Transcript Homeostasis – Chapter 1

Chapter 01
Lecture Outline*
Homeostasis:
A Framework for Human
Physiology
Eric P. Widmaier
Boston University
Hershel Raff
Medical College of Wisconsin
Kevin T. Strang
University of Wisconsin - Madison
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figures and tables pre-inserted into
PowerPoint without notes.
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Chapter 1 Homeostasis:
A framework for human physiology
1. Cells, the fundamental units of life, exchange nutrients
and wastes with their surroundings:
The intracellular fluid is “conditioned by”…
the interstitial fluid, which is “conditioned by” …
the plasma, which is “conditioned by” …
the organ systems it passes through.
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Homeostasis:
A framework for human physiology
Homeostasis refers to the dynamic mechanisms
that detect and respond to deviations in
physiological variables from their “set point” values
by initiating effector responses that restore
the variables to the optimal physiological range.
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Homeostasis
• Most of the common physiological variables of
the body are maintained within a predictable
range.
• Examples of such physiological variables:
– Blood pressure
– Body temperature
– Blood glucose levels
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Figure 1-1
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Muscle Cells and Tissues
• There are 3 types of muscle cells in the
human body: Cardiac, Skeletal and Smooth.
• Control of the cardiac and smooth muscle is
involuntary, while skeletal is voluntary.
• Muscle cells with be covered in depth in
Chapter 9.
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Neurons and Nervous Tissue
• A neuron is a cell of the nervous system that is specialized
to initiate, integrate and conduct electrical signals to other
cells.
• A collection of neurons forms nervous tissue (brain or
spinal cord).
• Axons from many neurons are packaged together along
with connective tissue to form a nerve.
• Neurons, nervous tissue, and the nervous system will be
covered in Chapter 6.
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Epithelial cells and epithelial tissue
• Epithelial cells are specialized for the selective secretion and absorption of
ions and organic molecules, and for protection.
• These cells are characterized and named according to their unique shapes,
including cuboidal (cube-shaped), columnar (elongated), squamous
(flattened) and ciliated.
• Epithelial tissue (known as an epithelium) may form from any type of
epithelial cell. Epithelia may be arranged in single-cell thick tissue, called a
simple epithelium, or a thicker tissue consisting of numerous layers of
cells, called a stratified epithelium.
• The type of epithelium that forms in a given region of the body reflects the
function of that particular epithelium. For example, the epithelium that
lines the inner surface of the main airway, the trachea, consists of ciliated
epithelial cells (see Chapter 13).
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Epithelial cells and epithelial tissue
• Epithelia are located at the surfaces that cover the body or individual
organs, and they line the inner surfaces of the tubular and hollow
structures within the body.
• Epithelial cells rest on an extracellular protein layer called the
basement membrane. The side of the cell anchored to the basement
membrane is called the basolateral side; the opposite side, which
typically faces the interior, is called the apical side.
• A defining feature of many epithelia is that the two sides of all the
epithelial cells in the tissue may perform different physiological
functions.
• In addition, the cells are held together along their lateral surfaces by
extracellular barriers called tight junctions Tight junctions enable
epithelia to form boundaries between body compartments and to
function as selective barriers regulating the exchange of molecules.
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Figure 1-2
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Connective tissue cells and connective tissue
• Connective tissue cells connect, anchor, and
support the structures of the body.
•
•
•
•
•
•
Types of connective tissues include:
Loose Connective
Dense Connective
Blood
Cartilage
Adipose
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What surrounds the cells?
• The immediate environment that surrounds each individual cell in the body is
the extracellular fluid and extracellular matrix (ECM).
• ECM consists of a mixture of proteins, polysaccharides, and in some cases,
minerals.
• The matrix serves two general functions: (1) It provides a scaffold for cellular
attachments, and (2) it transmits information, in the form of chemical
messengers, to the cells to help regulate their activity, migration, growth, and
differentiation.
• The proteins of the extracellular matrix consist of fibers—ropelike collagen
fibers and rubberband-like elastin fibers—and a mixture of nonfibrous
proteins that contain carbohydrate.
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Organs and Organ Systems
• Organs are composed of multiple tissue
types (example: blood vessels have layers
of smooth muscle cells, endothelial cells
and fibroblasts).
• Organ systems contain multiple organs that
work together (example: the urinary system
has the kidney, ureters, urethra, bladder).
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Table 1-1, on page 5 in the text, outlines
the structural components and functions of
the major organ systems in the body.
