The Human Body

A recent careful count puts the human body at roughly 30 trillion cells, most of them red blood cells. (You also carry a similar number of bacteria, mostly in your gut.) Those cells are organized in levels. For a closer look at the cell itself, see The Cell.

Textbooks usually divide the body into eleven organ systems. Some combine or split them differently (the lymphatic and immune systems are often treated together, as here), but the work is the same. Tap a system in the figure or choose one from the buttons.

Homeostasis means keeping conditions inside the body, such as temperature, blood glucose, water and pH, within a narrow range even when the outside world changes. Most of it works through feedback loops with three parts: a sensor that detects a change, a control center (often the brain) that compares it with a set point, and an effector (a muscle or gland) that responds.

Negative feedback

In negative feedback, the response pushes the change back the other way. It is how almost all homeostasis works.

• Body temperature. Core temperature sits near 37 °C, varying a little between people, through the day and with where it is measured. The hypothalamus in the brain acts as the control center. If you get too warm, you sweat and blood vessels in the skin widen to shed heat. If you get too cold, you shiver and those vessels narrow.
• Blood glucose. After a meal, glucose rises. Beta cells in the pancreas release insulin, which helps cells take up glucose and the liver store it as glycogen, so the level falls. Between meals, alpha cells in the pancreas release glucagon, which tells the liver to break glycogen down and release glucose. In type 1 diabetes, the immune system destroys the insulin-making cells; in type 2, the body responds poorly to insulin.

Positive feedback

In positive feedback, the response makes the change bigger, until some event ends the loop. The body uses it only for jobs that need to be finished quickly.

• Childbirth. As the baby presses on the cervix, stretch signals trigger the release of the hormone oxytocin, which makes the uterus contract harder. Stronger contractions cause more stretching and more oxytocin, until the baby is born.
• Blood clotting. Platelets that stick to a damaged vessel release chemicals that activate more platelets, which release more chemicals, until a plug seals the wound.

The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is all the nerves that branch out from them to the skin, muscles and organs. Sensory neurons carry information in; motor neurons carry commands out.

A neuron has a cell body, short branches called dendrites that receive signals, and a long axon that sends them on. Many axons are wrapped in a fatty myelin sheath that makes signals travel much faster, more than 100 meters per second in the fastest fibers (see Purves et al.).

At rest, the inside of a neuron is about 70 millivolts negative compared with the outside. When a neuron is stimulated enough, channels open and sodium ions rush in, briefly flipping the inside to positive. Potassium ions then flow out and restore the negative charge. This quick spike is an action potential. It is all-or-none: a stronger stimulus does not make a bigger spike, it makes spikes come more often.

Neurons do not quite touch. At a synapse, the arriving signal makes the axon ending release chemicals called neurotransmitters. They cross the tiny gap and bind to receptors on the next cell, which may then fire.

A reflex arc is a shortcut for speed. Touch a hot pan and a receptor in your skin fires a sensory neuron. In the spinal cord, an interneuron passes the signal straight to a motor neuron, and your arm muscles pull your hand away. The message reaches your brain too, but only after you have moved, which is why the pain seems to come second. (The knee-jerk reflex is even simpler: the sensory neuron connects directly to the motor neuron.)

The heart is a muscular pump with four chambers. The two upper chambers, the atria, receive blood. The two lower chambers, the ventricles, pump it out. Valves between them, and at the exits, keep blood moving one way; their closing makes the "lub-dub" sound of a heartbeat.

Blood makes two loops. The pulmonary circuit takes blood from the right side of the heart to the lungs and back to the left side. The systemic circuit takes it from the left side to the rest of the body and back to the right. Diagrams like the one below are drawn as if you are facing the person, so their right side is on your left.

Vessels. Arteries carry blood away from the heart. Their thick, elastic walls stretch with each beat; that stretch is the pulse you feel in your wrist. Veins carry blood back to the heart. They have thinner walls and valves that stop blood pooling in your legs. Capillaries join the two. Their walls are just one cell thick, so oxygen, nutrients and wastes can pass through.

