Human Reproduction

I

Introduction

Human Reproduction, process by which human beings are formed. Like nearly all other animal reproduction, that of humans is described as sexual, that is, it involves a male and a female.

Human reproduction is also said to be viviparous. This means that the new individual undergoes considerable development within the body of the mother and is not born until he or she is capable of surviving outside the mother’s body. Because of this, there are substantial differences in the reproductive structures of females and males.

II

Female Reproductive Organs

The organ in which the new individual grows to birth maturity is called the uterus or womb. This is a thick-walled, muscular bag which is capable not only of stretching, but also of actually growing during pregnancy. In addition, the female’s outlet passage—formerly known as the birth canal—is the vagina, which serves a double purpose in reproduction. Although capable of stretching sufficiently to allow a full-term baby to pass, it is normally much narrower and tighter so as to be suitable for its other purpose—the accommodation of the erect penis, the male sex organ, during sexual intercourse (see below).

The differentiated sex cells, which form the basis of a new individual, are called gametes. The female gametes—the ova (eggs)—are produced in a pair of organs known as ovaries. These are small, solid structures lying one on either side of the womb near the outer walls of the pelvis. Ova are normal body cells that have been modified during their final stages of replication so as to contain only one set of chromosomes, instead of the usual two sets. One ovum is released once a month from the surface of one or other ovary in the process known as ovulation. While it is usual for only one ovum to be released, it is also quite common for more than one to be produced.

Immediately above the ovaries are the Fallopian tubes. The open ends of these narrow tubes curve inwards to connect with the womb; the free ovum is wafted into a Fallopian tube by the movement of tiny, finger-like structures hanging from the edges of the open end of the tube. An ovum takes about four days to travel the full length of the Fallopian tube to reach the womb. This movement is driven by millions of beating microscopic hairs (cilia) that carry it along.

III

Male Reproductive Organs

The male reproductive organs are concerned with the production of sperm (sing. spermatozoon; pl. spermatozoa) and with their delivery into the vagina. Sperm are formed in long, coiled tubes in the testes (testicles). They, too, are normal body cells but with half the normal number of chromosomes—that is only one set. Sperm are active cells each with a head, a neck, and a lashing tail. The chromosomes containing the DNA are in the head and the sperm is propelled by its tail. At the tip of the head is a small sac called an acrosome that contains vital enzymes.

Many millions of sperm are constantly being formed and it is this, rather than their swimming action, that forces them in a kind of slow-moving sludge upwards from the testicles through a pair of tubes each known as the vas deferens (pl. vasa deferentes). The upper end of each vas deferens passes through the prostate gland (which lies under the bladder) and into the urethra—the tube that runs from the bladder through the prostate gland and along the centre of the penis. The urethra also carries urine from the bladder to outside the body. The prostate gland, together with two small adjoining sacs called the seminal vesicles, secretes an alkaline fluid that nourishes the sperm and provides a medium in which they can easily swim. Together with the sperm, this is known as seminal fluid, or semen. Pressure of fluid in the vesicles and upper ends of the vasa deferentes commonly leads to spontaneous orgasm and discharge of semen in young males, usually in the course of an erotic dream (“wet dream”).

IV

Sexual Intercourse

Male sexual arousal usually results in the penis becoming erect (tumescence). This happens under the direct control of the nervous system, which sends messages interpreted by the brain as pleasurable. These cause a widening of the arteries supplying blood to the spongy tissue bodies of the penis (the two corpora cavernosa and the single, central corpus spongiosum). Under the pressure of the blood, these bodies enlarge and stiffen so that the penis becomes relatively rigid. The pressure in the tissues compresses the veins that drain the organ, so preventing loss of the erection. There is often also some secretion of mucus and this appears at the penile opening.

Female sexual arousal also results in an increase in blood supply to the female genitalia, and the sensitive clitoris and other parts become engorged. This excitation also results in activity of two glands near the opening of the vagina that secrete clear mucus. This acts as a lubricant, making it easier for the penis to be inserted into the vagina. Once this is achieved there is an instinctive urge to stimulate the head of the penis and the clitoris by appropriate movements. In most cases, this results, after several minutes but possibly longer, if movements are not continuous, in the male orgasm: a pleasurable succession of spasmodic muscular contractions at the root of the penis that drive out several spurts of seminal fluid. While not necessary for reproduction, the female orgasm involves the same involuntary movements in the genitalia, voluntary muscle movements, and rapid tension release as the male—the main difference being that the male ejaculates semen. While men have only one orgasm, women may have more than one during intercourse, both before the male has had an orgasm or after he has climaxed. Either can, of course, climax without having intercourse.

Intercourse is an intimate form of communication, between two individuals. In humans, as distinct from most other animals, sexual intercourse often involves an emotional relationship and a preliminary period during which this may be established. Even though the improved availability of effective contraception has made sex less of a pregnancy risk, it still incurs both emotional and social responsibilities, particularly in light of the AIDS epidemic. Campaigns encouraging people to have safer sex (always using a condom in penetrative sex) in all instances have increased awareness of these responsibilities.

Semen is deposited high in the vagina and around the opening of the womb, known as the cervix. Within minutes sperm have entered the womb, partly by their own swimming efforts and partly by the action of cilia, as in the Fallopian tubes. If there is an ovum in one of the Fallopian tubes, the female hormones, chiefly oestrogen, will have ensured that the mucus in the cervical canal is watery, making it easy for sperm to pass through. At other times the mucus is stickier and harder to penetrate. Several hundred million sperm are deposited in the vagina but only a few hundred reach the Fallopian tube, and only one passes into the ovum. This happens several hours following intercourse, after which several changes occur in each of the sperm. An outer layer of material is stripped off and the cell membrane is altered so that it becomes capable of fusing with that of the ovum. The movement of the tail also changes to a forward-moving swimming action.