HOMEOSTASIS DR ROBERT RYAN BUENAVENTURA MEDICAL GROUP APRIL 24, 2006.

HOMEOSTASIS

DR ROBERT RYANBUENAVENTURA MEDICAL GROUP

APRIL 24, 2006

Homeostasis

From the Greek rootsFrom the Greek roots– Homo: alike or the sameHomo: alike or the same– Stasis: to stand or postureStasis: to stand or posture

Term coined by Walter Cannon in Term coined by Walter Cannon in 1932 1932

HomeostasisAbility or tendency of an organism/cell to maintain internal equilibrium by adjusting its physiological processes

– e.g., temp, pH, blood pressure, PCO2, etc.

Negative or Positive Feedback

Feedback

Negative Feedback– Slows down process to return to

equilibrium

Positive Feedback – Accelerates process away from

equilibrium

Feedback (cont’d)

Stock Market

Aircraft autopilot

Computer modeling

Biological systems

Genes

Feedback (cont’d)

Control triangle preprogrammed with desired rangesInput arrives and acted upon in control triangle to provide an outputOutput sampled and fed back to control triangle Output modified to make sure output stays within programmed range for center

Types of Feedback

Negative Feedback: change in an environment causes response that returns conditions to original state– ThermostatThermostat

Positive Feedback: response of the system is to change that variable even more in the same direction– ““Sim City” Sim City” – Two in human bodyTwo in human body

Nervous SystemNervous SystemBody’s surveillance system– Blood pressure,

temperature, pH, etc. Sensory centers – Brain or spinal cord– Barrage of input

integrated and response generated

Organs convert signal into some form of action– E.g., movement,

changes in heart rate, release of hormones, etc.

Reflex arc

Cardio-Respiratory System Maintain homeostasis and deliver oxygenated blood to cells

Negative Feedback

– Blood Pressure

– pH

– PCO2

Hyperventilation example

Endocrine SystemElegant feedback networks Affects activities of cells, tissues, and organs throughout the body Controls slower, longer lasting responses to internal stimuliSelf-Regulating Negative Feedback Mechanism– Hypothalamus – Anterior Pituitary – Other Endocrine End Organs

Endocrine System (cont’d)

Thyroid hormone

Endocrine System (cont’d)

Sex hormones

Endocrine System (cont’d)

Steroid hormones

Endocrine System (cont’d)

Positive Feedback Loop– Childbirth

Increased rate of contractions causes increased rate of stretch receptor stimulus (feedback)Continues positive feedback loop until baby is delivered

Endocrine System (cont’d)

Positive Feedback Loop– Name the other positive

system found in mammals

Endocrine System (cont’d)

Blood clotting – Self-catalytic, self-accelerating

effect – Clotting process begins

Runs faster and fasterUntil bleeding stops

– Positive feedback loop is part of a larger negative feedback loop

Activated by bleeding Ultimately works to stop the bleeding

Blood Clotting

SummaryHomeostasis is achieved by– Mainly negative feedback– Infrequent cases positive feedback

Negative feedback’s goal is to maintain equilibrium

Positive feedback’s goal is to accelerate away from equilibrium

Endocrine, Nervous and Cardio-Respiratory systems all use feedback to maintain life