Homeostasis
Homeostasis
• The maintenance of the internal environment
in a relatively stable state in the face of
changes in the external or internal environment.
• In multicellular organisms when we talk about
the internal environment we are considering the
interstitial (extracellular) fluid between its
cells.
• In unicellular organisms we are considering
the internal environment of the cell itself.
Intracellular and Extracellular
Fluids
• Both tissue fluid and plasma are located outside cells and can be
labelled as extracellular fluids (extra = outside).
• In contrast, the fluid, called cytosol, that is located within all cells can
be labelled as intracellular fluid (intra = within).
• Tissue fluid and plasma are separated from each other by the thin
flat cells that form the walls of the capillaries, the smallest vessels of
the blood circulatory system.
• Extracellular fluids are separated from intracellular fluids by the
partially permeable cell membranes. We can think of the various
intra- and extracellular fluids as ‘compartments’.
• It is the extracellular fluids, and in particular the tissue fluid, that
forms the internal environment for cells of the body.
Changing composition of
intracellular and extracellular fluids
•
Although the tissue fluid and
plasma that make up the
internal environment are
separated, continuous
exchange occurs between
them.
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Nutrients (such as glucose) and
gases (such as oxygen) pass
from the blood to the tissue
fluid.
Waste products (such as carbon
dioxide and urea) pass from
tissue fluid into the blood.
In addition, many substances
can move between cells and the
fluids of their environment.
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Purpose of blood tests
• Two-way exchanges of many substances can occur between cells,
tissue fluid and plasma.
• This means that the make-up of one fluid can be affected by that
of another fluid.
• So, the composition of tissue fluid is affected by the make-up of
blood plasma. Similarly, the composition of cell cytosol is affected by
the makeup of tissue fluid.
• Blood is a readily accessible body tissue, and the exchange of
fluids between compartments means that blood tests can
provide information about the state of cells in other parts of the
body.
• Alarm is raised if abnormally high (or low) levels of certain
substances are detected in a person’s blood. Why?
• These levels will not just be present in the blood cells
themselves, but also in the tissue fluid and the other cells of
the body which are bathed in tissue fluid.
Why maintain a stable internal
environment?
Why maintain a stable internal
environment?
• Biochemical reactions in living cells can only occur
when pH, various salts and nutrients, and physical
conditions are within certain limits.
• Concentrations of substances such as glucose, gases
such as oxygen and carbon dioxide, and hydrogen ions
in cells and tissue fluids all have an impact on the
way cells function.
• Movement of materials across the membrane is
affected by concentration of water, nutrient particles and
ions on either side of the membrane.
• Activity of enzymes responsible for metabolic
activities of cells is affected by temperature.
Homeostasis in different organisms
• The complexity of the mechanisms
involved in homeostasis varies according
to the complexity of the organism.
• All homeostatic mechanisms require
the ability to detect changes in the
external or internal environment and
respond to them.
Homeostasis in different organisms
• Simple unicellular organisms are able to sense and respond to
environmental conditions – generally moving away from heat and
light, and towards food.
• Multicellular organisms such as sponges rely on the cellular
membrane to regulate concentration of cytoplasmic contents
while being bathed directly in the fluids of the external environment.
• Homeostatic mechanisms in multicellular organisms such as
plants and animals are more complex, and the basic physiology
of multicellular organisms has evolved in order to provide and
appropriate internal environment. For example:
– Gas exchange: leaves, stomata, root hairs, lungs, gills, skin of
amphibians
– Nutrients: roots of plants, digestive systems of animals
– Transport within organisms: xylem and phloem in plants, circulatory
systems in animals
– Waste: excretory cells and organs
Conformity vs Regulation
• When an organism is faced with a change in its environment, it has
two broad categories of response:
1. conform to the change (because it is unable to maintain homeostasis
for internal conditions)
2. regulate their internal environment (i.e. maintain homeostasis) over a
broad range of external environmental changes by using biochemical,
physiological behavioral and other mechanisms (generally there is a
maximum ability to do this)
• The ability to regulate their internal environment enables
organisms to live in a wider range of conditions.
• Organisms with narrow tolerance limits are often those with little or
no ability to regulate their internal environment.
