Human Reproductive System and Menstrual Cycle
Male and Female Reproductive Systems
Female Reproductive System
- Gametes are produced and delivered.
- Male and female gametes each carry one ‘half-set’ of genetic information in their nuclei.
- One male and one female gamete join together.
- A zygote is formed when the male and female nuclei fuse. This is fertilization. The zygote has one ‘complete set’ of genetic information.
- The zygote divides many times to produce a large number of cells.
- The embryo is formed when the cells begin to take up the positions they will occupy in the young animal.
The baby is born and grows into a child.
At puberty, children become young adults capable of producing and delivering gametes.
The Menstrual Cycle
The two ovaries take turns releasing a mature female gamete approximately every 28 days. The cycle of producing and releasing mature ova is called the menstrual cycle. The menstrual cycle is a long-term process controlled by several hormones, which:
- Prepare the uterus to receive any fertilized ova.
- Control the development of mature ova.
Hormonal Effects on the Uterine Wall
During the menstrual cycle, the uterine wall undergoes four phases under the influence of two hormones: estrogen and progesterone.
- Menstruation: The uterine lining is shed, accompanied by blood loss. This is a woman’s period, or menstrual phase. It is triggered by a decrease in progesterone concentration. Menstruating women have a high iron requirement to replace lost blood and prevent anemia.
- Repair Phase: Blood vessels grow in the uterine lining, thickening and stabilizing it. This is triggered by an increase in estrogen concentration.
- Receptive Phase: The uterine lining and blood vessels are well-developed. If fertilization has occurred, the embryo can implant. Optimal conditions for implantation last 6-7 days after ovulation, maintained by increasing progesterone concentration.
- Premenstrual Phase: The uterine lining degenerates as progesterone concentration falls, unless implantation has occurred. In that case, progesterone maintains the lining to support pregnancy.
Ovulation occurs at peak estrogen concentration and is triggered by a pituitary gland hormone.
Hormonal Control of Ova Development
Ova develop from cells lining the ovary, triggered by follicle-stimulating hormone (FSH) from the pituitary gland. FSH causes a follicle to form around a cell in the ovary. When the fluid-filled follicle and ovum are mature, and estrogen concentration is high, it moves to the ovary’s surface and bursts, releasing the ovum into the oviduct (ovulation). The remaining Graafian follicle cells form the corpus luteum, producing progesterone. Progesterone maintains the uterine wall for zygote development if implantation occurs and inhibits FSH secretion via feedback, preventing further ova release. This ensures only one fertilized ovum develops at a time.
Male Reproductive System
Testes
Located in the scrotum, the testes produce testosterone, stimulating sperm production and secondary sex characteristics at puberty.
Prostate Gland
A muscular gland surrounding the urethra’s first inch. Its smooth muscle contracts during ejaculation, contributing to semen expulsion.
Bulbourethral Glands
Located below the prostate, emptying into the urethra. Their alkaline fluid neutralizes vaginal acidity, aiding sperm mobility.
Penis
An external genital organ with the glans penis at the distal end, covered by the prepuce (foreskin). It contains erectile tissue with blood sinuses.
Urethra
The largest part of the urinary tract, passing through the corpus spongiosum and opening at the glans penis tip. It serves for both urine and semen ejaculation.
Seminiferous Tubules
Each testis contains over 100 yards of tightly packed seminiferous tubules, the functional units where spermatogenesis occurs. Sperm move from the tubules to the epididymis for maturation.
Ductus Deferens
Extends from the epididymis through the inguinal canal into the abdominal cavity. Its smooth muscle contracts during ejaculation.
Seminal Vesicles
Paired glands posterior to the bladder, secreting fructose for sperm energy and alkalinity for mobility. Their ducts join the ductus deferens to form the ejaculatory duct.
Ejaculatory Ducts
Two ducts receiving sperm and seminal vesicle secretions, emptying into the urethra.