The document discusses homeostasis and how the human body maintains stable internal conditions. It defines homeostasis as the maintenance of stable internal conditions necessary for cell survival. Key systems like the circulatory, respiratory, and urinary systems act to regulate factors such as nutrient levels, pH, water concentration, and waste removal to maintain homeostasis. Negative feedback loops trigger responses to correct deviations from normal ranges. Disruptions in homeostasis can lead to pathophysiological states and even death if too severe.

1. Homeostasis
The Human body and control of the
internal and external environments.
(Introduced by Walter B. Cannon)

2. Cells as the living units of the body
• The basic living unit of the body is the cell.
• Each organ is an aggregate of many different cells
held together by intercellular supporting structures.
Each type of cell is specially adapted to perform one
or a few particular functions.
– For instance, the red blood cells, numbering 25 trillion in
each human being, transport oxygen from the lungs to the
tissues.
• Although the red cells are the most abundant of any
single type of cell in the body, there are about 75
trillion additional cells of other types that perform
functions different from those of the red cell.
– The entire body, then, contains about 100 trillion cells.

3. Cells as the living units of the body
• Although the many cells of the body often differ markedly
from one another, all of them have certain basic
characteristics that are alike.
– For instance, in all cells, oxygen reacts with carbohydrate, fat,
and protein to release the energy required for cell function.
– Further, the general chemical mechanisms for changing
nutrients into energy are basically the same in all cells, and all
cells deliver end products of their chemical reactions into the
surrounding fluids.
– Almost all cells also have the ability to reproduce additional
cells of their own kind.
• Fortunately, when cells of a particular type are destroyed
from one cause or another, the remaining cells of this type
usually generate new cells until the supply is replenished.

4. Intra and extracellular fluids
• Intracellular fluid (ICF) – the fluid collectively
contained within all body cells
• Extracellular fluid (ECF) – the fluid outside
cells, which is the internal environment of
human body. Note that the ECF is outside the
cells but inside the body.
• ECF – is made of Plasma and Interstitial fluid
• Inter – between
• Stitial – that which is standing

5. Cell
Interstitial fluid
Plasma
Blood vessel
ECF/Internal Environment
The internal and the
external environments
are in constant
interaction. E.g. the
digestive system carries
nutrients from external
sources into the
plasma. The respiratory
system also transfers
oxygen from the
external environment
into the plasma. The
circulatory system
distributes the nutrients
and oxygen throughout
the whole body.
Likewise waste products
aught to be eliminated
so they do not reach
toxic levels.

7. Homeostasis
(Homeo – the same; stasis – to stand/stay)
Homeostasis is
the maintenance
of a relatively
stable internal
environment. It is
essential for the
survival of cells,
body systems
maintain
homeostasis,
and cells make up
body systems.

8. Factors Homeostatically regulated
• Concentration of nutrients: cells need a constant
supply of nutrient molecules for energy production.
Energy, in turn, is needed to support life-sustaining
and specialized cell activities.
• Concentration of oxygen and carbondioxide: cells
need oxygen to carry out energy yielding chemical
reaction. The carbondioxide produced during these
reactions must be removed so that acid-forming
carbondioxide does not increase the acidity of the
internal environment.

9. Factors homeostatically regulated
• Concentration of waste products: some
chemical reactions produce end products that
have a toxic effect on the body’s cells if these
wastes are allowed to accumulate.
• pH: changes in pH of the ECF adversely affect
nerve cell function and wreak havoc with the
enzyme activity of all cells.

10. Factors homeostatically regulated
• Concentration of water, salt, and other
electrolytes: the relative concentrations of NaCl
and water in the ECF influence how much water
enters or leaves the cells.
• These concentrations are carefully regulated to
maintain the proper volume of the cell.
• Cells do not function normally when they are
swollen or shrunken.
• Other electrolytes (chemicals that form ions in
solution) perform a variety of vital functions.
– E.g. the rhythmic beating of the heart depends on a
relatively constant concentration of potassium ions in
the ECE.

