Reproduction in Humans - Biology Notes - Naba
Keywords:
Ovaries: organs that produce female gametes (eggs)
Oviducts: tubes leading from the ovaries to the uterus; also known as Fallopian tubes.
Uterus: the organ in which a fetus develops before birth; also known as the womb.
Cervix: a narrow opening leading from the uterus to the vagina
Vagina: opening from the uterus to the outside of the body
Testes: organs in which the male gametes (sperm) are made.
Scrotum: the sac that contains testes
Sperm duct: a tube that transports sperm the testis to the urethra
Penis: organ containing the urethra, through which urine and sperm are carried
prostate glands: organ that produces a nutritious fluid in which sperm are transported
Ovulation: the release of an egg from an ovary
Epidymis: part of the testis in which sperm are stored
Flagellum: a long, whip-like ‘tail’ structure found on sperm cells, used for swimming.
Acrosome: a structure containing digestive enzyme, in the head of a sperm cell.
Embryo: the ball of cells that is produced by repeated division of the zygote
Implantation: attachment of the embryo to the lining of the uterus
Amniotic sac: a tough membrane that surrounds a developing fetus in the uterus
Amniotic fluid: liquid secreted by the amniotic sac which supports and protects the fetus.
Placenta: an organ that connects the growing fetus to its mother, in which the blood of the fetus and mother are brought close together so that materials can be exchanged between them.
Fetus: an unborn mammal, in which all organs have been formed
Umbilical cord: a structure containing blood vessels that connects the fetus to the placenta.
Puberty: the time at which sexual maturity is reached
Follicle: a structure within an ovary, in which an egg develops
Menstruation: the loss of the broken down uterus lining through the vagina
FSH: follicle stimulating hormone, a hormone secreted by the pituitary gland which causes a follicle to develop in an ovary
LH: luteinising hormone, a hormone secreted by the pituitary gland that causes ovulation to happen.
Pituitary gland: a small endocrine gland attached to the underside of the brain
Corpus luteum: a structure that develops from the empty follicle after an egg has been released from an ovary; also known as a yellow body
Sexually transmitted infection (STI): a disease caused by pathogens that are transmitted during sexual contact
AIDS: acquired immune deficiency syndrome; a disease caused by HIV, which destroys white blood cells and therefore reduces the ability of the immune system to defend against other pathogens.
HIV: the human immunodeficiency virus.
The reproductive system
The female reproductive organs
The female gametes are called eggs (egg cells) and are produced in the two ovaries.
Each ovary is connected to an oviduct, also called a Fallopian tube, which carries the egg to the uterus.
The uterus, or womb, has thick muscular walls that can stretch to accommodate a developing fetus during pregnancy. At the lower end of the uterus is a narrow opening called the cervix, which leads into the vagina.
In front of the vagina is the opening of the urethra, which comes from the bladder, and behind it is the rectum, so all openings are separate.
The male reproductive organs
The male gametes are called sperm and are made in the two testes, which lie outside the body in the scrotum.
Sperm travel from the testes through the sperm duct, which joins the urethra just below the bladder.
The urethra continues down the penis and carries either urine or sperm at different times. When the sperm ducts meet the urethra, the prostate gland releases a fluid to form semen, which nourishes and transports the sperm. Sperm are stored in the epididymis, and because sperm production is sensitive to heat, the testes are positioned outside the body.
A sperm cell is adapted for movement and fertilisation. It has a flagellum for swimming, an acrosome at the head containing enzymes to digest the egg’s outer layer, and a nucleus; its mid-piece contains many mitochondria to release energy for movement.
Human gametes
Eggs and sperm are haploid cells, each containing 23 chromosomes produced by meiosis.
A girl is born with thousands of partly developed eggs in her ovaries, and these begin to mature once she reaches puberty.
When an egg is fully developed, it is released from the ovary into the funnel of the oviduct in a process called ovulation, which usually happens once a month.
The egg travels along the oviduct with the help of cilia and gentle muscular movements. If the egg is not fertilised within 24 hours, it dies.
Fertilisation
During sexual intercourse, semen is released into the vagina.
The sperm swim through the cervix and uterus into the oviduct, although only a small fraction of the millions of sperm reach the egg.
When a sperm reaches the egg, the enzymes in its acrosome digest the jelly coat, allowing the head to enter the egg’s cytoplasm.
The nuclei of the sperm and egg then fuse, and this is called fertilisation. Immediately afterwards, the jelly coat becomes impenetrable to prevent other sperm from entering.
Implantation
Once fertilisation occurs, the cell formed is the zygote.
It travels down the oviduct while dividing repeatedly, forming a ball of cells called an embryo.
When the embryo reaches the uterus and attaches to its lining, the process is known as implantation.
