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Physiology Class

Introduction to Human Physiology

What is Physiology?
• The study of body function; in particular in regards to regulation and
response to changes in the external environment
• Homeostasis is the body’s tendency to maintain a stable internal

Physiology is a broad field of study…
• Physiology can be understood at many levels of organization
• We will focus on physiology from the molecular to organismal levels

The Organ Systems
• Your book gives 10 organ systems
• All organ systems directly or indirectly interface with external environment
• An unstable external environment can create an unstable internal environment
• We must interface with external environment to live

Fluids of the Body
• The external environment interfaces with the extracellular fluid
• All living cells are surrounded by fluid

Review: Osmosis and Tonicity
• Hypotonic
• Low salt outside
• Isotonic
• Equal salt inside as out
• Hypertonic
• High salt outside

• Molarity (M) = Moles/Liter solvent
• Osmolarity (OsM) = Moles of dissolved solutes/Liter solvent
• E.g., NaCl dissociates to about 1.8 moles of solutes (0.9 moles Na+/0.9 moles
Cl-), 1mole of NaCl in 1L of water=1.8OsM
• Osmolarity only considers dissolved solutes, not individual  components
• E.g., 1 mole of NaCl and 1 mole of glucose in 1L = 2.8OsM
• A milliosmole=1/1000 of a OsM
• E.g., .01 moles of NaCl in 1 liter = 0.018 OsM = 18 mOsM

Osmolarity and Tonicity
• Osmolarity considers both penetrating and non-penetrating solutes
• Tonicity considers non-penetrating solutes only
• E.g., the salt is stuck on one side of the cell membrane
• You cannot assume relationships between osmolarity and tonicity
• A couple things to remember
• A cell has an osmolarity of 300mOsM
• Assume that intracellular solutes are non-penetrating
• NaCl is non-penetrating, Dextrose (glucose) is

Law of Mass Balance
• To maintain homeostasis as much substance that is output must be put back in
• E.g., as much blood that is lost needs to be replaced by IV drip

Homeostasis and Disease
• The body must compensate for external changes
• If not = illness and disease
• If compensated = wellness (not noticed)
• Pathophysiology studies the disordered functioning of the body during disease

Feedback Loops
• Allow for the body to compensate for changing environment
• Two major categories that lead to homeostasis
• Local control
• Reflex control
• Considered negative feedback


Return the body directly to homeostasis by reducing stimulus
• If effective, the  feedback loop terminates
• Images also illustrates parts of reflex control

• Maintaining room temperature at exactly 70oF is very challenging
• Most thermostats have a differential
• I.e., even if you have a setpoint of 70oF it allows for deviations of +/- 2oF
• Same for body
• E.g., that is why we have a range of body temperatures throughout the day

Pathophysiology and Body Temperature
• Inflammatory molecules released by WBCs during influenza infection affect hypothalamus of brain
• Convince brain that setpoint for body temperature should be higher
• Start shivering because body thinks you are “too cold”

Negative vs. Positive Feedback Loops
• Positive feedback loops don’t turn themselves off
• Reinforce stimulus → don’t directly lead to homeostasis

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