11. Factors homeostatically regulated
• Volume and pressure: the circulating
component of the internal environment, the
plasma, must be maintained at adequate
volume and blood pressure to ensure bodywide
distribution of this important link between the
external environment and the cells.
• Temperature: body cells function best within a
narrow temperature range. If cells are too cold,
their function slow down too much. If they get
too hot, their structural and enzymatic proteins
are impaired or destroyed

12. Measurement Normal Range
Arterial pH 7.35 – 7.45
Bicarbonate 24 – 28 mEq/L
Sodium 135 – 145 mEq/L
Calcium 4.5 – 5.5 mEq/L
Oxygen content 17.2 – 22.0 ml/100ml
Urea 12 – 35 mg/100ml
Amino acids 3.3 – 5.1 mg/100ml
Protein 6.5 – 8.0 g/100ml
Total lipids 400 – 800 mg/100ml
Glucose 75 – 110 mg/100ml

13. Body systems and homeostasis
• Circulatory system: (heart, blood vessels, and
blood) transports materials such as nutrients,
oxygen, carbondioxide, wastes, electrolytes, and
hormones from one part of the body to another.
• Digestive system: (mouth, oesophagus, stomach,
intestines, and related organs) breaks down dietary
food into small nutrient molecules that can be
absorbed into the plasma for distribution to the
body cells.
– It also transfers water and electrolytes from the external
environment into the internal environment. It eliminates
undigested food residues to the external environment in
the faeces.

14. Body systems and homeostasis
• Respiratory system: (lungs and major airways) gets
oxygen from and eliminates carbondioxide to the
external environment.
• By adjusting the rate of removal of acid-forming
carbondioxide, the respiratory system is also
important in maintaining the proper pH of the
internal environment.
• Urinary system: (Kidneys and associated organs)
removes excess water, salt, acid, and other
electrolytes from the plasma and eliminates them
in the urine, along with waste products other than
carbondioxide.

15. Body systems and homeostasis
• Skeletal system: (bones, joints) provides support and
protection for the soft tissues and organs. It also serves
as a storage reservoir for calcium ions.
• Calcium ions are electrolytes whose plasma
concentration must be maintained within very narrow
limits.
• Together with the muscular system, the skeletal system
also enables movement of the body and its parts.
• Also the bone marrow (soft interior portion of some
types of bones) is the ultimate source of all blood cells.

16. Body systems and homeostasis
• Muscular system: (skeletal muscles) moves the bones
to which the skeletal muscles are attached.
• This system enables an individual to move toward food
or away from harm.
• Heat generated through shivering is important in
temperature regulation.
• Because the muscular system is also under voluntary
control, one can use them to accomplish myriad other
movements.
• The movements ranges from the fine motor skills
required for delicate needlework to the powerful
movements involved in weight lifting. These are not
necessarily directed towards maintaining homeostasis.

17. Body systems and homeostasis
• Integumentary system: (skin and related
structures) serves as an outer protective
barrier that prevents internal fluid from being
lost from the body and foreign microorganisms
from entering.
• The system is also important in regulating body
temperature. The amount of heat lost from the
body surface to the external environment can
be adjusted by controlling sweat production
and by regulating the flow of warm blood
through the skin.

18. Body systems and homeostasis
• Immune system: (white blood cells, lymphoid organs)
defends against foreign invaders such as bacteria and
viruses and against body cells that have become
cancerous. It also paves the way for repairing or
replacing injured or worn-out cells.
• Nervous system: (brain, spinal cord, nerves and sense
organs) controls and coordinates bodily activities that
require swift responses.
• It detects changes in the external environment and
initiating reactions to them.
• It is responsible for higher functions that are not
entirely directed toward maintaining homeostasis, such
as consciousness, memory, and creativity.

19. Body systems and homeostasis
• Endocrine system: (all hormone-secreting glands) is
the major regulatory system.
• It regulates activities that require duration rather than
speed, such as growth.
• It is especially important in controlling the
concentration of nutrients and, by adjusting kidney
function, controlling the volume and electrolyte
composition of the ECF.
• Reproductive system: (male and female gonads and
related organs) is not essential for homeostasis and
therefore is not essential for survival of the individual.
It is essential however, for perpetuating the species.

20. Local and body-wide homeostatic
control systems
• Homeostatic control systems are grouped into two
classes.
• 1. Intrinsic/local controls: these are built into or are
inherent in an organ.
– E.g. an exercising skeletal muscle rapidly uses up oxygen to
generate energy to support its contractile activity.
– The oxygen concentration within the muscle falls.
– This local chemical change act directly on the smooth
muscle in the walls of the blood vessels that supply the
exercising muscle, causing the smooth muscle to relax so
that the vessels dilate, or open widely.
– As a result, increased blood flows through the dilated vessels
into the exercising muscle, bringing in more oxygen. This
local mechanism helps maintain an optimal level of oxygen in
the fluid immediately around the exercising muscle’s cells.