Osmoconformers and Osmoregulators
Hyas araneus – the intertidal decorator crab
• Is an osmoconformer
• Solute concentration of its body fluids is always
about the same as that of the surrounding water.
Carcinus maenas – common shore crab
• Is an osmoregulator
• Expends energy to maintain relatively constant
levels of ions and solutes in its body fluids.
Osmoconformers and Osmoregulators
• Animals that osmoregulate are more able to survive
the fluctuations of salinity encountered in seashore and
estuarine habitats.
• There is a cost in energy in terms of active pumping of
salts.
• In contrast, osmoconformers do not have this cost,
however, they suffer considerable stress if the
concentration of their body solutes decreases due to
environmental factors such as increased rain in their
rock pool.
• The tolerance limits of osmoconformers are set by
the ability of their tissues to function in dilute salt
solutions.
Detecting and Responding
• Ability to detect and respond to changes is
fundamental to all organisms.
• Changes serve as stimuli.
• Stimuli are detected by receptors.
• Intensity of stimulus must be sufficient to reach the
threshold of the receptor (weakest stimulus to which
receptor can respond).
• Receptors then stimulate effectors to produce a
response.
• Response of the effector influences the magnitude of the
stimulus and returns the variable to homeostasis.
• This is known the stimulus-response model.
Stimulus-Response Model
Stimuli
• Both internal and external factors act as stimuli for
animals in the maintenance of homeostasis.
• Changes in any of these factors may stimulate a
response in the animal that tends towards returning
the internal environment to its stable state.
• Internal conditions include:
– Chemical factors e.g. levels of oxygen, carbon dioxide, glucose,
ions, wastes and water.
– Physical factors such as temperature, blood pressure and
balance.
• External conditions include:
– Physical factors such as light, temperature, gravity, sound and
day length.
– Chemical factors such as food, oxygen, carbon dioxide, water
and specific chemicals.
Detecting and Responding
• Information comes from detectors functioning at
one of two different levels:
– Misalignment detectors – monitor a precise factor of
the internal environment that is being controlled and
detect when a particular factor is out of line.
– Disturbance detectors – detect the presence of
external or internal changes that are likely to cause a
change in the particular factor of the internal
environment being controlled. They warn of
problems before they actually arise.
• Regulation of body temperature involves both
misalignment and disturbance detectors.
Regulation of body temperature
Types of Receptors
• Chemoreceptors - detect chemicals
– Olfactory lining in nose; taste buds; oxygen concentration
receptor in aorta; osmoreceptors in hypothalamus; glucose level
receptors in pancreas; pH/CO2 receptors in medulla, aorta and
carotid arteries
• Mechanoreceptors – detect pressure and movement
– Ear; touch & pressure receptors in skin muscles, joints and
connective tissue; muscle length receptors in skeletal muscle;
muscle tension receptors in tendons; joint receptors; venous
pressure receptors; arterial pressure receptors; lung inflation
receptors; lung deflation receptors; lung irritant receptors.
• Photoreceptors – detect light
– Eye.
• Thermoreceptors - detect temperature
– Heat receptors and cold receptors in skin; body temperature
receptor in hypothalamus.
Feedback systems
• Homeostasis or regulation therefore involves
fluctuations around a set-point.
• The size of the fluctuations depends on the
sensitivity and location of the sensory receptors, the
tolerance of the control centre to variation from the
set-point, and efficiency of the response mechanism.
• Most biological feedback systems are negative
feedback systems which operate as proportional
control systems – the size of the response is
proportional to the size of the stimulus.
Feedback System
Negative Feedback
• Most common type of biological feedback system.
• Activity of effector opposes stimulus.
• Effector produces opposite effect of stimulus.
• Example:
– Home heating system. The temperature of the home is
monitored and heating will be turned off until the temperature
returns to set level.
• Biological examples: body temperature and blood
glucose levels
Positive Feedback
• Not particularly common in biological
systems.
• Activity of effector reinforces stimulus.
• Effector produces response in same
direction as stimulus
• Must have a mechanism to halt the cycle
• Biological examples: childbirth and blood
clotting
Detecting and Responding to Change
Body systems involved in regulation
of homeostasis
• Nervous system
– Receives and transmits information about both the external and internal
environment. Transmits electrical impulses to body cells that respond in
various ways.