Blood. A little over half of blood is plasma, a yellowish liquid of water, salts, proteins, nutrients and hormones. The rest is mostly red blood cells, packed with the protein hemoglobin that carries oxygen. Smaller amounts are white blood cells, which fight infection, and platelets, cell fragments that help blood clot.

Air enters through the nose or mouth, passes the throat (pharynx) and voice box (larynx), and goes down the trachea, a tube held open by rings of cartilage. The trachea splits into two bronchi, one for each lung, which branch again and again into tiny bronchioles. These end in clusters of air sacs called alveoli.

The lungs hold hundreds of millions of alveoli. Spread out, their surface is often estimated at around 70 square meters, though estimates vary (see OpenStax Biology). Each alveolus is wrapped in capillaries, and the wall between air and blood is extremely thin. Oxygen diffuses from the air, where it is more concentrated, into the blood. Carbon dioxide diffuses the other way.

The lungs have no muscles of their own. To breathe in, the diaphragm, a dome of muscle under the lungs, contracts and flattens, and the rib muscles lift the ribs. The chest gets bigger, the pressure inside drops, and air flows in. To breathe out at rest, these muscles relax and the lungs spring back.

Breathing and cellular respiration are not the same thing. Breathing moves air; cellular respiration is how your cells use that oxygen to release energy from glucose, making the carbon dioxide you breathe out. It is covered in Photosynthesis and Respiration.

Digestion turns food into molecules small enough to cross into the blood. Mechanical digestion breaks food into smaller pieces: chewing, and the churning of the stomach. Chemical digestion uses enzymes to break large molecules into small ones.

The pancreas sends digestive enzymes and bicarbonate, which neutralizes stomach acid, into the small intestine. The liver makes bile, stored in the gallbladder, which breaks fat into small droplets so lipase can work on them. Bile is not an enzyme. The liver also processes the nutrient-rich blood coming from the intestine, storing glucose as glycogen and breaking down toxins.

The lining of the small intestine is folded into millions of finger-like villi, each covered in even smaller microvilli. They give it a huge surface for absorption. Sugars and amino acids pass into capillaries in each villus; most fats go into small lymph vessels instead.

The innate immune system is the fast, general first response. It includes barriers like skin, mucus and stomach acid; inflammation, the redness, heat and swelling around an injury; fever; and white blood cells called phagocytes, such as neutrophils and macrophages, that swallow and destroy invaders.

The adaptive immune system is slower to start but targets one specific invader. It recognizes antigens, molecules on the surface of a germ.

• B cells make antibodies, Y-shaped proteins that lock onto one antigen. Antibodies can block a virus from entering cells and flag germs for destruction.
• Helper T cells coordinate the response, activating B cells and other immune cells.
• Cytotoxic (killer) T cells destroy body cells that are infected with a virus.

After an infection, some B and T cells stay on as memory cells. If the same germ returns, they respond so quickly that you may never feel ill. A vaccine trains this memory safely, by showing the immune system a weakened or inactivated germ, a piece of one, or instructions for cells to make a harmless piece of one (as in mRNA vaccines).

No system works alone. Think about what happens when you start running:

• Your muscles need far more ATP, so they use more oxygen and glucose and make more carbon dioxide and heat.
• Your nervous and endocrine systems respond: signals from the brain and the hormone adrenaline speed up the heart.
• Your respiratory system breathes faster and deeper to bring in oxygen and get rid of carbon dioxide.
• Your circulatory system pumps more blood with each beat and more beats per minute, and sends more of it to the working muscles and less to the gut.
• Your liver releases stored glucose, and your skin sweats and flushes to get rid of the extra heat, which is the temperature loop above in action.

When you stop, these responses wind down as feedback brings each value back toward its set point.