Doctor-Reviewed Health Guides: Conditions, Procedures & Health News

The shoulder joint, medically known as the glenohumeral joint, is a ball-and-socket joint where the upper arm bone (humerus) meets the shoulder blade (scapula). It has the greatest range of motion of any joint in the body. The rotator cuff muscles and tendons, along with cartilage and ligaments, give it the stability needed for lifting, reaching and rotating.

A scab is a hard, dried blood clot that forms over a cut or scrape to stop bleeding and protect the tissue underneath from infection. It is made of platelets, red and white blood cells and a sticky protein called fibrin. It falls off naturally once new skin forms, and picking it can cause scarring and slow healing.

Red blood cells (erythrocytes) are disk-shaped cells that deliver oxygen to the body's tissues and remove carbon dioxide, using the protein hemoglobin to bind oxygen in the lungs. They are made in the bone marrow, circulate for about 120 days and make up about 40% of the blood. A healthy count is vital for energy production.

Coronary perfusion pressure (CPP) is the pressure that pushes blood from the aorta into the coronary arteries and through the heart muscle. It matters because the heart uses about 70% to 80% of the oxygen in the blood passing through its arteries. If CPP drops too low, the heart lacks oxygen, which can cause ischemia or a heart attack.

The olfactory nerve (CN I) is the first pair of cranial nerves and the shortest sensory nerve, carrying signals from smell receptors in the upper nose to the brain. Damage can cause reduced, lost, distorted or phantom smells, from causes such as sinus infections, head injury, toxins and COVID-19. See a provider if your sense of smell changes suddenly.

The nipple is the small structure at the center of the breast, surrounded by a darker area called the areola. It contains smooth muscle, nerves and milk duct openings, and it delivers milk during breastfeeding and provides sensation. Nipples may protrude, be flat or inverted, but discharge, a rash or a new inversion needs medical evaluation to rule out cancer.

The optic chiasm is where the optic nerves from both eyes meet and partly cross at the base of the brain, with only the inner-half fibers crossing. This helps the brain combine both eyes' views and judge depth. Nearby tumors and aneurysms can compress it, causing blind spots and loss of side vision in both eyes. Report any vision change.

The oculomotor nerve (CN III) is the third of 12 pairs of cranial nerves, serving seven muscles in and around each eye. It helps open the eyelids, move the eyes, focus and adjust pupil size. Damage can cause a drooping eyelid, double vision and an enlarged pupil, from causes such as diabetes, trauma or stroke. Sudden changes need prompt evaluation.

Mucosa is the medical term for the mucous membranes that line internal body surfaces exposed to the outside environment, such as the digestive, respiratory and reproductive tracts. It lubricates organs, traps pathogens in mucus and shields tissues from abrasive particles, stomach acid and urine, while immune cells defend against invaders. It also helps absorb nutrients.

NAD is a small molecule found in every cell that carries electrons between chemical reactions, helping cells turn food into energy, repair DNA and resist stress. It exists as NAD+ and NADH. The body makes it from building blocks such as vitamin B3 and tryptophan. NAD levels naturally decline with age, and evidence for NAD+ supplements is still developing.

Nerves are bundles of fibers that act as the body's electrical wiring, carrying signals between the central nervous system and the rest of the body. Sensory nerves carry touch and sight to the brain, while motor nerves command muscles. Good nutrition, exercise and managing conditions like diabetes protect them, and persistent pain or numbness should be checked.

Muscles are pieces of soft tissue made of thousands of small fibers that move the body and its organs. There are more than 600, grouped into skeletal, cardiac and smooth types. Minor strains are common and often managed with rest, ice, compression and elevation, but pain lasting over a week needs checking, and trouble breathing or swallowing needs emergency care.

Motilin is a gastrointestinal hormone that regulates the movement of food from the small intestine to the large intestine. It triggers rhythmic contractions called the migrating motor complex, which clears undigested food and helps prevent bacterial overgrowth. Motilin levels rise during fasting and fall after eating fats or sugar, and it stimulates gastric emptying and insulin release.

Mucus is a slippery, gel-like fluid that shields the body by keeping germs out while letting oxygen and nutrients in. It is made mostly of water and a protein called mucin and contains antibodies and white blood cells. Healthy mucus is usually clear and thin, and dark, very bloody or unusual-colored mucus is a reason to see a provider.

Nail anatomy is the structure of the nail unit: the visible nail plate plus the tissues around and beneath it, including the matrix that produces the plate, the nail bed, the nail folds and cuticle, and the hyponychium. Nails protect fingertips and toes and aid touch. Unexplained nail changes, especially a dark or red line, should be checked.

Monocytes are the largest type of white blood cell and an essential part of the immune system. Made in the bone marrow, they circulate in the blood before maturing into macrophages or dendritic cells in tissues, where they destroy germs or alert other immune cells to infection. A healthy monocyte count helps defend against viruses and bacteria.

Metabolism is the continuous set of chemical reactions in your cells that sustain life by converting nutrients from food into energy for breathing, circulation and cell repair. Your total metabolic rate combines your basal metabolic rate (BMR), the energy used for digestion and the energy used for physical activity. The system regulates itself to maintain internal balance, known as homeostasis.

Menarche is the medical term for the first menstrual period, signaling the start of the reproductive years. In the United States, the average age is 12, though it typically occurs between ages 9 and 15, usually about two years after breast development begins. Common signs include light bleeding, cramping and mood swings, a natural part of puberty.

Mast cells are immune cells made in the bone marrow that mature in tissues throughout the body. When they detect pathogens, toxins or allergens, they release chemicals such as histamine to alert other immune cells. When overactive, they can cause allergies, allergic asthma and anaphylaxis, and rarely disorders such as mast cell activation syndrome (MCAS) and mastocytosis.

Lymphocytes are white blood cells that play a central role in the immune system, making up about 20% to 40% of white blood cells. The two main types are T cells, which control immune responses and attack infected cells, and B cells, which make antibodies. Memory cells remember past invaders, which is why vaccines work. Abnormal counts usually reflect infection.

Lymphatic capillaries are tiny, closed-ended vessels found in the tissues of most organs. Their one-way walls let fluid from around cells flow in but not back out. By returning this extra fluid, called lymph, to the bloodstream, they help maintain normal blood pressure and volume and prevent fluid buildup. Poor drainage can cause swelling such as lymphedema.

Lymph is a body fluid that flows through the lymphatic system to support immunity and fluid balance. It begins as plasma that seeps out of blood capillaries, delivers nutrients to tissues and collects debris, bacteria and viruses. It is filtered through lymph nodes, where white blood cells destroy invaders, before returning to the bloodstream. Blockages cause swelling.

Ligaments are connective tissues made mainly of collagen and elastin fibers that link bones to other bones. They reinforce joints, prevent excessive movement and secure internal organs in the abdomen. Sudden impacts or twists can overstretch or tear them, causing sprains. Because they have a limited blood supply, ligament injuries often need significant time, rest and rehabilitation to heal.

Lip anatomy describes the upper and lower lips, the folds of tissue around the mouth, including the surrounding skin, the reddish vermilion and the inner mucosa. The orbicularis oris muscles move the lips, and lips help with speaking, eating and sealing the mouth. Because the vermilion has no sweat or oil glands, lips are prone to chapping and sun damage.