The digestive system
Food passes through the mouth (mechanical breakdown, saliva starts starch digestion), esophagus, stomach (acid and enzymes break down proteins), small intestine (most digestion and nutrient absorption happens here, aided by enzymes from the pancreas and bile from the liver), and large intestine (water absorption), before waste leaves via the rectum and anus.
The respiratory system
Air travels through the trachea into the lungs, branching into bronchi and bronchioles, ending in millions of tiny air sacs called alveoli. Alveoli have thin walls and a large surface area, allowing oxygen to diffuse into the blood and carbon dioxide to diffuse out, into the air to be exhaled.
The circulatory system
The heart has four chambers: two atria (receive blood) and two ventricles (pump blood out). Blood travels away from the heart in arteries (thick, muscular walls, to withstand high pressure) and back to the heart in veins (thinner walls, with valves to prevent backflow). Capillaries are the tiny vessels where actual exchange of substances with body tissues happens, with walls just one cell thick.
Example 1: Explain why artery walls are much thicker and more muscular than vein walls.
Blood in arteries is under high pressure directly from the heart's pumping, so thick, elastic, muscular walls are needed to withstand that pressure and help push blood along. By the time blood reaches veins, pressure has dropped considerably, so thinner walls (aided by valves and surrounding muscle movement) are sufficient to return blood to the heart.
The excretory system
The kidneys filter blood, removing waste products (like urea, from protein breakdown) and excess water and salts to form urine, which travels via the ureters to the bladder for storage before being released. This filtering process, carried out by millions of tiny units called nephrons, is essential for maintaining the body's internal water and salt balance (homeostasis).