Cortisol is a hormone released by the adrenal glands, regulated by the Hypothalamic-Pituitary-Adrenal (HPA) axis. This regulation is influenced by stress and the body's circadian rhythm, which is the sleep-wake cycle. Cortisol can be converted into its inactive form, cortisone, in the kidneys and pancreas, with the majority of circulating cortisol existing in this inactive state. Most tissues in the body can convert cortisone back into cortisol, and the body adjusts this conversion based on need; increased stress can lead to greater conversion of cortisone back to cortisol.
What is Cortisol's Role in the Body?
Cortisol receptors are found throughout the body, allowing this hormone to influence several systems, including the respiratory, reproductive, immune, musculoskeletal, nervous, cardiovascular, integumentary, and gastrointestinal systems. Its effects are particularly significant during the stress response and are also linked to the circadian rhythm.
Stress Response
The body's nervous system comprises two main systems: the parasympathetic and the sympathetic. The parasympathetic system is responsible for resting and digestion, while the sympathetic system triggers the fight-or-flight response. The amygdala, a brain center for processing fear, activates the fight-or-flight response when a stressor is present. The amygdala signals the hypothalamus, which in turn communicates with the pituitary gland to initiate the release of adrenaline hormones, norepinephrine, and epinephrine. These hormones increase blood pressure, heart rate, and respiration. Approximately 15 minutes later, the amygdala activates the HPA axis, leading to cortisol production that influences inflammatory, immune, metabolic, hormonal, and gastrointestinal responses. It's important to recognize the evolutionary purpose of the stress response: survival. Consequently, these mechanisms prioritize the body's resources for immediate survival needs. Here are some specific effects:
Metabolic Function
Cortisol raises glucose levels by enhancing the pathways that produce glucose while inhibiting those that break it down. Glucose serves as the primary precursor to ATP, the body’s energy source. This increase in glucose is essential for fueling the brain and other tissues. While the exact mechanism is not fully understood, cortisol appears to influence blood pressure—potentially increasing it at high levels and lowering it at low levels. This effect can impact blood flow throughout the body, thereby affecting the delivery of oxygen and nutrients to tissues.
Immune and Inflammatory Response
Cortisol plays a role in the immune response through various mechanisms, including affecting the production of certain antibodies. It may also induce apoptosis, or cell death, in certain inflammatory cells.
Hormone Production
Cortisol can influence the production of gonadotropin-releasing hormone (GnRH), which regulates the menstrual cycle in women, as well as testosterone and sperm production in men. Consequently, high levels of cortisol can lead to menstrual irregularities, low sperm count, and low testosterone, all of which are often associated with infertility. During a survival state, reproduction is typically deprioritized. Additionally, cortisol affects levels of ghrelin and leptin—ghrelin is known as the "hunger hormone" because it stimulates appetite, while leptin induces feelings of satiety. Cortisol may lower leptin levels while increasing ghrelin levels, potentially leading to weight gain, especially around the abdominal area. This mechanism supports survival by ensuring the body has access to energy sources.
Gastrointestinal (GI) System
Cortisol can slow down food movement through the stomach, resulting in bloating and indigestion. It may also increase transit time in the colon, leading to diarrhea. Furthermore, cortisol can heighten sensitivities in the tissues of the esophagus, causing heartburn symptoms without increased stomach acid. It may negatively impact the microbiome of the GI tract, affecting digestion, absorption, and immune function. Additionally, cortisol can alter the permeability of the small intestine, leading to impaired digestion and absorption, as gastrointestinal function is not regarded as essential during acute survival situations.
Cortisol as a Circadian Rhythm Hormone
Cortisol also plays a significant role in regulating our circadian rhythm, independent of its function in the stress response. The body follows a 24-hour sleep-wake cycle, and during this cycle, cortisol levels rise sharply in the morning—an occurrence known as the Cortisol Awakening Response (CAR)—and gradually decline throughout the day, reaching their lowest levels at night. Due to this role, cortisol is often referred to as the "daytime hormone." However, while cortisol's circadian role is independent of its stress response function, the two can influence one another. Extended exposure to stress can lead to both chronically high and low cortisol levels, which in turn can disrupt the typical cortisol release pattern throughout the circadian rhythm, potentially resulting in fatigue, sleep difficulties, and more.
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