Introduction to Physiology
• Physiology is defined as
the study of function of
living organism – so
attempts to explain how
and why humans function.
• Physiology is where we
figure out how stuff
• How do muscles contract?
• How do we run?
• How does our heart beat?
Levels of Structure
• In order to understand how
something is built and how
something works, you must
look at all of its components
and analyze them both
individually and together.
• In doing these collective and
separate analyses, you must
examine things at multiple
structural levels, i.e., one must
break them down from large to
small – this is called
• An organism (such as a human
being) may be broken down as
illustrated on the left.
Levels of Structure
• The basic unit of life is the cell.
• All living organisms are composed of one or more
• The human body contains about 100 trillion cells.
• There are about 200 different types of cells in the
• The different types of cells have different features but
for the most part, all cells are made up of organelles
and various macromolecules (e.g., proteins, lipids,
carbohydrates and nucleic acids).
• Organelles themselves are made of these
macromolecules and macromolecules are polymers of
smaller molecules which consist of atoms of various
• Plasma Membrane → Separates the cell exterior from the
cell interior (cytoplasm).
• Nucleus → Membrane bound structure that contains
deoxyribonucleic acid (DNA) which is the set of
instructions for the synthesis of all the body’s proteins.
• Mitochondria → Structure bound by a double membrane
and the site at which the energy stored in sugars and other
organic molecules is transferred to ATP, the chemical
which acts as the “currency” for energy in the cell.
• Ribosomes → Not bound by a membrane. Sites of protein
synthesis. May be free – floating in the cytoplasm – or
bound to the endoplasmic reticulum.
• Rough Endoplasmic Reticulum →
Membranous set of tubes with
ribosomes studded along its surface.
Site of the synthesis of proteins that
are destined to be exported from the
• Smooth Endoplasmic Reticulum →
ER w/o the attached ribosomes.
Site of cellular lipid synthesis,
among other things.
• Golgi Apparatus → Membrane
bound organelle responsible for
determining the direction of proteins
synthesized in the rough ER.
• Lysosomes → Membrane bound
organelle that houses digestive
enzymes that can be used to break
down ingested toxins or worn out
More Levels of Structure
• Similar cells and cell
products come together to
• A structure made of 2 or
more tissue types that
perform a particular
function is an organ.
• A group of organs with a
unique collective function
is an organ system.
- Skin, hair, sweat and oil glands
- Forms the external body covering
- Protects deeper tissues from injury
- Involved in vitamin D synthesis
- Prevents desiccation, heat loss, and
- Site of pain and pressure receptors
– The 206 bones of the human body
– Protects and supports body organs
– Provides a framework that muscles can
use to create movement
– Hemopoiesis (synthesis of blood cells)
– Mineral storage
• Bone contains 99% of the body’s store of
– The 600+ muscles of
– Maintaining posture
(generation of heat)
– Brain, spinal cord, and
– Fast-acting control
system of the body
– Monitoring of the
internal and external
necessary) by initiating
muscular or glandular
– Hormone-secreting glands
• Pituitary, Thyroid, Thymus,
Pineal, Parathyroid, Adrenal,
Pancreas, Small Intestine,
Stomach, Testes, Ovaries,
– Long-term control system of
– Regulates growth,
reproduction, and nutrient use
among other things.
– Heart, Blood vessels (arteries,
veins, and capillaries)
– The heart pumps blood thru the
– Blood provides the transport
medium for nutrients (glucose,
amino acids, lipids), gases (O2,
CO2), wastes (urea, creatinine),
signaling molecules (hormones),
– Lymphatic vessels, Lymph nodes,
Spleen, Thymus, Red bone
– Disposal of debris
– Attacking and resisting foreign
invaders (pathogens i.e., disease-
– Nasal cavity, pharynx,
trachea, bronchi, lungs
– Constantly supply the
blood with O2, and
– Regulate blood pH
– Oral cavity, esophagus, stomach, small
intestine, large intestine, rectum, salivary
glands, pancreas, liver, gallbladder
– Ingestion and subsequent breakdown of food
into absorbable units that will enter the blood
for distribution to the body’s cells
– Kidneys, ureters,
– Removal of
– Regulation of body’s
levels of water,
• Testes, scrotum, epididymis,
vas deferens, urethra, prostate
gland, seminal vesicles, penis
• Ovary, uterine tube, uterus,
cervix, vagina, mammary
– Production of offspring
• Your body has about 100 trillion cells in it.
