How hormones and negative feedback keep blood glucose steady
TEAS 7 outline: Endocrine system: anatomy and physiology
Also covers HESI A2 Anatomy & Physiology (Endocrine system)
- Hormones travel in blood but act only on cells with matching receptors.
- Most hormone levels are held steady by negative feedback.
- High blood glucose: beta cells release insulin, which lowers it.
- Low blood glucose: alpha cells release glucagon, and the liver releases glucose.
- TRH, then TSH, then T3 and T4; rising T3 and T4 lower TSH.
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After a meal, glucose absorbed during digestion enters the blood, and blood glucose rises above its normal range.
Key facts
- Endocrine glands are ductless and release hormones into the surrounding fluid and blood; exocrine glands, such as sweat glands, release their products through ducts.
- Beta cells of the pancreatic islets secrete insulin when blood glucose rises; insulin lowers blood glucose.
- Alpha cells of the pancreatic islets secrete glucagon when blood glucose falls; glucagon makes the liver break glycogen down to glucose (glycogenolysis) and build new glucose from amino acids (gluconeogenesis).
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- Negative feedback, the most common control, slows further release of a hormone once its effect has been reached.
- Oxytocin during labor is positive feedback: contractions stretch the cervix, which triggers release of more oxytocin.
- The hypothalamus releases TRH, the anterior pituitary releases TSH, and the thyroid releases T3 and T4; rising T3 and T4 decrease TSH secretion.
- ADH and oxytocin are made in the hypothalamus but stored and released by the posterior pituitary.
Quick check1 question · 2 more at the end
Which choice correctly matches a pancreatic islet cell type with the hormone it secretes and that hormone's effect on blood glucose?
Correct answer: D.
The pancreatic islets contain two main hormone-secreting cell types with opposite jobs. Beta cells release insulin when blood glucose rises, and insulin lowers blood glucose by helping cells take it up and by having the liver store it as glycogen. Alpha cells release glucagon when blood glucose falls, and glucagon raises blood glucose. Glycogen is a stored form of glucose, not a hormone.
A Insulin does lower blood glucose, but it comes from beta cells; alpha cells secrete glucagon.
B Glucagon does raise blood glucose, but it comes from alpha cells; beta cells secrete insulin.
C This confuses glycogen (stored glucose in the liver) with glucagon (the hormone alpha cells secrete).
D Correct: beta cells secrete insulin in response to rising blood glucose, and insulin lowers it.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: The endocrine system uses hormones, chemical messengers carried by the blood to target cells with matching receptors. Most hormone levels are held steady by negative feedback. When blood glucose rises, beta cells release insulin, which moves glucose into cells and liver glycogen. When it falls, alpha cells release glucagon, which makes the liver release glucose.
1 What is a hormone, and how is it different from a nerve signal?A hormone is a chemical messenger that an endocrine cell releases into the fluid around it. 1 min
A hormone is a chemical messenger that an endocrine cell releases into the fluid around it. The blood carries it all over the body, but it only changes target cells that have a matching receptor. Think of a radio broadcast: everyone can hear it, but only cells with the right receiver respond.
Endocrine glands have no ducts. They release hormones straight into the surrounding fluid and blood. Exocrine glands, such as sweat and oil glands, send their products through ducts. The pancreas does both jobs: some of its cells send digestive juice through ducts, while its islet cells release insulin and glucagon into the blood.
The nervous system and the endocrine system also differ in speed and reach. A neuron signals across a tiny gap (the synapse), the target responds within milliseconds, and the response stops quickly once the signal stops. Hormones can act over long or short distances and are slower. Epinephrine is released within seconds, but some reproductive hormones take up to 48 hours to produce a response.
