The human body is a marvel, to say the least. It took 3.8 billion years for it to evolve from a single cell into what it is today. Since we first achieved self-awareness, our curiosity about our body and our origins has helped us understand much and aided the field of medicine immeasurably. It is no wonder that no matter how much we learn about our body, there are always things about it that will amaze and astonish us. So, here are some amazing facts about human body that we are sure you would enjoy reading about.

The brain itself cannot feel pain. It has no pain receptors.
Receiving and processing sensory input such as vision, smell, touch, or taste are part of all the innumerable complex functions that brain performs. Among these sensory inputs is pain which can come from any part of the body. These inputs and signals are interpreted by the brain as �pain�� which indicates that something bad is happening.  The brain then initiates actions which it determines are required to deal with it. However, the brain itself cannot feel pain if anything happens to it because it has no pain receptors. So, if you are experiencing headache, the pain is probably coming from the membranes surrounding the brain, the connective tissue over your skull or the scalp.

If you donate or damage a part of your liver, it will grow back to its full size.
Liver Regeneration: The liver is the only internal organ that has regenerative abilities. If the liver is either surgically removed or suffers chemical damage, it can regenerate. It is said that as little as 25% of the liver is enough for it to regenerate itself back to full size, though for donation there are several criteria that has to be met. The donors must have a matching or compatible blood type, should be between 18 and 60 years old and healthy. However, the regeneration process in mammals is not true regeneration. The liver can only restore its function and mass, but not its original form. The liver lobes do not grow back. True regeneration, that is restoration of function, mass, and form, only happens in lower species such as fish.

The acid in our stomach is strong enough to dissolve razor blades.
The stomach is the organ that digests the food we eat and along with that has to take care of the bacteria and other harmful things that enter it. The gastric acid it releases is composed of hydrochloric acid, potassium chloride, and sodium chloride. It kills the bacteria and activates the digestive enzymes. It has a pH between 1.5 and 3.5 which makes it a very strong acid. Scientists experimented on razor blades, disc batteries, and pennies by putting them in simulated gastric juice to find out how the best way to deal with them when they are accidentally swallowed. They found that after twenty-four hours the blades weighed 63% of their original weight and became fragile. The disc batteries, however, showed no leakage, and the pennies were not affected by the acid.

We shed differently structured tears depending on the reason we're crying.
When put under the microscope, tears due to grief, hope, or onions all show unique structure. We produce three different types of tears. Basal tears are those produced naturally by any healthy mammal to keep the eyes wet and lubricated. Reflex tears are those produced as a result of irritation due to foreign particles, chemicals, onions, when eating hot or spicy foods, or sudden exposure to light. Crying or weeping tears are those that we shed due to emotional stress, pain or even pleasure, like during uncontrollable laughing. All these types of tears have a different composition because they are meant for different purposes. Basal tears contain oils, salts, liquids and antibacterial elements, whereas crying and weeping tears contain protein-based hormones relevant to the emotion you are experiencing. Photographer Rose-Lynn Fisher took microscopic photographs of her tears during various situations as part of the project �The Topography of Tears��. These photographs show how uniquely these tears are structured.

Nobody knows the exact number of muscles in a human body.
Depending on how you count, it varies between 656 to 850 to over 50 billion!. There are essentially three types of muscles in our body: skeletal, cardiac, and smooth muscles. The reason we don't know the exact number of muscles is because there are varied opinions about what could be called a muscle. The skeletal muscles are those that could be voluntarily controlled by us and are what we use to move our body. Skeletal muscles are always in pairs, and there are 320 pairs of them. However, considering there are different ideas about how they are grouped, there is a total of anywhere from 640 to 850 skeletal muscles in the human body. The second type of muscle, the cardiac muscle, is just a single piece of muscle. Smooth muscles are those that are found in our intestines, the urinary tract, trachea and in the linings of arteries and blood vessels, the iris of the eye, and skin. These muscles function involuntarily and exist in huge numbers. In total, it could be said that there are over 50,100,000,701 (over 50 billion) muscles in our body.

There's actually a blind spot in our vision, but our brain fills the gap.
The blind spot is a very common phenomenon among vertebrates. It is because the light-detecting photoreceptor cells are absent in the area where the optic nerve passes through the retina. Because of that, the corresponding field of vision becomes invisible as no light is detected from there by our eyes. The phenomenon was first discovered in the 1600s by Edme Mariotte in France. Until then, the point where the optic nerve enters the eye was thought to be the most photosensitive part of the eye.

Our brain filters out quite a lot of sensory information it receives.
Your nose, for example, is something your eyes always see but your brain just ignores it. Sensory �gating�� or filtering is a neurological process which filters out all the unnecessary or redundant information from the environmental stimuli we receive in our brain. Information such as the blinking of your eyes, the clothes on your skin, breathing, swallowing, or your tongue moving to a comfortable position in your mouth without any conscious input from you, is always filtered out unless you are consciously trying to experience them to prevent overload in the higher cortical centers of the brain. All this filtering work is done by the tiny part of our brain known as the �thalamus��, which is the point from where the sensory and motor neurons relay signals and which also is thought to be involved in a person's consciousness.

At the beginning of a dangerous or stressful situation, your blood thickens.
The body does this to encourage clotting in case of any physical injury. This is why stress often causes heart attacks. At the onset of anxiety and stress, a varied set of responses begins in our body to help us deal with the situation. Among them is the activation of blood coagulation. A study was conducted a few years ago involving a group of people suffering from anxiety disorder and a group of healthy people. After taking a round of blood samples from these people, they were asked to take a number of tests on the computer. A second round of blood samples was taken upon finishing the tests. What the scientists found was that the group with an anxiety disorder had a much more highly activated coagulation system. The scientists believe that this could explain why anxiety patients have a statistically higher risk, three to four times higher, of dying from heart diseases.

Human skin is actually covered in stripes called Blaschko's Lines.
They cover our body from head to toe, we just can't see them. In the early 1900s, German dermatologist Alfred Blaschko found that the rashes and moles on the skin of many of his patients followed specific patterns. Those patterns, however, did not follow the nerves, blood vessels or any other known body system. He traced these patterns on the body. However, it was later found that they are far more extensive than the number Blaschko had thought. These patterns are a result of how the embryonic cells migrate to accommodate for the formation of new cells on the skin of a quickly growing body. They follow a �V�� shape over the back, �S�� shaped whirls over the chest and sides, and wavy shapes on the head.(1, 2)

It takes around thirteen milliseconds for your brain to perceive images after they actually happen. So, you are basically always living in the past.
During an MIT research project, many subjects were shown a series of six to twelve images, each image lasting for thirteen to eighty milliseconds, gradually decreasing the exposure period from eighty to thirteen milliseconds. They were asked to look for a particular type of image among them. Professor Mary Potter, one of the researchers in the study, conducted a similar study previously and found that humans could recognize images that were seen for as little time as one hundred milliseconds. But, in the new study, she found that the brain can recognize them for as less as thirteen milliseconds, which was the fastest rate at which the computer monitor they used could display the images.