When you’re walking, the sensation of a popping hip joint can be very unnerving. Fortunately, a pain-free snapping or clicking in the hip region is rarely of immediate concern. In many cases, popping in your hip is the result of muscle and tissue imbalances that are often correctable by using stretches and exercises.

The parasympathetic nervous system activates the relaxation, or "rest-and-digest," response. Roughly the opposite of the "fight-or-flight" response of the sympathetic nervous system, the relaxation response involves restoration of energy reserves and other "peace time" functions, such as repair and reproduction.

Diseases and illnesses target all body systems, such as the circulatory, digestive, reproductive, endocrine, neurological, skeletal and muscular systems. Sources of diseases include genetics, bacteria, poor nutrition, viruses, parasites and fungi.

Bones might be the first thing that comes to mind when you think of calcium, but calcium performs other tasks in the body besides building strong bones. Calcium also helps transmit nerve impulses and regulate muscle contractions.

The human hand is a picture of complexity and balance. The muscles that control the hand allow it to be strong and refined. Muscles that influence hand movement either originate on bones outside or inside the hand.

Bacteria reside in great numbers in many places, both inside and on the surface of the human body. In many cases, like in the gut, the presence of bacteria is beneficial. Bacteria in the mouth causes the formation of cavities, and can increase the chance of other disease and illness.

New studies appear almost daily on the benefits of exercise, from lowering your risk of heart disease to improving memory. Whether you exercise for strength, endurance, or flexibility, the functioning of the body is related to physiological functioning.

From single-celled organisms such as bacteria to humans, cells are the basic structural and functional units of life. An adult human body consists of more than 37 trillion cells, according to an estimate published in the November-December 2013 issue of the "Annals of Human Biology."

Nucleic acids are large biomolecules, and include both deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA carries your cells' genetic information. RNA uses this genetic information, and helps the cells make proteins.

Nematodes are highly abundant, non-segmented worms that are present in a variety of habitats. Many are free-living (such as those that live in the soil), while others are parasitic and must attach themselves to a plant, insect, animal or human host to survive and reproduce.

The human body has 11 major organ systems, each with specific functions and energy needs. The cardiovascular system, or CVS, serves all of them, delivering vital nutrients, picking up and distributing metabolic products and carrying away waste for elimination.

The carotid artery carries blood from the heart to the head and brain. There are two carotid arteries, one on each side of the neck. Over time, carotid arteries can become clogged with plaque as a result of coronary artery disease.