Human Reproduction: Anatomy, Life Stages, and Sexual Health

Posted on Oct 1, 2025 in Biology

1. Reproduction and Sexuality Fundamentals

Reproduction is a key function: the birth of new individuals. Sexual reproduction is characterized by two main features:

• The new individual is formed from a single cell, called a zygote.
• A zygote originates from the union (fertilization) of two gametes (sex cells).

While male and female reproductive systems are different, they both have gonads. Humans have two kinds of gametes: sperm and ova.

Key characteristics of human sexual reproduction:

1. Maturation into an adult capable of producing gametes.
2. Internal fertilization, which takes place inside the female reproductive system.
3. The development of the embryo takes place inside the mother’s body (viviparous).
4. During sexual reproduction, genetic information from both parents is united; the new being will not be identical to their parents.

2. Changes During Our Reproductive Life

The stages of reproductive life include puberty, adolescence, and the end of reproductive life.

2.1 Puberty: Onset of Reproductive Capability

Puberty begins in males with the first ejaculation (spermarche) and in females when the first menstruation takes place (menarche). Hormones such as Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) stimulate the production of gametes. Hormone secretion produces anatomical, physiological, and even psychological changes that transform the boy or girl into an adolescent.

2.2 Adolescence: Physical and Psychological Development

Adolescence involves significant physical and psychological changes. Individuals develop their sexuality and become capable of reproducing. In females, hormones (Estrogen and Progesterone) are released from the ovaries. In males, the release of sperm and sexual hormones (Testosterone) occurs in the testicles.

Physical changes include:

• An increase in the size of the sexual organs.
• Pubic and armpit hair begins to grow; males also develop facial hair.
• Females’ breasts develop and their pelvises widen, while males’ pelvises narrow and shoulders broaden.
• The larynx and vocal cords of males thicken, leading to a deeper voice.

2.3 The End of Reproductive Life (Menopause & Andropause)

Ovaries normally stop functioning completely when women reach around 50 years of age. The activity of men’s testicles decreases from that age onwards. These physical and psychological changes are known as the menopause (for women) and the andropause (for men).

3. Anatomy of the Reproductive Systems

3.1 The Male Reproductive System

The male system is formed by the testicles, penis, reproductive tract, and the male accessory glands.

3.1.1 The Testicles

The testicles are the organs that produce male gametes (sperm). The seminiferous tubules contain the cells that produce sperm and male sexual hormones (androgens). The testicles are covered by a layer of skin known as the scrotum.

3.1.2 The Reproductive Tract

• Epididymis: Formed by the seminiferous tubules in each testicle.
• Ductus Deferens (Vas Deferens): Stores sperm that remain there until they exit the body.
• Urethra: Connects the bladder to the exterior, allowing the release of both sperm and urine.

3.1.3 The Penis

The penis is the male copulatory organ, used to deposit the sperm.

3.1.4 Accessory Glands

• Seminal Vesicles: Secrete spermatic fluid containing glucose, a nutrient source for sperm.
• Prostate: Secretes prostatic fluid, which consists of substances to protect sperm from the acidity of the vagina.
• Cowper’s Glands (Bulbourethral glands): Secrete a lubricating fluid that facilitates sexual intercourse.

3.2 The Female Reproductive System

The female system is formed by the ovaries and reproductive tracts: the fallopian tubes, the uterus, the vagina, and the vulva.

3.2.1 The Ovaries

The gonads of the female reproductive system are the ovaries. Female gametes (ova) are produced in the ovaries. Ovaries also produce female sex hormones, estrogen and progesterone, which are essential for adequate sexual development and reproductive ability.

3.2.2 The Reproductive Tract

• Fallopian Tubes: These tubes collect the ovum released by the ovaries. This is normally where fertilization occurs.
• Uterus: The lower part is called the cervix. The gestation of a new human being takes place in the uterus.
• Vagina: It secretes lubricating substances. Sperm are placed inside the vagina during sexual intercourse.
• Vulva: The female external genital organ.

4. The Reproductive Process Stages

4.1 Gametogenesis: Sperm and Ova Formation

The formation of ova is known as oogenesis, and the formation of spermatozoa is known as spermatogenesis.

4.1.1 The Ovarian Cycle

The purpose of the ovarian cycle is the formation, maturation, and release of an ovum.

• Follicular Phase: FSH (Follicle-Stimulating Hormone) stimulates the development of one or several follicles.
• Ovulation: The release of an ovum from the ovary, triggered by LH (Luteinizing Hormone).
• Secretory Phase: Once the ovum is released, the remaining follicle forms the corpus luteum, which produces and secretes the hormone progesterone.

4.1.2 The Uterine or Menstrual Cycle

Its purpose is to prepare the uterus to host the embryo in case the ovum is fertilized.