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Body Fluids and Compartments
• The term “body fluids,” is used to refer to the watery solution of
dissolved substances (oxygen, nutrients, etc.) present in the body.
• The fluid in the blood and surrounding cells is called
extracellular fluid (i.e., outside the cell).
• About 20–25 percent is in the fluid portion of blood (plasma) and
the remaining 75–80 percent of the extracellular fluid lies around
and between cells and is known as the interstitial fluid.
• The space containing interstitial fluid is called the interstitium.
Therefore, the total volume of extracellular fluid is the sum of the
plasma and interstitial volumes.
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Body Fluids and Compartments
• Intracellular fluid is the fluid located inside the cells.
• The composition of the extracellular fluid is very different from
that of the intracellular fluid.
• Maintaining differences in fluid composition across the cell
membrane is an important way in which cells regulate their own
activity.
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Figure 1-3
ICF
ISF
plasma
organs
internal environment
external
environment
Exchange and communication are key concepts
for understanding physiological homeostasis.
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Homeostasis
• Homeostasis is a dynamic, not a static, process.
• Physiological variables can change dramatically
over a 24-hr. period, but the system is still in
overall balance.
• When homeostasis is maintained, we refer to
physiology; when it is not, we refer to
pathophysiology.
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Figure 1-4
Blood glucose levels
increase after eating.
Levels return to their set
point via homeostasis.
This is an example of
dynamic constancy.
Levels change over short
periods of time, but
remain relatively
constant over long
periods of time.
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Figure 1-5
Interpret the arrows
in
textbook’s flow charts as
“leads to” or “causes.”
(e.g., decreased room
temperature causes
increased heat loss
from the body, which
leads to a decrease in
body temperature, etc.)
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System Controls
• Feedback loops or systems are a common
mechanism to control physiological processes.
• A positive feedback system (also called a feed
forward) enhances the production of the
product.
• A negative feedback system shuts the system
off once the set point has been reached.
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Figure 1-6
Negative
Feedback
“Active product” controls the sequence of chemical reactions
by inhibiting the sequence’s rate-limiting enzyme, “Enzyme A.” 23
A strategy for exploring homeostasis
(see Tables 1-2 & 1-3)
• Identify the internal environmental variable.
example: concentration of glucose in the blood
• Establish the “set point” value for that variable.
example: 70 to 110 mg glucose/dL of blood
• Identify the inputs and outputs affecting the variable.
example: diet and energy metabolism
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A strategy for exploring homeostasis
(see Tables 1-2 & 1-3)
• Examine the balance between the inputs and outputs.
example: resting versus exercising
• Determine how the body monitors/senses the variable.
example: certain endocrine cells in the pancreas
“sense” changes in glucose levels
• Identify effectors that restore the variable to its set point.
example: a hormone that increases glucose
synthesis by the liver
Many homeostatic mechanisms utilize neural
communication.
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Reflexes
• A reflex is a specific involuntary,
unpremeditated, unlearned “built-in” response
to a particular stimulus.
• Example: pulling your hand away from a hot
object or shutting your eyes as an object rapidly
approaches your face.
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Reflexes
• The pathway mediating a reflex is known as the reflex arc.
• An arc has several components: stimulus, receptor, afferent (incoming)
pathway, integration center, efferent (outgoing) pathway, and effector.
• A stimulus is defined as a detectable change in the internal or external
environment. A receptor detects the change. The pathway the signal
travels between the receptor and the integrating center is known as the
afferent pathway. The pathway along which information travels away
from the integration center to the effector is known as the efferent
pathway
• An integrating center often receives signals from many receptors, some
of which may respond to quite different types of stimuli. Thus, the
output of an integrating center reflects the net effect of the total
afferent input; that is, it represents an integration of numerous bits of
information.
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Figure 1-7
Afferent and efferent pathways in temperature homeostasis.
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Figure 1-8
Communication systems use signals that bind to receptors.
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Non-nerve Reflexes
• Almost all body cells can act as effectors in homeostatic
reflexes.
• There are, however, two specialized classes of tissues—muscle
and gland—that are the major effectors of biological control
systems.
• In the case of glands, the effector may be a hormone secreted
into the blood.
• A hormone is a type of chemical messenger secreted into the
blood by cells of the endocrine system (see Table 1–1).
Hormones may act on many different cells simultaneously
because they circulate throughout the body.
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Types of Signals
• Hormones are produced in and secreted
from endocrine glands or in scattered cells
that are distributed throughout another
organ.
