It's Good to Have Soapy Lungs The tiny air-sacs in your lungs known as alveoli are coated with a surfactant, a soap-like substance. It keeps the alveoli's delicate membranes--which measure less than half a millimeter thick--from sticking to each other when you exhale, emptying the oxygen from your body. You wouldn't want to use it to do laundry, but without it, you'd take exactly one breath--your last. What's Inside Your Bones? Metaphyseal Vessels. Your bones may just seem like scaffolding, but they do far more than simply hold up the weight of your body. Most people know that bone marrow actively produces crucial red and white blood cells and platelets. But less attention goes to the special blood supply for the bones. The vessels that provide blood enter into the bone through the thin membrane that covers it, which is known as the periosteum. Without this circulation, you'd lack any immune cells--and without an immune response, any exposure to everyday germs would prove fatal. Those Magical Stomach Lining Cells Some of the most important players in digestion are hormones released by the cells in your stomach lining. When your stomach senses it's not empty, for instance, your brain signals cells in your stomach lining to release gastrin, a hormone that stimulates the release of gastric juices containing hydrochloric acid to dissolve your food. Gastrin also contains pepsin, which breaks down proteins and stimulates your liver to produce bile, which allows you to digest fats. Without gastrin, you wouldn't be able to glean much nourishment from a steak or an omelette. The Beauty of Blood Buffering To stay healthy, your body needs to keep the pH level of your blood within a pretty narrow range: between 7.35 and 7.45. To maintain this level, your blood creates its own buffer system consisting of molecules that absorb excess hydrogen ions if things get too acidic, or release them if the blood is too basic. Without this system, your blood would become either too basic or too acidic for your body's enzymes to go about their business, toxins would accumulate at warp speed, and after a short time, you'd die. Keeping the Heart in Check: The Pericardium The pericardium--the thin membrane that forms a sac around the heart--helps hold the fist-sized muscle in place, and keeps it from overexpanding. You won't even know the pericardium is there unless you get pericarditis: when it becomes inflamed due to anything from infection to injury. When this condition occurs, the membrane lining becomes thicker, compressing the fluid that lies between the sac and your heart and making it harder for your heart to pump blood. A Pillow for Your Brain Cerebrospinal fluid bathes the entire central nervous system--the brain and spinal cord--and acts like a pillow, cushioning the brain if you bang your head. It also transports certain hormones to your brain, and shunts toxins away from it by mixing with your blood once it leaves your head. And by helping to support your noggin, this fluid also reduces the pressure that your heavy head puts on your brain stem. Zapping Your Heart Into Action Ever wonder what keeps your heart beating? That would be a bundle of nerves called the sinoatrial node. It produces the electrical signal that makes your heart contract and relax like an angry milkmaid's fist. About 60 to 70 times a minute when you're resting, this node jolts the rest of the heart into action, sending blood through the heart's chambers and re-oxygenating it before sending it on its way through the rest of the body. Feel the Burn: Thermoreceptors Thermoreceptors are the nerve fibers in your skin that let you know if, say, a body part is on fire, or if that stovetop you're touching is still warm. Without them, life-threatening burns would be a major risk. There are also thermoreceptors that sense cold, and while it's good that we can tell it's bad to hang out for long in icy water, it's safe to say that short bursts of heat pose a greater risk. Insulin-Pumping Islets You have between one million and three million islets of Langerhans in your pancreas, the spongy, six-inch-long organ positioned behind your stomach. Without these clumps of cells, you'd be unable to produce insulin, the hormone that is released after you eat to mop up the sugar released by your food. Most cases of type 1 diabetes occur when the body mistakenly sees the islet cells as a foreign invader and wrecks the thing.