Immunology is one of the Biology Topics focused on understanding the immune system and its response to pathogens and diseases.
Reproduction in Human Beings – Definition, Diagram & Facts
Humans use a sexual mode of reproduction. The organs associated with the process of reproduction in human males (men) and human females (women) are different, so the reproductive systems in males and females are different which are known as the male reproductive system and the female reproductive system, respectively. The reproductive systems in human beings become functional (or start functioning) at a definite age called puberty. We will now describe the human male reproductive system and female reproductive system in detail, one by one.
Male Reproductive System: Organs, Structure & Function
The human male reproductive system consists of the following organs: Testes, Scrotum, Epididymis, Vas deferens (or Sperm duct), Seminal vesicles, Prostrate gland, and Penis. The human male reproductive system is shown in Figure. Since the human male is called man, we can also say that it is the reproductive system of man.
Testes are the oval-shaped organs that lie outside the abdominal cavity of a man (see Figure). A man has two testes (singular of testes is testis). Testes are the primary reproductive organs in men (or males). The function of the testes is to make the male sex cells (or male gametes) called sperm peniS and also to make the male sex hormone called testosterone.
Please note that the testes of a man make the sex gametes (or sperms) from puberty onwards, throughout his life. The testes of a man lie in a small muscular pouch called the scrotum, outside the abdominal cavity (see Figure). The testes are outside the abdominal cavity of the body (and not deep inside the body), because sperm formation requires a lower temperature than the normal body temperature. Being outside the abdominal cavity, the temperature of the scrotum is about 3°C lower than the temperature inside the body. In this way, the testes provide an optimal temperature (most suitable temperature) for the formation of sperms.
The male reproductive system in humans (side view).
The sperms formed in the testes come out and go into a coiled tube called epididymis (see Figure). The sperms get stored temporarily in the epididymis. From the epididymis, the sperms are carried by a long tube called vas deferens (or sperm duct) which joins with another tube called urethra coming from the bladder (see Figure). Along the path of the vas deferens, the glands called seminal vesicles and prostrate glands add their secretions to sperms so that the sperms are now in a liquid. This liquid plus the sperms it contains is called semen (which is a thick liquid). The secretions of seminal vesicles and prostrate glands provide nutrition to the sperms and also make their further transport easier. The urethra forms a common passage for sperm and urine. Urethra carries the sperm to an organ called the penis which opens outside the body. The penis passes the sperm from the man’s body into the vagina in the woman’s body during mating for the purpose of reproduction. Please note that in man (or human male) there is only one opening for the urine and sperm to pass out of the body.
Female Reproductive Organ Anatomy, Parts, and Function
The human female reproductive system consists of the following organs: Ovaries, Oviducts (which are also called Fallopian tubes), Uterus, and Vagina. The human female reproductive system is shown in Figure. Since the human female is called a woman, we can also say that it is the reproductive system of a woman.
Ovaries are the oval-shaped organs that are inside the abdominal cavity of a woman near the kidneys (see Figure). A woman has two ovaries. Ovaries are the primary reproductive organs in a woman (or female). The function of ovaries is to make mature female sex cells (or female gametes) called ‘ova’ or ‘eggs’, and also to make the female sex hormones (called oestrogen and progesterone). Each ovary is composed of several thousand follicles (which are a kind of unripe eggs or unripe ova). At puberty, these follicles mature to form the ripe eggs or ripe ova (required for fertilisation).
Just above the ovaries are the tubes called oviducts (which are also known as fallopian tubes). The oviducts are not directly connected to the ovaries but have funnel-shaped openings that almost cover the ovaries (see Figure). The ovum (or egg cell) released by an ovary goes into the oviduct through its funnel-shaped opening. The fertilisation of an egg (or ovum) by sperm takes place in the oviduct.
The two oviducts connect to a bag-like organ called the uterus (or womb) at their other ends (see Figure). The growth and development of a fertilised ovum (or fertilised egg) into a baby take place in the uterus. The uterus is connected through a narrow opening called the cervix to another tube called the vagina which opens to the outside of the body (see Figure). Vagina receives the penis for putting sperm into the woman’s body. The vagina is a tubular structure. The vagina is also called the ‘birth canal’ because it is through this passage that the baby is born after the completion of development inside the uterus of the mother. Please note that in a woman (or human female) the opening for passing out urine (called urethra) and the vaginal opening are separate.
The female reproductive system in humans (front view).
It is obvious from the above discussion that the female reproductive system in humans is more complex than that of the male reproductive system. The complexity in structure and function of the female reproductive system is necessary for the union of sperm and ovum (or eggs) inside the female body and the development of the baby in the mother’s uterus.
