The endocrine and nervous systems are two major systems involved in the control and regulation of body functions. The endocrine system consists of endocrine glands and endocrine cells in other organs (kidney, placenta, etc.) that secrete biologically active substances called hormones. In contrast to exocrine glands, which have secretory ducts, the endocrine glands are “ductless” and secrete hormones into the blood. Hormones are distant chemical messengers that are transported via the circulating blood to the target cells where they exert their specific effects. Most hormones are derivatives of cholesterol (steroid hormones) or are amino acids (thyroid hormones, adrenaline) and polypeptide derivatives. Depending on the nature and size of the hormone molecule, hormones bind to specific receptors that are either on the surface or in the cytoplasm/the nucleus of target cells, triggering sequences of intracellular changes that produce their physiological effects. The main endocrine glands in the body are the pituitary (hypophysis), thyroid, parathyroid, and adrenal glands, the pancreas, ovaries, and testes. The rate of hormone secretion is often regulated by a homeostatic negative feedback control mechanism. The major hormones and their effects are presented in tables (see picture box).
Male and female reproductive systems
The differences between male and female organisms depend primarily on the sex chromosomes (XY or XX) and a pair of endocrine structures, the testes in the male and the ovaries in the female. The differentiation of gonads in utero is genetically determined, but the formation of male genitalia depends upon the presence of testes, secreting testosterone, and if there is no testicular tissue the development is female. After birth the gonads remain inactive until adolescence when they are activated by gonadotrophins (LH and FSH) from the anterior pituitary. In adolescence, as a result of the action of the gonads-secreted hormones, the secondary sexual characteristics develop and the final maturation of the reproductive system occurs, including mammary development and onset of menstrual cycles in the female.
In both sexes, the gonads have gametogenic (production of germ cells) and endocrine (secretion of sex hormones) functions. Steroid hormones secreted by the gonads (androgens, principally testosterone, from testes and estrogen and progesterone from ovaries) promote the sex-specific physical characteristics and initiate and maintain reproductive function. Androgens are steroid hormones with masculinizing effects and estrogens are steroid hormones with feminizing effects. Both types of sex hormones are normally secreted in males and females, but there are general differences in hormone concentrations between sexes. There is a slow decline in the functions of male reproductive system with advancing age, but the ability to father children persists. The functions of female reproductive system regress after a number of years and the menstrual cycles cease (menopause).
Note: Some hormones reach the target cells by extracellular fluid and have local, paracrine effects (prostaglandins, endothelins, etc.).
Male and female reproductive systems
The differences between male and female organisms depend primarily on the sex chromosomes (XY or XX) and a pair of endocrine structures, the testes in the male and the ovaries in the female. The differentiation of gonads in utero is genetically determined, but the formation of male genitalia depends upon the presence of testes, secreting testosterone, and if there is no testicular tissue the development is female. After birth the gonads remain inactive until adolescence when they are activated by gonadotrophins (LH and FSH) from the anterior pituitary. In adolescence, as a result of the action of the gonads-secreted hormones, the secondary sexual characteristics develop and the final maturation of the reproductive system occurs, including mammary development and onset of menstrual cycles in the female.
In both sexes, the gonads have gametogenic (production of germ cells) and endocrine (secretion of sex hormones) functions. Steroid hormones secreted by the gonads (androgens, principally testosterone, from testes and estrogen and progesterone from ovaries) promote the sex-specific physical characteristics and initiate and maintain reproductive function. Androgens are steroid hormones with masculinizing effects and estrogens are steroid hormones with feminizing effects. Both types of sex hormones are normally secreted in males and females, but there are general differences in hormone concentrations between sexes. There is a slow decline in the functions of male reproductive system with advancing age, but the ability to father children persists. The functions of female reproductive system regress after a number of years and the menstrual cycles cease (menopause).
Crosstalk:
Reproductive and endocrine system
In man, acute alcohol ingestion in small amounts may improve erections and increase libido as alcohol has vasodilatory and anxiety-suppressing effects; however, large amounts can cause central sedation, decreased libido and transient erectile dysfunction. Studies have found that moderate or heavy alcohol consumption results in reduced testosterone levels in the blood. In the pituitary gland, alcohol can decrease the production and release of LH and FSH. It has also a direct inhibitory effect on the testosterone secretion by Leydig cells. In addition, alcohol may increase the aromatization of testosterone and androstenedione into estrogens in peripheral tissues. Spermatogenesis is impaired as a result of decreased FSH and testosterone levels, and the direct toxic effect of alcohol or its metabolites on Sertoli cells. Chronic alcoholism may cause feminization, hypogonadism and polyneuropathy, which can additionally affect penile nerve function. Impotence, testicular atrophy, loss of libido, infertility, reduced male secondary sexual characteristics and gynecomastia (as a result of elevated estrogen levels) are observed in almost all chronic male alcoholics.
Acute alcohol ingestion in higher doses can decrease sexual functioning in women by interfering with normal processes of sexual stimulation, and blocking orgasmic response. In a minority of women, moderate alcohol intake has been shown to result in anovulation. In some female heavy drinkers, an increase in the serum prolactin concentration and serum levels of adrenal androgens has been noted. Maternal gestational alcoholism is associated with fetal alcohol syndrome in the baby.
Another alcohol effect on the endocrine function is a decrease of ADH release, leading to water diuresis. Acute ingestion of alcohol may increase serum cortisol in normal subjects. Increased glucocorticoid secretion and pseudo-Cushing’s syndrome have been reported in alcoholics. Acute administration of ethanol to normal subjects results in decrements in serum levels of ionized calcium and parathyroid hormone and augments urinary calcium and magnesium excretion increasing the risk of osteoporosis and fractures in alcoholics. Alcohol by itself may cause hypoglycemia. Alcoholic hypoglycemia can occur in otherwise normal people on an alcohol binge. In patients with alcoholic cirrhosis, reduced peripheral conversion of T4 to T3 and increased aldosterone concentration with fluid retention has also been observed.
Acute alcohol ingestion in higher doses can decrease sexual functioning in women by interfering with normal processes of sexual stimulation, and blocking orgasmic response. In a minority of women, moderate alcohol intake has been shown to result in anovulation. In some female heavy drinkers, an increase in the serum prolactin concentration and serum levels of adrenal androgens has been noted. Maternal gestational alcoholism is associated with fetal alcohol syndrome in the baby.
Another alcohol effect on the endocrine function is a decrease of ADH release, leading to water diuresis. Acute ingestion of alcohol may increase serum cortisol in normal subjects. Increased glucocorticoid secretion and pseudo-Cushing’s syndrome have been reported in alcoholics. Acute administration of ethanol to normal subjects results in decrements in serum levels of ionized calcium and parathyroid hormone and augments urinary calcium and magnesium excretion increasing the risk of osteoporosis and fractures in alcoholics. Alcohol by itself may cause hypoglycemia. Alcoholic hypoglycemia can occur in otherwise normal people on an alcohol binge. In patients with alcoholic cirrhosis, reduced peripheral conversion of T4 to T3 and increased aldosterone concentration with fluid retention has also been observed.
