The human reproductive systems
The male reproductive system produces sperm in the testes, which travel through the sperm duct during ejaculation. The female reproductive system produces eggs in the ovaries; a released egg travels down the oviduct (fallopian tube) towards the uterus, where a fertilized egg can implant and develop.
The menstrual cycle
The menstrual cycle (roughly 28 days) prepares the female body for a possible pregnancy each month. An egg is released from an ovary around the middle of the cycle (ovulation); the uterus lining thickens to prepare for a fertilized egg. If fertilization doesn't happen, the thickened lining breaks down and is shed as menstruation, and the cycle restarts.
Fertilization and gestation
Fertilization happens when a sperm cell fuses with an egg cell, usually in the oviduct, forming a zygote that implants in the uterus lining. Gestation (pregnancy) in humans lasts about 9 months, during which the fetus develops and is nourished via the placenta, which lets nutrients and oxygen pass from mother to fetus (and waste pass back) without their blood mixing directly.
Nutrition and balanced diet
A balanced diet supplies carbohydrates and fats (energy), proteins (growth and repair), vitamins and minerals (specific body functions), fibre (digestion), and water. Lacking a specific nutrient causes a deficiency disease: for example, kwashiorkor results from too little protein (common in children with diets high in starch but low in protein), scurvy results from too little vitamin C, and rickets results from too little vitamin D or calcium (causing weak, poorly formed bones).
Example 1: A child with a swollen belly, thin limbs, and poor growth is diagnosed with kwashiorkor. Which nutrient deficiency causes this, and why does the belly swell?
Kwashiorkor is caused by a severe protein deficiency. The swollen belly happens because low blood protein levels disrupt the normal balance that keeps fluid inside blood vessels, allowing fluid to leak into the abdomen (a symptom called edema).