6 The human eye is sensitive enough that if the Earth was flat and there was total darkness, it could spot a candle flame flickering up to 30 miles (48 kilometers) away.

The ability of a human eye to perceive an object something depends on how many photons are either emitted or reflected by that object. We can see the stars that are millions and billions of light-years away from Earth, the nearest being the Andromeda galaxy 2.6 million light-years away. In 1941, scientist Selig Hecht and his colleagues at Columbia University tried to find out the minimum number of photos that must fall on the retina for the eye to register the light. Initially, they found that it was between 54 and 148 photons. But when they measured how much the retina actually absorbed, they found that it was just five to 14 photons or rather the activation of five to 15 rod cells.

It takes the eye around four seconds to adapt to uninterrupted darkness and half an hour for 80% of the rod cells to fully adapt. A complete adaption, however, depends on the health of and blood flow to the eyes. When the eyes are adapted, they can easily spot a candle about 30 miles away in absolute dark. (source)

7 If a pregnant woman suffers organ damage, like a heart attack, the fetus can send stem cells to help repair the damage.

While treating pregnant women or women who had recently given birth for heart conditions, doctors observed better recovery rates than any other group of patients. When researchers experimented on mice to understand what is happening, they found that the mice that had a heart attack had eight times as many cells from the fetus in their hearts as the mice that didn’t have a heart attack. They also found that the embryo’s stem cells became various types of heart tissue, including cardiomyocytes, the muscle cells that are responsible for the heartbeat.

There have also been other curious clinical observations in which the hearts of two human mothers who were suffering from heart weakness were found to have cells derived from male fetus cells years after giving birth to their sons. What’s more, it isn’t just the damaged heart that the fetal cells try to repair. They also show up in the brain, lung, and liver when a pregnant woman suffers damage in any of these organs. (source)

8 The human brain consumes over 20% of the body’s energy despite accounting for only 2% of the total body’s weight, or on average 1.4 kilograms.

The brain is the central organ of our nervous system and controls most of the bodily functions, both voluntary and involuntary. Its primary source of energy is glucose, though when glucose levels are low it can also use ketone bodies as fuel. Though the amount of energy the brain consumes does not vary over time, the active regions consume more energy than the inactive regions, a fact that is the basis of MRI and PET scans. Most of the brain’s energy consumption is due to neuronal activity, that is neuronal computation and information processing. The brain receives 15% of the cardiac input, 20% of the total oxygen consumption, and 20% of total glucose consumption, that is approximately 5.6 milligrams of glucose per 100 grams of human brain tissue per minute. (1, 2)

9 Our immune system is so advanced that it actually kills cancerous cells every single day before they lead to cancer.

Two of the three types of lymphocytes, a type of white blood cell, present in our body are B-cells and T-cells. B-cells develop in the bone marrow and T-cells develop in the thymus gland, hence their names. The function of T-cells is to detect and kill virus-infected cells, as well as to help B-cells create antibodies to fight any infection. To generate these antibodies, the B-cells undergo an immense number of mutations and, unfortunately, can sometimes turn cancerous. In a healthy individual, these cancerous B-cells are detected and eliminated by the T-cells every single day protecting us from non-Hodgkin’s lymphomas. It is also the reason why non-Hodgkin’s lymphomas are rare when compared to other types of cancers unless the individual has a suppressed immune system as in the case of HIV/AIDS. (source)

10 The liver is the only internal organ in the human body that can regenerate itself. As little as 25% is enough to regenerate back into a whole liver.

One of the key functions of the liver is to remove toxins which can damage cells in the body. The liver also produces bile to digest fats and stores carbohydrates in complex forms known as glycogen to preserve homeostasis making it essential to survival. However, unlike other internal organs, the liver is capable of rebuilding itself in case of chemical injury or following partial surgical removal because of fibrosis or tumor.

The cells known as hepatocytes are replicated first after which the bile duct cells and other cells are formed. The regeneration of the liver is a very rapid process, and in doing so, it prevents its own failure. Another interesting and astonishing fact is that, even while the regeneration process is going on, the liver’s primary functions such as albumin and bile production are barely affected, although there is a decrease in drug metabolism. (source)