• Endocrine (hormonal) system
– Produces hormones that are secreted directly into the bloodstream and
transported throughout the body where they regulate cell activities.
• Respiratory system
– Obtains oxygen from air and eliminates carbon dioxide which is a waste
product of metabolism of cells. Assists in regulation of pH through
removal of carbon dioxide.
• Circulatory system
– Transports O2 to cells, CO2 away from cells, and hormones, wastes and
nutrients such as glucose, amino acids and fatty acids throughout the
body. Has a central role in maintaining the environment of cells
within desirable limits.
Body systems involved in regulation
of homeostasis
• Digestive system
– Obtains nutrients, water and salts
from the food we eat. These are
transferred to blood and lymph
vessels in the intestine wall.
Undigested residue is eliminated.
• Excretory system
– Removes waste such as urea, excess
water, salt and other ions from the
blood and eliminates them from the
body in the form of urine. Important in
regulating pH.
• Integumentary system
– Barrier between body and external
environment. Evaporation of sweat is
important in temperature regulation.
Also inhibits entry of micro-organisms.
Some systems controlled by
homeostasis
Control of
Requires regulation of
nutrient levels
(e.g. glucose)
•nutrient intake
•digestive and circulatory system functions
•storage and mobilisation of nutrients
•behaviour
body temperature
•general metabolism
•blood flow to tissues
•muscle activity and sweating
•behaviour
water and salt
balance
•excretion of water and salts to maintain correct
osmotic concentration of internal body fluids
•behaviour
metabolic rate
•lung ventilation and circulation to deliver adequate
oxygen to tissues
•nutrient intake and storage
•behaviour
Homeostasis in the Newborn
• Research has shown that
the regulatory systems
of newborn babies
mature at different
rates.
• Premature babies need
particular care as they
have poor temperature
control and brain and
breathing functions are
underdeveloped.
Regulatory role of the liver
• Eating a meal results in a considerable disturbance to the internal
environment.
• Blood leaving the gut suddenly contains very high concentrations of
nutrients such as simple sugars and amino acids. If this blood
simply passed into the circulation, blood composition would be
drastically altered and impair normal function.
• Instead the blood passes via the hepatic portal vein to a second
network of capillaries in the liver, which is one of the most important
homeostatic organs in the body.
• The composition of blood is regulated by the liver before it passes
into the hepatic vein and then back to the heart.
• The liver plays and important role in storing and mobilising
nutrients in a regulated manner so that blood levels are
maintained within relatively narrow limits.
Regulatory functions of the liver
Controlling Blood Glucose
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The pancreas produces two hormones, insulin and glucagon — each
produced in special cells in the pancreas.
These hormones are involved in the control of glucose in the blood. The
hormone insulin controls the uptake by cells of glucose from the
blood. The hormone glucagon acts on the liver to release more glucose
into the blood.
If the blood glucose level falls below normal, the pancreas usually responds
in two ways:
1. Some cells, called alpha cells, increase their production of the hormone
glucagon which acts on the liver to convert stored glycogen to glucose.
Glucose passes from the liver into the bloodstream.
2. Other cells, called beta cells, decrease their production of insulin.
Less insulin in the blood results in less glucose being taken from the
blood by cells of the body.
These events cause the blood glucose level to rise. This is detected by the
pancreas which then responds by increasing the insulin and decreasing the
glucagon it produces.
A steady state is achieved, although there are small fluctuations. The
steady state of glucose in the blood is produced as a result of
negative feedback mechanisms involving both insulin and glucagon.
Controlling Blood Pressure
Challenges to Homeostasis
• Homeostatic mechanisms must operate to coordinate
appropriate integrated responses to different
situations.
• Examples of challenges for this integrated coordination
include:
– Exercise: results in transient changes in oxygen demand, blood
glucose levels, blood pressure and distribution, body
temperature and water and salt. It can also produce an oxygen
debt.
– Pregnancy: particularly during the latter stages, increases
nutritional requirements, the physical workload associated with
weight gain, and the demands on kidney function.
– Space: The lack of gravity experienced by astronauts reduces
the work done by the cardiovascular and musculoskeletal
systems. This weakens bones and muscles and reduces
cardiovascular fitness.