The figure depicts the interaction between the hypothalamus and the pituitary glands. The hypothalamus is connected to the posterior pituitary, and sends nerve impulses to stimulate the release of hormones. The hypothalamus is connected to the anterior pituitary by the hypothalamic-pituitary portal system, an array of blood vessels with two capillary beds located in series, one after the other. This system ensures direct delivery of hormones to target cells, without diluting or degrading the hormones. 

While these structures stimulate various hormones, our focus is on those involved in the stress response.

Hormones involved in the Stress Response

Hormones	Actions	Glycemia	BP
Cortisol	Counteracts insulin by breaking down glycogen, lipids and proteins. Mobilizes amino acids and ketone bodies. (Leads to increased glycemia via gluconeogenesis. Suppresses inflammation	Up	X
Aldosterone	Increases sodium absorption, which increases water retention	X	Up
Vasopressin	Increases Water retention; vasoconstrictor	X	Up
Thyroid Hormones	Elevate metabolism, gastrointestinal motility, and cerebration	Up	X
Catecholamines	Liberate nutrient stores for fuel; increase heart rate and contractility, respiration, and perspiration	Up	Up

First, the sympathetic nervous system (SNS) innervates organs; therefore, its effects are immediate. For instance, it has nerve endings on the heart, which act to accelerate cardiac muscle contractions. Thus, the immediate excitement felt during a stressful situation is caused by the sympathetic nervous system. The nerve endings of the sympathetic nervous system secrete the catecholamines norepinephrine and epinephrine; the SNS also stimulates the adrenal medulla to release catecholamines. However, the response of the adrenal medulla is not as fast as the direct effect of the sympathetic nervous system. The sympathetic nervous system also causes piloerection (stimulates the hair to stand up), dilates the pupils, and increases concentrations in the plasma of the protein fibrinogen, a key component in blood clotting.

The hypothalamus starts a hormonal cascade during the stress response. The release of corticotropin releasing hormone from the hypothalamus triggers the release of adrenocorticotrophic hormone (ACTH) from the anterior pituitary, which triggers the adrenal cortex to release glucocoritcoids and mineralcorticoids, the primary ones being cortisol and aldosterone, respectively.

The adrenal gland is located directly above the kidneys. It has two hormonal centers: the adrenal medulla and adrenal cortex. The medulla is the core of the adrenal glands, and secretes catecholamines, 80% of which is epinephrine, which has a more potent effect on the cardiovascular system; and the remaining coming from norepinephrine, which has a more potent effect on the vascular system. The adrenal cortex (crust) has three zones. Zone one, called the zona glomerulosa, releases mineralcorticoids, which regulate the mineral salts sodium and potassium in the extra cellular fluid. Aldosterone is the most abundant mineralcorticoid, representing 95% of its class. Zone 2, called the zona fasciculate, secretes glucocorticoid hormones, primarily cortisol.

The hypothalamus also releases thyrotropin releasing factor, which triggers the anterior pituitary to release thyroid stimulating hormone (TSH; or TTH, which stands for thyroid tropic hormone), which triggers the thyroid gland to secrete two hormones: triiodothyronine (T3) and thyroxine (T4), T3 being the most active hormone.

Further, the hypothalamus produces the hormone vasopression, also known as anti-diuretic hormone (ADH). ADH is packaged and transported to the posterior pituitary for storage. Then, during times of stress, the hypothalamus stimulates its release through nerve impulses sent to the posterior pituitary.  

Actions of these hormones

Cortisol is a hyperglycemic hormone - it raises blood glucose. Its actions include shifting substrate utilization away from carbohydrates and towards fats and proteins. Cortisol stimulates gluconeogenesis (the conversion of non-carbohydrate substrates to carbohydrates), typically of proteins, as well as deaminiation of amino acids by the liver (this involves removing the nitrogen group of the amino acid, and then using the remaining molecule for energy). Further, it suppresses the inflammatory response through degradation of white blood cells.

Collectively, catecholamines act to increase blood pressure, blood glucose, heart rate, respiration, and perspiration. Thyroid hormones elevates metabolism, respiratory rates, internal temperatures, oxygen consumption, gastrointestinal motility, and cerebration (thoughts).

Vasopressin acts on the kidney to retain water, which increases blood volume; it also is a powerful vasoconstrictor (narrows blood vessel diameter). All of this results in higher blood pressure.

Aldosterone acts on the kidneys to reserve sodium, which further has the effect of reserving water. This also results in increased blood pressure.  

Summary

Stress stimulates the flight-fight response. Through the hypothalamus, SNS, anterior pituitary, and adrenal gland, stress triggers the release of various hormones that have the effect of increasing heart rate, blood volume, blood pressure, energy liberation for fuel, and many other effects all of which prepare the organism for action.

©2005 American Author, a division of Cevado Technologies. All rights reserved.