Earth Science: Humans – Development
Human development begins when two gametes from the parents fuse to form the zygote. A new individual develops from the zygote over the course of about nine months.
After puberty, humans are sexually mature and can reproduce. As the body ages, the efficiency and function of the organs gradually diminishes.
Earth Science: Biology – Humans – Reproductive Organs
Human reproduction is governed by a complex system of chemicals known as hormones. Patterns of behavior and specific male and female characteristics stimulate sexual arousal and lead to fertilization.
Human reproduction is ensured by the sex drive, which can be stimulated by, among other things, sex hormones, specific male or female sexual characteristics, and behavior.
Male reproductive process
Most adult men produce over 250 million sperm cells daily. These cells carry the genetic materials that can fertilize a human egg. In humans—and most other mammals-sperm is produced in testes located outside the body in the scrotum.
Normal body temperature, 98.6°F (37°C), is too warm for sperm production, but the slightly cooler temperature in the scrotum is ideal. The sperm ripen in a long tube called the epididymis.
When ejaculation occurs, the sperm exit the body by passing through the vas deferens and the urethra within the erect penis. The sperm cells are deposited inside the vagina during sex.
Female reproductive process
The female reproductive organs include two ovaries and fallopian tubes, the uterus, vagina, clitoris, and labia. Before birth, about 400,000 eggs develop in the ovaries. These eggs periodically mature after a woman reaches puberty.
Multiple eggs, each covered with several layers of follicle cells for nourishment, develop during the female’s monthly cycle, but generally only one grows to full size and is released into the fallopian tube during ovulation.
If sperm enters the fallopian tube and fertilizes the egg, a pregnancy can occur. As the egg cell travels to the uterus, the remaining follicle forms the corpus luteum and secretes estrogen and progesterone.
These prepare the walls of the uterus either for implantation of a fertilized egg or for breakdown and menstruation.
MENSTRUAL CYCLE
A woman’s menstrual cycle lasts about 28 days. During the first five days of the cycle the lining of the uterus is shed via bleeding (menstruation) if fertilization has not occurred.
The follicular phase lasts from the fifth to the 14th day, during which the pituitary gland releases a follicle-stimulating hormone (FSH) to ripen the follicle around an egg Around the 14th day, the follicle produces estrogen to increase a luteinizing hormone (LH) that triggers the egg’s release (ovulation).
As the egg travels to the uterus, the woman’s body temperature rises. From the 14th to the 28th day, the follicle left by the egg becomes the corpus luteum. It releases progesterone and prepares the lining of the uterus for either a fertilized egg or menstruation.