2 Which glands make which hormones?The hypothalamus and pituitary work together as the "command center" of the endocrine system. 1 min
The hypothalamus and pituitary work together as the "command center" of the endocrine system. Here are the hormones worth knowing cold:
| Gland | Key hormone(s) | Main job |
|---|---|---|
| Hypothalamus | Releasing hormones (TRH, CRH, GnRH); makes ADH and oxytocin | Tells the anterior pituitary what to release |
| Anterior pituitary | GH, TSH, ACTH, FSH, LH, prolactin | GH: growth. TSH: thyroid. ACTH: adrenal cortex. FSH and LH: gonads. Prolactin: milk production |
| Posterior pituitary | ADH, oxytocin (stores and releases them) | ADH: kidneys reabsorb water. Oxytocin: uterine contractions, milk ejection |
| Thyroid | T3 and T4; calcitonin | T3/T4 raise basal metabolic rate; calcitonin lowers blood calcium |
| Parathyroid | Parathyroid hormone (PTH) | Raises blood calcium |
| Adrenal cortex | Aldosterone; cortisol | Aldosterone raises blood sodium; cortisol raises blood glucose during long-term stress |
| Adrenal medulla | Epinephrine, norepinephrine | Fight-or-flight response |
| Pancreatic islets | Insulin (beta cells); glucagon (alpha cells) | Insulin lowers blood glucose; glucagon raises it |
| Gonads | Testosterone (testes); estrogens and progesterone (ovaries) | Male and female sex traits, sperm production, preparing the body for childbirth |
| Pineal | Melatonin | Regulates sleep cycles |
3 What is negative feedback, and when is feedback positive?Most hormones are controlled by negative feedback: the effect of a hormone switches off further release of that same hormone. 1 min
Most hormones are controlled by negative feedback: the effect of a hormone switches off further release of that same hormone. It works like a thermostat, keeping blood levels in a narrow range. Example: as cortisol from the adrenal cortex rises in the blood, the hypothalamus and pituitary send fewer signals to the adrenal glands, so cortisol release slows.
Positive feedback is less common. Here the effect causes more of the hormone to be released, pushing a process to a finish. In labor, oxytocin makes the uterus contract, the contractions push the baby against the cervix and stretch it, and that stretch signals the pituitary to release more oxytocin. The loop ends after birth.
4 How do insulin and glucagon keep blood glucose steady?This is the classic negative feedback pair. 1 min
This is the classic negative feedback pair. The two hormones come from different cells of the pancreatic islets and have opposite effects.
After a meal (glucose rising): beta cells sense the rise and release insulin. Insulin helps body cells, mainly skeletal muscle and fat cells, take glucose out of the blood. It also has the liver store extra glucose as glycogen and stops the liver from making new glucose. Blood glucose falls, and as it falls, insulin release slows.
Between meals (glucose falling): alpha cells release glucagon. Glucagon tells the liver to break glycogen back into glucose (glycogenolysis) and to build new glucose from amino acids (gluconeogenesis). It also has fat tissue break down triglycerides, and the liver turns the glycerol into glucose. Blood glucose rises, and as it rises, glucagon release slows.
A few tissues, including the brain and red blood cells, take up glucose without insulin, but muscle and fat cells depend on it. In type 1 diabetes, an autoimmune disease, the immune system attacks the beta cells, so they make no insulin to signal those cells. In type 2 diabetes, cells become resistant to insulin. Either way, glucose stays high in the blood (hyperglycemia).
5 How does the brain control the thyroid?The adrenal cortex follows the same pattern (CRH, then ACTH, then cortisol). 1 min
The thyroid shows how the hypothalamus and pituitary run a chain of command:
- Low T3 and T4 trigger the hypothalamus to release TRH.
- TRH makes the anterior pituitary release TSH.
- TSH makes the thyroid release T3 and T4, which raise the basal metabolic rate.
- Rising T3 and T4 decrease TSH production, so the chain slows down.
The adrenal cortex follows the same pattern (CRH, then ACTH, then cortisol). A useful rule: when the final hormone in a chain drops, the hormones upstream of it rise.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Glucagon is the hormone that raises blood glucose; glycogen is the stored form of glucose in the liver, not a hormone.
Watch out: Insulin lowers blood glucose by moving glucose out of the blood and into cells, not raises it.
Watch out: ADH (posterior pituitary) makes the kidneys reabsorb water; aldosterone (adrenal cortex) makes the body retain sodium, not the other way around.
Watch out: The posterior pituitary stores and releases ADH and oxytocin; the hypothalamus makes them, not the posterior pituitary.
Watch out: The pancreas is both exocrine (digestive juice through ducts) and endocrine (insulin and glucagon into the blood), not only one or the other.
How to remember it
Memory hook: Insulin lets glucose IN to cells. Glucagon is called when the glucose is gone. And glycogen is the glucose "generator" in storage, not a hormone.
Self-check in 3 steps:
- Blood glucose is high: which islet cell responds, with which hormone, and where does the glucose go?
- Blood glucose is low: name the two ways the liver raises it.
- T3 and T4 levels are rising: what happens to TSH, and why is that negative feedback?