Human Reproduction and Development

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Human Reproduction and Development

Human reproduction is the set of complex biological and biochemical processes that allow two human beings of different sexes (male and female) to conceive a new individual belonging to the species, that is, to reproduce.

To do this, humans have a specialized reproductive system in their bodies, which matures and develops throughout puberty and is physically and biochemically distinct depending on the sex of each individual. Therefore, in order to reproduce, two fertile, adult, and fully developed human beings are required.

Human reproduction is sexual: it occurs through the union of germ cells from both parents (called gametes: female eggs and male sperm), which contain half of each individual's genetic information, assigned entirely at random.

These cells are produced in the sexual organs. Thus, when the two halves join together, they create a completely new, unique, and unrepeatable genetic information or genome, which will be that of the new member of the species.

From that moment on, the new human being grows within the mother's womb until it has developed enough to begin living independently. It is then expelled from the uterus through the birth canal, in what we commonly call birth.

The stages of human reproduction are: intercourse, fertilization, pregnancy, and childbirth.

Male Reproductive System

Male members of the human species possess a reproductive system composed of the following organs:

Penis: A cylindrical, external organ, erectile in nature, meaning it can be filled with blood and expand its size several times over, until it reaches a hard consistency, ideal for entering the vagina, in what we know as penetration. Its mission is to deposit the sex cells there, so that fertilization can occur.

The testes: Two large glands located below the penis, also on the outside of the body, and connected to it by a series of ducts. They produce the sex cells, the sperm, which are very active cells and equipped with a flagellum, or a tail for swimming. In addition, the testicles produce testosterone, the male hormone that, during puberty, triggers the physical and organic changes necessary for the male body to mature sexually. As if that weren't enough, that same hormone is responsible for male sexual desire.

Prostate: A walnut-sized gland located very close to the bladder in the male body, whose function is to produce the different compounds that make up semen, a whitish liquid, a mixture of biochemical substances, in which sperm travel and from which they take everything they need for their nutrition and sustenance.

Seminal vesicles: Also called seminal glands, they are located above the prostate, to which they are attached, and are responsible for producing around 60% of the liquid that makes up semen, called seminal fluid.

Seminal ducts and urethra: These are the ducts that connect everything and allow, when the time comes, the semen full of sperm to flow out through the urethra, culminating in the tip of the penis.

Female Reproductive System

The female reproductive system is also composed of several organs, all internal unlike those of men.

Labia majora and minora: These are the folds of skin and mucosa visible to the naked eye from the outside, covering and protecting the entrance to the vagina and the woman's body.

Clitoris: The head or glans of the vulva, located between the labia, is an organ whose sole function is to provide sexual pleasure to women. It has thousands of nerve endings and extends along the labia majora, the perineum, and the lower third of the vagina.

Vagina: The passage that connects the outside of the female body with the entrance to the uterus. It is a muscular region, normally narrow, whose function is to receive the penis and communicate the discharge of semen to the internal regions where fertilization occurs.

Cervix: It is the point where the vagina enters the uterus, located at the end of the vagina. It is a flexible, thin region, approximately three centimeters long.

Uterus: Also called the womb, it is the space where fertilization occurs and the zygote attaches to the walls, giving way to the development of an embryo, that is, what will later become a baby. It is lined by the endometrium, its inner mucosa, which is renewed monthly, thus giving rise to menstruation. The uterus is composed mostly of muscle, is roughly pear-shaped, and its size changes as more space is needed to accommodate the fetus during pregnancy.

Ovaries: There are two of them, one on each side of the uterus, and they are the female equivalent of the testicles. They produce the sex hormones that allow development (estrogen and progesterone, in particular) and also the sex cells that meet with the male cells inside the uterus, the eggs. An egg is released from the ovaries each month and descends to the uterus, where it may or may not be fertilized and therefore may become a zygote, or it may be discarded with menstruation.

The fallopian tubes: Also arranged in pairs, these are the tubes that connect the ovaries and the uterus, through which an egg descends each month.