Fertilisation
In human beings, internal fertilisation takes place. The sperms (or male gametes) made in the testes of man are introduced into the vagina of the woman through the penis during copulation (or mating). In this way, millions of sperm are released into the vagina at one time. The sperms are highly active and mobile (moving). The sperms move up through the cervix into the uterus. From the uterus, the sperms pass into the oviducts (see Figure).
Fertilisation in human to form a zygote (fertilised egg).
One of the oviducts contains an ovum (or egg cell) released by the ovary during ovulation. Only one sperm fuses with the ovum (or egg) in the oviduct to form a zygote. This is called fertilisation. Thus, the fertilisation of the ovum (or egg) takes place in the oviduct.
Development of Embryo
When the ovum (or egg) is fertilised in the oviduct, then a zygote is formed. The zygote divides rapidly by mitosis as it moves down slowly in the oviduct and forms a hollow ball of hundreds of cells. This hollow ball of cells now called an embryo, sinks into the soft and thick lining of the uterus and gets embedded in it (see Figure). The embedding of the embryo in the thick lining of the uterus is called implantation.
Implantation of the embryo in the uterus.
After implantation, a disc-like special tissue develops between the uterus wall (called a uterine wall) and the embryo (or foetus), which is called the placenta (see Figure) (The foetus is connected to the placenta in the mother’s body through the umbilical cord). It is through the placenta that all the requirements of the developing foetus like nutrition, respiration, excretion, etc., are met by the mother’s body. In other words, the exchange of nutrients, oxygen, and waste products between the embryo and the mother takes place through the placenta.
Placenta links the embryo to the mother through the umbilical cord. In the placenta, the embryo’s blood vessels are close to the mother’s blood vessels but they are not joined. Because the two sets of blood vessels are close to each other, substances (like oxygen, nutrients, and wastes) can pass between the two blood supplies.
The embryo grows and develops inside the uterus (or womb) of the mother and becomes foetus. This picture shows the side view of a developing foetus a few weeks before birth.
This baby has just been bom. We can see in this picture that the umbilical cord is still attached to the baby. The umbilical cord will be just tied and cut to separate the new bom baby from the mother.
The time period from the fertilisation up to the birth of the baby is called gestation. The average gestation period in humans (or the average duration of human pregnancy) is about nine months (or about 38 weeks). During the gestation period, the foetus grows to become a baby (see Figure). Birth begins when the strong muscles in the walls of the uterus start to contract rhythmically. The rhythmic contraction of uterus muscles gradually pushes the baby out of the mother’s body through the vagina. This is how a baby is born (see Figure). All of us were born from our mother in this way.
Differences between Zygote, Embryo, and Foetus
A zygote is formed after fertilisation. A zygote develops and becomes an embryo. And finally, an embryo develops and becomes a foetus. The main differences between zygote, embryo, and foetus are given below:
| Zygote | Embryo | Foetus |
| 1. A zygote is formed by the fusion of male and female gametes (sperm and egg) | 1. An embryo is formed by the repeated cell division of a zygote. | 1. A foetus is formed by the growth and development of an embryo. |
| 2. A zygote is the beginning of the formation of a baby. | 2. An embryo is an unborn baby in the uterus in the early stages of development (up to 8 weeks after fertilisation) | 2. A foetus is an unborn baby in the uterus in the later stages of development (after 8 weeks till birth). |
| 3. A zygote is a single cell. It is smaller than a full stop (.) | 3. An embryo is multicellular. The body features of a growing baby in the embryo are not much developed. | 3. A foetus is also multicellular. The body features of a developing baby (like hands, legs, head, eyes, ears, etc.) can be identified. |
These pictures show the difference between a human zygote, an embryo, and a foetus.
Sexual Cycle in Females: Menstruation
We will now describe the sexual cycle in human females (or women). Please note that when a girl child is born, her ovaries already contain many thousands of immature ova (or eggs) which are contained in immature follicles. When a girl reaches the age of puberty, then one follicle develops at a time to form a mature ovum (or egg). On maturing, the follicle bursts, and the ovum (or egg) shoots out of the ovary. This is called ovulation. Thus, the release of an ovum (or egg) from an ovary is called ovulation.
