16.4) Sexual reproduction in humans
Male reproductive system:
- Testes: It is a male gland which produces sperms and the male sex hormone testosterone
- Scrotum: it is the sac which contains the testicles
- Sperm Ducts: They are two muscular tubes, each connected to a testis. They carry the sperms from the testis to the urethra
- Prostate Gland: It secretes a nutritive fluid to the sperms to form a mixture called semen
- Urethra: It is a tube inside the penis which is the pathway of semen and urine out of the body
- Penis: It is the male sex organ which ejaculates semen into the vagina during sexual intercourse
- Epididymis: coiled tubes in which sperms are stored
- Seminal vesicle: it is another gland like the prostate gland. It also secretes nutritive fluids for sperms to feed from and swim in forming semen
Female reproductive system:
- The Ovaries: They contain follicles where eggs are produced
- Oviducts (Fallopian Tube): They are two tubes, one on each side connected to an ovary. They are where fertilization occurs and they provide a pathway for the eggs to travel to the uterus by sweeping them by cilia on its walls
- Uterus (Womb) : Where the fetus develops,
- Cervix: A muscular tissue which separates the vagina from the uterus
- Vagina: it receives the male penis during sexual intercourse
Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete, producing a new cell called a zygote. This then matures into an embryo.
- Sperm are much smaller than eggs and are produced in much larger numbers.
- The tip of the cell carries an acrosome, which secretes enzymes capable of digesting a path into an egg cell, through the jelly coat, so the sperm nucleus can fuse with the egg nucleus.
- The cytoplasm of the midpiece of the sperm contains many mitochondria. They carry out respiration, providing energy to make the tail (flagellum) move and propel the sperm forward.
- The egg cell is much larger than sperm cell and only one egg is released each month while the woman is fertile.
- It is surrounded by a jelly coat, which protects the contents of the cell and prevents more than one sperm from entering and fertilising the egg.
- The egg contains a large amount of cytoplasm, which is rich in fats and proteins.
- The fats act as energy store. Proteins are available for growth if the egg is fertilised.
Pregnancy and development:
After fertilisation, the newly-formed zygote divides repeatedly to form a ball of cells called an embryo. This becomes implanted in the wall of the uterus.
After eight weeks of development, the embryo is called a fetus. The amniotic sac produces amniotic fluid, which surrounds and protects the developing embryo.
A placenta, connected by an umbilical cord, develops from the embryo.
The placenta anchors the embryo in the uterus. It also allows:
- nutrients and oxygen to move from the mother to the embryo
- waste materials and carbon dioxide to move from the embryo to the mother
There is no physical connection between the circulatory systems of the embryo and its mother, so their blood doesn’t mix. These materials pass from one to the other by diffusion.
- The placenta can prevent some harmful substances in the mother’s blood from reaching the embryo. It cannot prevent all of them, however: alcohol and nicotine can pass to the developing fetus.
- Some pathogens such as the rubella virus and HIV can pass across the placenta.
- The placenta produces hormones, including oestrogens and progesterone which are essential to keep the uterus in good condition and stimulate milk-producing tissues in the mother.
‘Antenatal” or “prenatal” refers to the period before birth. Antenatal care is the way a woman should look after herself during pregnancy, so that the birth will be safe and her baby healthy.
- Eat properly, take more iron and folic acid (a vitamin) to prevent anaemia.
- Drinking or smoking are more likely to cause babies with low birth weights. These babies are more likely to be ill than babies of normal birth weights.
Labour and birth:
- The period from fertilization to birth takes about 38 weeks in humans. This is called the gestation
- A few weeks before the birth, the fetus has come to lie downwards in the uterus, with its head just above the cervix.
- When birth starts, the uterus begins to contract rhythmically. This is the beginning of what is called ‘labour’.
- These regular rhythmic contractions become stronger and more frequent.
- The opening of the cervix gradually widens (dilates) enough to let the baby’s head pass through and the contractions of the muscles in the uterus wall are assisted by muscular contraction of the abdomen.
- The amniotic sac breaks at some stage in labour and the fluid escapes through the vagina.
- Finally, the muscular contractions of the uterus wall and abdomen push the baby head-first through the widened cervix and vagina.
- The umbilical cord, which still connects the child to the placenta, is tied and cut. Later, the placenta breaks away from the uterus and is pushed out separately as the ‘afterbirth’.
- The best food for a newborn is breast milk. This is because breast milk contains all essential nutrients for the baby like proteins, fats, sugars, vitamins and minerals in easily digestible form.
- The mother’s milk also contains antibodies which are needed by the baby since their immune system has not developed yet.
- Moreover, breastfeeding builds a close bond between the mother and her baby.
- There is no risk of an allergic reaction to breast milk.
- Breast milk is produced at the correct temperature.
- There are no additives or preservatives in breast milk.
- Breast milk does not require sterilisation since there are no bacteria present that could cause intestinal disease.
- There is no cost involved in using breast milk and does not need to be prepared.
- Breastfeeding triggers a reduction in the size of the mother’s uterus.
- If the mother cannot breastfeed for any reason, there is another alternative which is formula milk powder. Formula milk powder is mixed with boiled water and fed to the babies in bottles.
- Formula milk however contains nutrients in harder digestible form which is a disadvantage.
- Formula milk also lacks of antibodies which are needed by the baby which makes a bottle feeding baby in a greater risk of infection than a breastfeeding baby.