Human Biology and Homeostasis
What to know
Homeostasis is the maintenance of stable internal conditions despite external change, achieved largely through negative feedback: a deviation triggers a response that reverses it (e.g., body temperature, blood glucose, water balance). Positive feedback amplifies a change and is rarer (e.g., childbirth contractions). Know the major organ systems and their roles: digestive (breakdown and absorption of nutrients), circulatory (transport of gases, nutrients, wastes), respiratory (gas exchange), excretory (removal of wastes, water balance), nervous (rapid signaling), endocrine (hormonal signaling), immune (defense), skeletal and muscular (support and movement), and reproductive.
Understand specific regulatory examples: insulin lowers blood glucose and glucagon raises it; the kidneys regulate water and solute balance; the nervous system uses neurons and neurotransmitters for fast responses, while the endocrine system uses hormones for slower, longer-lasting effects. Know the roles of the small intestine (absorption), lungs (oxygen in, carbon dioxide out), and the heart's chambers and circulation. Recognize how systems interact - for instance, the respiratory and circulatory systems cooperate to deliver oxygen to cells for cellular respiration. Disease results when regulation fails, as in diabetes.
Traps
- Negative vs. positive feedback. Negative feedback reverses a change; positive feedback amplifies it. Most homeostasis is negative feedback.
- Insulin vs. glucagon. Insulin lowers glucose; glucagon raises it.
- Nervous vs. endocrine. Nervous is fast and short-lived; endocrine is slower and longer-lasting.
- System interaction questions. Trace oxygen and glucose to cells and wastes away from them.
Rule of thumb
When a question describes a change and asks for the response, identify the sensor, the control center, and the effector, then check whether the response reverses or amplifies the original change.
Worked example
After a meal, blood glucose rises. The pancreas releases insulin, which signals cells to take up glucose and the liver to store it, so blood glucose falls back toward normal - a negative feedback loop.