Title: The Reproductive System
Key words: Reproduction, testes, ovaries, sperm, ova, uterus, hormonal cycle, menstruation, gestation, placenta, foetus, lactation
Date: July 2000
Category: 6. The Body
Author: DJE Candlish
The Reproductive System
The function of the reproductive system is the reproduction of the individual and the continuation of the human race. In the female, the system is capable of nurturing the developing foetus until it can exist independently outside the mother's body.
In the male, the reproductive system consists of the testes, epididymis, penis, vas deferens and prostate gland. In the female it consists of the ovaries, fallopian tubes, uterus and vagina.
Although there are many emotional and moral aspects to human sexuality, this section will focus only on the anatomy and physiology involved.
The male reproductive organs
The testes or testicles are oval shaped organs held outside the body in a sac called the scrotum. The testes have an outer membrane of tough connective tissue. Within this they are divided into sections or lobules containing tightly coiled seminiferous tubules. The male gametes or spermatozoa are produced in these tubules. Around the seminiferous tubules are interstitial cells or Leydig cells. These produce the male sex hormone testosterone.
Around 200 million spermatozoa are produced every day. Once mature, these sperms move down the seminiferous tubules to the efferent ducts of the epididymis, at the base of the testicle. The epididymis itself is 5 metres long but is normally tightly coiled. It leads into the spermatic duct which enters the abdomen and eventually leads into the urethra.
The spermatozoa are discharged through the urethra and out of the penis during the sexual act, as part of the semen, a fluid secreted by the seminiferous and prostate glands. This provides protection and nutrition for the spermatozoa as they enter the female reproductive system.
The penis is the copulatory organ for the male, but it is also part of the urinary system, as the urethra also discharges urine from the bladder to the outside. The penis contains erectile tissue, which becomes rigid and inflated with extra blood during sexual arousal. This enables the penis to enter the vagina and deliver the spermatozoa as close as possible to the entrance to the womb, to increase the possibility of fertilisation.
The female reproductive organs
The ovaries are located within the abdomen, in contrast to the testes. Female egg cells or ova are produced in the ovaries, then released into funnel-shaped ducts called Fallopian tubes. These are lined with ciliated epithelium which helps move the egg along towards the uterus.
The uterus is a small, pear shaped muscular organ between the ovaries. The lower end of the uterus has a ring of muscle known as the cervix. This can dilate or contract the opening to the uterus. Beyond the cervix is the vagina, which leads to the vulva or external female sex organs, the labia and clitoris.
The entire female reproductive tract is lined with sensitive tissues and is governed by a complex hormonal cycle. The conditions within the tract are designed to provide an ideal environment for both spermatozoa and eggs and, later for the developing embryo. Unfortunately, the sex act itself and the moist, warm environment of the vagina also provide an ideal breeding ground for bacteria, fungi (thrush) and viruses. As a result, women are far more vulnerable to sexually transmitted diseases and infection of the tract than males.
The gynaecological infections that can result include:
The menstrual cycle
Even in the developing female embryo, there are millions of potential egg cells in the ovaries. After puberty these start to mature. Cells around an ovum start to divide rapidly, forming a fluid filled cavity known as a Graafian follicle. This develops during the first 12-14 days of the menstrual cycle.
At the same time, the ovary produces oestrogen, in response to follicle stimulating hormone (FSH) from the pituitary gland. Oestrogen prepares the uterine lining, increasing the blood supply and the thickness of the lining and stimulating other glandular activity.
Around day 14, ovulation occurs. The ovum is released into the Fallopian tube as the Graafian follicle bursts. The remaining cells rapidly fill the space left by the ova, forming the corpus luteum. This starts to produce progesterone in response to luteinising hormone (LH). This further prepares the uterine lining to receive the ovum if fertilisation takes place. It also begins to prepare breast tissue for lactation (milk production).
When sperm stored in the epididymis are released by the male into the vagina during the sexual act, combined with fluid from the seminal vesicles and prostate gland, they become active and swim over the cervix and uterus to the Fallopian tube where fertilisation can take place. Sperm are attracted to the ovum, eventually surrounding it. They release enzymes that remove the covering of follicle cells and the outer membrane from the ovum.
Eventually, one sperm will penetrate the inner membrane of the ovum, entering the cytoplasm. The nuclei of the male and female gametes now fuse, forming a new individual.
If fertilisation does take place, progesterone continues to be secreted. This maintains the uterine lining, ready for implantation of the fertilised ovum, as it moves down from the Fallopian tube. If fertilisation does not occur, however, within two weeks of ovulation, the corpus luteum breaks down, progesterone production ceases. The lining of the uterus is shed during the menstrual period that follows.
Development of the foetus
After fertilisation, the ovum starts to divide rapidly. It passes down the Fallopian tube to the uterus where it becomes implanted in the lining. Here, the embryo begins to develop into a foetus as cell division continues. A specialised mass of tissue called the placenta develops to secure the foetus and provide it with nutrition for growth.
The placenta secretes hormones, particularly human chorionic gonadotrophin (HCG). This stimulates the corpus luteum to continue secreting oestrogen and progesterone to maintain the endometrium or lining of the uterus.
The placenta extracts oxygen and nutrients from the mother's blood but is not connected directly to it. Adult blood pressure would be too great for an embryo and toxins in the mother's blood could damage it. Instead, a network of capillaries form where the placenta and endometrium meet. Here, gases and nutrients can diffuse from one circulation to the other. The waste products of the foetus are also removed by diffusion into the mother's circulation.
The placenta forms a barrier, protecting the foetus from exposure to harmful substances at this vulnerable stage. Some substances, such as drugs, can cross the barrier so doctors must be very careful when prescribing medication for pregnant women. The women, too, have to be careful not to take any medications or other drugs that might cause damage to their developing child.
The birth process
The development of the human foetus (gestation period) takes about 280 days from fertilisation to birth. The process of giving birth, labour, is triggered by hormones. The corpus luteum degenerates as the foetus grows, so the level of progesterone falls. The swelling or distension of the uterus restricts blood flow and this also inhibits hormone production. Around this time, the pituitary begins to secrete oxytocin.
At a given moment, when hormonal conditions are right, rhythmic contractions of uterus begin and the cervix dilates. The contractions increase until the baby is forced through the birth canal, the cervix and vagina, to the outside world. After the birth the contractions continue long enough to expel the placenta as the 'afterbirth'.
Bleeding may continue from the placental attachment point for some time, while the uterus itself rapidly shrinks to its original size. The mother is at risk from infection during this early post partum period and mortality used to be high. Modern medical practices have reduced this risk.
In the last stages of pregnancy, the mother's breast tissue will have increased in size and started preparing to produce milk. After the birth, most women are able to breastfeed in response to the infant's suckling, which releases oxytocin and stimulates lactation or milk production. Breast milk contains many substances, including antibodies to protect the child, as well as providing the ideal nutrition for healthy growth and development.