Your body carries about 38 trillion bacteria alongside 30 trillion cells
Cells build tissues, tissues build organs, and organs lock into systems that keep the body alive. An adult carries roughly 30 trillion of its own cells and an estimated 38 trillion bacteria, and each human cell suffers tens of thousands of DNA damages every day that must be repaired.
Hydrogen, oxygen, carbon, calcium and phosphorus dominate the elemental recipe. Not everything is cellular: cells sit in a matrix of proteins such as collagen, bathed in fluid. An adult man is about 60 percent water, some 42 litres. Roughly 23 litres sit inside cells, where potassium and phosphates dominate, while about 19 litres lie outside them, rich in sodium and chloride, including about 8.4 litres of fluid between cells and 3.2 litres of blood plasma.
Inside each nucleus, DNA is copied into RNA, which cells use to build the proteins that run them. Not every cell keeps its DNA; mature red blood cells discard their nucleus. Histologists sort tissues into four main types: epithelia that line surfaces, connective tissue, nerve and muscle. Linings range from flat single layers to the beating cilia of the airways and the columnar cells of the stomach, and they police what passes through.
The heart sits between the lungs, slightly left, wrapped in the protective pericardium. Its two atria keep venous blood flowing in while the ventricles contract, lifting output by roughly 75 percent, and a pacemaker signal from the sinoatrial node slows briefly at the atrioventricular node so the ventricles can fill. Coronary artery disease, the narrowing of the heart's own arteries by plaque, causes about 16 percent of deaths worldwide. Cancer of the heart is extremely rare because heart cells soon stop dividing.
Smaller organs matter too. The pear-shaped gallbladder, tucked beneath the liver, stores bile for digesting fat; stones that block its ducts cause sudden upper-right pain and usually lead to its removal. Anatomists, physiologists, clinicians and artists all study the body, which varies in known ways from textbook averages.
Source: Human body
