McKenna's Pharmacology for Nursing, 2e

599

C H A P T E R 3 9 Introduction to the reproductive system

Fertilisation of the ovum and implantation in the uterine wall results in the production of human chori- onic gonadotropin (HCG). This hormone stimulates the corpus luteum to continue to produce oestrogen and progesterone until the placenta develops and becomes functional, producing these hormones at a level high enough to sustain the pregnancy. If pregnancy does not occur, the corpus luteum invo- lutes and becomes a white scar on the ovary. This scar is called the corpus albicans. Initially, the rising levels of oestrogen and progesterone produced by the corpus luteum act as a negative feedback system to the hypo- thalamus and the pituitary, stopping the production and secretion of GnRH, FSH and LH. Later in the cycle, the corpus luteum atrophies, the falling levels of oestrogen and progesterone stimulate the hypothalamus to release GnRH and the cycle begins again. Factors influencing control mechanisms Because of its position in the brain, the hypothalamus is influenced by many internal and external factors. For example, high levels of stress can interrupt the reproduc- tive cycle. Tremendous amounts of energy are expended in reproduction, and if the body needs energy for “fight or flight”, the hypothalamus shuts down the reproduc- tive activities, stopping the release of GnRH, which results in no FSH or LH release and no stimulation of the follicles. This saves a tremendous amount of energy in the body, energy that the body will use for fight or flight. In addition to stress, starvation, extreme exercise and emotional problems are all associated with a decrease in reproductive capacity related to the controls of the hypothalamus. The menstrual cycle The cyclical nature of the female sex hormones on the body produces the menstrual cycle . The onset of the menstrual cycle at puberty is called the menarche . Each cycle starts with release of FSH and LH and stimula- tion of the ovarian follicles. For about the next 14 days, the developing follicles release oestrogen into the body. Thus the woman may notice the many effects of oestro- gen, such as breast tenderness and water retention. In addition, oestrogen thins cervical mucosa and increases susceptibility to infections. By about day 14, the oestrogen levels have caused the LH surge and ovulation occurs. The woman experi­ ences increased body temperature, increased appetite, breast tenderness, bloating and abdominal fullness and constipation, among others—the effects associated with progesterone, which is released into the system when the follicle ruptures. The uterus becomes thicker and more vascular as the cycle progresses and develops a prolif- erative endometrium. After ovulation, the lining of the uterus begins to produce glucose and other nutrients

■■ BOX 39.2  Effects of progesterone

Decreased uterine motility (to provide increased chance that implantation can occur) Development of a secretory endometrium (to provide glucose and a rich blood supply for the developing placenta and embryo) Thickened cervical mucus (to protect the developing embryo and keep out bacteria and other pathogens; this is lost at the beginning of labour as the mucous plug) Breast growth (to prepare for breastfeeding) Increased body temperature (a direct hypothalamic response to progesterone, which stimulates metabolism and promotes activities for the developing embryo; this increase in temperature is monitored in the “rhythm method” of birth control to indicate that ovulation has occurred) Increased appetite (this is a direct effect on the satiety centres of the hypothalamus and results in increased nutrients for the developing embryo) Depressed T-cell function (again, this protects the non- self cells of the developing embryo from the immune system) Anti-insulin effect (to generate a higher blood glucose concentration to allow rapid diffusion of glucose to the developing embryo)

CNS

Hypothalamus

GnRH

Anterior pituitary

LH FSH LH

surge

Follicles

Oestrogen

Corpus luteum

Oestrogen Progesterone

the fallopian tube and then into the uterus. The ruptured follicle becomes a functioning endocrine gland called the corpus luteum . It will continue to produce oestrogen and progesterone for 10 to 14 days unless pregnancy occurs. FIGURE 39.2  Interaction of the hypothalamic, pituitary and ovarian hormones that underlies the menstrual cycle. Dotted lines indicate negative feedback surge. CNS, central nervous system; FSH, follicle- stimulating hormone; GnRH, gonadotropin-releasing hormone; LH, luteinising hormone.