The reproductive systems

Name the two types of human gametes, state in which organs each is produced, and cite one observable difference between them.

Explain what gametes are and how they differ from the rest of the body's cells in terms of their function.

Look at the compared characteristics of the two human gametes:

• Sperm cell: length of about 0.05 mm, no nutrient reserves, motile via flagellum, continuous production of about 100 million per day.

• Egg cell: diameter of about 0.15 mm (a volume hundreds of times greater), non-motile, loaded with nutrient reserves, maturation of 1 every 28 days.

1. a) Justify why the sperm cell requires such a light design and such a massive production, relating it to the physical obstacles and the journey it faces within the female reproductive system.
2. b) Explain why it would be biologically unfeasible and detrimental for a woman to mature millions of egg cells at once, taking into account the energy cost of producing reserves and the type of mammalian gestation.

In the image of the female reproductive system, four structures have been marked with numbers. Write the name of the marked structures and state in which of them egg cells (ova) are formed.

Name in order the structures of the female reproductive system through which the egg cell (ovum) passes from where it is formed until it reaches the uterus.

Explain what function the Fallopian tubes have and reason why they are not the right place for a pregnancy to develop.

Explain where fertilisation usually takes place in the female reproductive system and what path the zygote takes afterwards until it becomes implanted in the place where it will develop.

After ejaculation, sperm must travel a long way inside the female reproductive system. Describe this journey and explain why, of the millions that set out, only a few hundred reach the vicinity of the egg cell.

Explain the difference between the vulva and the vagina, and reason why knowing the correct names of the parts of one's own body helps to look after health.

A woman goes to the emergency department with acute abdominal pain at 7 weeks of pregnancy. The ultrasound confirms that the embryo is not in the uterus, but has implanted in the wall of one of the Fallopian tubes (tubal ectopic pregnancy).

1. a) Knowing that the embryo normally takes between 5 and 7 days to travel from the site of fertilization to the uterus propelled by cilia and muscular contractions, deduce what functional or anatomical failure in the tube could have caused it to become trapped there.
2. b) Compare the structure and distensibility of the uterine myometrium with the thin muscular layer of the Fallopian tube. Based on this comparison, justify why this pregnancy is non-viable and represents a surgical emergency due to the risk of rupture and severe hemorrhage.

In the cross-sectional image of the male reproductive system, four structures have been marked with numbers. Write the name of the marked structures and describe the path sperm cells travel from where they are formed in the testes until they leave the body, naming the structures they pass through.

Explain what semen is and which glands, besides the testes, provide the fluids that form it.

The testes are located outside the abdominal cavity, in the scrotum, because sperm production requires a temperature slightly lower than that inside the body. Reason what might happen to sperm production if the testes were always kept at internal body temperature.

With age, the prostate can grow and make it difficult for urine to leave the body. Taking into account the anatomy of the male reproductive and urinary systems, explain why a reproductive gland affects urination.

The complete formation and maturation process of a sperm cell (spermatogenesis) lasts between 65 and 75 days in humans. Analyze the following case:

• February 1: A man experiences four days of very high fever (above 39 °C) due to a viral illness. The excessive heat destroys a large portion of the cells that were forming in his seminiferous tubules.

• February 10: A semen analysis is performed, and the results are completely normal.

• April 15: The semen analysis is repeated, and the count shows a drastic drop in sperm count and a high proportion of abnormal forms.

1. a) Justify why the thermal damage from the fever did not manifest in the February analysis and took more than two months to become visible.
2. b) If this man wishes to restore the quality of his semen to seek a pregnancy, when would his semen analysis be expected to return to optimal values? Explain your reasoning.

State whether these statements are true or false and justify your answer: a) the ovaries produce egg cells and sex hormones; b) the male urethra only carries urine; c) the embryo implants and develops in the uterus.

In addition to producing gametes, the ovaries and testes produce sex hormones. Explain what type of organs ovaries and testes are, therefore, and state two changes that these hormones cause during puberty.

The female reproductive system works in a cyclic way and the male one continuously. Relate this difference to the way gametes mature in each case, and explain what consequence it has for the fertile days of each cycle.

During childhood, the reproductive hormonal axis remains "dormant". Upon reaching puberty, the pituitary gland begins to release gonadotropins, which induce the testes to secrete testosterone and the ovaries to secrete estrogens and progesterone.

1. a) Imagine an adolescent whose ovaries or testes respond normally to hormonal signals, but the cells in the rest of their body lack functional receptors for estrogens or testosterone. Predict what will happen to the maturation of their gametes and what will happen to the appearance of their secondary sexual characteristics (hair, breasts, voice change).
2. b) Explain, from the point of view of the body's energy expenditure, why puberty is triggered in the second decade of life rather than during early childhood.

Analyse the following comparative table between two vertebrate species:

1. a) Justify why internal fertilisation was an essential adaptation for vertebrates to colonise the terrestrial environment away from water, explaining what would happen to human gametes if they were released into the open air.
2. b) Explain the energetic trade-off shown in the table: why does the human species not become extinct while producing so few female gametes, and why, on the other hand, would the trout quickly become extinct if it adopted the same production rate as women?

Barrier contraceptive methods, such as the condom, prevent sperm from reaching the uterus. Explain at which point of the journey they act and what additional protection the condom offers compared with other methods.

A classmate holds two mistaken ideas: "After a vasectomy, a man can no longer ejaculate semen and loses his male characteristics because he has no testosterone," and "a woman with tubal ligation stops having periods because the egg does not reach the uterus."

1. a) Reasonably refute the statement about the man: explain where most of the volume of ejaculated semen comes from and clarify why a vasectomy does not prevent testosterone from reaching the blood or alter its effects.
2. b) Refute the statement about the woman: explain why she continues to have a menstrual cycle and ovulate normally despite having her fallopian tubes severed.

A couple with fertility issues turns to in vitro fertilization (IVF). In the laboratory, 6 oocytes are successfully fertilized with the partner's sperm and, after observing that the embryos divide well for three days, the doctors transfer the best-looking embryo into the maternal uterus. However, two weeks later, the pregnancy test is negative.

1. a) State precisely which natural physiological processes were artificially replaced in the laboratory dish and which fundamental biological process depended exclusively on the uterus and failed to be successfully completed.
2. b) Propose two independent biological hypotheses explaining why the technique might fail despite having transferred a living and apparently healthy embryo.

A couple is unable to achieve pregnancy after two years of trying. The man's semen analysis shows healthy, motile, and abundant sperm, but medical tests reveal that the woman has a complete and irreversible obstruction in both Fallopian tubes due to a past infection. The doctor completely rules out intrauterine artificial insemination and proposes turning directly to in vitro fertilization (IVF).

1. a) Following the path traveled by the gametes, justify why artificial insemination (introducing semen with a cannula directly into the uterus) would certainly fail in this patient.
2. b) Explain how IVF bypasses this specific anatomical problem and enables the woman to carry her own child.