The placenta
The cells in the embryo continue to divide while being buried in the soft wall of the uterus.
A placenta grows as the embryo grows and connects it to the wall of the uterus.
It has finger-like projections called villi that fit closely to the uterus wall.
It supports the embryo’s life and growth. The placenta is where the substances are exchanged between the mother’s blood and the embryo’s blood.
After 11 weeks, the embryo has developed organs and is called a fetus.
The placenta is joined to the fetus by the umbilical cord. They take the blood from the fetus to the placenta.
The placenta contains capillaries filled with blood.
The mother’s blood does not mix with the fetus’s blood due to the wall of the placenta but are brought close because the wall of the placenta is very thin.
Oxygen and nutrients diffuse across the placenta to the fetus’s blood and are carried through the umbilical cord to the fetus.
Carbon dioxide and excretory products are carried away from the fetus through the mother’s blood.
Toxins like alcohol and carbon monoxide, viruses and pathogens can cross the placenta.
The placenta grows alongside the fetus. The placenta is a flat disc about 12 cm in diameter and 3 cm thick by the time the baby is born.
Amniotic sac
The fetus is surrounded by a strong membrane (amniotic sac) that produces a liquid called the amniotic fluid.
The amniotic fluid helps support the embryo and protect it from injuries. The fetus floats along the fluid and can move its limbs to help it develop correctly.
The fetus is ready to born after nine months and the muscles in the uterus walls start contracting, often breaking the amniotic sac, causing the amniotic fluid to flow out of the vagina.
Testosterone and oestrogen
Testosterone is the main reproductive hormone in men and oestrogen and progesterone are the main for women.
When a person reaches sexual maturity, sperm production begins in a boy and ovulation in a girl.
In boys, testosterone is secreted by the testes. During adoloscence, secondary sexual characteristcs to develop:
Deepening voice
Facial hair
Pubic hair
Broadening hair
Muscular development
In girls, oestrogen is secreted by the ovaries causing:
Breast development
Hips become wider
Pubic hair
The menstrual cycle
Eggs develop in structures called follicles in the ovaries.
An egg is released from the follicle every month.
This causes the lining of the uterus to thicken, ready to receive a zygote.
If the egg is not fertilised, the lining gradually breaks down and is lost through the vagina. This is called menstruation.
It usually lasts for 5 - 10 days and the process repeats.
These events are controlled by the hormones secreted by the pituitary glands, called the FSH and LH
During the beginning of the cycle, following menstruation, FSH and LH are produced.
FSH causes a follicle to develop inside the ovary with an egg inside it. It secrets oestrogen in increasing amounts which causes the uterus to become thicker and supplied with blood.
When the follicle is developed, LH is produced in a great amount that causes ovulation to take place and oestrogen levels to drop.
The follicle from which the egg was developed develops into the corpus luteum that secrets progesterone. Progesterone keeps the lining of the uterus thick and spongy.
Progesterone inhibits the pituitary gland and FSH and LH levels decrease.
The corpus luteum breaks down if the egg is not fertilised, so the progesterone stops being secreted and menstruation occurs.
If the egg is fertilised, the corpus luteum remains in the ovary and progesterone levels stay high.
When the placenta develops, it starts to release progesterone instead of the corpus luteum and does so throughout pregnancy.
These high levels of progesterone causes the FSH to inhibit, so that no more follicles develop.
Sexually transmitted diseases.
STIs are caused by bacteria/viruses that can be passed through one person to another secxaully.
AIDS is caused by HIV.
HIV infects white blood cells and over a period of time, destroys T cells. Several years after infection, the number of T cells are so low, they are unable to fight against the pathogens.
After 10 years of infection, an untreated person may develop symptoms of AIDS and can become vulnerable to pneumonia or even may develop cancer.
Many cells get damaged due to this including brain cells and hence a person with AIDS may die with several illnesses.
Researchers are working on the production of vaccine against HIV infection.
Preventing HIV transmission
HIV can not survive outside the body and wil not be spread through indirect contact. It can be transmitted during sexual contact or through blood contact.
Sexual contact:
HIV can live in the fluid of the vagina, rectum and urethra.
During sexual intercourse, fluids from one partner can come in contact with the other and the virus can be passed this way.
The more sexual partners a person has, the higher percentage it is for them to develop AIDS. Hence, the best way to avoid this is to not have more than one sexual partner or using condoms can also help.
It is important to know that they have HIV.
Blood contact
During blood transfusions, AIDS can be passed and so nowadays, blood is screened for HIV before transfusion.
It can also be passed from from a mother to her child during childbirth.
Blood can also be transferred if they share hypodermic needles.
Through breast feeding
HIV can pass through a woman’s breast milk to her baby and so the Mum must take antiretroviral drugs during pregnancy and breast-feeding.
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