• Neurotransmitters are chemical messengers
that are released from the endings of
neurons onto other neurons, muscle cells, or
gland cells.
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Chemical Messengers
• Chemical messengers participate not only in reflexes,
but also in local responses.
• Communication signals in three categories:
Endocrine: signal reaches often-distant targets after
transport in blood.
Paracrine: signal reaches neighboring cells via the ISF.
Autocrine: signal affects the cell that synthesized the
signal.
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Points to Remember
• A neuron, endocrine gland cell, and other cell
types may all secrete the same chemical
messenger.
• In some cases, a particular messenger may
function as a neurotransmitter, as a hormone, or as
a paracrine/autocrine substance.
• Example: Norepinephrine is a neurotransmitter in
the brain and is also produced as a hormone by
cells of the adrenal glands.
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Figure 1-9
A given signal can
fit into all 3 categories:
(e.g., the steroid
hormone cortisol
affects the very cells in
which it is made,
the nearby cells that
produce other hormones,
and many distant targets,
including muscles and
liver.)
Multi-factorial control of
signal release adds
more complexity.
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Other Types of Cell Communication
• There are two important types of chemical communication
between cells that do not require secretion of a chemical
messenger.
1. Gap junctions (physical linkages connecting the cytosol
between two cells) allow molecules to move from one cell to
an adjacent cell without entering the extracellular fluid.
2. Juxtacrine signaling is the chemical messenger not actually
being released from the cell producing it, but rather is located
in the plasma membrane of that cell. When the cell encounters
another cell type capable of responding to the message, the
two cells link up via the membrane-bound messenger.
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Adaptation and Acclimatization
• The term adaptation denotes a characteristic that
favors survival in specific environments.
• Acclimatization refers to the improved
functioning of an already existing homeostatic
system based on an environmental stress.
• In an individual, acclimatizations are reversible;
adaptations are not.
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Biological Rhythms
• Many body functions are rhythmical
changes.
• Example: circadian rhythm, which cycles
approximately once every 24 h.
• Waking and sleeping, body temperature,
hormone concentrations in the blood, the
excretion of ions into the urine, and many
other functions undergo circadian variation.
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Figure 1-10
asleep
asleep
A full analysis of the
hormone cortisol requires
not only knowledge of the
signals that cause its
synthesis and secretion
but also consideration
of biological rhythms.
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What have biological rhythms to do with homeostasis?
• They add an anticipatory component to homeostatic
control systems and in effect are a feed-forward system
operating without detectors.
• The negative-feedback homeostatic responses are
corrective responses. They are initiated after the steady
state of the individual has been perturbed.
• Biological rhythms enable homeostatic mechanisms to
be utilized immediately and automatically by activating
them at times when a challenge is likely to occur but
before it actually does occur.
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Balance in the Homeostasis of
Chemical Substances in the Body
• Many homeostatic systems regulate the balance
between addition and removal of a chemical substance
from the body.
• Two important generalizations concerning the balance
concept: (1) During any period of time, total-body
balance depends upon the relative rates of net gain and
net loss to the body; and (2) the pool concentration
depends not only upon the total amount of the substance
in the body, but also upon exchanges of the substance
within the body.
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Figure 1-11
Some of the potential inputs and outputs that can
affect the “pool” of a material (like glucose) that is a
dynamically regulated physiological variable.
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Figure 1-12
Sodium homeostasis: Consuming greater amounts of dietary
sodium initiates a set of dynamic responses that include greater
excretion of sodium in the urine. Though not shown here, the
amount excreted would likely exceed the amount ingested until
the “set point” is restored.
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Clinical Correlation
• A 64-year-old, fair-skinned man in good overall health spent a very
hot, humid summer day gardening in his backyard. After several hours
in the sun, he began to feel dizzy and confused as he knelt over his
vegetable garden. Although he had earlier been perspiring profusely,
his sweating had eventually stopped. Because he also felt confused and
disoriented, he could not recall for how long he had not been
perspiring, or even how long it had been since he had taken a drink of
water. He called to his wife, who was alarmed to see that his skin had
turned a pale blue color. She asked her husband to come indoors, but
he fainted as soon as he tried to stand. The wife called for an
ambulance, and the man was taken to a hospital and diagnosed with a
condition called heat stroke. What happened to this man that would
explain his condition, and how does it relate to homeostasis?
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You don’t have a figure for this
but it would be really helpful to
have a flow chart diagram here
with the information for the
clinical correlation
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The End.
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