• For your life to NOT end abruptly, these cells
need to have the correct amount of:
• Waste removal
• Ions (sodium, calcium, etc.)
• Lots of other stuff
The Cell’s Environment
• In order to keep the right amount of stuff in the cell,
we’ve got to make sure that all the fluid surrounding
our cells (i.e., the extracellular fluid) has the right
assortment of nutrients, ions, etc.
• We keep both our cells and the fluid surrounding our
cells in a dynamically stable environment via a process
• Defined as the body’s ability to maintain stable
internal conditions in spite of the changing
• Our body needs to have the right amount of stuff
(i.e., temperature, blood glucose, pH etc.) at all
times in order to function properly.
• First, let’s refer to all this stuff as “different
Let’s use a thermostat
as an example
• In order to keep the temperature in my house at
the right level, the thermostat must first measure
the current temperature in the house.
• After the thermostat measures the temperature, it
compares the current value to a preset standard
– If there is no difference then there’s nothing to do.
– However, if it’s too hot or too cold, the thermostat has
to send a signal to the furnace or air conditioner to
change the temperature of the house so that it equals the
Let’s clarify some stuff.
• In the previous example we had a:
1. Variable temperature
2. Measuring implement thermostat
3. Control center also the thermostat
4. A preset or standard value for the variable
5. Effectors the air conditioner and furnace
• Similar situations arise in the human body
where there are lots of variables that we
want to maintain at certain precise levels
• BP is a variable that we’ve got to
maintain at a certain level
• We have sensory receptors that
measure the BP in the body. They’re
located in the aorta (the big blood
vessel coming out of the heart) and in
the carotid arteries (the large vessels
that bring blood to the brain).
• These pressure receptors measure BP
and then send the info (we can call
this input) to a control center in the
brain – the particular BP control
center is in the medulla oblongata of
• We call the connection btwn the receptor
and the control center the afferent
• In the control center, the input BP is
compared with a set value.
• If there is a difference between the current
BP value and the reference BP value then
we’ve got an error.
• And we’ve got to fix that error!
• The control center will signal effector
organs – such as the heart in this case – to
alter their activity. This process is called
• The connection between the control center
and the effector organ is called the efferent
• Suppose the current BP is too high.
• The effector must act in a way to decrease it – so
the medulla oblongata (the control center) would
signal the heart to decrease the force and rate of its
contractions; this would decrease BP.
• Notice that the original stimulus was an Increase
in BP and the body’s response was to act so as to
• The stimulus is opposite the response!
• The movement of a variable in one
direction causes the body to enact processes
that cause the variable to move in the
opposite direction (so as to return the value
to the correct level) – we call it negative
• Let’s look at BP again:
Sensed by pressure
receptors in aortic arch
and carotid sinus
Input sent via
set point and
Output sent along
efferent pathway to
heart and blood
Heart rate & force
Why is Negative Feedback so
common in the body?
• Every time a variable starts changing too
much, we’ve got to bring it back to normal.
We’ve got to counteract its change.
• THAT’S NEGATIVE FEEDBACK
• Other examples you will encounter:
– Maintenance of blood [Ca2+
], blood [Glucose],
blood pH, and many others
When does a negative feedback process end?
• A negative feedback
process begins when a
particular variable leaves
its homeostatic range.
• The process ends when
that variable is back
within its normal range.
• Negative feedback
processes (or loops) are
Homeostasis is Important!
• Most of the physiological processes that
occur in your body are designed to maintain
What this means is that the homeostatic variables are NOT kept
rigidly fixed upon a single value. They are kept within a certain
range, and when they exit that range – that’s when negative
feedback loops turn on to bring them back.
What about Positive Feedback?
• Positive feedback occurs when the response
magnifies the stimulus that produced it.
• In other words, a variable is altered and
then the body’s response alters that variable
even more in the same direction.
Water, water everywhere!
• About 60% of the human body is water
• 2/3 of this water is found within your
cells so we refer to it as intracellular
• The other 1/3 is outside your cells so
we call it extracellular fluid (ECF)
• The 2 main types of ECF are:
1. The fluid that surrounds the cells – the
tissue fluid or interstitial fluid
• Minor types of ECF include
cerebrospinal fluid and intraocular fluid
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