Right now your body temperature is almost certainly within a degree of 37°C, whether you are sitting in a cold room or walking down a hot street. The saltiness of your blood, its acidity and its sugar level are all being held inside narrow ranges at this moment, without a single conscious decision from you. This course is about how a body manages that feat: how roughly 37 trillion cells, none of which knows anything about you, cooperate to keep you alive.
Anatomy, physiology and an old mistake
Anatomy is the study of the body's structure: what is there and where. Physiology is the study of function: how those parts work. For more than a thousand years, European and Islamic medicine leaned heavily on Galen, a Greek physician who worked in Rome in the second century AD. Galen was a brilliant observer, but human dissection was rarely allowed in his world, so he worked mostly on apes, pigs and other animals. In 1543 the Flemish anatomist Andreas Vesalius published On the Fabric of the Human Body, based on his own dissections of human corpses. He showed that many of Galen's errors came from describing animal bodies rather than human ones. The deeper lesson was about method: look at the thing itself rather than trusting an honored authority.
Levels of organization
Biologists describe the body as a ladder of levels. At the bottom are cells, the smallest living units. Similar cells working together form tissues. Several tissues combine into organs, and organs that share a job form organ systems, such as the digestive or circulatory system. So the order runs cells, tissues, organs, organ systems, and finally the whole organism.
Despite the body's variety, there are only four basic tissue types: epithelial, connective, muscle and nervous. Epithelial tissue covers surfaces and lines hollow spaces: your skin, the inside of your gut, the walls of the tiny air sacs in your lungs. Connective tissue supports and binds, and it comes in surprising forms, including bone, cartilage, fat and even blood. Muscle tissue contracts. Nervous tissue carries signals. In the stomach, for instance, all four work together.
The constant inner world
In the middle of the nineteenth century the French physiologist Claude Bernard noticed something remarkable. Our cells live bathed in fluid, and the body keeps that fluid strikingly stable even as the outside world changes. He called it the milieu intérieur, the inner environment, and wrote in 1878 that "the fixity of the internal environment is the condition of free life." In the 1920s the American physiologist Walter Cannon coined the word homeostasis, from Greek roots meaning roughly "staying similar," for this active balancing.
Feedback loops
Homeostasis works through feedback loops with three parts: a sensor that detects a value, a control center that compares it with a target called the set point, and an effector that makes a change. Most loops use negative feedback, which reverses a change and pulls a value back toward its set point, much as a home thermostat switches the heater off once the room is warm. Body temperature is the classic case. A small region at the base of the brain called the hypothalamus acts as the control center. When you overheat, you sweat, and blood vessels in the skin widen so warm blood can lose heat at the surface. When you are cold, those vessels narrow and you shiver, since working muscles produce heat.
A few loops use positive feedback, which amplifies a change until some job is finished. In childbirth, pressure on the cervix triggers release of the hormone oxytocin, which strengthens contractions, which raise the pressure and release still more oxytocin, until the baby is born. Rising contractions in childbirth and blood clotting are the standard examples. Positive feedback is good for finishing a process, but it would be disastrous for everyday control.
Set points can move
A fever is not a failure of temperature control. During many infections the hypothalamus raises the body's temperature set point, so you shiver and feel cold even while your temperature climbs. Higher temperatures seem to help some immune defenses and make life harder for some microbes. Real failure looks different: in heatstroke the cooling system is overwhelmed and core temperature rises above about 40°C, which can damage organs.
Worked example: a marathon on a hot day
A runner starts a marathon at 28°C. Within minutes her leg muscles are producing many times their resting heat. Sensors in her skin and blood report the rising temperature to the hypothalamus, which responds through negative feedback: skin vessels widen and her face flushes, and sweat glands pour out fluid that cools her as it evaporates. Sweat also carries off water and salt, so a second loop begins. As her blood grows more concentrated she feels thirsty and her kidneys hold back water, a process Level 7 explains. If she drinks too little, both loops strain at once. Her blood volume falls, her heart must beat faster to keep pressure up, and less blood can be spared for the skin, so cooling weakens just when she needs it most. That is how dehydration and overheating feed each other.