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

The diaphragm is a thin, dome-shaped muscle that separates the chest cavity from the abdomen and drives breathing: when it contracts, it flattens and moves down, pulling air into the lungs. It also helps increase abdominal pressure for bodily functions and helps prevent acid reflux by exerting pressure on the esophagus. Its health is vital for respiratory efficiency.

The dermis is the middle and thickest layer of the skin, between the outer epidermis and the inner hypodermis, and serves as its structural foundation. It supports the epidermis, enables sensations like touch and pain, regulates temperature through sweat and keeps skin hydrated through oil. It contains collagen and elastin, which give skin strength and elasticity.

The complement system is part of the immune system made of roughly 50 plasma proteins produced in the liver. They circulate inactive until triggered by injury or pathogens. Once activated, they initiate a chain reaction known as a “cascade,” that targets invaders, triggers inflammation, and removes damaged cells. An overactive or deficient system can lead to autoimmune or inflammatory conditions.

The common hepatic artery is a small artery in the upper abdomen, branching from the celiac trunk. It delivers oxygenated blood to the liver, gallbladder, pancreas, stomach, and duodenum. About 38 millimeters long and 5.5 millimeters wide, it divides into the gastroduodenal artery and the proper hepatic artery. Narrowing or blood clots can occur, particularly after liver transplant surgery.

The coccyx is a small, curved bone at the base of the spine, below the sacrum. It is formed by three to five fused vertebrae. It is part of the body’s support system, acting as one leg of a tripod when seated. It also anchors the gluteus maximus and pelvic floor muscles. Falls can injure it, causing pain (coccydynia).

The clavicle (collarbone) is a horizontal bone approximately 6 inches long at the base of the neck. It connects the arm to the body’s axial skeleton. It supports the upper body, holds the shoulder in position, and rotates for full arm movement. It is prone to fractures from falls or contact sports, accounting for roughly 5% of adult bone breaks.

The circumflex artery (LCx) is one of two main branches of the left main coronary artery. It travels in a groove between the left atrium and left ventricle. It delivers oxygen-rich blood to the heart's left pumping chambers and, in some people, to the SA node. Blockages from plaque buildup can lead to angina or heart attacks.

The central nervous system (CNS) is the body's main processing center. It consists of the brain and the spinal cord and manages everything from thoughts, feelings, and movements to automatic functions such as breathing and digestion. By collecting and interpreting sensory information, the CNS coordinates how your body responds to the world around it.

The circulatory system is the body’s transport network: the heart and more than 60,000 miles of blood vessels. It pumps blood to supply organs, muscles, and tissues with oxygen and nutrients and remove waste like carbon dioxide. The heart circulates about 2,000 gallons of blood every day. Any disruption in flow can lead to serious health issues.

The bronchi are the large tubes that carry air from the windpipe to the lungs, with the left and right main bronchi each dividing into smaller bronchi inside the lungs. A lining of mucus and tiny hairlike cilia moistens the air and traps viruses, bacteria and dust, while cartilage and smooth muscle keep the airways open and control airflow.

The Circle of Willis (CoW) is a pentagon-shaped structure of arteries at the base of the brain that acts as a roundabout for blood flow. It connects the internal carotid artery pathway, which supplies the front, with the vertebral artery pathway, which supplies the back. If an artery is blocked, blood can be rerouted, potentially limiting brain damage.

The brachiocephalic veins are two major veins of the upper body, one on each side, that collect oxygen-poor blood from the head, neck and arms. They join to form the superior vena cava, which empties into the heart's right atrium. Unlike many veins, they have no valves, and providers often use them for central venous access.

The cerebellum is a region of the brain at the back of the head that coordinates voluntary movements, balance, and posture. It also helps with language learning, judging distances, and emotional regulation. Although it occupies only 10% of the brain's volume, it contains more than 50% of the body's total neurons.

The brachial artery is the major blood vessel that carries oxygen-rich blood to the upper arm, elbow, forearm and hand. It continues from the axillary artery near the armpit, runs down the front of the upper arm and divides into the radial and ulnar arteries at the inner elbow. It is also a standard pulse point and blood pressure site.

Antibodies, also called immunoglobulins, are protective proteins made by the immune system that attach to antigens such as bacteria, viruses, allergens and toxins so the body can neutralize or remove them. They are made by B cells, which clone into plasma cells that release millions of antibodies. The five main classes are IgG, IgA, IgM, IgE and IgD.

The biliary tract, also called the biliary tree or system, is the network of organs and ducts that make, store and move bile, including the liver, bile ducts and gallbladder. It helps digest fat and fat-soluble vitamins and removes waste such as bilirubin in stool. Because its parts are linked, a blocked duct can affect the liver, gallbladder and pancreas.

The axillary nerve, also called the circumflex nerve, is one of the five main peripheral nerves of the arm, branching from the brachial plexus to the shoulder. It moves the deltoid and teres minor muscles and carries sensation from the outer shoulder. Injury causes arm-lifting weakness and numbness; most improve with rest and physical therapy, though severe cases need surgery.

The ampulla of Vater is the junction where the common bile duct and pancreatic duct join and empty into the duodenum, the first part of the small intestine. It holds bile from the liver and enzymes from the pancreas and releases them through the major duodenal papilla to help digest food, with flow controlled by the sphincter of Oddi.

The axillary artery is a major blood vessel deep in the armpit that delivers oxygen-rich blood to the shoulder, chest wall and upper arm. It begins at the first rib as a continuation of the subclavian artery and becomes the brachial artery. It is also a useful access point for devices such as heart-lung machines during complex surgery.

The amniotic membrane, or amnion, is a thin, strong layer of tissue forming the inside of the amniotic sac. It holds the amniotic fluid, which cushions the fetus, helps regulate temperature and lets the fetus move freely, supporting healthy growth. Its anti-inflammatory, anti-bacterial and anti-scarring properties are being explored for eye treatments and wound healing.

The axilla, or armpit, is the region between the chest wall and the upper arm, just below the shoulder joint. It is the main passage through which blood vessels and nerves reach the arm. It also contains sweat glands for temperature control and lymph nodes that filter fluid from the breast and arm, making it important for immune health.

The AC (acromioclavicular) joint is where the collarbone connects to the top of the shoulder blade, acting as a bridge that supports shoulder stability and arm movement. Common problems include sprains, separations and arthritis, often from falls, sports collisions or car accidents. After an injury, keep the shoulder still and never try to force it back into place.

The axial skeleton is the central axis of the human body, made up of 80 bones in the head, neck, back and chest. It shields critical organs such as the brain, heart and lungs and gives muscles a stable anchor. It differs from the appendicular skeleton, which includes the limbs; together they form the 206-bone adult skeleton.

The ascending aorta is the first and shortest portion of the aorta, the body's largest blood vessel, carrying oxygen-rich blood from the heart's left ventricle. It lies in the chest behind the breastbone and includes the aortic root and the tubular ascending aorta. It is essential for circulation but can be affected by life-threatening aneurysms and dissections.