1. Vomit

When the vomit center triggers an upchuck, you begin to produce more saliva (to protect your teeth from the harsh stomach acids). At the moment of barfing, there is a strong, downward contraction of the diaphragm, while simultaneously the abdominal muscles tighten against a relaxed stomach. As a result, the contents of the stomach are propelled up and out – sometimes with astonishing forward range!

2. Peeing

Your bladder is normally doing only one of two things – filling or emptying – thanks to the coordinated effort of the bladder itself and the “urethral sphincter.” During storage mode, the bladder acts as a low-pressure receptacle, while the sphincter stays “sphinct” to keep the bladder outlet closed. When you pee, the bladder contracts to expel urine and the sphincter relaxes to allow the flow. Your bladder function is automatic, but it is controlled by your brain, which makes the final decision on whether to pee or not to pee. In other words, you have the ability to stop and start peeing on command and if necessary, delay urination until you find a more socially acceptable time and place.

Normal urine flow rates vary by age and gender. Up until age 8, the flow rate for boys and girls is the same. Interestingly, it is only between the ages of 14 and 45 that the male flow rate is greater than that of females. During this period, males pee at an average rate of 21 ml per second, which is equivalent to filling a liter bottle in 47.6 seconds. It would take the average female 56 seconds to do the same, with an average flow rate of 18 ml per second.  

Ever get a shiver after you pee? It’s called “post-micturition convulsion syndrome,” and it occurs in more than 80% of men, but only around 55% of women. 

“A burp is but a gust of wind that comes straight from the heart. But if it takes a downward turn, it comes out as a fart.” Charming, but anatomically incorrect. The gas, or “flatus,” you expel below the belt is generally a by-product of the digestive process. Flatulence prevents this gas from building up in your intestines. It moves through the intestines in the same way that poop moves through, and causes similar sensations of urgency or discomfort. Nerve endings in the rectum USUALLY allow you to differentiate between “just gas” and poop – but not always.

Just about everyone has the ability to burp (belch) expelled gas from the stomach out through the mouth. But it’s not as simple as you think. Burping requires the coordination of the larynx, both esophageal sphincters (upper and lower), and the diaphragm. The larynx must be closed off so any liquid or food (beer or pizza) that might return with the air won’t get into the lungs. During burping, the larynx is voluntarily raised, which also relaxes the upper esophageal sphincter to allow air to pass into the throat. Obviously, the lower esophageal sphincter must be open as well, to allow air out of the stomach. At the same time, the diaphragm descends, similar to an inhalation, which increases abdominal pressure and decreases chest pressure. These changes in pressure encourage the flow of air from the stomach up and out.

5. Sneezing

Sternutation or sneezing is an involuntary response that causes a sudden, forceful burst of air (and sometimes milk) through the nose and mouth. Like vomiting and burping, it’s a tricky physical process involving the brain, nose, mouth and chest. A sneeze is your body’s way to get rid of upper respiratory irritants such as dust, pollen or smoke. Some people (photic sneezers like yours truly) also have the urge to sneeze when exposed to bright light – for still unknown reasons.

A sneeze begins when the nose encounters a particularly irritating stimulus, sending a message to the brain’s sneeze center that something needs to be done immediately. The brain then instant-messages several muscle groups which must contract in a particular order for the sneeze to occur: abdominal muscles, chest muscles, diaphragm, vocal cords, muscles in the back of the throat, and finally, muscles in the eyelids (try sneezing with your eyes open – it’s nearly impossible). According to James Strauss, instructor of biology at Penn State, people may close their eyes as a response to an allergic reaction. “It probably helps to protect the eyes and squeezes out a little bit of tears,” he says.

What does labor feel like? To get the idea, some women say you should imagine squeezing a watermelon out of your yin-yang. Labor is the process in which contractions of the uterine muscle bring about birth. Just to review, the uterus is a hollow muscle with an opening at one end (the cervix). During labor, the uterine contractions cause the cervix to open up (or dilate) and lower the head of the baby into the birth canal. The contractions continue to strengthen until the baby completes its journey through the birth canal and pops out into the world.  

It’s no surprise that labor pains are indeed painful. The force of the muscle contraction is extremely powerful, on the order of about 43 pounds per square inch. To put that into perspective, your car tires are inflated to about 32 pounds per square inch.

And speaking of pressure, babies are exposed to greatly differing pressures through the birth process. The pressure inside the uterus is greater than the ambient atmospheric pressure outside. A vaginal birth helps the baby transition from the differing pressure. Plus the pressure exerted as the baby passes through the birth canal expels amniotic fluid from the lungs in preparation for the first breath. Babies born via C-section sometimes suffer complications because of the extreme change in pressure experienced by their bodies - kind of like a scuba diving accident.