Module 2 of 5

Your Body

Where peptides are made and how they communicate with your cells

This course is education, not medical advice.

Your Body's Peptide Factory

Your body produces many different peptides that regulate everything from hunger and mood to immune response and blood pressure. Click on different areas to discover what peptides are made where.

Interactive

How Peptides Talk to Cells

Many peptides act from outside the cell. They work like a key fitting into a lock, binding to specific receptors on the cell surface to trigger a relay of signals inside the cell. Several steps of that relay amplify the signal along the way.

Interactive
Signal amplification: When a single peptide binds its receptor, that one receptor can activate many G-proteins, each of which switches on an enzyme, and each enzyme turns out a large number of second messenger molecules. This is why tiny amounts of peptides can have large effects.

Peptides You Already Know

You encounter peptides every day without realizing it. Here are some of the most important ones your body produces naturally, and what happens when their levels are disrupted.

Insulin

51 amino acids

Made by the pancreas. Controls blood sugar by telling cells to absorb glucose. Disruption causes diabetes.

Oxytocin

9 amino acids

Made by the hypothalamus. Involved in bonding, trust, and childbirth. Often called the "love hormone."

Endorphins

16-31 amino acids

Made by the pituitary gland. Natural painkillers that work by binding to opioid receptors, especially the mu subtype.

GLP-1

30 amino acids

Made by the gut after eating. Signals fullness and regulates blood sugar. Semaglutide, the drug in Ozempic and Wegovy, acts on the same receptor.

Melatonin

Not a peptide (made from tryptophan)

Made by the pineal gland in darkness and suppressed by light. Regulates your sleep-wake cycle. Often listed alongside peptides, but it is a small molecule.

Angiotensin

Cut from a larger precursor

Its precursor comes from the liver, and kidney enzymes start the cuts that activate it. Raises blood pressure, partly by constricting blood vessels. ACE inhibitors target this pathway.

Three Families of Peptides

Not all peptides do the same job. A simple way to sort them is into three families, each with distinct roles in keeping you alive and healthy.

Hormones
Long-distance messengers, released into the bloodstream to reach distant organs. Insulin, oxytocin, and GLP-1 are peptide hormones. They regulate metabolism, growth, reproduction, and mood.
Neuropeptides
Brain signals, used by neurons to communicate. Endorphins and substance P are the best-known examples, shaping how pain is felt; others shape appetite, sleep, and mood. Often co-released with neurotransmitters.
Antimicrobial
Immune defenders that can act on microbes directly. Defensins and cathelicidins (like LL-37) are the best-known families. Your skin, gut, and airways produce them as a first line of defense.
Overlap is common: many peptides fall into multiple categories. Oxytocin is both a hormone (released into blood) and a neuropeptide (used in the brain). GLP-1 acts as a gut hormone, but some of its effects may run through receptors in the brain.

Common Questions About Peptides in the Body

Short, sourced answers to the questions people ask most about what peptides do in the body, how they signal cells, and where they come from.

What do peptides do in the body?

Your body produces many different peptides that act as chemical messengers. They regulate everything from hunger and mood to immune response and blood pressure. Peptides work by binding to receptors on the surface of your cells, triggering signals that tell those cells what to do.

How do peptides signal your cells?

Many peptides act from outside the cell, like a key fitting into a lock: they bind specific receptors on the cell surface and trigger a cascade of signals inside the cell. Several steps of that cascade amplify the signal, which is why tiny amounts of a peptide can have large effects.

Where are peptides made in the body?

Peptides are produced all over the body. Insulin is made by the pancreas, oxytocin by the hypothalamus, endorphins by the pituitary gland, GLP-1 by the gut after eating, and angiotensin via the liver and kidneys. Each site releases peptides suited to the job it controls, from blood sugar to blood pressure.

Are peptides hormones?

Some are. Peptides are often sorted into three families: hormones, neuropeptides, and antimicrobial peptides. Peptide hormones like insulin, oxytocin, and GLP-1 travel through the bloodstream to reach distant organs. Many peptides overlap categories, though; oxytocin acts as both a hormone and a neuropeptide.

What are some natural peptides in the body?

You encounter peptides every day without realizing it. Common natural ones your body makes include insulin for blood sugar, oxytocin for bonding and childbirth, endorphins as natural painkillers, GLP-1 for fullness and blood sugar, and angiotensin for blood pressure.

  1. Weiss M, Steiner DF, Philipson LH. "Insulin Biosynthesis, Secretion, Structure, and Structure-Activity Relationships." Endotext. 2014. NBK279029
  2. Walter MH, Abele H, Plappert CF. "The Role of Oxytocin and the Effect of Stress During Childbirth: Neurobiological Basics and Implications for Mother and Child." Front Endocrinol (Lausanne). 2021;12:742236. PMID 34777247
  3. Osilla EV, Patel P, Sharma S. "Oxytocin." StatPearls. 2025. NBK507848
  4. Holst JJ. "Glucagon-like peptide 1: a newly discovered gastrointestinal hormone." Gastroenterology. 1994. PMID 7958701
  5. Alberts B et al. "Signaling through G-Protein-Linked Cell-Surface Receptors." Molecular Biology of the Cell, 4th ed. 2002. NBK26912
  6. Sprouse-Blum AS et al. "Understanding Endorphins and Their Importance in Pain Management." Hawaii Med J. 2010;69(3):70-71. PMID 20397507
  7. Chaudhry SR, Rahimi N. "Biochemistry, Endorphin." StatPearls. 2025. NBK470306
  8. Kojima M, Hosoda H, Date Y, et al. "Ghrelin is a growth-hormone-releasing acylated peptide from stomach." Nature. 1999;402(6762):656-660. PMID 10604470
  9. Kreymann B et al. "Glucagon-like peptide-1 7-36: a physiological incretin in man." Lancet. 1987;2(8571):1300-4. PMID 2890903
  10. Kaur J, Rout P. "Physiology, Renin Angiotensin System." StatPearls. 2026. NBK470410
  11. Ilahi S, Beriwal N, Ilahi TB. "Physiology, Pineal Gland." StatPearls. 2023. NBK525955
  12. Novo Nordisk. "OZEMPIC (semaglutide) injection, US prescribing information." Revised 05/2026. PDF as served on the fetch date. novo-pi.com ozempic.pdf
  13. Novo Nordisk. "WEGOVY (semaglutide) injection, US prescribing information." PDF as served on the fetch date. novo-pi.com wegovy.pdf

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Practice Exercises

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How They're Used