The adrenal glands are located along the superior border of each kidney. Because of this location, they are also known as suprarenal glands.
Each gland weighs about 7.5 grams, but can vary according to secretory demand.
Each gland has two parts:
- the medulla
- the cortex
The medulla is considered an extension of the SNS (sympathetic nervous system – or the “fight or flight” system). Therefore, the hormones it secretes are called sympathomimetic hormones.Two hormones are secreted by the medulla:
- makes up 75 to 80 percent of the secretions from the medula;
- increases the heart rate and the force of its contraction;
- dilates the coronary, muscle, kidney, and lung arterioles;
- relaxes muscles in the lungs, digestive system, and the wall of the bladder;
- increases secretion of hormones and digestive enzymes from the pancreas;
- decreases blood sugar levels;
- increases blood fat levels
- makes up 20 to 25 percent of the secretions from the medula;
- increases sweating and thickness of saliva;
- dilates pupils;
- constricts coronary arteries and the smaller arteries (arterioles) in the skin, muscle, kidney, and digestive system;
- constricts the small veins (venules);
- helps move food through the digestive system;
- contracts the sphincter muscles of the anus to regulate bowel movements;
- causes ejaculations in men;
- decreases the secretion of hormones and digestive juices by the pancreas;
- increases alertness, fearfulness, and anxiousness.
Epinephrine and norepinephrine are classified as amines; and, because they contain a chemical group called a catechol group, they are also referred to as catecholamines.
The cortex is comprised of three main tissue regions:
- zona glomerulosa
- zona fasciculata
- zona reticularis
The cortex produces more than two dozen different steroid hormones, which are collectively known as adrenocorticol steroids, or simply steroids.
These hormones are vital, for without the glands, a person would die without supplementation. Measurement must be precise: too much or too little will have a dramatic effect on glucose metabolism.
The cortex secretes three important steroid hormones:
- Glucocorticoids (cortisol, corticosterone, and cortisone)
- control carbohydrate, fat, and protein metabolism;
- convert amino acids into glucose;
- regulate blood glucose levels;
- possess anti-inflammatory action by preventing phospholipid release.
Cortisol (hydrocortisone) is the main glucocorticoid secreted and in greater amounts during stress. Secretion also involves the hypothalamus and the anterior pituitary, as well as the adrenal glands. The hypothalamus secretes CRH (corticotropin-releasing hormone), which then stimulates the anterior pituitary into releasing ACTH (adrenocorticotropic hormone), which then stimulates the adrenal cortex into secreting cortisol. Negative feedback reverses the process to decrease the amounts when the crisis is over.
- Mineralocorticoids are a class of steroid hormones responsible for regulating electrolyte (sodium and potassium) and water levels in the body. They also regulate blood volume and blood pressure.The primary mineralocorticoid is aldosterone which:
- targets the kidney cells, regulating ion composition of the urine;
- helps maintain blood pressure by causing the kidneys to retain water;
- reduces or retains sodium and water losses in sweat glands, salivary glands, and along the digestive tract.
- Androgens are the third steroidal hormones secreted by the cortex. Secreted in small amounts, androgens are responsible for the appropriate gender sex characteristics. In the male, they are called androgens (testosterone). In the female, these hormones are called estrogen (progesterone).