The endocrine system is a network of glands that release hormones into the bloodstream to control body functions. This cheat sheet helps students connect each major gland with the hormones it produces and the body responses it regulates. It is useful for studying homeostasis, growth, metabolism, reproduction, and stress responses.
Understanding hormones also helps explain diseases such as diabetes and thyroid disorders.
The most important idea is that hormones are chemical messengers that act only on target cells with matching receptors. Many endocrine pathways use negative feedback, where the body reduces a signal after the desired condition is restored. Blood glucose regulation is a key example, with insulin lowering blood sugar and glucagon raising it.
Hormones can be steroid-based or protein-based, and their structure affects how they enter cells and trigger responses.
Key Facts
- Hormones are chemical messengers released by endocrine glands into the blood to affect target cells with specific receptors.
- Negative feedback means a change triggers responses that reverse the change, such as high blood glucose causing insulin release to lower blood glucose.
- Positive feedback means a change triggers responses that increase the change, such as oxytocin strengthening uterine contractions during childbirth.
- The pancreas releases insulin from beta cells to lower blood glucose and glucagon from alpha cells to raise blood glucose.
- The hypothalamus controls the pituitary gland, and the pituitary gland releases hormones that regulate growth, water balance, reproduction, and other endocrine glands.
- The thyroid gland releases thyroxine, also called T4, which helps regulate metabolic rate, growth, and development.
- Steroid hormones are lipid-soluble, so they can cross cell membranes and often change gene expression inside the nucleus.
- Protein and peptide hormones are water-soluble, so they usually bind to receptors on the cell membrane and activate signaling pathways inside the cell.
Vocabulary
- Endocrine gland
- An organ that releases hormones directly into the bloodstream instead of through ducts.
- Hormone
- A chemical messenger that travels through the blood and changes the activity of target cells.
- Target cell
- A cell that has the correct receptor for a specific hormone and can respond to that hormone.
- Receptor
- A protein on a cell membrane or inside a cell that binds a specific hormone and starts a response.
- Homeostasis
- The maintenance of stable internal conditions, such as body temperature, blood glucose, and water balance.
- Feedback loop
- A control pathway in which the result of a process affects whether the process continues, increases, or decreases.
Common Mistakes to Avoid
- Confusing endocrine glands with exocrine glands is wrong because endocrine glands release hormones into blood, while exocrine glands release substances through ducts.
- Thinking every cell responds to every hormone is wrong because only target cells with matching receptors can detect and respond to a hormone.
- Reversing insulin and glucagon is wrong because insulin lowers blood glucose after a meal, while glucagon raises blood glucose when levels are low.
- Calling all feedback positive is wrong because most hormone regulation uses negative feedback to return conditions toward a set point.
- Assuming steroid and protein hormones act the same way is wrong because steroid hormones can cross membranes, while most protein hormones bind to cell surface receptors.
Practice Questions
- 1 A student's blood glucose rises from 90 mg/dL to 150 mg/dL after lunch. Which pancreatic hormone should increase, and what effect should it have on blood glucose?
- 2 If blood glucose drops from 85 mg/dL to 60 mg/dL during exercise, which hormone should the pancreas release to help restore homeostasis?
- 3 A gland releases a hormone that causes another gland to reduce its hormone output once blood levels are high enough. Is this negative feedback or positive feedback?
- 4 Why can two different body cells be exposed to the same hormone but only one cell responds to it?
Understanding Endocrine System & Hormones
Hormone effects depend on more than the amount released. A target cell must have the right receptor, and the number of receptors can change over time. Cells may reduce receptor numbers after long exposure to a high hormone level.
This is called down regulation. They may increase receptor numbers when stimulation stays low. Receptors can be blocked by drugs or damaged by disease.
This helps explain why a blood test can show a normal hormone level while body tissues still respond poorly. Type 2 diabetes is one example. Many body cells become less sensitive to insulin, so the pancreas must release more insulin to keep glucose in a safe range.
The route a hormone takes inside a cell affects the speed of its action. Peptide hormones often trigger a chain of reactions at the cell surface. One hormone molecule can activate many molecules inside the cell, making a small signal produce a large response.
These effects can begin quickly, sometimes within seconds or minutes. Steroid hormones usually act more slowly because they influence which proteins a cell makes. Their effects can last longer.
Cortisol, made by the adrenal cortex, helps the body manage longer periods of stress by changing how stored fuel is used. Adrenaline acts much faster during sudden danger, increasing heart rate and directing more blood to skeletal muscles.
The hypothalamus and pituitary form a control pathway with several steps. The hypothalamus releases signals that tell the pituitary whether to stimulate another gland. The pituitary can then signal glands such as the thyroid, adrenal glands, or ovaries and testes.
Hormones made by those glands feed back to the brain and pituitary. This arrangement allows fine control, but it makes endocrine problems harder to trace.
A low thyroid hormone result could come from the thyroid itself, the pituitary, or the hypothalamus. Doctors often measure more than one hormone to identify where a pathway is failing.
Daily life changes hormone levels in predictable ways. Sleep affects melatonin, growth hormone, and cortisol patterns. Eating changes insulin and glucagon levels.
Exercise makes muscles use more glucose and can increase their sensitivity to insulin. Puberty involves rising sex hormone levels that affect reproductive organs, bones, skin, and mood. When studying a hormone pathway, track four details.
Identify the stimulus, the gland that responds, the target tissue, and the final body change. Then decide whether the final change reduces or strengthens the original stimulus. It helps to draw arrows between each step and label whether each arrow means release, stimulation, or inhibition.