Secreted Products

Aldosterone 

The primary function of aldosterone is to allow the reabsorbtion of sodium ions in the kidney. As a result of this less water will be excreted by the kidney in order to maintain the concentration of tissues in the body. The net effect of this is an increase in blood volume and therefore, an increase in blood pressure. The release of aldosterone is a well regulated system involving complex feedback. Aldosterone levels are regulated by the renin-angiotensin system.  Blood pressure is detected by baroreceptors, such as those in the aortic arch and carotid sinus. In response to low pressure the liver produces the precursor of angiotensin which is called angiotensinogen. This product is cleaved by renin; an enzyme released by the juxtaglomerular apperatus of the kidney. The cleavage results in the formation of angiotensin I. This is further modified by angiotensin converting enzyme (ACE). ACE is primarily produced by the vascular endothelium of the lung. ACE is able to convert angiotensin I into angiotensin II. The presence of angiotensin II stimulates the release of aldosterone from the suprarenal gland. Aldosterone is also released when low sodium levels are detected. As well as this, activation of the sympathetic nervous system can result in aldosterone release, which is involved in the fight and flight response. Summary of the Renin-Angiotensin system

Cortisol

Cortisol can be found in the blood under normal circumstances, with its concentration varying throughout the day. Generally the highest levels of cortisol are found in early morning, and lowest concentration during early evening. However, in response to the presence of a stressor, the concentration of cortisol in the body can be increased rapidly. The effects of cortisol are wide ranging, and generally act to maintain or restore homeostasis when the body is reacting to stress. Primarily cortisol acts to increase the concentration of sugar in the blood which may be required during the fight or flight reaction. Cortisol affects the liver, and stimulates glycogenolysis. In addition gluconeogenesis occurs. To support this, levels of circulating amino acids are increased. The increase of amino acids occurs due to a decreased ability of muscles to take up amino acids, and inhibition of formation of proteins. Cortisol also acts to impair the formation of bones, and reduces the activity of the immune response and inflammatory response, which in the short term are favourable in the fight or flight reaction, however long term exposure can have serious negative effects. Cortisol has been shown to increase the sensitivity of cells to both adrenalin and noradrenalin.

Due to the diverse effects of cortisol, its release must be carefully monitored. As a result cortisol negatively affects its own release in a negative feedback loop. The release of cortisol is stimulated by hypothalamic activity, which can be induced by stress. Such stresses are identified by the cerebral cortex. When triggered, the hypothalamus releases corticotropin-releasing hormone (CRH). CRH stimulates the anterior pituitary to release Adrenocorticotropic hormone (ACTH) which in turn causes the release of cortisol from the suprarenal gland. In normal conditions, cortisol downregulates the production of both CRH and ACTH, thereby inhibiting its own release. This is summarized by the diagram below. ACTH is also responsible for increasing levels of androgens secreted by the suprarenal gland.

image adapted from wikicommons found under the creative commons licence

Androgens

In a normal suprarenal gland both the Zona Fasiculata and the Zona Reticularis secrete androgens (also known as sex steroids), as well as cortisol. These androgens include testosterone. The levels of androgens secreted by the adrenal cortex are generally too low to cause any great changes in the male adult, as much greater quantities of testosterone are exuded by the testes. However some disorders can lead to excess androgen production, these tend to particularly affect women. The result of this can lead to excess facial hair growth and amenorrhoea in women, along with behavioral changes.   

Adrenalin

Adrenalin is a hormone which induces many different reactions in many different tissues. It is able to do this, as it can act via many different subtypes of receptor. There are five main classifications of adrenergic receptor. These are α₁, α2, β1, β2 and β₃. These subtypes are able to elicit different responses, and are expressed on specific tissues. The primary responses to activation of each receptor type can be seen below.

Each of these effects is crucial to survival, particularly when under stresses such as those induced by a fight or flight reaction. Contraction of smooth muscle occurs in organ systems which are not essential for survival when under stress. By reducing blood flow to organ systems (such as the gastrointestinal tract and reproductive system) which are not crucial to survival, more blood can be redirected to systems involved in fight or flight (such as the skeletal muscle). To add to this effect, increased heart rate and contractility allows greater perfusion to areas which require blood. As metabolism increases the body would require more oxygen and glucose to meet respiratory demands. As such, bronchi dilate to increase ventilation and gluconeogenesis is stimulated.

All of these effects are stimulated by adrenalin in order to allow the body to react to stressful situations.

Noradrenalin 

Noradrenalin acts as a hormone in a similar way to adrenalin, via the same receptors. However noradrenalin binds to these receptors with different affinities. As a result, the action of noradrenalin leads to much greater increase in vasoconstriction in the gastrointestinal (GI) tract then adrenalin. As a result of this, peripheral resistance can increase to a point where blood pressure can become dangerously high. In order to compensate for this the body will decrease heart rate. Noradrenalin is unable to bind to β2 and β₃ receptors.

As well as its role as a hormone, noradrenalin can act as a neurotransmitter in the central nervous system. As a neurotransmitter noradrenalin found in a large proportion of the brain, particularly in areas involved in arousal and alertness. Due to the fact that noradrenalin does act as a neurotransmitter, it is not solely produced in the adrenal medulla. Noradrenalin is also produced by noradrenergic neurons.