Human body

• 1. Introduction

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  The human body holds roughly 30 trillion cells, and outnumbering them, about 38 trillion bacteria. That is the scale of a single living person. Every one of those cells sits in a chemical bath that is watched and adjusted constantly, keeping sugar, iron, and oxygen at safe levels in the blood. The body is the entire structure of a human being, built from cells into tissues, tissues into organs, organs into whole systems. Most of those systems hide inside cavities you never see. So how does a structure this complicated hold itself together? What keeps every cell supplied while waste is carried away? Who first cut the body open to understand it, and what did they find?

• 2. Built From Elements

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  Oxygen makes up 65 percent of the human body by mass, the single largest share. Carbon follows at 18.5 percent, then hydrogen at 9.5, and nitrogen at 3.2. Calcium and phosphorus each sit around 1 to 1.5 percent. Every trace element below that comes in at less than 0.1 percent. These elements live inside trillions of cells and in the non-cellular material between them.

  Water accounts for about 60 percent of the adult male body, some 42 litres in total. Of that, roughly 23 litres sit inside cells, while about 19 litres are extracellular, including around 3.2 litres of blood plasma and about 8.4 litres of interstitial fluid. The acidity and composition of this water are kept under tight control. Sodium and chloride dominate the fluid outside cells, while potassium and phosphates dominate within them.

  Not every part of the body is made from cells. Cells rest in an extracellular matrix of proteins such as collagen, surrounded by fluid. The cells themselves run because of DNA, which sits inside the nucleus. Parts of that DNA are copied and carried out by RNA, which is then used to build proteins. Proteins dictate what a cell does and how its genes are expressed. Yet some cells abandon their DNA entirely; mature red blood cells lose their nucleus as they grow up. Each cell also faces tens of thousands of DNA damages every single day, and a repair that goes wrong can leave a mutation behind.

• 3. The Four Tissues

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  Histology is the study of tissue, and it is usually done with a microscope. Tissue is defined as cells that act together with a specialised function. The body builds everything from four main kinds: lining cells called epithelia, connective tissue, nerve tissue, and muscle tissue.

  Lining cells come in a startling range of shapes. Some are single layers of flat cells. Some carry tiny beating hair-like cilia in the lungs. Others stand like columns along the wall of the stomach. Endothelial cells line internal cavities, including blood vessels and glands. These lining cells decide what can pass through them, shield the structures behind them, and act as sensory surfaces.

  Organs are structured collections of cells with a specific function. Most sit inside the body, with skin the notable exception. The heart, lungs, and liver are examples, and many organs nest within cavities. The abdomen holds the stomach. The pleura holds the lungs. That arrangement of sheltered organs sets up the next question: how does each one do its particular job?

• 4. Two Organs Up Close

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  The heart sits in the thoracic cavity between the lungs and slightly to the left. The pericardium surrounds it, holding it in place in the mediastinum, protecting it from blunt trauma and infection, and lubricating its movement with pericardial fluid. The heart pumps blood around the body, carrying oxygen, nutrients, waste, hormones, and white blood cells. It is built from two atria and two ventricles. The atria let venous blood keep flowing in during ventricular systole, which allows a cardiac output roughly 75 percent greater than would otherwise be possible. An electrical signal starts in the sinoatrial node, spreads through the atria, then reaches the atrioventricular node, which slows it just enough to let the ventricles fill before they pump.

  Coronary artery disease is the leading cause of death worldwide, making up 16 percent of all deaths. Plaque builds in the arteries feeding the heart until too little blood reaches the myocardium, the condition known as a heart attack. Cancer in the heart, by contrast, is exceedingly rare, because heart cells quickly stop dividing and grow only by getting larger.

  The gallbladder is a hollow pear-shaped organ tucked behind the lower middle of the right lobe of the liver. It stores bile, then releases it into the small intestine through the common bile duct to help digest fats. Its blood arrives through the cystic artery, which in most people branches off the right hepatic artery. Gallstones are common, and most people never feel them, but a stone blocking the biliary tract triggers a gallbladder attack of sudden pain in the upper right or center of the abdomen. The usual fix is removing the organ in a cholecystectomy.

