Anatomy Terms Starting With B

Bile Duct is a tubular channel that conveys bile from the liver and gallbladder into the duodenum of the small intestine, where bile salts emulsify dietary fats to prepare them for enzymatic digestion.

Within the liver, tiny bile canaliculi drain into progressively larger ducts that merge into the right and left hepatic ducts, which unite to form the common hepatic duct. Hepatocytes produce 0.5 to 1 liter of bile per day, containing bile salts, cholesterol, phospholipids, and bilirubin. The cystic duct branches from the common hepatic duct and leads to the gallbladder, where water is reabsorbed to concentrate bile up to tenfold before storage.

When fatty food enters the duodenum, the hormone cholecystokinin triggers gallbladder contraction, and bile flows through the common bile duct, merges with the pancreatic duct at the hepatopancreatic ampulla, and enters the duodenum through the sphincter of Oddi; obstruction of this pathway by gallstones raises bilirubin levels in the blood, causing the yellowing of skin and eyes called jaundice.

Bioluminescence is the production and emission of light by a living organism through a chemical reaction, found in fireflies, deep-sea fish, jellyfish, bacteria, fungi, and many other organisms, used for attracting mates, luring prey, communicating, or defense.

Bioluminescence typically involves a light-emitting molecule called luciferin that reacts with oxygen in the presence of an enzyme called luciferase to produce visible light and very little heat. Different organisms have evolved chemically distinct luciferin-luciferase systems independently, with bioluminescence arising at least 40 separate times across the history of life, making it one of the most repeatedly evolved traits known. In the deep ocean, where no sunlight penetrates, an estimated 90% of species produce some form of bioluminescent light.

Because so little energy escapes as heat, researchers often call this output cold light to distinguish it from incandescent sources.

Blood pressure is the force exerted by circulating blood against the walls of blood vessels, measured in millimeters of mercury and typically recorded as two numbers, such as 120 over 80, in a healthy adult human.

Blood pressure is determined by cardiac output multiplied by total peripheral resistance, expressed as mean arterial pressure equals cardiac output times peripheral resistance. Systolic pressure, the higher value near 120 mmHg in healthy adults, occurs when the left ventricle contracts and ejects blood into the aorta, while diastolic pressure near 80 mmHg occurs when the ventricle relaxes and arterial walls recoil. Pressure decreases progressively from large elastic arteries through muscular arteries, arterioles, and capillaries, approaching zero in the veins near the right atrium.

Sympathetic nervous system activation, blood volume, vessel compliance, and blood viscosity all influence how pressure is regulated moment to moment.

Blood Vessel blood vessel is a tubular structure composed of muscle and connective tissue that transports blood between the heart and the tissues of the body, classified into arteries, veins, and capillaries based on direction of flow, wall structure, and function.

Arteries carry blood away from the heart under high pressure and have thick, elastic walls reinforced with smooth muscle to withstand that force. Veins return blood toward the heart at lower pressure and rely on one-way valves and skeletal muscle contractions to move blood against gravity. Capillaries, the smallest vessels, consist of a single layer of endothelial cells and are the sites where oxygen, nutrients, hormones, and metabolic wastes move between blood and surrounding tissues.

Bone marrow is the soft, vascular tissue filling the internal cavities of bones that produces all blood cell lineages from hematopoietic stem cells throughout adult life.

Red marrow, found in flat bones such as the sternum and ilium and in the epiphyses of long bones, actively generates erythrocytes, platelets, and all classes of leukocyte from multipotent stem cells housed in a specialized niche. Yellow marrow, which predominates in the shafts of long bones in adults, consists mainly of adipocytes but retains the capacity to revert to red marrow during prolonged anemia or severe blood loss. Bone marrow transplantation reconstitutes the entire hematopoietic and immune system and offers curative potential for leukemia, aplastic anemia, sickle cell disease, and severe combined immunodeficiency.

A healthy adult produces roughly 200 billion red blood cells every day through this tissue.

Bowman's capsule is the double-walled, cup-shaped epithelial structure at the beginning of each nephron in the kidney that surrounds the glomerular capillaries and collects the fluid filtered from the blood.

Bowman’s capsule and the glomerulus together form the renal corpuscle, the filtration unit of the nephron. Blood pressure drives water, ions, glucose, urea, and other small solutes across the glomerular filtration barrier and into the capsular space, while large proteins and blood cells are normally retained in the capillaries. The inner visceral layer of the capsule is lined by specialized cells called podocytes, whose interdigitating foot processes create filtration slits that determine what passes through.

Fluid collected in the capsular space, called the glomerular filtrate, then flows into the proximal convoluted tubule for further processing.

Bronchi are the paired, cartilage-reinforced airways that branch from the lower end of the trachea and conduct air into the left and right lungs.

Each main bronchus enters its respective lung at the hilum and then divides repeatedly into lobar and segmental bronchi, distributing air to every region of lung tissue. Cartilaginous rings and plates within the bronchial walls prevent collapse under the pressure changes of breathing. Ciliated epithelium lining the bronchi sweeps mucus and trapped particles upward toward the throat in a process called mucociliary clearance.

The right main bronchus in adult humans is wider, shorter, and more vertically oriented than the left, which is why inhaled foreign objects most often lodge in the right bronchial tree.

Bronchiole bronchiole is a small airway in the lung that branches from the bronchi, lacks cartilage in its wall, and conducts air toward the alveoli where gas exchange occurs.

Bronchioles range from about 1 mm down to less than 0.5 mm in diameter and are lined with ciliated epithelium that grades into simple cuboidal epithelium in the smallest terminal branches. Smooth muscle encircling their walls contracts or relaxes in response to neural signals, hormones, and local chemical mediators, adjusting airway diameter and airflow resistance. During an asthma attack, widespread bronchiolar smooth muscle contraction, combined with mucosal swelling and excess mucus secretion, sharply reduces airflow and produces the characteristic wheeze.

Terminal bronchioles give rise to respiratory bronchioles, which bear scattered alveoli in their walls and mark the transition from purely conducting to gas-exchanging airways.