Hormonal Coordination in Animals

Lessons

Senior 6 · Biology

Hormonal Coordination in Animals

4 lessons

Lesson 3 of 4

Negative feedback and the need for hormonal balance

30 min

Steady, whatever you eat

A plate of rice, beans and sweet potatoes pours glucose into the blood within an hour. A long walk to school on an empty stomach uses glucose up. Yet in a healthy person the glucose concentration of the blood stays within narrow limits all day. Too little glucose and the brain, which depends on it, cannot work; too much and water is drawn out of cells and glucose is lost in the urine. Keeping it steady is the job of two hormones working against each other, controlled by negative feedback. This lesson explains that principle and why the balance between hormones matters.

Definition

Homeostasis: keeping the internal environment (such as blood glucose, temperature, water potential and pH) within narrow limits, despite changes inside and outside the body.

Set point: the value around which a factor is kept, such as a blood glucose concentration of about 5 mmol dm⁻³.

Negative feedback: a control mechanism in which a change in a factor triggers a response that reverses the change, returning the factor towards its set point.

Positive feedback: a mechanism in which a change triggers a response that increases the change further.

Antagonistic hormones: two hormones with opposite effects on the same factor, such as insulin and glucagon.

Regulating blood glucose

How insulin lowers blood glucose. Insulin binds to receptors on liver, muscle and fat cells. This makes vesicles carrying glucose transporter proteins fuse with the cell-surface membrane, so the cells take up glucose faster by facilitated diffusion. Insulin also activates the enzymes that turn glucose into glycogen (glycogenesis) and into fat, and it increases the rate of respiration.

How glucagon raises blood glucose. Glucagon binds to receptors on liver cells and, through the second messenger cAMP, activates enzymes that break glycogen down to glucose (glycogenolysis) and that make new glucose from amino acids and glycerol (gluconeogenesis). Glucagon has little effect on muscle. Adrenaline also raises blood glucose in an emergency.

Notice that the β and α cells are both the receptors (they detect the glucose concentration of the blood flowing through the islets) and the coordinators (they secrete the hormone). Because insulin and glucagon are antagonistic, the concentration can be corrected in either direction, and the level oscillates only slightly around the set point.

Regulating thyroxine

If the thyroxine level falls, less inhibition reaches the hypothalamus and pituitary, so more TRH and TSH are released and the thyroid secretes more thyroxine. If the level rises, TRH and TSH secretion is inhibited. The same pattern controls cortisol (CRH → ACTH → cortisol) and the sex hormones.

Positive feedback

In positive feedback, a change is amplified instead of reversed, so it cannot keep a factor steady. It is useful when a process must be completed quickly once it has begun. In childbirth, pressure of the baby's head on the cervix sends nerve impulses to the hypothalamus, so more oxytocin is released from the posterior pituitary; oxytocin makes the uterus contract harder, which pushes the head further, which releases still more oxytocin, until the baby is born and the stimulus ends. A surge of LH before ovulation (unit 8) is another example.

The need for hormonal balance

Many factors are controlled by pairs of antagonistic hormones (insulin and glucagon; parathyroid hormone and calcitonin), so the body can correct a change in either direction.
Hormones act in very small concentrations and are amplified in their target cells, so a small excess or shortage has large effects.
Hormone levels depend on a balance between secretion and breakdown (mostly in the liver, with removal by the kidneys); disease of either can upset the level.
Feedback depends on every link working: the gland, the hormone, and the receptors on target cells. If any link fails, the result is a hormonal imbalance (lesson 4).

Worked example 1: a glucose tolerance test

After fasting overnight, a healthy student drinks a solution containing 75 g of glucose. Her blood glucose is measured every 30 minutes (illustrative data).

Time / min	0	30	60	90	120
Blood glucose / mmol dm⁻³	4.8	8.2	7.0	5.9	5.0

Describe: glucose rises from 4.8 to a peak of 8.2 mmol dm⁻³ at 30 minutes, then falls back almost to the fasting value by 120 minutes.
Calculate the mean rate of fall from 30 to 120 minutes: $\frac{8.2 - 5.0}{120 - 30} = \frac{3.2}{90} = 0.036\,\mathrm{mmol\,dm^{-3}\,min^{-1}}$ .
Explain: glucose absorbed from the gut raised the blood level; β cells detected the rise and secreted insulin; liver and muscle cells took up glucose and stored it as glycogen, so the level fell back towards the set point: negative feedback.

Worked example 2: too many thyroxine tablets

A patient whose thyroid works poorly takes thyroxine tablets. By mistake she takes a much larger dose than prescribed for several weeks. Predict and explain what happens to (a) her TSH level, (b) her own thyroid's secretion of thyroxine, (c) her metabolic rate.

