The body has two stress responses, not one. There is the fast, electric jolt of "fight or flight," driven by the nervous system and over within minutes. And there is a slower, more sustained response, driven by hormones, that unfolds over a longer timescale. That second response runs through a three-part circuit known as the HPA axis.

A Circuit of Three

The letters stand for the three structures the circuit connects:

The hypothalamus, a control center in the brain.
The pituitary, a small gland just beneath it.
The adrenal glands, which sit atop the kidneys.

The HPA axis is the communication line linking these three. When the brain perceives stress, a signal travels down this chain: the hypothalamus signals the pituitary, the pituitary signals the adrenal glands, and the adrenal glands respond by releasing the hormone cortisol.

This relay is the heart of the body's slower, hormonal stress response.

The Role of Cortisol

Cortisol, the end product of the axis, is a powerful and broadly acting hormone. In the context of a stress response, it works to mobilize the body's resources—making energy available and adjusting various systems to help the body cope with a demand.

In the short term, this is genuinely adaptive. A real challenge calls for mobilized resources, and the HPA axis provides them. The axis is not a flaw; it is a vital survival system.

The Brake: Negative Feedback

A stress system that could only switch on would be dangerous. The HPA axis therefore has a built-in brake.

Cortisol, once released, is itself detected by the brain. When the brain senses sufficient cortisol, it dials back the signal that started the cascade. This is a negative feedback loop: the output of the system (cortisol) acts to shut the system down.

In a healthy axis, this means the stress response is self-limiting. The challenge passes, cortisol does its job, the feedback brake engages, and the system returns to baseline.

When the System Stays On

The problems associated with the HPA axis arise not from the response itself, but from its failure to switch off.

Under chronic stress, the axis may be activated repeatedly or continuously, and the normal feedback braking can become less effective. A stress system designed for short bursts is then left running for extended periods. Prolonged activation of the HPA axis is associated with a range of effects on the body and mind, which is why chronic stress is taken seriously as a health concern, and why it is connected to both mental health and physical wellbeing.

Supporting a Healthy Axis

The aim is not to eliminate the stress response—that would be neither possible nor desirable—but to support the axis's natural rhythm of activation and recovery. Practices that promote recovery, adequate sleep, and a genuine return to a calm baseline all help the axis do what it is designed to do: respond, and then reset.

A Circuit Worth Understanding

The HPA axis is the body's central stress circuit—a three-part hormonal relay that mobilizes resources in the face of a challenge and, in health, switches itself off afterward. Understanding it clarifies why short-term stress is normal and survivable, while chronic, unrelenting stress is genuinely harmful. It is a foundational concept in endocrinology, and a key to understanding the deep connection between the brain, the body, and stress.