Humans reproduce through sexual mode of reproduction where male and female gametes or sex cells are involved.

Male gametes called sperms, and female gametes called eggs or ovum,   fuse together to form zygote. Later,  the zygote will develop into an embryo and later develop into a fetus.

There are certain characteristic features that appear during puberty in both males and females that do not have  a direct reproductive function. They  are called secondary sexual characteristics. Some of these changes are common to both girls and boys and some are not. 

Common Changes in both boys and Girls  at the time of puberty

• Thick Hairs begin to grow in different parts of the body like armpits, pubic region.
• Thinner hair appears on legs, arms and face.
• Skin becomes oily and may lead to growth  acne and pimples.

Changes  in boys at the time of puberty

• Thick hair begins to grow on face.
• Testes and scrotum grows
• Deepening or cracking of voice 
• Penis occasionally becomes enlarged and erect, either in day dreams or night.
• Oil glands secrete more oil making the skin oily. This may lead to pimples and acne.
• Growth of muscles, broad shoulders.

Changes in Girls at the time of puberty

• Breast size begins to increase.
• The body grows taller and curvier, and the hips widen.
• Develops high pitched voice 
• Beginning of menstrual cycle 
• Maturation of female reproductive organs
• Darkening of skin near nipples.

Gametes produced by male and female

The males and females who are sexually matured can produce male and female gametes respeectively. The sperm is the male gamete and the ovum or egg is the female gamete. The fusion of both the gametes result in an offspring. 

Sperm

It is the male reproductive cell or gamete highly specialized for the function of fertilization.

Ovum

It is a female reproductive cell or gamete  released from either of the female reproductive organs, the ovaries, which has the capacity to develop into an individual after fertilization.

Offspring

Offspring is the product of the reproductive process. 

Also Check – 4 Important Significance of Sexual Reproduction 

Human reproductive system 

The actual transfer of germ cells or gametes need special organs for sexual act. The system of organs required for the process of sexual reproduction is called reproductive system. The human reproductive system includes male reproductive system and the female reproductive system. They both are needed for reproduction. The male reproductive system is different from the female reproductive system, both in structure and in function.  

Male Reproductive System

The male gametes or sperms are produced in the male reproductive system. The male reproductive system produces sperm, transferring it to the female reproductive organ. It also  produce and secrete male sex hormones. It contains the external genitals (the penis, testes and the scrotum) and internal parts, including the prostate gland, vas deferens and urethra. 

Parts of the male reproductive system 

Testicles (testes): 

• They are a pair of oval shaped organs located in a pouch called the scrotum. 
• They consist of seminiferous tubules ,where the sperms are produced. 
• They also produce male sex hormone called  testosterone.

Scrotum: 

• It is a loose sac like organ  that hangs below the penis and behind it. 
• It contains testis and holds it in place.
• It is present outside the abdominal cavity. 
• It maintains a temperature that is required for the production of sperm by it.

Epididymis:

• It  is a highly coiled tube that lies at the back of the testes.
• It carries and stores sperm cells that are created in the testes.

Vas deferens: 

•  It is a thick-walled tube joined to the epididymis.
• It connects testes to urethra in order to allow the passage of semen. 

Urethra : 

• It is a tube that extends from the bladder to the external opening at the end of the penis.
• It serves as the common passage for both the urine and sperm. 
• However, it never carries both at the same time. 
• It also secretes seminal fluids and nutrients. 

Prostate Gland : 

• It is a walnut-sized gland that sits in the middle of the pelvis.
• It surrounds the top portion of urethra.
• It produces the fluid secretions that support and nourish the sperm.

Seminal vesicles:

• They are 2 small glands located above the prostate gland.
• They store and make the majority of the fluid that makes up semen.
• It produces alkaline fluid which is rich in fructose and is a major part of seminal fluid. 

Penis : 

• It is a  cylindrical tube which serves as both reproductive organ and an excretory organ. 
• It has spongy tissue which can fill with blood to cause an erection.
• It transfers sperms into the vagina during sexual intercourse. 

