Hydrogen is vital for life, the third most common element in the human body and a component of water, DNA and most other organic molecules. Macronutrients, such as carbohydrates, proteins and fats, contain hydrogen as part of the chemical structure, and water contains hydrogen as well.

It eventually happens to almost all contact lens wearers: You fall asleep while still wearing your contacts. With approximately 10 percent of Americans wearing contact lenses regularly, according to the American Academy of Ophthalmology, concerns about sleeping with contact lenses are common.

Bacteria are the oldest and most abundant life form on Earth. These microscopic, single-celled organisms live in all environments -- including in and on the human body. A study published in May 2016 in the "Proceedings of the National Academy of Sciences of the U.S.A."

The typical human skeleton consists of 206 bones in adults. More bones are present at birth, which gradually fuse together as the body matures. The skeleton is divided into two parts. The axial skeleton includes the bones of the skull, face and spine along with the ribs and breastbone.

There are many different roles that nucleic acids, which include DNA and RNA, play in the human body and in other living organisms. Scientists continue to identify new and different functions of nucleic acids on a regular basis.

Bacteria and fungi occur in all environments. In humans, these lifeforms can be harmless passengers, participate in beneficial biological processes, or cause disease. Bacterial and fungal infections affect people of all ages and range from minor to potentially life-threatening.

You are at the beach when a gust of wind blows sand into your eye. Your eye is immediately tearing and irritated. The next steps you should take include flushing the eye with water and planning a visit to your eye doctor for careful inspection.

Your individual degree of symmetry may give off signals that others perceive unconsciously. Humans have bodily symmetry, similar to other animals and organisms in nature, which may be a factor that helps us control and interact with other people and organisms.

When you eat a meal, your body gets to work digesting it using enzymes. The job of these substances is to break up the food into particles that can be absorbed by your body. Several of these enzymes -- amylase, lipase and proteases -- are produced in an inactive form in the pancreas, an organ in your abdomen.

Sunburns can range from tender, pink skin to painful blisters accompanied by nausea and fever. They're known to contribute to premature aging and cancer. Vitamins, when combined with sunscreen and protective clothing, can lessen the sun's damaging effects.

Sometimes, chemical compounds can have wildly different structures and properties despite similar names.

Physiology encompasses the tissue, organ and cellular functions of the human body. An optimally healthy body has normal physiology. Conversely, human disease represents an aberration in normal physiology--something in the body fails to function as it should.