When levels reach a certain point, adrenal gland receptors signal the two adrenal glands to produce the final chemical of the HPA axis – cortisol. Sitting on top of the kidneys like two hats The two adrenal glands (suprarenal glands) sit on top of the kidneys, one on the right and one on the left kidney. Of these, only ACTH is part of the HPA axis definition.ĪCTH is secreted into the blood circulation. As with the hypothalamus, this gland produces many different hormones. On its own, vasopressin has a much weaker effect in combination with CRH, ACTH production is greater. When the CRH and AVP receptors of the pituitary have detected the presence of these two hypothalamic neurohormones, the pituitary gland is stimulated to produce adrenocorticotropic hormone (ACTH). Section through the pituitary gland, attached to the hypothalamus These neurohormones are transported from the hypothalamus via connecting blood vessels – the distance from the hypothalamus to the pituitary gland is very short. HPA axis pituitary tissue has many receptors for corticotropin-releasing hormone and vasopressin. Our sleep and wake patterns (circadian rhythms) can heighten and dampen stress responses, as do certain levels of hormones in the bloodstream. Stress is not only psychological – infection, medication, pregnancy, and our metabolism can produce an HPA axis stress response, as do thousands of other autonomic and somatic stimuli. The trigger that causes the hypothalamus to increase production and release of CRH and AVP is stress.
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