21. Local and body-wide homeostatic control systems
• 2. Extrinsic/systemic controls: initiated outside an
organ to alter the organ’s activity.
– accomplished by the nervous and endocrine systems,
the two major regulatory systems.
– Extrinsic control permits coordinated regulation of
several organs toward a common goal; in contrast,
intrinsic controls serve only the organ in which they
occur.
– To restore blood pressure to the proper level when it
falls too low, for example, the nervous system acts
simultaneously on the heart and blood vessels
throughout the body to increase the blood pressure to
normal.

22. Local and body-wide homeostatic
control systems
• To stabilize the physiological factor being
regulated, homeostatic control systems must
be able to detect and resist change.
• The term Feedback refers to responses made
after a change has been detected.
• The term feedforward is used for responses
made in anticipation of a change.

23. Negative feedback
• Homeostatic control mechanisms operate
primarily on the principle of negative feedback.
• In negative feedback, a change in a
homeostatically controlled factor triggers a
response that seeks to restore the factor to
normal by moving the factor in the opposite
direction of its initial change.
• That is, a corrective adjustment opposes the
original deviation from the normal desired level.

24. Room temperature is
a controlled variable.
The thermometer is
the sensor, which
monitors the
magnitude of the
controlled variable.
The thermostat
setting provides the
desired temperature
level, or set point.
The thermostat acts
as an integrator, or
control centre.
The furnace is the
effector, the
component of the
control system
commanded to bring
about the desired
effect.

25. A negative-feedback control system detects a change
away from the ideal value in a controlled variable,
initiates mechanisms to correct the situation, then
shuts itself off . In this way, the controlled variable
does not drift too far above or below the set point.
When temperature-monitoring nerve cells detect
a decrease in body temperature below the
desired level, these sensors signal the
temperature control centre in the brain, which
begins a sequence of events to raise the
temperature to the proper level.
When the set point is reached the temperature
monitoring nerve cells turn off the stimulatory
signal to the skeletal muscles. As a result, the
body temperature does not continue to increase
above the set point.
When the temperature-monitoring nerve cells
detect a rise in body temperature above normal,
cooling mechanisms such as sweating are called
into play to lower the temperature to normal.

26. Positive feedback
• In Positive feedback, the output enhances or amplifies
a change so that the controlled variable continues to
move in the direction of the initial change.
• Because the major goal in the body is to maintain
stable, homeostatic conditions, positive feedback does
not occur nearly as often as negative feedback.
• E.g. Birth of a baby: the hormone oxytocin causes
powerful contractions of the uterus.
• As the contractions push the baby against the cervix
(exit from the uterus), the resultant stretching of the
cervix triggers a sequence of events that brings about
the release of even more oxytocin, which causes even
stronger uterine contractions, triggering the release of
more oxytocin, and so on.

28. Positive feedback
• This positive feedback cycle does not stop until the
baby is finally born.
• Blood clotting occurs as a result of a sequential
activation of clotting factors. The activation of one
clotting factor results in activation of many in a
positive feedback.
• In this way, a single change is amplified to produce
a blood clot.
• Formation of the clot, however, can prevent further
loss of blood, and thus represents the completion
of a negative feedback loop.

29. Feedforward mechanisms
• In addition to feedback mechanisms, which bring about a
reaction to a change in a regulated variable, the body
less frequently employs feedforward mechanisms, which
respond in anticipation of a change in a regulated
variable.
• For example, when a meal is still in the digestive tract, a
feedforward mechanism increases secretion of a
hormone (insulin) that will promote the cellular uptake
and storage of ingested nutrients after they have been
absorbed from the digestive tract.
• This anticipatory response helps limit the rise in blood
nutrient (glucose) concentration after nutrients have
been absorbed.

30. Eating
Blood glucose
Islets of Langerhans
Insulin
Cellular uptake of
glucose
Blood glucose
-ve
Negative feedback control of insulin secretion and blood glucose
concentration. Mechanisms such as this maintain homeostasis

31. Disruptions in homeostasis
• When one or more of the body’s systems
malfunction, homeostasis is disrupted, and all the
cells suffer because they no longer have an optimal
environment in which to live and function.
• Various pathophysiological states develop,
depending on the type and extent of the disruption.
• The term Pathophysiology refers to the abnormal
functioning of the body (altered physiology)
associated with disease.
• When a homeostatic disruption becomes so severe
that it is no longer compatible with survival, death
results.