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Grade 10-11 (age 15-17)

high school Homeostasis

Homeostasis is the body's tendency to keep its internal environment, such as body temperature, blood glucose and the osmotic pressure of body fluids, within a steady range. It rests on negative feedback: when a value drifts away from normal, the body responds in the direction that undoes the change. When blood glucose rises, the pancreas releases more insulin, and when it falls, more glucagon, bringing glucose back; body temperature is set by the hypothalamus, which controls the skin's blood vessels, sweating and shivering. In the diagram you can move blood glucose from 40 to 180 mg/dL and see how the secretion of the two hormones, and what happens in the liver and body cells, changes.

What Is Homeostasis?

🌡️ The Body Balances Itself
①You shiver in the cold and sweat in the heat, and both are responses that bring body temperature back
②The tendency to keep the internal environment (temperature, blood glucose, pH, osmotic pressure) within a steady range is called homeostasis
③Homeostasis is maintained mostly by negative feedback: when something changes, the response works to reduce that change

Blood Glucose Regulation

100 mg/dL
💡 Changing Blood Glucose
①Raise glucose above the normal range and insulin secretion from the pancreatic beta cells rises sharply, while glucagon secretion from the alpha cells almost stops
②Lower glucose below normal and the reverse happens: glucagon rises and insulin falls
③Even inside the normal range both hormones are released in small amounts, and their balance holds glucose at around 100 mg/dL
④Once glucose is back to normal, the stimulus for secretion is gone and secretion drops too; that is negative feedback

How Homeostasis Works

Negative Feedback
detect a change → respond to undo it → return to baseline → the stimulus fades and so does the response
the principle behind most homeostatic control, including body temperature, blood glucose and osmotic pressure

Blood Glucose Regulation

Insulin and Glucagon
ItemInsulinGlucagon
Secreted bypancreatic beta cellspancreatic alpha cells
Secretion rises whenblood glucose is highblood glucose is low
Actionlowers glucose (glucose → glycogen; cells take up more glucose)raises glucose (glycogen → glucose)
Main targetsliver, muscle, fat tissueliver

Body Temperature Regulation

Temperature Regulation

Responses When Body Temperature Rises or Falls
ItemWhen temperature risesWhen temperature falls
Skin vesselsdilate (more heat loss)constrict (less heat loss)
Sweat glandsmore sweatless sweat
Heat productionmetabolism slowsmuscles shiver, metabolism speeds up
Control centerhypothalamushypothalamus

Worked Examples

Example 1
After a meal, blood glucose rises to 140 mg/dL. Which hormone is secreted more, which cells secrete it, and what does it do?
1
Glucose is above the normal range (fasting about 70–110 mg/dL), so secretion of the hormone that lowers it increases.
2
Pancreatic beta cells release insulin, which makes the liver convert glucose into glycogen for storage and makes body cells take up more glucose. As a result, blood glucose falls.
▸ insulin (pancreatic beta cells), which lowers blood glucose
With glucose at 140 in the diagram, the insulin bar is mostly full and the glucagon bar sits at the bottom. As glucose comes back down to normal, insulin secretion falls with it; that is negative feedback.
Example 2
When you step out into the cold, name two responses your body makes to keep its temperature, and identify the control center.
1
When body temperature falls, the body responds by losing less heat and making more.
2
The skin's blood vessels constrict, so less blood reaches the skin and less heat is lost, and skeletal muscles shiver to make heat. The control center is the hypothalamus.
▸ skin blood vessels constrict and muscles shiver; control center: hypothalamus
Temperature and glucose are handled by different organs, but both are controlled by the same negative feedback: a response that undoes however far the value has strayed from normal.

Summary

The Path of Homeostatic Control
stimulus → receptor → control center → effector → return to baseline
negative feedback keeps the internal environment within a steady range
CSAT-style problem, adapted
Which statement about human homeostatic regulation is INCORRECT?
①When blood glucose rises, insulin is secreted.
②Insulin is secreted by pancreatic beta cells.
③The control center for body temperature is the hypothalamus.
④Most homeostasis is regulated by negative feedback.
⑤When blood glucose falls, insulin is secreted to lower it further.
▸ ⑤ When blood glucose falls, insulin is secreted to lower it further.
1
When glucose is low, glucagon from the alpha cells of the pancreas breaks down liver glycogen into glucose and raises blood glucose.
2
Insulin is secreted more when glucose is high and lowers it, so ⑤, which has insulin released at low glucose to push it even lower, runs in the wrong direction.
🎯 Exam Points
①Homeostasis is the tendency to keep the internal environment, such as temperature, glucose, pH and osmotic pressure, within a steady range
②Negative feedback triggers a response that undoes a change and eases off once the baseline is restored
③High glucose → insulin from beta cells lowers it; low glucose → glucagon from alpha cells raises it
④The control center for body temperature is the hypothalamus
⑤Diabetes is a breakdown of glucose homeostasis because insulin is lacking or does not work properly
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