables stability.
• Stability takes place as part of a dynamic equilibrium. A dynamic equilibrium will
have many small changes that sum together to create no net change.
Forward reaction = reverse reaction = steady state
• Regulated variables such as body temperature have a sensor which will detect the
stimuli. Once detected, a signal is sent to the control centre where the information is
processed. A decision can then be made, and a signal is sent to an effector where the
valid response occurs.
• Controlled variables such as ventilation and perspiration allow the maintenance of
regulated variables such as arterial blood oxygen and body temperature.
Examples within physiology:
• The control of body temperature in humans is a good example of homeostasis
physiologically. Normal body temperature fluctuates around the value of 37 degrees,
but various factors can disturb this trend.
• Exposure, hormones, metabolic rate and disease can all lead to excessively high or
low body temperatures.
• Body temperature is controlled by a region in the brain known as the hypothalamus.
Feedback about body temperature is carried to the brain via the nervous system and
compensatory adjustments are then made to regulate body temperature.
• These compensatory
adjustments include
perspiration, hormone
secretion, decreased
circulation, and cultural
modifications such as
taking shelter.
,Arterial oxygen pressure – a continued oxygen supply from the blood is required for cell
survival, if this is insufficient, catastrophic effects will occur.
Blood glucose – glucose is an important energy source. If this is low (hypoglycaemic) there
will be insufficient fuel for cells and organs to function. If glucose is too high
(hyperglycaemic) complications such as nerve damage and kidney disease can occur.
The body’s communication systems:
1. The Nervous System – sends signals through nerve cells called neurons to (afferent)
and from (efferent) the central nervous system. Afferent fibres carry signals to the
CNS, efferent fibres carry signals away from the CNS.
2. The Endocrine System – Secretes hormones (chemical messages) from glands into
the blood, which travel to and effect on distant organs.
Exercise and homeostasis:
Exercise poses a threat to homeostasis as it requires a coordinated response from multiple
bodily systems to maintain a steady state. Exercise increases body temperature, the use of
oxygen, the production of carbon dioxide and the body also uses more blood glucose. These
all need to be regulated and returned to their stable limits to ensure safety.
Disease and homeostasis:
Disease can affect homeostasis. It can cause an altered sensitivity of the sensors, failure at
the control centre and an impaired ability of the effector to respond. Type 1 diabetes is an
example of this. The sensors in the pancreas can no longer detect abnormal changes in
blood glucose levels and so its steady state cannot be maintained effectively. A disruption of
homeostatic mechanisms can also lead to disease and effective therapy must be directed at
re-establishing these homeostatic conditions.
, 2. Cardiovascular Response to Exercise
• There are four fundamental mechanisms that are responsible for cardiovascular
changes during physical activity, these are mechanical, metabolic, autonomic, and
hormonal.
• At the onset of exercise, cardiac output increases before neurohormonal
mechanisms are activated.
• The initial increase in cardiac output results from the skeletal muscle pump system,
which increases venous return.
• The rhythmic contraction-relaxation cycle of the muscles during exercise pushes
more blood through the venous system into the atria, thus increasing filling pressure.
• When the filling pressure in the ventricles is increased during diastole, the greater
tension causes an increase in stroke volume. This is termed preload.
Autonomic nervous system changes:
The increase in venous return causes an increase in preload via the increase in end-diastolic
volume. The increase in the force produced by the heart per contraction increases stroke
volume and reduces end-systolic volume (the volume of blood left in the heart after
contracting.
Metabolic Mechanisms:
Within seconds of the initiation of physical activity, metabolic mechanisms in the contracting
muscle dilate resistance vessels and increase blood flow. Nitric Oxide is also produced in
response to an increase in arterial stress due to increased blood flow through arteries and
arterioles. This increases vasodilation and decreases vasoconstriction.
During exercise, there is an overall increase in sympathetic nervous system activity. In active
muscles, the local factors (metabolites, nitric oxide) are able to overcome sympathetic
activity and vasodilation occurs.
In less active tissue (kidney, liver) there is a limited increase in metabolic activity and
vasoconstriction occurs. This helps redistribute blood to where it’s needed most.
What initiated the increase in sympathetic nervous system activity during exercise?
, When we perform exercise, neural signals are sent to activate skeletal muscle. Concurrently,
commands are sent to the cardiovascular control centre in the medulla, influencing
sympathetic and parasympathetic activity in an anticipatory feedforward manner.
Within the muscles, receptors exist which are sensitive to mechanical tension
(mechanoreceptors, group III afferents) and metabolic disturbances (metaboreceptors,
group IV afferents.)
When activated by exercise, these afferents transmit signals to the cardiovascular control
centre, evoking alterations in parasympathetic and/or sympathetic outflow to elicit the
appropriate cardiovascular response.
Hear rate and stroke volume response to incremental exercise:
The distribution of cardiac output is altered during exercise. At rest around 1000 mL (20%) of
blood reaches the muscle whereas during exercise around 21,000 mL (84%) of blood reaches
the muscle and organs such as the liver and kidneys receive less blood.
Systolic blood pressure increases during exercise despite total peripheral resistance
decreasing (due to vasodilation) as cardiac output increases. This means mean arterial blood
pressure is increased during exercise.
1. Homeostasis
• The maintenance of a stable internal environment while adjusting to changing
external conditions. It works within a negative feedback loop and if it’s
successful, life continues.