Intercourse and Fertilization

The encounter between a man and a woman to begin the reproductive process is called coitus or sexual intercourse. Both individuals must be in a state of sexual arousal: the penis must be erect and the vagina lubricated, for penetration to occur easily and painlessly.

There, a series of movements occur that stimulate the abundant nerves of each sexual organ, leading to climax and orgasm, which is a set of intense sensations of pleasure. During orgasm, semen is expelled from the man's body, producing ejaculation.

Sperm travel in the semen and, aided by the vaginal contractions of climax, enter the uterus and are received by the waiting egg. Only one of the millions of sperm present in a single ejaculation enters the egg, thus fertilization occurs, which is the beginning of reproduction.

Eventually, the same egg can be fertilized by two sperm, thus producing a twin pregnancy.

Pregnancy

Pregnancy is the stage during which the fertilized egg, now called a zygote, begins to grow. From the beginning, it subdivides rapidly, going through different stages of complexity, such as the morula, blastula, or gastrula, through continuous processes of mitosis.

Eventually, the set of cells is sufficient to begin a delicate process of specialization, until an embryo, that is, a potential human being, is formed.

Embryos are composed of three layers of cells: endoderm, ectoderm, and mesoderm, each of which will give rise to different parts of the new individual's human body over the nine months of pregnancy.

Throughout this stage, menstruation in the female body stops and the swollen abdomen begins, as more and more space is required for the embryo, which from a certain point onwards will take on a defined human form and be called a fetus.

Throughout this process, the embryo is kept alive by the mother's body through the umbilical cord. This affects the mother's metabolism, and as the pregnancy nears term, she prepares her body for birth and the beginning of lactation, filling her breasts with milk to nourish the newborn.

Childbirth

Pregnancy ideally ends with childbirth: the expulsion of the newborn through the birth canal, that is, through the vagina, which is capable of widening and readjusting to allow passage.

This process can be more or less painful, and more or less rapid, and culminates with the exit of the baby and subsequently the sac that surrounded it inside the uterus, called the placenta. Once the umbilical cord is cut, the new human being will have taken the first step toward an independent and totally new life outside the maternal body.

Activity: Answer the following questions

Remember to review the answers to the open-ended questions at the bottom of this page.

1. What do the testicles produce besides sperm?

2. Where does fertilization normally occur?

3. Embryos are made up of three layers of cells. What are their names?

4. What happens during birth?

5. What female organ is covered by the endometrium?

Una vez que des click a este botón, los reactivos se cerraran y no podrás cambiar tu respuesta.

6. ¿Que son los gametos?

7. ¿Cuál es la función principal de los ovarios?

8. ¿Qué diferencia principal existe entre el aparato reproductor masculino y el femenino en cuanto a localización?

9. ¿Qué función cumple el cordón umbilical durante el embarazo?

10. ¿Qué ocurre si un óvulo no es fecundado?

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Answers to open-ended questions

6. These are the sex cells (sperm and eggs) that contain half of the genetic information of each parent.

8. The male sex hormone is primarily external, while the female sex hormone is internal.

9. It connects the embryo/fetus to the mother and allows it to receive nutrients and oxygen.

1. Equipo editorial Etecé. (2021, 16 julio). Reproducción Humana - Información, aparato reproductor, etapas y más. Concepto. https://concepto.de/reproduccion-humana/ https://concepto.de/reproduccion-humana/

2. Harrison, & J, R. (2025, 31 julio). Human reproductive system | Definition, Diagram & Facts. Encyclopedia Britannica. https://www.britannica.com/science/human-reproductive-system https://www.britannica.com/science/human-reproductive-system

6. CrashCourse. (2015, 23 noviembre). Reproductive System, Part 4 - Pregnancy & Development: Crash course Anatomy & Physiology #43 [Vídeo]. YouTube. https://www.youtube.com/watch?v=BtsSbZ85yiQ https://www.youtube.com/watch?v=BtsSbZ85yiQ