Reproduction in humans
Reproduction is the process in which parent species produce new individuals (offspring). In human reproduction the two sexes, the male species produce the male sex cells (sperm or spermatozoa) and the female species produce the female sex cells (ova, singular = ovum)
Male Reproductive System
Sperm are produced in the male reproductive organs the testes (singular = testis). The testes lie outside the abdominal cavity in a special sac called the scrotum. The sperm are then stored in the epididymis until they are mature. The epididymis then leads into a muscular sperm duct (vas deferens). The two sperm ducts, one from each testis, lead into the top of the urethra. The urethra passes through the penis and may conduct either urine or sperm at different times.
Function of testes
- produces the male sex cells known as sperms
- produces male sex hormones called testosterone
Function of the epididymis
- stores sperms until they are mature
Function of the sperm duct
- connects the epididymis with the urethra.
Function of male reproductive glands
- produce the fluid which nourishes the sperm and helps them to swim through. The mixture of this fluid together with the sperm is called semen.
Function of the urethra
- carries urine and semen at different times through the penis
Function of the penis
- carries urine and semen out of the body
- becomes stiff and erect so that copulation may occur and release the sperm into the vagina
The testes produce sperm and the sperm are stored in the epididymis until they are mature. During copulation, the epididymis and sperm ducts contract to force sperm out through the urethra.
The prostate gland and seminal vesicle then add fluid to the sperm. This fluid plus the sperm it contains is called semen, and the ejection of sperm through the penis is called ejaculation.
Female Reproductive System
The eggs (ova) are produced from the female reproductive organs called ovaries. The ovaries are in the lower half of the abdomen, one on each side of the uterus. Close to each ovary is the funnel-shaped opening of the oviduct (fallopian tube), the path through which the ova pass when released from the ovary. The oviducts lead into the uterus or womb. The uterus then leads to the outside through a narrow muscular passage called the vagina. Between the uterus and the vagina is the cervix which is a ring of muscle closing the lower end of the uterus.
Function of the ovaries
- releases a female sex cell (egg or ovum) once every 28 days into the oviduct (fallopian tube)
- produces female sex hormones – oestrogen and progesterone
Function of the oviduct
- to direct the ovum towards the uterus
- fertilisation of the egg usually occurs here
Function of the cervix
- closes and helps keep the foetus until it is ready to be born
Function of the vagina
- opening for the entry of the penis during copulation
- outlet for the birth of baby during delivery
The egg cells (ova) are present in the ovary from the time a female is born. During puberty the egg cells start to mature and are released, one at a time about every 28 days from alternate ovaries. The ovum is released into the funnel of the oviduct. This process is called ovulation. The ovum is wafted down the oviduct by the action of cilia in the lining of the oviduct. If the ovum meets sperm cells in the oviduct, it may be fertilised by one of them.
After being deposited in the vagina, the sperm swim through the cervix and into the uterus by wriggling their tails. They pass through the uterus into the oviduct. Although a single ejaculation may contain over three hundred million sperm, only a few hundred will reach the oviduct and only one will fertilise the ovum.
If there is an ovum in the oviduct, one of the sperm may bump into it and enter the cytoplasm of the ovum. The male nucleus of the sperm then fuse with the female nucleus. This is fertilisation.
Fertilisation is the fusion of the male sex cells and the female sex cells to produce a zygote.
The product of fertilisation of an ovum by a sperm is called a zygote. A zygote will then grow by cell division to produce first an embryo and then a fully formed baby.
Pregnancy and development
The zygote first divides into two cells. Each of these two cells divides again, so producing four cells and so on until it becomes an embryo. The embryo then travels down the oviduct to the uterus where it implants into the lining of the uterus. This process called implantation.
As the embryo grows, the uterus enlarges to contain it. Inside the uterus the embryo becomes enclosed in a fluid-filled sac called the amnion or water sac. The fluid is called amniotic fluid. This protects the embryo from damage.
The oxygen and food needed to keep the embryo alive and growing are obtained from the mother’s blood through the placenta. After 8 weeks, when all the organs are formed, the embryo is called a fetus.
Some of the cells of the embryo, develop into a disc-like structure called the placenta. The placenta becomes closely attached to the lining of the uterus and is attached to the embryo by a tube called the umbilical cord.
After a few weeks of development, the embryo’s heart is developed enough to circulate blood through the umbilical cord and placenta as well as through its own tissues.
Functions of the placenta
- Provides nutrients (glucose, amino acids and mineral salts) and oxygen for the embryo.
- Removes waste materials such as urea and carbon dioxide.
- Allows antibodies to diffuse from maternal blood into embryonic blood.
- Provides a barrier preventing the mother’s blood and the embryo’s blood from mixing because maternal blood pressure is much higher than embryonic blood pressure and would damage vital tissues.