Internal environment External environment
Core body temperature External temperature
Blood pressure Exercise
Arterial oxygen content Feeding
Blood glucose Fasting
Blood volume Hydration status
• All regulated variables must be kept within stable limits. They don’t have a definitive
limit, rather a definitive range which enables stability.
• Stability takes place as part of a dynamic equilibrium. A dynamic equilibrium will
have many small changes that sum together to create no net change.
Forward reaction = reverse reaction = steady state
• Regulated variables such as body temperature have a sensor which will detect the
stimuli. Once detected, a signal is sent to the control centre where the information is
processed. A decision can then be made, and a signal is sent to an effector where the
valid response occurs.
• Controlled variables such as ventilation and perspiration allow the maintenance of
regulated variables such as arterial blood oxygen and body temperature.
Examples within physiology:
• The control of body temperature in humans is a good example of homeostasis
physiologically. Normal body temperature fluctuates around the value of 37 degrees,
but various factors can disturb this trend.
• Exposure, hormones, metabolic rate and disease can all lead to excessively high or
low body temperatures.
• Body temperature is controlled by a region in the brain known as the hypothalamus.
Feedback about body temperature is carried to the brain via the nervous system and
compensatory adjustments are then made to regulate body temperature.
• These compensatory
adjustments include
perspiration, hormone
secretion, decreased
circulation, and cultural
modifications such as
taking shelter.
,Arterial oxygen pressure – a continued oxygen supply from the blood is required for cell
survival, if this is insufficient, catastrophic effects will occur.
Blood glucose – glucose is an important energy source. If this is low (hypoglycaemic) there
will be insufficient fuel for cells and organs to function. If glucose is too high
(hyperglycaemic) complications such as nerve damage and kidney disease can occur.
The body’s communication systems:
1. The Nervous System – sends signals through nerve cells called neurons to (afferent)
and from (efferent) the central nervous system. Afferent fibres carry signals to the
CNS, efferent fibres carry signals away from the CNS.
2. The Endocrine System – Secretes hormones (chemical messages) from glands into
the blood, which travel to and effect on distant organs.
Exercise and homeostasis:
Exercise poses a threat to homeostasis as it requires a coordinated response from multiple
bodily systems to maintain a steady state. Exercise increases body temperature, the use of
oxygen, the production of carbon dioxide and the body also uses more blood glucose. These
all need to be regulated and returned to their stable limits to ensure safety.
Disease and homeostasis:
Disease can affect homeostasis. It can cause an altered sensitivity of the sensors, failure at
the control centre and an impaired ability of the effector to respond. Type 1 diabetes is an
example of this. The sensors in the pancreas can no longer detect abnormal changes in
blood glucose levels and so its steady state cannot be maintained effectively. A disruption of
homeostatic mechanisms can also lead to disease and effective therapy must be directed at
re-establishing these homeostatic conditions.
, 2. Cardiovascular Response to Exercise
• There are four fundamental mechanisms that are responsible for cardiovascular
changes during physical activity, these are mechanical, metabolic, autonomic, and
hormonal.
• At the onset of exercise, cardiac output increases before neurohormonal
mechanisms are activated.
• The initial increase in cardiac output results from the skeletal muscle pump system,
which increases venous return.
• The rhythmic contraction-relaxation cycle of the muscles during exercise pushes
more blood through the venous system into the atria, thus increasing filling pressure.
• When the filling pressure in the ventricles is increased during diastole, the greater
tension causes an increase in stroke volume. This is termed preload.
Autonomic nervous system changes:
The increase in venous return causes an increase in preload via the increase in end-diastolic
volume. The increase in the force produced by the heart per contraction increases stroke
volume and reduces end-systolic volume (the volume of blood left in the heart after
contracting.
Metabolic Mechanisms:
Within seconds of the initiation of physical activity, metabolic mechanisms in the contracting
muscle dilate resistance vessels and increase blood flow. Nitric Oxide is also produced in
response to an increase in arterial stress due to increased blood flow through arteries and
arterioles. This increases vasodilation and decreases vasoconstriction.
During exercise, there is an overall increase in sympathetic nervous system activity. In active
muscles, the local factors (metabolites, nitric oxide) are able to overcome sympathetic
activity and vasodilation occurs.
In less active tissue (kidney, liver) there is a limited increase in metabolic activity and
vasoconstriction occurs. This helps redistribute blood to where it’s needed most.
What initiated the increase in sympathetic nervous system activity during exercise?
, When we perform exercise, neural signals are sent to activate skeletal muscle. Concurrently,
commands are sent to the cardiovascular control centre in the medulla, influencing
sympathetic and parasympathetic activity in an anticipatory feedforward manner.
Within the muscles, receptors exist which are sensitive to mechanical tension
(mechanoreceptors, group III afferents) and metabolic disturbances (metaboreceptors,
group IV afferents.)
When activated by exercise, these afferents transmit signals to the cardiovascular control
centre, evoking alterations in parasympathetic and/or sympathetic outflow to elicit the
appropriate cardiovascular response.
Hear rate and stroke volume response to incremental exercise:
The distribution of cardiac output is altered during exercise. At rest around 1000 mL (20%) of
blood reaches the muscle whereas during exercise around 21,000 mL (84%) of blood reaches
the muscle and organs such as the liver and kidneys receive less blood.
Systolic blood pressure increases during exercise despite total peripheral resistance
decreasing (due to vasodilation) as cardiac output increases. This means mean arterial blood
pressure is increased during exercise.
