The HPO and HPA axes

The hypothalamic-pituitary-ovarian axis (HPO) is a critical interplay of hormonal events that controls the menstrual cycle, while the hypothalamic-pituitary-adrenal (HPA) axis is part of our stress response, affecting our nervous system. They are two distinct systems, but they affect each other considerably.

     The HPO axis consists of:

  1. Hypothalamus – part of the brain that secretes gonadotropin releasing hormone (GnRH)
  2. Pituitary gland – part of the brain, just underneath the hypothalamus, that secretes follicle stimulating hormone (FSH) and luteinising hormone (LH)
  3. Ovaries – found in the pelvis, which secrete oestrogen and progesterone

     The HPA axis

This interconnected system becomes activated during stress, and involves your adrenal glands and your nervous system. Adrenaline is released into the system by the HPA axis when it is ‘told’ to. The HPA axis switches ‘on’, and releases the needed hormones to get us through a stressful situation.

     The HPA axis consists of:

  1. Hypothalamus – part of the brain
  2. Pituitary gland – part of the brain
  3. Adrenal gland – found sitting just on top of the kidneys in the trunk

     How the HPA axis affects the HPO axis

When you are stressed, the HPA axis is activated, which affects your whole body. The HPA axis activation effectively halts the HPO – reproductive – axis (and many other body systems) to conserve energy. Stress can thus effectively stop you from ovulating. Chronic stress may impair ovulation and fertility.

Some states/conditions are related to HPA axis regulation, such as polycystic ovarian syndrome (PCOS), anorexia (the eating disorder), and the third trimester of pregnancy are all conditions whereby the HPA axis is activated.

Conversely, the HPA axis is suppressed during the period of time immediately after giving birth (postpartum), and is a possible reason for post-birth depression. The hormones that keep the HPA axis in check are also essential to foetal development and a baby’s development, which is why switching off the HPA axis at birth is required.