(a) The high level of thyroxine in the blood inhibits the hypothalamus (less TRH) and the anterior pituitary, so TSH falls.
(b) With little TSH to stimulate it, her own thyroid secretes even less thyroxine.
(c) The total thyroxine is still high, so her metabolic rate rises: she may lose weight, feel hot and have a fast heartbeat. Doctors use a low TSH as a sign that the dose is too high.

Warning

Common mistakes

Glucagon, glycogen and glycerol are different: glucagon is a hormone; glycogen is the storage carbohydrate; glycerol is part of a fat molecule. Spell them carefully.
"Insulin turns glucose into glycogen." Insulin is not an enzyme; it activates the enzymes that do this and increases glucose uptake.
"Negative feedback means the effect is bad." "Negative" means the response reverses the change.

Kathryn White: "Homeostasis of blood glucose (a negative feedback loop)". Watch for the roles of insulin and glucagon in returning blood glucose to normal.

✦

Practice: feedback and hormonal balance

Define negative feedback and give one hormonal example.
State the effect of insulin on (a) uptake of glucose by muscle cells, (b) glycogen in liver cells.
Distinguish between glycogenesis, glycogenolysis and gluconeogenesis.
Explain why the islets of Langerhans act as both receptor and coordinator.
Blood glucose falls from 7.4 to 5.0 mmol dm⁻³ in 60 minutes. Calculate the mean rate of fall.
Describe how thyroxine secretion is kept constant.
Explain why oxytocin release during childbirth is an example of positive feedback, and why positive feedback cannot be used for homeostasis.
Suggest why it is an advantage to control blood glucose with two antagonistic hormones instead of one.

Show answersHide answers

A change in a factor triggers a response that reverses the change, returning it to its set point. Examples: insulin and glucagon controlling blood glucose; thyroxine inhibiting TRH and TSH.
(a) Insulin increases glucose uptake, by adding glucose transporter proteins to the membrane. (b) Insulin activates enzymes that convert glucose to glycogen, so glycogen increases.
Glycogenesis: making glycogen from glucose. Glycogenolysis: breaking glycogen down to glucose. Gluconeogenesis: making new glucose from non-carbohydrates such as amino acids and glycerol.
The α and β cells detect the glucose concentration of the blood flowing through them (receptor) and respond by secreting glucagon or insulin (coordinator).
$\frac{7.4 - 5.0}{60} = \frac{2.4}{60} = 0.04\,\mathrm{mmol\,dm^{-3}\,min^{-1}}$ .
The hypothalamus secretes TRH, which makes the anterior pituitary release TSH, which makes the thyroid secrete thyroxine. A high thyroxine level inhibits the release of TRH and TSH, so secretion falls; a low level removes the inhibition, so secretion rises.
Each contraction causes more oxytocin release, which causes stronger contractions: the change is increased, not reversed. Positive feedback drives a factor further away from its starting value, so it cannot hold it steady; it ends only when an event (birth) removes the stimulus.
With two hormones, the level can be actively pushed in either direction, so corrections are faster and more precise; with one hormone, a rise could be corrected only by stopping secretion and waiting for the hormone to be broken down.

Key points

Negative feedback reverses a change and keeps a factor near its set point; positive feedback amplifies a change and ends with an event.
High blood glucose → β cells secrete insulin → glucose uptake, glycogenesis, respiration → glucose falls. Low blood glucose → α cells secrete glucagon → glycogenolysis, gluconeogenesis → glucose rises.
TRH → TSH → thyroxine; high thyroxine inhibits the hypothalamus and pituitary.
Balance depends on antagonistic hormones, secretion matching breakdown, and working receptors.

Sources and credits

7.5.4: Regulation of Body Processes and 7.5.5: Regulation of Hormone Production, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the control of blood glucose, thyroid hormones and feedback. Adapted by Ganzaa.
7.1.4: Homeostasis, from the same book and licence. Used for set points and negative and positive feedback. Adapted by Ganzaa.
Photo: (20250417) Blood glucose meter 09, Roy Zuo, licensed under CC BY-SA 4.0, via Wikimedia Commons.
Video: "Homeostasis of blood glucose (a negative feedback loop)", Kathryn White, embedded from YouTube.

Figures 1 and 2, the worked examples (with illustrative data) and the practice questions were written by Ganzaa. This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Steady, whatever you eat

2Regulating blood glucose

3Regulating thyroxine

4Positive feedback

5The need for hormonal balance

Steady, whatever you eat

Regulating blood glucose

Regulating thyroxine

Positive feedback

The need for hormonal balance