The Male Reproductive System
The male reproductive system consists of testes, glands, sperm ducts, urethra and the penis. The two testes (singular testis) are contained in the scrotal sac (scrotum) and they produce male sex cells (sperm) and make male sex hormones which affect the way a man's body develops. The sperm pass through the sperm ducts and mix with fluids produced by the glands. The fluids provide the sperm cells with nutrients and the mixture of sperm and fluids is called semen. The penis has two functions: to pass urine out of the body and to pass semen into the vagina of the female during sexual intercourse. The urethra is the tube inside the penis that can carry urine or semen. A ring of muscle makes sure that there is no mixing between urine and semen. The penis becomes erect as blood rushes into the erectile tissue.
The Female Reproductive System
The female reproductive system consists of ovaries, egg tubes (fallopian tubes or oviducts), uterus (womb), cervix and vagina. The two ovaries contain hundreds of under developed female sex cells called egg cells or ova. Female have these cells in their bodies from birth, whereas male produce new sperm continually. Each ovary is connected to the uterus by an egg tube (oviduct or fallopian tube). The oviduct is lined with cilia which are tiny hairs on cells. Every 28 days, an egg develops and becomes mature, and is released from one of the ovaries. The cilia waft the egg along inside the oviduct and into the uterus. The uterus is also called the womb and it is a muscular bag with a soft lining. The uterus is where a baby develops until its birth. The cervix is a ring of muscle at the lower end of the uterus, keeping the baby in place while the female is pregnant. The vagina is a muscular tube that leads from the cervix to the outside of the female's body. The outer vagina has folds of skin called labia that meet to form a vulva. The urethra, separate from the vagina, opens into the vulva to pass urine from the body.
Fertilisation
During sexual intercourse sperm pass through the penis from the male into the vagina of the female. Sperm cells travel in semen enter the uterus through the cervix and travel to the egg tubes. If there is an egg in the tubes, fertilisation may take place. Only one sperm breaks through the membrane of the egg and fertilises the egg. Once break through, the tail is left behind and the head of the sperm and the egg nucleus fuses.
The fertilised egg (called a zygote) travels to the uterus and attaches itself to the lining. Here the fertilised egg divides to form a ball of cells called an embryo. The embryo develops into a foetus and finally into a baby. Identical twins are formed if the fertilised egg divides into two parts and each part develops into a baby. They will have the same genes and are either both male or female. Non-identical twins are formed when two eggs are released from the ovary and both are fertilised. They do not have the same genes and can be both male, both female or one male and one female.
Fertilisation can only happen in a very short period in each menstrual cycle. Ovulation is when an egg is released from the ovary and occurs around day 14. Copulation is the act of sexual intercourse. Fertilisation is only possible during the ovulation period and copulation has taken place. Fertilisation is when an egg is fertilised it became a zygote and start cell division to become an embryo in about 4 days with 32 cells. Implantation occurs about a week after fertisilisation when the embryo embed itself into the wall of the uterus.
If fertilisation does not take place, the uterus lining breaks down and is lost as part of the 28 days menstrual cycle.
Embryo and Foetus Development
The embryo/foetus relies upon its mother for oxygen, nutrients (food and water), waste removal and protection. The placenta is responsible for providing oxygen and nutrients, and removing waste substances. It grows into the wall of the uterus and is joined to the embryo/foetus by the umbilical cord. In the placenta the embryo/foetus's blood vessels are close the the mother's but their blood do not mix and the placenta lets substances pass between the two blood supplies: oxygen and nutrients diffuse across the placenta to the embryo/foetus, and waste substances, such as carbon dioxide, diffuse across the placenta to the mother. The embyro/foetus is protected by the uterus and the amniotic fluid, a liquid contained in a bag called the amnion.
About 4 days after fertilisation the fertilised egg has divided into 32 cells and become an embryo. The embryo will have a brain, heart, eyes, ears and legs after a month. At 9 weeks, the body is completely formed and beomes a foetus. The foetus looks much like a baby at 3 months and starts kicking at 5 months. The foetus is vaiable at 7 months (i.e. it has a fair chance of surviving if born) and the baby is fully developed at 39 weeks ready to be born through the vagina. At birth the cervix relaxes and muscles in the wall of the uterus contract, pushing the baby out.
Puberty
The reproductive system of a child is not mature and needs to change as the children develop into adults, so that the system is fully functional. These changes begin between the ages of ten and fifteen and the time when the changes happen is called puberty. The changes happen because of sex hormones produced by the testes in boys and by the ovaries in girls. Some changes happen in boys and girls, while others just happen in boys or girls.
Some changes that happen to both sexes are the growth of underarm and pubic hairs, stronger body odour, increase of growth rate and emotional changes. The time when the physical changes and emotional changes happen is called adolescence.
Changes that happen only to boys are production of sperm cells and a hormone called testosterone by the testes, both testes and penis grow larger, growth of chest and facial hair, voice break (grows deeper) and shoulders get wider. Changes that happed to girls are the production of eggs and a hormone called oestrogen, ovaries start to release egg cells, periods/menstrual cycle starts , breasts develop and hips get wider.
The Menstrual Cycle
From the age of puberty females starts the menstrual cycle preparing the uterus (womb) in case it receives a fertilised egg. If the egg is not fertilised, the egg and uterus lining break down and are lost from the body through the vagina over a period of 3 to 4 days. The menstrual cycle lasts 28 days and it starts at day 1 with the period - bleeding starts as the lining of the uterus breaks down and passed out of the vagina. At day 4, the lining of the uterus starts to build up again - it thickens into a spongy layer full of blood vessels ready for implanation. On Day 14 an egg is released from one of the ovaries and it is the most likely time that a female become pregnant. The uterus lining is maintained awaiting the arrival of a fertilised egg until Day 28, after while the lining breaks down and the menstrual cycle starts again.