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ECE 593- 728
Chapter One
What is physiology?
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Physiology is the study of how living
organisms work. Physiologist are interested in
function and integration- how parts of the body
work together at various levels of organization
and, most importantly, in the entire organism.
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The human body is organized into cells,
tissues, organs, and organ systems.
What are cells?
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Cells are the simplest structural units into which a
complex multicellular organism can be divided and
still retain the functions characteristic of life. Cells
can be classified broadly into four types namely
muscle cells, nerve cells, epithelial cells, connective
tissue cells.
Muscle cells: these are specialized to generate the
mechanical forces that produce movement. There are
three types of muscle cells—skeletal, cardiac, and
smooth. These muscles differ from each other in
shape and in their location in the various organs of
the body.
Cont…
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Nerve cells: these are specialized to initiate and conduct
electrical signals, often over long distances. A signal may
initiate new electrical signals in other nerve cells, or it may
stimulate a gland cell to secrete or a muscle cell to contract.
Thus, nerve cells provide a major means of controlling the
activities of other cells.
Epithelial cells: these are specialized for the selective secretion
and absorption of ions and organic molecules, and for
protection. Epithelial cells are located mainly at the surfaces
that cover the body or line the walls of various tubular and
hollow structures within the body.
Connective tissue cells: these are cells that connect, anchor
and support the structures of the body. Examples include; bone
cells, fat storing cells, red blood cells and white blood cells.
What are Tissues, Organs and Organ
systems?
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A tissue is a group of specialized cells that have
similar structure and function together as a unit.
There are four main tissue types in the body: muscle,
nervous, epithelial and connective tissue. Each is
designed for specific functions.
An organ is a group of tissues that perform a specific
function or group of functions. Examples of organs
include; the heart, lungs, kidneys etc
Cont…
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An organ system is composed of two or more
different organs that work together to provide a
common function. There are 10 major organ
systems in the body, they are the: skeletal,
muscular, circulatory, nervous, respiratory,
digestive, excretory, endocrine, reproductive
and lymphatic system.
Homeostasis
This is the property of living organisms; it is
defined as a state of reasonably stable balance
between physiological variables.
What Characterizes the homeostatic control
system
Homeostatic control system is characterized by
a feedback system; there are basically two
types of feedback system— negative and
positive feedback system.

Negative feedback system

This is a system in which an increase or
decrease in the variable being regulated brings
about responses that tend to move the variable
in the direction opposite the direction of the
original change. A good illustration of the
negative feedback mechanism can be seen on
the next slide.
Negative feedback of Cortisol
Cont…
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Cortisol is one of the hormones in the human body
that raises the level of glucose in the blood.
When there is low glucose in the blood, the
hypothalamus stimulates corticotrophin releasing
hormone, which in turn stimulates the anterior
pituitary gland.
When this gland is stimulated it releases ACTH
which then stimulates the adrenal cortex. The adrenal
cortex then releases cortisol, cortisol raises the blood
glucose level by stimulating the liver to produce
glucose from stored non-carbohydrate sources such as
proteins and lipids and to release it into the blood.
Cont…
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On the contrary, when there is excess glucose in the
blood, cortisol can inhibit its production by either a
short loop or a long loop negative feedback system.
In the short loop, cortisol inhibits the anterior
pituitary gland and prevents it from secreting ACTH,
thus slowing the production of cortisol. In the long
loop, cortisol can directly inhibit the hypothalamus
and prevent it from secreting CRH also shutting down
the production of cortisol and thus maintaining
homeostasis.
Positive Feedback Mechanism

This is a system in which an increase in the
variable being regulated brings about
responses that tend to move the variable in the
direction same as that of the original change.
The end result of a positive feedback is often
amplifying and explosive.
Example of positive feedback

A good illustration of positive feedback
mechanism can be seen with parturition (
birth). As the uterine muscles contract and its
walls are stretched during labor, signals from
the uterus are relayed via nerves to a gland at
the base of the brain called the posterior
pituitary gland. This gland responds by
secreting the hormone oxytocin, which is a
potent stimulator of further uterine
contractions.
Components of Homeostatic
Control Systems
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Reflexes:
A reflex is a specific involuntary,
unpremeditated, unlearned response to a
particular stimulus. Examples of such reflexes
include pulling your hand away from a hot
object or shutting your eyes as an object
rapidly approaches your face. The pathway
mediating a reflex is known as the reflex arc.
Components of Homeostatic
Control Systems
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The reflex arc compose of several components namely;
stimulus, receptor, integrating center, afferent pathway
effector and efferent pathway.
Stimulus: a stimulus is defined as a detectable change in
the internal or external environment, such as change in
temperature, or blood pressure.
Receptor: a receptor detects the environmental change;
it is usually acted upon by the stimulus to produce a
signal that is sent to the integrating center.
Intercellular Chemical Messengers
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There are three categories of intercellular
chemical messengers they are: hormones,
neurotransmitters and paracrine and autocrine
agents.
Hormones: these are chemical messengers that
carries signal from one hormone secreting cell
or group of cells to another via the blood.
Examples are epinephrine, dopamine, gastrin
etc.
Cont…
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Neurotransmitters: these are chemical messengers
that aid communication among nerve cells. Nerve
cells are able to alter the activities of other nerve cells
by releasing from its ending a neurotransmitter that
diffuses through the extra cellular separating the two
nerve cells and acts upon the second. Examples are
acetylcholine, nitrogen monoxide etc.
Paracrine agents: these are chemical messengers that
exert its effect on cells near its secretion site.
Autocrine agents: these are chemical messenger
secreted into extra cellular fluid that acts upon the cell
that secreted it.