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

Leptin is a hormone released by body fat that helps maintain long-term body weight by regulating hunger and signaling fullness, acting mainly on the brain's hypothalamus. Higher fat stores raise leptin levels, but chronic elevation can cause leptin resistance, in which the brain ignores fullness signals, leading to persistent hunger and a lower metabolic rate despite adequate energy stores.

The linea nigra is a vertical streak of dark pigmentation on the abdomen during pregnancy, usually running from the pubic bone to the belly button. Pregnancy hormones increase melanin and darken the linea alba, a faint line everyone has. It usually appears around week 20, is harmless and typically fades over several weeks or months after delivery.

Lanugo is the first hair produced by fetal hair follicles, forming a protective layer during development in the uterus. It helps regulate temperature and holds vernix in place to protect skin from amniotic fluid. Most fetuses shed it in the final eight weeks of pregnancy. In older children and adults, lanugo is often linked to severe weight loss and malnutrition.

The leg muscles are skeletal muscles that work with bones, tendons and ligaments to produce movement and support posture. Upper leg muscles include the quadriceps, which extend the knee, and the hamstrings, while lower leg muscles such as the gastrocnemius and soleus move the foot and toes. Common problems like strains and cramps are managed with rest, ice and stretching.

Knee ligaments are tough bands of soft tissue that connect the femur (thigh bone) to the tibia and fibula (lower leg bones). They stabilize the knee joint, hold bones in position and prevent unnatural twisting or collapsing. The four main ligaments, the ACL, PCL, MCL and LCL, work together, and warm-ups, supportive footwear and a healthy weight help protect them.

A joint is any place in the body where two bones meet, ranging from immovable fibrous joints in the skull to highly mobile synovial joints such as the shoulder and knee. Arthritis, bursitis and injuries can damage them. Staying active with low-impact exercise, eating well, wearing protective gear and seeing a provider for new joint pain support joint health.

Insulin is a naturally occurring hormone made by the pancreas that lets your body use glucose for energy by moving it from the bloodstream into cells, lowering blood sugar. Without enough working insulin, glucose builds up in the blood, leading to high blood sugar and diabetes. Manufactured insulin, from rapid-acting to ultra long-acting, helps people manage these conditions.

Inguinal lymph nodes are immune system filters in the groin crease, where the thighs meet the lower abdomen. They remove germs from lymph fluid draining from the lower body and are organized into superficial and deep layers. They can enlarge during an immune response, and a hard or persistent lump should be checked by a professional.

Immunoglobulin D (IgD) is one of the five major classes of antibodies, found on B cells and in the airways, mouth, tonsils, blood and tears. It acts as a B-cell receptor, recognizing antigens and triggering an immune response. Low IgD usually causes no symptoms, while high levels can point to chronic infection, autoimmune disease, smoking or certain blood cancers.

Human chorionic gonadotropin (hCG) is the pregnancy hormone made by the placenta after conception. It stops menstruation and signals production of progesterone and estrogen needed for pregnancy. hCG rises quickly in early pregnancy, almost doubling every three days, and peaks around 10 weeks. Providers also use it in fertility treatment and as a tumor marker for some cancers.

The human body is a complex collection of about 30 trillion cells that form tissues, organs and organ systems. Its 12 organ systems, including the cardiovascular, nervous and skeletal systems, depend on one another to maintain internal balance, called homeostasis. Together they let you digest food, move, breathe and filter waste, and disease happens when systems aren't working properly.

The clitoris is a complex network of erectile tissue and nerves, partly outside and partly inside the body, and the most sensitive part of the vulva. Only the glans is visible; the rest branches around the vagina and urethra. It enables sexual pleasure. Soreness, pain, itching or skin changes that don't go away are worth discussing with a gynecologist.

Genes are sections of DNA that give cells specific instructions for traits and body functions, acting as a code for making proteins that control everything from hair color to heart function. Humans have approximately 20,000 coding genes, inherited in pairs from biological parents. Genes interact in patterns such as dominant and recessive traits to shape physical characteristics and health.

Ghrelin is a hormone made mainly in the stomach that signals the brain's hypothalamus that it is time to eat. It peaks right before meals and drops after eating, and also promotes fat storage and growth hormone release. Dieting and weight loss raise it, which may explain weight loss plateaus. Good sleep and balanced meals may help.

Granulocytes are white blood cells containing small, enzyme-filled granules that rush to sites of infection or inflammation to fight bacteria, parasites and allergens. The four main types are neutrophils, eosinophils, basophils and mast cells, each with unique roles. A healthy granulocyte count typically ranges between 1,500 and 8,500 cells per microliter of blood.

Gastrin is a hormone produced by G-cells in the stomach lining that triggers the release of gastric acid, activates digestive enzymes and stimulates stomach contractions that move food along. It is essential for healthy digestion, but excessive levels, often from certain conditions or medications, can cause acid overproduction, leading to painful ulcers, chronic reflux and damage to the digestive lining.

Fimbriae are small, finger-like extensions at the outer end of the fallopian tubes near the ovaries. They capture the egg released during ovulation and guide it into the fallopian tube using hair-like cilia. Because they are vital for moving the egg toward the uterus, damage or blockage can significantly affect a person's ability to conceive.

The fibula is the smaller of the two lower leg bones, commonly called the calf bone, running along the outer leg parallel to the tibia. It gives the calf structure, stabilizes the ankle and anchors muscles, tendons and ligaments. Although it bears little weight, it can fracture or weaken from osteoporosis, so diet and exercise help protect it.

A fetal pole is the earliest visible sign of an embryo on prenatal ultrasound, appearing as a curved structure within the gestational sac. It is often seen by week six with a transvaginal ultrasound but may take up to nine weeks. Its crown-to-rump length helps date the pregnancy and monitor growth, and a heartbeat usually becomes detectable around six weeks.

The femur is the thigh bone and the longest and strongest bone in the body, running from the hip joint to the knee joint. It bears body weight, stabilizes movement and connects hip and knee muscles, tendons and ligaments. Fractures usually need severe trauma, but osteoporosis can weaken it, so good nutrition, exercise and checkups help protect it.

Fallopian tubes are a pair of hollow, muscular ducts connecting the ovaries to the uterus that are essential for conception. They catch the egg after ovulation, host fertilization and move the embryo toward the uterus using cilia and muscle contractions. Up to 1 in 3 infertility cases stem from tubal problems such as blockages or infection scarring.

Facial muscles are about 20 flat skeletal muscles that attach to the skull and insert into the skin. Their two major jobs are chewing and making facial expressions, and they also help you talk and protect your eyes. The facial nerve controls them, and sudden facial weakness or drooping needs prompt attention as a possible stroke.

Eustachian tubes are small passages of bone and cartilage that connect each middle ear to the back of the throat. They balance air pressure and drain fluid, opening when you yawn or swallow so the eardrum can vibrate properly. When swollen, they can cause discomfort, muffled sound and balance problems, which usually resolve with home remedies.

Estrone (E1) is a naturally occurring female sex hormone and the weakest of the three main types of estrogen. It is produced mainly in the adrenal glands, body fat and ovaries and supports bone strength and blood vessel health. After menopause, it is the body's primary natural estrogen source, and obesity can cause overproduction that may raise certain cancer risks.