Sperms :

•  They are tiny and motile bodies that use their long tail to move through the female  reproductive system . 
• Its function is to move and carry genetic information to the egg and fertilize it. 

Also Check – What are STDS- Sexually transmitted Diseases 

Female reproductive system 

The female gametes or ovum are produced in the female reproductive system. The female reproductive system is made up of internal organs and external structures. The  female reproductive system  is active before, during and after fertilization as well. The female reproductive system provides several functions. They produce ovum or eggs and female sex hormones that maintain the reproductive cycle. 

Parts of the female reproductive system 

Ovaries:

• They are paired oval shaped organs located in the abdominal cavity near the kidneys.
• They  produce and store ovum in them.
• They also produce a female hormone called estrogen and progesterone.

Oviduct/ fallopian tube: 

• These are narrow tubes that are attached to the upper part of the uterus.
•  They connect ovaries with the uterus.
• It has a funnel shaped opening near the ovary.
• They serve as  the site of fertilization .

Uterus: 

•     It is a hollow, pear bag like structure 
• It holds and nourishes a developing fetus.
• Uterus is the site of development for the embryo.

Cervix: 

•  It is the lower and narrower part of the uterus which opens into the vagina. 
• It acts as the door to the female reproductive system.

Vagina: 

• it is a canal that joins the cervix (the lower part of uterus) to the outside of the body. 
• It is also called as birth canal.
• It receives the sperm from the male partner during an intercource. 

Fertilization and post fertilization changes

The process of fusion of male and female gametes resulting in the  formation of a diploid zygote is called fertilization.

The steps of fertilization is as follows:

The zygote that is formed sinks down and gets embedded in the thick lining of the uterus. The embedding of the zygote in the lining of the uterus is called implantation. The zygote will develop and become a foetus. 

Post Fertilization Changes: 

Post fertilization changes or events refers to all the events of sexual reproduction taking place after fertilization , that is after the formation of zygote. Post fertilization basically involves: 

1. Early embryonic development
2. Establishing multicellularity
3. Formation of blastula
4. Formation of embryonic germ layer. 

Pregnancy

The fertilization results in the pregnancy . During pregnancy a small disc-like tissue called placenta is formed which grows between the uterus wall and embryo. 

• Placenta forms finger-like structures called villi towards the embryo which facilitate the exchange of glucose and oxygen between mother and the embryo.
• The developing embryo also generates waste products. These are transferred into the mother’s blood for the purpose of removal .
• Strong rhythmic contractions of the uterus facilitate childbirth. 

How does an embryo get nourishment inside the mothers body ?

The embryo gets nourishment inside the mother’s body through a special tissue called placenta. Placenta is a disc-shaped temporary organ that develops during pregnancy. It forms a connection between the growing fetus and the mother via the umbilical cord. Placenta forms finger-like structures called villi towards the embryo which facilitate the exchange of glucose and oxygen between mother and the embryo.

• Placenta provides oxygen and nutrients to the growing fetus.
• It also provides some antibodies (IgG) present in maternal blood to the embryo.
• It also helps in the removal of carbon-dioxide and other excretory waste products produced by the fetus.

What happens when the egg is not fertilized ?

• When the egg is not fertilized, it lives for about one day. 
• Since the ovary releases one egg every month, the uterus prepares itself for implantation of the fertilized egg by becoming thick and spongy. 
• However, if the fertilization does not occur it is shredded off from the body and the menstrual cycle takes place. 
• This results in the discharge of blood and mucus through the vagina. 
• This cycle takes place once every month. 

Menstrual cycle

The cyclic changes in the lining of the uterus in response to ovarian hormones constitute the menstrual cycle. It is a regular natural change that occurs in the female reproductive system. This results in the discharge of blood and mucus through the vagina. It lasts for about 3-5 days. The average amount of blood loss during menstruation is about 50-60ml.

Also Check – Why does Menstruation occur

It is divided into three phases: 

1. Proliferative phase or follicular phase: lasts for 14 days in a 28 days cycle. 
2. Secretory phase or luteal phase: this phase lasts for about  14 days in a 28 days cycle. 
3. Menstrual phase: this lasts for about 3-5 days.