Reproduction, Excretion and the Skeletal System — O/L Science

The human body is a complex integrated system. Key subsystems at O/L Science level include the reproductive, excretory, and skeletal/muscular systems.

The reproductive system enables species continuation. In human females: ovaries produce eggs (ova) monthly — the menstrual cycle (~28 days) is regulated by hormones (FSH, LH, oestrogen, progesterone). Fertilisation occurs in the fallopian tube; the fertilised egg implants in the uterus. In males: testes produce sperm.

The excretory system removes metabolic waste. The kidneys filter blood, reabsorb useful substances, and excrete urea, excess salts, and water as urine. The lungs excrete CO₂; the skin excretes sweat (salts + water). The liver converts ammonia (toxic) to urea (less toxic) — urea enters the blood and is filtered by the kidneys. The skeletal system provides support, movement (joints), and protection; muscles work in antagonistic pairs (bicep contracts ↔ tricep relaxes).