Anatomy Fertilisation and sex determination Contraception

Anatomy
The human male reproductive organs (testes) are located in the scrotal sacs hanging beneath the pelvis because the male sex cells (sperm) need to be kept at slightly less than 37°C for viability.

Sperm are produced in the testes under the influence of the male sex hormone testosterone. The sperm travel to the urethra where they are mixed with nutrient-rich fluids from the seminal vesicles and prostate gland, to produce semen.

The urethra in the penis can carry either semen during sexual intercourse or, separately, urine from the bladder.

During sexual intercourse (copulation), the stiffened penis enters the female's vagina and the semen is released (ejaculation). The millions of sperm swim in the seminal fluid high up into the female's oviduct. If no mature egg (ovum) is present, the sperm are simply broken down and absorbed.

Fertilisation and sex determination
If a mature ovum is present, one sperm cell can fertilise the ovum and their nuclei join (fuse) to create the full complement of genetic information needed for the developing new individual. This first cell of the new individual is called a zygote.

Every cell of a human has 23 sets of chromosomes made up of protein and DNA. The DNA determines all the characteristics of the individual. (See Inheritance) One of each chromosome pair is inherited from each parent so that the child carries a mixture of features from both parents. Twenty two pairs are the same in both males and females.These are called autosomes. The 23rd set are called the sex chromosomes. A female carries two identical sex chromosomes called XX. A male carries two different sex chromosomes called XY. Half the sperm cells produced carry the X chromosome (plus the other 22 autosomes) and half a Y chromosome. All the female ova carry an X.

If a sperm cell carrying an X fertilises the ovum then a girl is produced (XX).
If a sperm cell carrying a Y fertilises the ovum then a boy is produced (XY).

Contraception
The reproductive systems are designed to ensure the best conditions for fertilisation and development. However, to prevent having too many children, fertilisation of the ovum can be stopped by:

• preventing ovulation by hormone manipulation, e.g. contraceptive pill.
  
  
• preventing semen entering the vagina by use of a condom, or withdrawal before ejaculation (coitus interruptus).
  
  
• preventing the sperm entering the oviduct, e.g. using a diaphragm which covers the
  opening of the cervix
  
  
• killing the sperm by spermicidal creams introduced into the vagina.
  
  
• preventing sperm and egg meeting by ceasing intercourse for the few days around the time of ovulation when fertilisation is possible (Rhythm method).
  
  
• causing changes in the uterine lining that disrupts the normal environment of the egg by using an intra-uterine device (IUD).
  
  
• sterilisation by blocking off the oviducts so that the eggs cannot get through ("tying the tubes"), or in the male by cutting the sperm ducts so that the sperm cannot reach the urethra (vasectomy).

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