Things Your Body Does Every Day Without You Noticing

You woke up this morning already in the middle of a long shift. While you slept, something kept your temperature steady, timed your sleep in cycles, cleaned your blood and kept you breathing. Nobody asked you to supervise, and nobody will send you a report.

One line from my book The Human Body sums up the arrangement:

Most of what keeps you alive happens without your permission.

That sentence opens the chapter on the autonomic nervous system, the part of the nervous system that runs the organs automatically: heart rate, blood pressure, breathing rhythm, digestion, sweating, the size of the pupils and the emptying of the bladder. Its commanders sit in the hypothalamus and the brainstem. You are not consulted.

What follows is a short tour of some of that unasked work. None of it is exotic. All of it is happening as you read.

Your heart starts itself

It is natural to assume the brain tells the heart to beat. It does not. In the wall of the right atrium sits the sinoatrial node, a small cluster of specialised muscle cells about the size of a grain of rice. These cells fire on their own. Their membranes slowly leak positive ions until they reach a threshold and discharge, then they reset and start again. Each discharge becomes one beat.

At rest this natural pacemaker fires around 60 to 100 times a minute in most adults. The nervous system can speed it up or slow it down, but it does not start it.

The clearest proof comes from transplant surgery. A transplanted heart arrives with its nerves cut. It still beats. In fact, with no vagal brake to hold it back, it typically beats faster at rest, often around 90 to 110 times a minute, and it speeds up during exercise only as adrenaline reaches it through the blood. The heart, in other words, was never waiting for instructions.

Your plumbing leaks on purpose, then cleans up

Here is a design decision that sounds like a fault.

Blood vessels leak, and they are meant to.

As blood passes through the capillaries, pressure pushes fluid out between the cells, bathing the tissues in nutrients. Most of it seeps back in at the far end of the capillary, but not all. Every day roughly three litres are left behind, along with stray proteins and cells. Without a way to collect it, the tissues would swell like sponges.

The mopping up falls to the lymphatic system, a second network of vessels that runs alongside the blood vessels almost everywhere. Most people could name the heart and the arteries. Far fewer could say that they have a second plumbing system, and that it has been quietly returning three litres a day for their whole lives.

It is also a reminder that the map is not finished. Lymphatic vessels in the membranes around the brain were described only in 2015, overturning the textbook view that the brain had none.

The filters are just as busy. Tuck your hands on your waist with thumbs pointing back and they rest just below your kidneys. Each one is about 11 to 12 centimetres long, the size of a computer mouse, and weighs around 150 grams. Together they make up less than half a per cent of body weight, yet they receive about a fifth of all the blood the heart pumps.

What they do with it is hard to picture:

At around 180 litres a day, the kidneys filter the whole volume of your blood plasma about 60 times every day.

The liver keeps similar hours. In an adult it weighs about 1.5 kilograms, and about 1.5 litres of blood flow through it every minute, around a quarter of everything the heart pumps. Roughly three-quarters of that blood arrives not from an artery but from the hepatic portal vein, carrying everything just absorbed from the gut. The liver gets first look at every meal.

It is also unusually forgiving. The liver can regrow to its full size from as little as a quarter of its original mass, which is what makes it possible for a living donor to give part of theirs.

Your throat and lungs run on reflex

Swallowing looks simple and is anything but. In under a second, food must cross the one place in the body where the routes for air and food meet, the throat, without a crumb going the wrong way.

Most adults swallow somewhere around 500 to 700 times a day, mostly saliva, and each time dozens of muscles in the tongue, palate, throat and neck fire in a strict order. You begin a swallow on purpose. After that first voluntary push comes a reflex seal, then a wave of muscle that carries the food down the oesophagus.

That wave is why gravity is optional. Because the oesophagus squeezes food along rather than letting it fall, you can swallow while standing on your head, and astronauts eat normally in orbit.

Breathing has its own surprise. Take a deep breath and hold it. Within a minute or so most untrained adults find the urge to breathe irresistible. The surprise is the cause. It is not a lack of oxygen that forces you to give up. The main alarm is the rising level of carbon dioxide in the blood.

Carbon dioxide dissolves to form acid, and sensors on the surface of the brainstem detect the resulting change in the acidity of the fluid around the brain. Smaller sensors in the neck watch oxygen as well, but under normal conditions carbon dioxide is the trigger. Your body is less worried about running out of fuel than about the exhaust building up.

The quantities at rest are modest. An adult takes up about 250 millilitres of oxygen a minute and gives off about 200 millilitres of carbon dioxide. Nothing pumps the gases across. Each one simply diffuses from where its pressure is higher to where it is lower.

There is one more thing your breath carries out. When you lose fat, most of its mass leaves your body as carbon dioxide, breathed out through your lungs. Only a smaller share leaves as water.

You hold a temperature, keep the time and track your limbs

Outside, the weather can swing from frost to heatwave. Inside, your core temperature stays within about a degree of 37 °C. This steadiness is called homeostasis, and temperature is only the most familiar example. The same holds for blood sugar, for the acidity of the blood, which is held between pH 7.35 and 7.45, and for water, salt, oxygen and carbon dioxide.

Even the famous number has a history. The 37 °C figure comes from Carl Wunderlich, who in 1868 published averages from around a million armpit readings. Modern measurements put the typical figure slightly lower, at roughly 36.6 to 36.8 °C.

You also carry a clock. Just above the point where the two optic nerves cross lies a pair of tiny clusters of about 20,000 nerve cells each. Together they form the body's master clock. Left in a cave or a windowless lab, it keeps running on its own, with a period of on average about 24 hours and 10 minutes. Light resets it each day.

In 1962 the French cave explorer Michel Siffre tested this the hard way. He lived underground without a clock for about two months. When he emerged, he thought it was around 25 days earlier than it really was.

And each night the same quiet scheduling continues. An adult passes through four to six sleep cycles, each lasting roughly 90 minutes. Sleep is not the brain switching off but switching modes.

One last piece of background work. Close your eyes and touch your nose. You can do it because of proprioception, the sense of where the parts of your body are and how they are moving. Stretch sensors in muscle and tension sensors in tendon report constantly, and you never hear a word of it.

It is often called the sixth sense, which also corrects a myth. Humans have far more than five senses, with balance, temperature, pain and the sense of a full bladder among them.

What to take from this

• Your heartbeat starts in the heart itself, not in the brain.
• Your blood vessels leak roughly three litres of fluid a day, and a second network returns it.
• Your kidneys filter about 180 litres a day, and your liver sees around a quarter of the heart's output every minute.
• The urge to breathe comes mainly from rising carbon dioxide, not falling oxygen.
• Your temperature, blood chemistry and daily rhythm are held steady without any effort from you.
• You have more than five senses, and the one that tells you where your limbs are works with your eyes shut.

None of this needs your attention, which is rather the point. But it is pleasant to know what the staff are doing.

If you would like the rest of the manual, The Human Body: How we are built and how we work covers the body from a single cell to a whole lifespan in 100 chapters, each with a detailed labelled plate. It is a 306-page PDF and EPUB, and it costs £9 on Gumroad: matsiems.gumroad.com/l/the-human-body.