BIO 304 · Human Anatomy & Physiology

Homeostasis & Feedback Loops

Foundations of Anatomy & Physiology · Module 1

A reference sheet for the Homeostasis video. Negative feedback returns the body to its set point; positive feedback amplifies a change until a definite endpoint stops the loop. Almost every physiological control system you will study is one of these two.

How to use this sheet Toggle the toolbar above. Notes prints the full reference for review. Study prints as a fill-in-the-blank worksheet , print it, then write each definition while you watch the video or read your book. Quiz me is on-screen typing practice; type the term, click Reveal to check yourself.

Open spaced recall

By the end

Define homeostasis, set point, and normal range using a real physiological example.
Identify the four components of a negative feedback loop (stimulus, sensor, control center, effector) and trace one through the body.
Contrast negative and positive feedback, naming one normal example of each.

OpenStax Figure 1.10: panel A is a general negative feedback flow chart with stimulus, sensor, control center, effector, and response inhibiting the stimulus. Panel B applies the same loop to body temperature regulation: rising body temperature is detected by nerve cells in the skin and brain, the temperature center triggers sweat glands, sweating cools the body and inhibits the original stimulus. — Fig 1.10 · Negative feedback

OpenStax Figure 1.11: a positive feedback loop for childbirth. The baby pushes against the cervix, stretch-sensitive nerve cells signal the brain, the pituitary releases oxytocin, oxytocin triggers stronger uterine contractions, the cycle amplifies until the baby is delivered. — Fig 1.11 · Positive feedback

Click any image to enlarge. Images: OpenStax Anatomy & Physiology 2e, CC BY 4.0.

The Four-Part Loop

1 · Stimulusa variable moves outside the normal range · body temp rises to 37.8°C
Your example
2 · Sensor (receptor)detects the change and reports it · nerve cells in skin and brain
Your example
3 · Control centercompares incoming value to the set point · hypothalamic temperature center
Your example
4 · Effectoracts to reverse the change · sweat glands open, skin vessels dilate
Your example
Responsethe variable returns toward set point · body temp drops back to 37°C
Your example

Vocabulary

Set pointthe target value the body defends · ~37°C, ~90 mg/dL glucose, pH 7.4
Your note
Normal rangethe narrow window around set point where function is preserved
Your note
Homeostasisthe maintained dynamic equilibrium between competing influences
Your note

Negative Feedback Examples

Body temperaturesweating, shivering, vasodilation, vasoconstriction
Your note
Blood glucoseinsulin (after a meal), glucagon (between meals)
Your note
Blood pressurebaroreceptors → ANS adjusts heart rate & vessel tone
Your note
Blood pHrespiratory rate, kidney bicarbonate handling
Your note

Positive Feedback

Less common than negative. Amplifies a change rather than reversing it. Only normal when a clear endpoint stops the cascade.

Childbirth (oxytocin)cervical stretch → oxytocin → stronger contractions → more stretch · endpoint: delivery
Your note
Blood clottingvessel injury → platelet plug → more platelet recruitment · endpoint: sealed clot
Your note
Action potentialNa+ opens more Na+ channels until peak · endpoint: depolarization
Your note
LH surge at ovulationrising estrogen flips into a brief positive loop · endpoint: egg released
Your note

How to tell them apart

Direction of correctionNegative reverses the stimulus. Positive amplifies it.
Your note
Frequency in the bodyNegative: nearly every system, all day. Positive: a handful of moments.
Your note
Stop conditionNegative: variable inside normal range. Positive: definite endpoint event.
Your note

Dr. Sharilyn Rennie BIO 304 · Module 1 · Homeostasis & Feedback Loops