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

The sternocleidomastoid is the largest muscle in the front of the neck, running from the base of the skull to the breastbone and collarbone. It turns the head and tilts the chin, and it also stabilizes the neck, assists breathing in and supports jaw function. Keeping it healthy helps prevent chronic neck stiffness and referred pain.

The superior mesenteric artery is a large artery branching off the front of the abdominal aorta that supplies the midgut, including parts of the pancreas, small intestine and colon. Steady flow through it lets the intestines digest food and absorb nutrients, and reduced flow can quickly damage the bowel. It can also compress nearby structures, causing Nutcracker and SMA syndromes.

Sphincters are circular bands of muscle that open and close to regulate the flow of food, bile, urine and feces between parts of the body. Like tiny gates, they relax to allow passage and contract to seal openings. The body has nine major sphincters in the digestive and urinary systems, plus microscopic ones in blood vessels and the eyes.

Sperm are tadpole-shaped male sex cells, also called gametes or spermatozoa, that carry genetic material to an egg. They are produced in the testicles starting at puberty. A healthy sperm uses its tail to swim through the female reproductive tract, where it can survive for up to five days, and only one needs to penetrate an egg for conception.

Smooth muscle is one of the body's three main muscle types, working involuntarily and appearing non-striated under a microscope. It lines the walls of hollow organs and passages, including the stomach, intestines, bladder, blood vessels and airways. It moves substances through the body and controls passage width, contracting slowly and holding tension for long periods.

Sleep is a complex physiological state in which the body and brain slow down to carry out maintenance tasks, supporting energy storage, cell repair and memory organization. Adults typically need seven to nine hours of quality sleep a night, cycling through four to five periods of 90 to 120 minutes that include light, deep and REM sleep.

The shoulder muscles are about eight voluntary skeletal muscles around the top of the arm that move and support the glenohumeral (shoulder) joint. They hold the ball-and-socket joint in place, protect it and allow movement in many directions. The most important are the four rotator cuff muscles, which provide support and help raise and rotate the arm.

Sebaceous glands are tiny glands, usually attached to hair follicles, that release an oily substance called sebum. Made of fats such as cholesterol and fatty acids, sebum forms a protective coating that keeps skin from drying out, reduces friction and helps defend against bacteria and fungi. The glands are found everywhere except the palms and soles.

Seminal vesicles, also known as seminal glands, are two coiled sacs about two inches long located behind the bladder. They produce a nutrient-rich fluid that makes up 50% to 80% of semen volume. This fluid contains fructose for sperm energy, alkaline substances to neutralize vaginal acidity and prostaglandins that protect sperm, helping them survive on the way to fertilization.

Opsonization is the process of coating a pathogen or damaged cell with proteins called opsonins, such as antibodies and complement factors, to make it recognizable to the immune system. These proteins act as bridging molecules that help phagocytes such as macrophages and neutrophils bind to and ingest the target through phagocytosis, making it easier to destroy.

An oncogene is a mutated gene that causes cancer by driving abnormal cell growth. It starts as a normal proto-oncogene that manages the cell cycle and becomes an oncogene through point mutations, gene amplification or chromosomal rearrangements. Most develop during life from environmental factors or carcinogens. Identifying specific oncogenes allows targeted treatments that block cancer-driving signals.

Neutralizing antibodies are Y-shaped proteins that protect against infection by attaching to pathogens such as viruses and toxins and preventing them from entering healthy cells. By blocking the key a germ uses to enter a cell's receptor, they stop infection at its earliest stage. Producing them is the primary goal of most vaccinations, reducing the chance of becoming symptomatic.

Natural killer cells are a type of lymphocyte, or white blood cell, that patrols the body to identify and destroy virus-infected or cancerous cells. Part of the innate immune system, they use a receptor system to distinguish healthy cells from dangerous ones, then release chemical agents to neutralize the target, providing a rapid response essential for early disease prevention.

Montgomery glands are small oil-producing glands in the areola, the dark skin around the nipple. They secrete a lubricating fluid that protects the skin from chafing, maintains an acidic pH that deters bacteria and gives off a natural scent that aids infant feeding. They often appear as tiny bumps and become more prominent during pregnancy and breastfeeding.

Melanin is a natural substance produced by specialized cells called melanocytes. Its primary role is to provide pigmentation to the body, but it also acts as a powerful antioxidant and UV absorber. By neutralizing reactive oxygen species and shielding cellular DNA from sun damage, melanin serves as one of the body's key protective mechanisms against environmental stressors and premature aging.

Meconium is a newborn baby's first stool, characterized by its tar-like, sticky consistency and blackish-green color. Unlike later bowel movements, meconium is odorless and composed of materials swallowed in the womb, such as water, mucus, and skin cells. Most infants pass this substance within the first 24 to 48 hours of life, a sign their intestines are working.

Macrophages are specialized white blood cells that live in all the body's tissues. They find and destroy harmful germs, damaged cells and cancer cells by swallowing them, a process called phagocytosis. They also help repair tissue, release signals that heal wounds and reduce inflammation, and activate other immune cells such as B cells and T cells.

Luteinizing hormone (LH) is a reproductive hormone made by the brain's pituitary gland that signals the sex organs to perform essential functions. In women, an LH surge triggers ovulation, while in men it stimulates testosterone and sperm production. Healthy LH levels are crucial for puberty, sexual development and fertility, and imbalances may signal underlying conditions.

Lactic acid (lactate) is a natural product of glucose metabolism that cells make all the time, and in much larger amounts when oxygen supply cannot keep up with energy demand. Produced primarily by muscles and red blood cells, it serves as a temporary fuel source during intense physical activity. The liver and kidneys eventually recycle lactic acid back into glucose.

Labial hypertrophy is enlargement of the labia minora or majora, often resulting in the inner vaginal lips being longer than the outer ones. This condition is a natural anatomical difference rather than a disease. Many have no symptoms; others have irritation or discomfort during physical activity, which can be managed through lifestyle adjustments or, in persistent cases, surgery.

The jejunum is the middle portion of the small intestine, an eight-foot-long coiled tube in the upper left abdomen between the duodenum and ileum. Its main job is absorbing nutrients, including proteins, fats, carbohydrates, vitamins and minerals, along with water. Through wave-like contractions called peristalsis, it keeps moving food toward the final section of the small intestine.

The internal thoracic vein, formerly the internal mammary vein, is a paired blood vessel running vertically on either side of the breastbone. It drains oxygen-poor blood from the chest wall, breast, diaphragm and pericardium back to the heart. Its predictable anatomy makes it an ideal choice for connecting blood vessels during complex reconstructive surgeries.

Immunoglobulin G (IgG) is a protein made by the immune system to identify and neutralize invaders such as viruses and bacteria. As the most abundant antibody in the bloodstream, it provides long-term protection by remembering past infections. IgG alerts other immune cells to destroy pathogens or binds to them directly, enabling rapid defense on later exposure.

Hemodynamics is the set of physical principles governing blood flow through the arteries and veins, involving the interplay between the heart's pumping force and blood vessel resistance. Proper hemodynamics deliver oxygenated blood to organs and return deoxygenated blood to the lungs. Vessel size, heart rate and cardiac output must stay balanced to prevent cardiovascular strain.