The human body: digestion, circulation and immunity

The human body runs on three cooperating systems: one to take in fuel, one to distribute it, and one to defend against invaders. Digestion begins in the mouth, where chewing and saliva start breaking down food, and continues in the stomach, where acid and enzymes break proteins apart. Most absorption happens in the small intestine, whose inner surface is covered in tiny finger-like villi that hugely increase its area. Nutrients pass through the villi into the blood, while the large intestine absorbs water and the remainder leaves the body. The whole process is a chemical factory: enzymes, each specialised for one kind of molecule, dismantle food into sugars, amino acids, and fatty acids small enough to use.

Circulation distributes those nutrients, along with oxygen, to every cell, and carries waste away. The heart is a double pump: one side sends blood to the lungs to collect oxygen and release carbon dioxide, the other sends oxygen-rich blood around the body. Arteries carry blood away from the heart, veins return it, and capillaries — vessels so narrow that blood cells pass single file — are where exchange actually happens. The English physician William Harvey described this circulation of blood in 1628, overturning the older idea that blood was continuously made and consumed. Blood also carries hormones that coordinate distant organs, and platelets and clotting factors that seal wounds.

Immunity defends the body at several levels. Skin and mucous membranes form a physical barrier. If a microbe gets past, innate immune cells attack broadly and quickly, causing inflammation — redness, warmth, and swelling — as part of the response. The adaptive immune system is slower but specific: it recognises particular invaders and remembers them, so a second encounter is met faster and harder. This memory is the principle behind vaccination, which trains the immune system with a harmless form of a germ or part of it, so that the real infection is met by a prepared defence.

Vaccination is among the most consequential applications of biology. Smallpox, a disease that killed millions, was certified eradicated by the World Health Organization in 1980, after a global vaccination campaign — the only human disease eradicated so far. The lesson generalises: understanding the body's systems lets medicine work with them rather than against them, and public health succeeds when it combines biology with organisation, trust, and access.