• 5. Systems In Motion

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  The circulatory system runs on the heart and the blood vessels: arteries, veins, and capillaries. Blood splits into two circuits. The pulmonary circuit sends blood to the lungs to collect oxygen and shed carbon dioxide. The systemic circuit carries blood from the heart out to the rest of the body.

  Digestion begins in the mouth, where food is chewed into smaller pieces before being swallowed down the esophagus. In the stomach, gastric acids mix with food to pull out nutrients, leaving a substance called chyme. The small intestine absorbs the nutrients and water, the large intestine dries what remains into feces, and the rectum stores it until it leaves through the anus. Peristalsis, the systematic expansion and contraction of muscles, pushes everything along.

  The respiratory system runs from the nose and trachea down to the lungs. The diaphragm pushes down to create a vacuum that pulls air in, storing it briefly in tiny sacs called alveoli. Capillaries wrapped around each alveolus draw oxygen out of the air and into the blood. Smoking can damage the connections between alveoli, producing emphysema.

  The urinary system is the body's filter. The kidneys process about 170 litres of blood daily, yet only 1 to 2 litres leave as urine. Smooth muscle in the ureter walls forces small amounts of urine toward the bladder every 10 to 15 seconds. A normal bladder can comfortably hold half a litre for 3 to 5 hours before the brain signals it to empty. Other systems work less visibly: the endocrine glands send hormone signals, the immune system tells the body's own cells from invaders, the lymphatic system moves the fluid found between cells, and the musculoskeletal system gives the body its frame, with the larger bones producing blood cells in their marrow.

• 6. Brain And Reproduction

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  The nervous system is built from neurons and glial cells that form nerves, ganglia, and gray matter, and ultimately the brain. The brain handles thought, emotion, memory, and sensory processing, drawing on vision, hearing, taste, and smell gathered by the eyes, ears, tongue, and nose. Structurally it splits into the central nervous system, the brain and spinal cord, and the peripheral nervous system of nerves and ganglia beyond them. Whether the central nervous system directly gives rise to consciousness remains a matter of debate. Functionally it splits again, into the somatic system handling voluntary acts like speaking, and the autonomic system running involuntary work like digestion. In multiple sclerosis the immune system attacks the nerve linings. Amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, gradually reduces movement.

  The reproductive system exists to make and nurture offspring, and its organs mature at puberty. Female puberty generally arrives between the ages of 9 and 13, marked by ovulation and menstruation. The female inner sex organs are the two ovaries, the fallopian tubes, the uterus, and the cervix. At birth there are about 70,000 immature egg cells, which dwindle to around 40,000 by puberty, and no more are ever produced. Sperm cells follow the opposite rule. The testicles, held in the scrotum, produce them throughout a man's whole life.

• 7. Reading The Body

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  Andreas Vesalius, who lived from 1514 to 1564, pioneered the modern study of human anatomy through dissection, writing the influential book De humani corporis fabrica. Long before him, the Hippocratic Corpus of Ancient Greece had described the skeleton and muscles. The 2nd century physician Galen of Pergamum gathered classical knowledge into a text used throughout the Middle Ages. Later the microscope opened up the cellular structure of tissue, and modern anatomy now reaches into the body with magnetic resonance imaging, computed tomography, fluoroscopy, and ultrasound.

  Physiology asks not what the body is but how it works. Its study began with Hippocrates around 420 BCE and with Aristotle, who lived from 384 to 322 BCE and stressed the link between structure and function. The French physician Jean Fernel, 1497 to 1558, introduced the word physiology itself. William Harvey, 1578 to 1657, described the circulatory system by joining close observation with careful experiment. In 1838 Matthias Schleiden and Theodor Schwann proposed the cell theory, that organisms are made of cells. Claude Bernard, 1813 to 1878, created the idea of the milieu interieur, the internal environment, which Walter Cannon, 1871 to 1945, later described as held to a steady state in homeostasis. That steady state is the thread tying all of it together, from a single dividing cell to the whole 30-trillion-cell structure of a living person.