Written by:

Dr Aynsley Cresswell


Dr Cresswell is a general practitioner in the North East having recently moved to Yorkshire from Oxford. Aynsley has a special interest in women's and sexual health, and she has gained diplomas in both of these specialist areas. She has a keen interest in teaching and also enjoys caring for patients with long-term health conditions in her day-today practice.

The Reproductive System

In this article:

  • Essential human biology (male and female)
  • The female reproductive system
  • The male reproductive system

The Reproductive System


Essential human biology (male and female)

Whether you are already trying to get pregnant or have just made the big decision, it’s a good idea to refresh your understanding of the female and male reproductive systems. Here we will cover the essential basics to help you understand what the reproductive organs are and how they work

The female reproductive system

The female reproductive system

The female reproductive system is actually made up of internal and external reproductive organs. The external female reproductive organs (those outside the body) include the labia majora and minora, the mons pubis, the clitoris, the urethral opening (where urine passes out) and the perineum (the area of skin between the vaginal opening and the anus). The internal female reproductive organs are all found within the pelvis (see diagram above). Together the reproductive organs work to receive sperm from a man, to allow an egg to be fertilised and to carry a pregnancy.


Females have two ovaries, one on each side of the uterus (womb). Did you know that before you were even born, your ovaries already contained all the eggs you would ever produce? At birth, women have about 2 million eggs in each ovary. Most of these don’t make it to puberty, at which point there are only 400,000 and 500,000 remaining. We don’t use all of these eggs and actually only 400 to 500 mature and are released at ovulation – think one egg each month for the duration of your reproductive years (see menstrual cycle). As a woman gets older, and particularly over the age of 35, the majority of remaining eggs are gone and those that are left may be poorer quality (they’ve been around since birth!). This means that although it is still possible, it can take longer and be more difficult to get pregnant naturally. This is a particular concern for women over 40 years. At menopause, all the eggs are gone and periods will stop.


There are also two fallopian tubes on either side of the uterus, one for each ovary. The end closest to the ovary is shaped like a funnel with fringing (called fimbriae) to catch the egg when it is released at ovulation. Inside the tube, there are lots of little hairs called cilia which waft the egg in wave-like motions towards the uterus. Once released, the egg can only survive for 12-24 hours. At this point, any sperm that have made it into the fallopian tube can fertilise the egg. The fallopian tubes are quite long (about 10cm) are very delicate and can be easily damaged resulting in reduced fertility. Possible  causes of damage include endometriosis and pelvic inflammatory disease. In rare instances, the fertilised egg remains in the fallopian tube resulting in a serious condition called ectopic pregnancy.


Each month, the lining of the womb thickens and gets ready for a potential pregnancy. If the egg is fertilised and forms an embryo, it will implant in the lining of the uterus. This is then where the developing baby will spend the next nine months. The uterus is a strong muscle and about the size of a small pear. By month nine of pregnancy, it is more like the size of a watermelon, reaching all the way to the woman’s ribs. At childbirth, this strong muscle contracts rhythmically (“contractions”) to push the baby through the birth canal. If the egg is not fertilised, then the lining of the uterus sheds as a period.


The cervix is effectively a passage (about 3-4cm long) between the uterus and the vagina. The lining of the cervix produces a mucus which varies in consistency throughout the menstrual cycle. In the fertile window before ovulation, the mucus becomes thinner and clearer like egg-whites. This makes it easier for sperm to pass through the cervix to reach the fallopian tubes. You can actually track ovulation by checking your cervical mucus regularly.  Even more amazingly, the mucus becomes a thick plug (the mucus plug) in pregnancy to prevent bacteria and infections entering the womb. At the time of delivery, this clever organ dilates to 10cm to create a continuous passage between the uterus and the vagina to deliver the baby.


The vagina is the opening to the internal reproductive organs. It is actually a muscular tube between the outside world and the cervix. It receives the penis during intercourse and acts as a passage for a baby or a period to exit the womb.

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The male reproductive system

The male reproductive system


There are two main parts to the penis: the head and the shaft. The head is covered in a sleeve of skin known as foreskin. Some men have this removed in a surgical procedure called circumcision. Urine and semen both come out of the penis in a single tube called the urethra which runs down the centre of the shaft – though not at the same time! Thankfully, the prostate closes off the bladder during ejaculation so that only semen is released. During an erection, the penis fills with blood causing it to grow longer, widen and harden. The foreskin becomes stretched back so that the head of the penis is exposed.


Within the scrotum (bag of skin) there are two individual testes (collectively known as testicles) which are the factories for sperm production. They hang outside of the body as the lower temperature is better for producing sperm. Unlike women who are born with their eggs, men don’t start producing sperm until puberty and then can continue to make sperm for the rest of their lives, i.e. there is no male equivalent of menopause.

Within each testicle there are very tightly coiled tubes known as the seminiferous tubules where millions of sperm are being produced at any one time. If you uncoiled these tubules it would stretch for a quarter of a mile! It takes a long time to make a mature sperm, about 70-80 days. Therefore, it’s worth thinking about ways to boost sperm production 2-3 months before you even start trying to have a baby. The sperm that are released today are reflective of a man’s lifestyle a few months ago i.e. if they were smoking or if the testicles were overly hot at that time.

The hormone which has the biggest effect on sperm production is testosterone which is also made in the testicles.


Sperm that are nearly ready to be released are stored in the epididymis, a structure that sits around the top and back of each testicle.  After a few weeks here, mature sperm then move on and are stored in the spermatic cord, also known as the vas deferens. These are long tubes that start at the epididymis and will meet with the urethra for ejaculation.


When the time comes, sperm that have been waiting in the sperm ducts pass into the urethra and exit the body. As it travels, fluid made from the seminal vesicles and prostate gland is added to create semen. This helps transport the sperm, keep it healthy and provide energy on the way to the egg. This fluid gives semen its white appearance.

Each ejaculation is approximately 2-4ml of semen, about a teaspoon’s worth. This small amount can contain up to a mighty 300 million sperm! Although initially quite thick, the ejaculate will become waterier inside the vagina and you may even find that some leaks out. Don’t worry, only one sperm is needed to fertilise the egg so there is no reason to lie in bed with your legs in the air.

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