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Human Physiology and
Homeostasis
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Cells Are the Living Units of the Body
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Body Organization: from cells to systems.
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Body Organization: from cells to systems.
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Body Organization: from cells to systems.
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Body Organization: from cells to systems.
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The “Internal Environment”
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About 60 percent of the adult human body is fluid, mainly a
water solution of ions and other substances.
Although most of this fluid is inside the cells and is called
intracellular fluid, about one third is in the spaces outside the
cells and is called extracellular fluid.
In the extracellular fluid are the ions and nutrients needed by the
cells to maintain life. Thus, all cells live in essentially the same
environment—the extracellular fluid. For this reason, the
extracellular fluid is also called the internal environment of the
body.
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Cells are capable of living and performing their
special functions as long as the proper
concentrations of oxygen, glucose, different ions,
amino acids, fatty substances, and other
constituents are available in this internal
environment.
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Homeostasis—Maintenance of a Nearly
Constant Internal Environment
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Homeostasis
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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Essentially all organs and
tissues of the body
perform functions that
help maintain these
relatively constant
conditions.
For instance, the lungs
provide oxygen to the
extracellular fluid to
replenish the oxygen used
by the cells, the kidneys
maintain constant ion
concentrations, and the
gastrointestinal system
provides nutrients.
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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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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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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 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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Most control systems
of the body act by
negative feedback
“Active product” controls the sequence of chemical reactions
by33inhibiting the sequence’s rate-limiting enzyme, “Enzyme A.”
Type of Signals
• Hormones are produced in and secreted from
endocrine glands or in scattered cells that are
distributed throughout another organ.
Hormones travel through the blood to their
target cells.
• Neurotransmitters are chemical messengers
that are released from the endings of neurons
onto other neurons, muscle cells, or gland
cells.
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Each functional structure contributes its share to the
maintenance of homeostatic conditions in the extracellular
fluid, which is called the internal environment.
As long as normal conditions are maintained in this internal
environment, the cells of the body continue to live and
function properly.
Each cell benefits from homeostasis, and in turn, each cell
contributes its share toward the maintenance of homeostasis.
This reciprocal interplay provides continuous automaticity of
the body until one or more functional systems lose their
ability to contribute their share of function. When this
happens, all the cells of the body suffer. Extreme dysfunction
leads to death; moderate dysfunction leads to sickness.
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Review of Cellular Biology
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Organization of the Cell
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The major parts of a cell are the
nucleus and the cytoplasm.
The nucleus is separated from the cytoplasm by a nuclear
membrane, and the cytoplasm is separated from the surrounding
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fluids by a cell membrane, also called the plasma membrane.
The different substances that make up the cell
are collectively called protoplasm. It is made of
5 substances:
1. Water constitutes 70 percent to 85 percent of most cells.
2. Ions/electrolytes provide inorganic chemicals for cellular
reactions. Some of the most important ions in the cell are
potassium, magnesium, phosphate, sulfate, bicarbonate, and
small quantities of sodium, chloride, and calcium.
3. Proteins normally constitute 10 to 20 percent of the cell
mass. They can be divided into two types: structural proteins
and globular (functional) proteins, which are mainly enzymes.
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4.
Lipids constitute about 2 percent of the total cell mass. Among
the most important lipids in the cells are phospholipids, cholesterol,
triglycerides, and neutral fats. In adipocytes (fat cells), triglycerides
account for as much as 95 percent of the cell mass and represent the
body’s main energy storehouse.
5.
Carbohydrates play a major role in nutrition of the cell. Most
human cells do not store large amounts of carbohydrates, which
usually average about 1 percent of the total cell mass but may be as
high as 3 percent in muscle cells and 6 percent in liver cells. The
small amount of carbohydrates in the cells is usually stored in the
form of glycogen, an insoluble polymer of glucose.
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