An Introduction to Anatomy
and Physiology
Chapter 1
Homeostasis
Slides 77-88
Chapter 1 Introduction to Anatomy and
Physiology
Start at 13:54. Stop at 21:00
https://www.youtube.com/watch?v=TK-dxtvDC9k
Homeostasis
 A condition of equilibrium (balance) in the
body’s internal environment
Dynamic condition
 Narrow range is compatible with maintaining
life
 Example

 Blood glucose levels range between 70 and 110 mg
of glucose/dL of blood
 Whole body contributes to maintain the internal
environment within normal limits
Homeostasis and Body Fluids
 Maintaining the volume and composition of body
fluids are important
 Body fluids are defined as dilute, watery
solutions containing dissolved chemicals inside
or outside of the cell
 Intracellular Fluid (ICF)
 Fluid within cells
 Extracellular Fluid (ECF)
 Fluid outside cells
 Interstitial fluid is ECF between cells and
tissues
ECF and Body Location
 Blood Plasma
 ECF within blood vessels
 Lymph
 ECF within lymphatic vessels
 Cerebrospinal fluid (CSF)
 ECF in the brain and spinal cord
 Synovial fluid
 ECF in joints
 Aqueous humor and vitreous body
 ECF in eyes
Interstitial Fluid and Body Function
 Cellular function depends on the regulation of
composition of interstitial fluid
 Body’s internal environment
 Composition of interstitial fluid changes as it
moves
 Movement back and forth across capillary
walls provide nutrients (glucose, oxygen,
ions) to tissue cells and removes waste
(carbon dioxide)
Homeostasis
• A relatively constant state maintained by the body
• Ability of the body to maintain its internal environment (cellular environment)
as the external environment constantly changes
• Internal environment (ie. body temp, pH, glucose level)
• External environment (ie. Weather, fluid surrounding cells)
• Every regulatory mechanism of the body exists to maintain homeostasis of the
body’s internal fluid environment
• Set point/set point range – normal reading or range
ie. 80-100 mg glucose / mL blood
body temp 37°C (98.6°F)
• Regulatory mechanisms control homeostasis
7
Homeostasis (homeo= similar + stasis = condition): ability to
maintain a relatively stable internal environment in response to an
ever-changing outside world
All body systems work together to
maintain a stable internal environment
in the face of various challenges
(temperature, fluid balance, energy
availability) The internal environment
of the body is in a dynamic state of
equilibrium
EX: temperature regulation: sweat,
shiver
Failure to function within a normal
range for a number of different
variables results in disease
EX: hypothermia, hyperthermia
and diabetes, cancer
1. Challenge (stimulus) produces a change in body status that
shifts it out of balance
2. Physiology responds to maintain return the variable to balance
(homeostasis)
Negative and Positive Feedback Systems
 Negative Feedback systems
 Reverses a change in a controlled condition
 Regulation of blood pressure (force exerted by
blood as it presses again the walls of the blood
vessels)
 Positive Feedback systems
 Strengthen or reinforce a change in one of the
body’s controlled conditions
 Normal child birth
Homeostatic Control Mechanisms
 Challenge produces a change in body status
 Physiology responds to maintain homeostasis
 Components:
 Variable: Regulated factor/event
 Receptor/Detector: Monitors the environments and
responds to a challenge (stimulus)
 Control center: Determines the set point at which the
“variable” is maintained and compares the current level of
the variable to the regulated “set-point”.
 Intrinsic: cell or tissue auto regulates
 Extrinsic: nervous system or endocrine system
 Effector: Provides the means to respond to stimulus to
return variable to “set-point”
Homeostatic Control Mechanisms
• Feedback control loop – highly complex and integrated
communication control system in order to accomplish selfregulation ie. blood CO2 level, temp, heart rate, sleep cycle,
thirst
• 4 basic components to every loop:
1. sensor mechanism (ie. nerve cells, hormone producing
glands)
2. control center (ie. hypothalamus)
3. effector mechanism (ie. organs)
4. feedback
• Afferent vs. Efferent
Afferent – signal travels toward reference point
Efferent – signal travels away from reference point
Control of Homeostasis
 Homeostasis is constantly being disrupted
 Physical insults
 Intense heat or lack of oxygen
 Changes in the internal environment
 Drop in blood glucose due to lack of food
 Physiological stress
 Demands of work or school
 Disruptions
 Mild and temporary (balance is quickly restored)
 Intense and Prolonged (poisoning or severe
infections)