In a normal, healthy girl (or woman), ovulation takes place on the 14th day of the beginning of the menstrual cycle of 28 days. This means that ovulation takes place in the middle of the menstrual cycle (because the 14th day is the middle of 28 days). In human females (or girls), the ovaries start releasing the ovum or egg (female gamete) once every 28 days from the age of puberty. That is, in girls, ovulation starts when they attain puberty. Please note that ovulation does not take place every day after puberty. It takes place after a period of every 28 days (which is almost once a month). Before every ovulation, the inner lining of the uterus becomes thick and soft with a lot of blood capillaries (or blood vessels) in it (see Figure).
A thick lining grows in the uterus to receive the fertilised egg cell (if any).
These changes in the uterus are necessary because in case the ovum (or egg) released by the ovary gets fertilised by the sperm, then the uterus has to keep this fertilised ovum (or egg) for further development and supply it with food and oxygen, etc., so that it may grow into a baby in due course of time. If, however, sperm is not available at the time of ovulation, then fertilisation of the ovum (or egg) does not take place. Since the ovum (or egg) is not fertilised, the thick and soft uterus lining having a lot of blood capillaries in it is not required. Thus, the unfertilised ovum (or egg) dies within a day and the uterus lining also breaks down. Since the thick and soft uterus lining contains a lot of blood vessels, the breaking (or disintegration) of the uterus lining produces blood alongwith other tissues. This blood and other tissues come out of the vagina in the form of ‘bleeding’ (see Figure 76).
In case the egg cell is not fertilised, the thick uterus lining breaks down leading to bleeding. This is called menstruation (or periods).
We can now say that the breakdown and removal of the inner, thick, and soft lining of the uterus alongwith its blood vessels in the form of vaginal bleeding is called menstrual flow or menstruation. Menstruation occurs if an ovum (or egg) released by the ovary of a woman does not get fertilised due to the non-availability of sperm at the time of ovulation. Since the process of menstruation in a woman occurs again and again after a fixed period of 28 days (to 30 days), it is also known as the menstrual cycle. Menstruation occurs every 28 days because ovulation (release of ovum or egg by the ovary) occurs every 28 days. In everyday language, menstruation is called ‘periods’. We will now describe the menstruation (or menstrual cycle) point-wise which can be reproduced in the examination.
The sexual cycle in females (or women) is called menstruation or menstrual cycle. This is described below:
- When a girl reaches puberty at the age of about 10 to 12 years, the sex hormones released into her blood cause some of the ova (or egg cells) in her ovaries to become mature (or ripe).
- Usually, one mature ovum (or egg) is released from the ovary into the oviduct once every 28 days. This is called ovulation.
- Before ovulation (or release of ovum), the inner lining of the uterus becomes thick and spongy, and full of tiny blood vessels (or blood capillaries), and prepares itself to receive the fertilised ovum or egg (in case it gets fertilised by sperm).
- If the ovum (or egg) does not get fertilised (due to the non-availability of sperm in the female body) then the thick and soft inner lining of the uterus is no longer needed and hence it breaks. So, the thick and soft inner lining of the uterus alongwith the blood vessels and the dead ovum (or egg) comes out of the vagina in the form of bleeding called menstruation.
- Menstruation usually occurs 14 days after ovulation and usually lasts for about 3 to 5 days.
- After menstruation is over, the inner lining of the uterus starts building up again so that it may become ready to receive the next ovum (or egg) in case it gets fertilised.
- If the ovum (or egg) does not get fertilised even now, then menstruation takes place again. This cycle of menstruation is repeated again and again in women after every 28 days (till the time ovum gets fertilised). The menstrual cycle is controlled by hormones.
Menstruation stops temporarily when the ovum (or egg) gets fertilised and the woman gets pregnant. This is because in this case the thick and soft lining of the uterus containing a lot of blood vessels is needed for the growth and development of the fertilised ovum (or fertilised egg cell) to form a baby. Menstruation restarts after the birth of the baby.
Menarche and Menopause
The first menstruation (or menstrual flow) begins at puberty (when the girl or woman is around 10 to 12 years of age). The first occurrence of menstruation (or periods) at puberty is called menarche. Menarche is the beginning of the reproductive life of a girl (or woman). In other words, menarche is the time from which a girl (or woman) becomes capable of having a baby. Menstruation stops permanently when a woman reaches the age of about 45 to 50 years. With the permanent stoppage of menstruation, a woman loses her ability to bear children. She becomes infertile. The permanent stoppage of menstruation (or periods) in a woman is called menopause. Menopause occurs in women at the age of about 45 to 50 years. A woman stops ovulating at menopause and can no longer become pregnant. Menopause is the end of the reproductive life of a woman. We can now say that the reproductive life of a woman starts at menarche and ends at menopause.