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

The male reproductive system is a network of internal and external organs involved in reproduction and urination. The penis and scrotum protect and house the testicles, which produce sperm and testosterone, while the prostate and seminal vesicles add fluids to create semen. Hormones such as FSH and LH regulate sperm maturity and transport.

The interstitium is a body-wide network of small, fluid-filled spaces between cells and tissues. Made of interstitial fluid and a mesh of collagen and elastin, it supports and cushions organs and helps transport nutrients, proteins and waste. It is found in nearly every part of the body and is vital for cell communication and the early stages of lymphatic filtering.

The humerus is the single bone of the upper arm, extending from the shoulder joint to the elbow. The longest bone in the body after those in the legs, it supports many muscles and nerves and enables arm movements like flexion and rotation. Weight-bearing exercise, good nutrition and bone density screenings help prevent fractures and support skeletal health.

The brain is a major organ that regulates movement, memory, emotion and automatic processes such as breathing and heart rate. Its three main parts are the cerebrum for conscious thought, the cerebellum for balance and the brainstem for survival functions. It keeps maturing until the mid to late 20s, and staying healthy takes activity, engagement and enough sleep.

The inferior epigastric artery is a major blood vessel that supplies oxygenated blood to the abdominal muscles and belly skin. It branches from the external iliac artery just above the inguinal ligament and curves up toward the belly button, where it meets the superior epigastric artery. Because of its location, surgeons map it carefully during abdominal procedures to avoid bleeding.

The kidneys are bean-shaped organs that filter all of the blood approximately once every hour, removing waste and excess fluid. They manage the body's pH, regulate blood pressure through renin and make hormones such as erythropoietin for red blood cell production. Most people have two, just below the rib cage. Managing diabetes and high blood pressure helps protect them.

The navel, or belly button, is a pushed-in scar on the abdomen that forms after the umbilical cord falls off after birth. It serves as a landmark for dividing the abdomen during exams and as a surgical access point. Navels are innies or outies. Gentle cleaning with soap and water prevents infections, yeast buildup and belly button stones.

The hepatic veins are a group of three major blood vessels in the liver: the right, middle and left hepatic veins. They collect oxygen-poor blood the liver has already processed and send it to the inferior vena cava, which returns it to the heart. When they work properly, blood leaves the liver without backing up, protecting liver function and circulation.

The nervous system is a network of neurons that transmits electrical signals between the brain and the rest of the body to regulate physical and mental activity. It comprises the central nervous system, the brain and spinal cord, and the peripheral nerves. It controls movement, processes sensory information and maintains automatic functions. Protective gear helps protect it.

The hypothalamus is an almond-sized structure deep in the brain that serves as the body's command center for homeostasis. It manages the autonomic nervous and endocrine systems, regulating body temperature, blood pressure, hunger, thirst and sleep cycles. Through its close link with the pituitary gland, it controls the release of hormones governing growth, metabolism and reproduction.

The nasopharynx is a box-shaped muscular passageway at the top of the throat that connects the nasal cavity to the respiratory system, letting air flow from the nose to the windpipe while filtering dust. It contains the adenoids, which support immunity, and the eustachian tubes, which regulate middle ear pressure. Common problems include colds, enlarged adenoids and rare tumors.

The hypoglossal nerve (CNXII) is the 12th pair of cranial nerves and controls all motor functions of the tongue. Running from the brainstem down the neck to the underside of the tongue, it enables clear speech, safe swallowing and chewing. Providers test it by checking tongue protrusion and strength, since dysfunction may signal conditions like stroke or ALS.

The lungs are the primary organs of the respiratory system, responsible for exchanging oxygen and carbon dioxide. Located within the rib cage, these spongy organs move air through bronchial tubes into tiny air sacs called alveoli. Breathing 12 to 20 times per minute, they deliver oxygen to the blood and expel waste gases so all body tissues can function.

The foreskin, or prepuce, is an outer layer of skin that protects the head of the penis and the urethral opening, made of sensitive mucosal tissue and keratinized skin. In infants it is naturally tight and non-retractable but usually loosens with growth. Gentle cleaning once retractable and safe sex practices help reduce the risk of infections.

The gut-brain connection is a two-way communication network in which signals pass constantly between the digestive system and the central nervous system. It involves the enteric nervous system, often called the second brain, the vagus nerve and the gut microbiome. This crosstalk influences hunger, digestion, stress and mood, so a healthy balance supports digestive and psychological well-being.

The cornea is the transparent, dome-shaped covering at the front of each eye. It keeps out debris and germs and bends light so you can focus. It has six layers and is prone to scratches, infections, burns and diseases such as keratoconus, though minor injuries heal within hours. Any eye pain, vision change or injury should be checked.

The amniotic sac is a thin, tough pair of membranes that surrounds the fetus, amniotic fluid, placenta and umbilical cord, protecting the fetus from infection and injury while its fluid cushions bumps. Providers prefer it stays intact until at least 37 weeks. When it ruptures, you may feel a gush or trickle of fluid, so tell your provider right away.

The follicular phase is the stage of the menstrual cycle in which the body prepares an egg for release, starting on day one of your period and ending at ovulation. Follicle-stimulating hormone prompts ovarian follicles to grow, and a dominant follicle releases estrogen that thickens the uterine lining. A surge in luteinizing hormone then triggers egg release.

The great vessels are five major blood vessels that connect directly to the heart: the aorta, pulmonary trunk, pulmonary veins, superior vena cava and inferior vena cava. They manage blood flow between the heart, lungs and body. Wider than other vessels, they handle large blood volumes and withstand high pressure for efficient oxygen delivery and waste removal.

The eye lens is a clear, curved structure that focuses incoming light onto the retina at the back of the eye. Controlled by ciliary muscles and elastic fibers, it automatically changes shape, becoming flatter or rounder, to focus at different distances. Over time, aging and environmental factors can make it cloudy or less flexible, requiring corrective treatment or surgery.

The adrenal cortex is the outer layer of the adrenal glands, which sit on top of the kidneys. Its three layers make different hormones: aldosterone for salt and water balance, cortisol for metabolism and stress, and androgens for puberty and sexual function. Damage usually happens slowly, so symptoms such as fatigue, weight changes and muscle weakness may appear gradually.

The autonomic nervous system is the part of the peripheral nervous system that regulates involuntary functions necessary for life, connecting the brain to internal organs through cranial and spinal nerves. By balancing the sympathetic fight-or-flight response with the parasympathetic rest-and-digest functions, it keeps organs such as the heart, lungs and intestines working without conscious effort.

The conjunctiva is a thin, transparent membrane that lines the inside of your eyelids and covers the white of the eye. It acts as a barrier against irritants and makes the mucus layer of tears. Pink eye and subconjunctival hemorrhage are usually minor, but see a specialist for lasting pain, light sensitivity or vision changes.

The glossopharyngeal nerve is the ninth of twelve cranial nerves, providing motor, sensory and parasympathetic support to the mouth and throat. It helps you swallow safely, provides taste to the back third of the tongue and helps regulate blood pressure through the carotid sinus. Problems can cause swallowing difficulty or sharp neuralgia pain needing medical evaluation.