• The Menstrual Phase or Period: Due to a decrease in the ovary’s hormonal secretions, the thick lining (endometrium) of the uterus breaks off and is released to the exterior along with blood from the blood vessels.
• Proliferation Phase: The uterine lining is reconstructed thanks to the action of the estrogens produced by the ovaries.
• Secretory Phase: The uterus reaches its maximum thickness and is prepared to host and nourish the fertilized ovum.

4.2 Fertilization

Fertilization takes place inside the female reproductive system. When the first sperm penetrates the ovum’s membrane, the gametes fuse. A zygote forms.

4.3 Gestation and Birth

4.3.1 Gestation: Embryo and Fetus Development

Gestation refers to the time that occurs between fertilization and birth. Seven days after fertilization, the zygote arrives at the uterus and implants in the endometrium.

During the first weeks, the cell mass is called an embryo. When the embryo develops a human shape, it is known as the fetus. The development of the embryo has three stages:

1. Germinal: After a series of divisions, the zygote transforms into a cell mass known as the morula, which then becomes hollow and creates another structure known as the blastula.
2. Embryonic: Three layers of cells are formed from the blastula: the inner layer (endoderm), a middle layer (mesoderm), and an outer layer (ectoderm).
3. Fetal: All body structures are formed, and the embryo becomes a fetus.

The embryo develops in the interior of a membrane known as the amnion, which fills up with amniotic fluid that protects the fetus from bumps and external pressures. The fetus feeds through the placenta. The fetus is attached to the placenta through the umbilical cord.

4.3.2 The Stages of Birth

Birth typically occurs in three stages:

1. Dilatation of the Cervix: The cervix dilates. The myometrium experiences a series of contractions, pushing the fetus towards the vagina. Oxytocin is responsible for these uterine movements. The amnion ruptures, releasing the amniotic fluid. This is known as breaking water.
2. Fetus Release: The fetus pushes its head out first. If complications arise, the fetus may be extracted via a Caesarean section.
3. Delivery of the Placenta: After the baby is born, the uterus suffers more contractions in order to release the placenta and umbilical cord. This marks the end of the birth process.

The first days after birth, the mother’s breasts produce a very nutritious liquid called colostrum, which is rich in antibodies. From the third day onwards, the mother starts to produce breast milk.

Types of twins:

• Fraternal Twins: Result from two ova that are fertilized by two different sperm (double ovulation). Their sexes can be different, and they look no more alike than siblings from different pregnancies.
• Identical Twins: Result from a single zygote that has divided itself into two cells. They are genetically identical individuals, are the same sex, and possess similar characteristics.

5. Contraceptive Methods

5.1 Natural Contraception Techniques

• Basal Temperature Method: Measures the woman’s body temperature, given that after ovulation (and during the next few days) it increases by 0.2ºC to 0.5ºC.
• Ogino Method (Rhythm Method): Involves observing menstrual cycles for several months and then calculating the approximate day that ovulation will take place.
• Billings Method (Cervical Mucus Method): Estimates the approximate time of ovulation by observing changes in the vaginal discharge from the cervix.
• Symptothermal Method: A combination of the methods described above: basal temperature is measured, cervical mucus is observed, and the fertile days are then calculated.
• Coitus Interruptus (Withdrawal): Removing the penis from the vagina before ejaculation happens in order to avoid semen penetrating it.

5.2 Artificial Contraception Methods

5.2.1 Barrier Methods

These methods stop sperm from reaching the ovum.

• Male Condom: Placed on the erect penis. It collects ejaculated semen.
• Female Condom: Placed inside the vagina and stops sperm from reaching the uterus.
• Diaphragm: A rubber cap that is placed at the entrance of the uterus in order to stop sperm.
• IUD (Intrauterine Device): A plastic and copper device placed in the uterus to stop the implantation of the blastula.

5.2.2 Chemical Methods

• Spermicides: Applied to the vagina, these substances are able to destroy sperm.
• Hormone Contraceptives: Incorporate female sexual hormones in order to alter the ovarian cycle and stop ovulation.
• Morning-After Pill: Contains a high concentration of hormones to prevent implantation of the blastula in the lining of the uterus.
• 5-Day Morning-After Pill: Prevents the nesting of the ovum and alters the endometrium.

5.2.3 Surgical Methods

• Tubal Ligation: The fallopian tubes in females are cut and joined to prevent the ovum from reaching the uterus.
• Vasectomy: The ductus deferens (vas deferens) in males are cut and joined in order for sperm not to be able to reach the semen.

7. Maintaining Reproductive Health and Sexuality

To maintain a healthy reproductive system and sexuality:

• Accept and value yourself. Avoid feeling guilty or embarrassed about your body.
• Respect everyone’s sexual orientation.
• Research the symptoms and consequences of sexually transmitted diseases (STDs). Visit a professional medical center if necessary.
• Use condoms. Avoid having sexual relationships without a condom, especially if you or your partner suffer from a sexually transmitted disease.

Common Sexually Transmitted Diseases (STDs)