Hormones are the chemical messengers employed by the body to carry specific signals to a particular organ or system. Hormones are produced at various centers located within the body, known as glands.
The hypothalamus is an organ that asserts control over many glandular functions. The hypothalamus is a crucial command center in the brain that is continually engaged in a number of processes that direct responses by the body to address external stimuli such as heat, a need for greater blood volume, or a reaction to stress. The most important of the glands controlled by the hypothalamus is the function of the pituitary gland. The pituitary gland functions both as a generator of hormones for particular physical and chemical response functions as well as those that regulate other glands in their production of hormones. Hormones have a regulatory purpose in relation to human function, as they are intended to act as a prompt, or triggering, mechanism to direct certain systems to act in a particular way. When produced by a gland, hormones are secreted directly into the bloodstream for transport to the desired center. The study and the science of hormone function is known as endocrinology.
Exercise has profound effects on the function of a number of glands and their corresponding impact on a number of systems through the release of specific hormones. The first gland to exhibit a response to exercise is the pituitary gland. This structure produces and secretes a number of different hormones for different purposes, of which the human growth hormone (HGH) is the most important. Exercise will trigger the generation and release of human growth hormone by the pituitary gland to stimulate the body to increase its production of bone, muscle, or connective tissue cells. Human growth hormone was synthesized for use as a muscle-building supplement as a direct result of the desire by some athletes to become bigger and stronger. The pituitary gland also regulates the function of two other glands whose hormones are released in response to exercise—the thyroid gland and the adrenal gland.
The thyroid gland is located at the base of the neck. Once stimulated by the pituitary gland, the thyroid produces two distinct hormones: triiodothyronine (T3) and thyroxine (T4). These hormones are responsible for raising the level of activity in the systems essential for exercise performance. The stimulus provided to the thyroid gland to generate T3 and T4 hormone production and release into the bloodstream creates corresponding increases in the heart rate and blood pressure of the cardiovascular system. These hormones will also influence the thermoregulatory system, particularly with respect to body temperature during exercise. These thyroid hormones also elevate the level of alertness present in the brain and the central nervous system during exercise: the concepts of concentration and reaction time are more focused as a result of the various systems being directed to work at a higher level.
The adrenal glands are constructed as a pair, with a single gland, built with two parts, located above each kidney. The adrenal will produce a number of hormones in response to the receipt of a hormone from the pituitary gland, adrenocorticotropin (ACTH), which signals the adrenal gland to take action. Three hormones are the most important of the chemicals commonly produced by the adrenal gland in response to exercise. The first is cortisol, one of a class of substances known as the glucocorticoids. Cortisol performs a number of functions when it is released into the bloodstream: it causes the blood pressure of the cardiovascular system to rise, it will trigger an increase in the level of glucose in the bloodstream, and it acts as an anti-inflammatory agent.
The second main exercise-induced adrenal hormone is aldosterone, which causes a bodily response to anticipated dehydration, through its impact upon kidney function. Aldosterone causes the kidneys to increase levels of sodium and water retention as a result of a signal that is first sent from the hypothalamus regarding the overall fluid levels of the body. Aldosterone correspondingly produces less urine, while releasing greater amounts of potassium into the renal system. The thirst mechanism, the physical sensation experienced by people that they have a need for water, is slower and much inferior to the internal chemical regulation-monitoring process maintained by the hypothalamus and the related glandular responses; by the time thirst is sensed, the body may be significantly dehydrated.
The third important adrenal hormone is adrenaline, which is produced when the body, through the hypothalamus, determines that it is now exposed to circumstances of excitement or self-defense. This release of adrenaline is often described as the "fight or flight" mechanism. Adrenaline is a stimulant, similar in its chemical construction to ephedrine. The release of adrenaline has the immediate effect of both increasing the strength and the frequency of heart contractions, as well as speeding the breakdown of the stored carbohydrate glycogen into glucose for immediate conversion into muscle energy.
Erythropoietin (EPO) is a protein hormone produced by the kidneys when the body senses that the level of available erythrocytes (red blood cells) is too low to properly transport the oxygen and nutrients required for the production of energy, particularly through the aerobic energy system. Training at high altitude is a common technique to naturally stimulate EPO production. The effects of increased EPO and the correspondingly higher levels of red blood cells within the bloodstream will continue, in diminishing amounts, for between one to three months after the altitude stimulation that created a greater presence of EPO is discontinued.