Babinski sign: The reflexive extension and abduction of the toes after the medial, plantar surface of the foot is stroked; positive reflex (Babinski sign) is normal up to age 1.5 years; thereafter, a positive reflex indicates damage to descending tracts.
bacteria: Single-celled microorganisms, some pathogenic, that are common in the environment and in and on the body.
Bainbridge reflex: See atrial reflex.
baroreception: The ability to detect changes in pressure.
baroreceptor reflex: A reflexive change in cardiac activity in response to changes in blood pressure.
baroreceptors: The receptors responsible for baroreception.
basal metabolic rate (BMR): The resting metabolic rate of a normal fasting individual under homeostatic conditions.
base: A compound whose dissociation releases a hydroxide ion (OH-) or removes a hydrogen ion (H+) from the solution.
basement membrane: A layer of filaments and fibers that attach an epithelium to the underlying connective tissue.
basilar membrane: The membrane that supports the organ of Corti and separates the cochlear duct from the scala tympani in the inner ear.
basophils: Circulating granulocytes (white blood cells) similar in size and function to tissue mast cells.
B cells: Lymphocytes capable of differentiating into plasma cells, which produce antibodies.
benign: Not malignant.
beta cells: Cells of the pancreatic islets that secrete insulin in response to elevated blood sugar concentrations.
beta oxidation: Fatty acid catabolism that produces molecules of acetyl-CoA.
beta receptors: Membrane receptors sensitive to epinephrine; stimulation may result in the excitation or inhibition of the target cell.
bicarbonate ions: HCO3-; anion components of the carbonic acid–bicarbonate buffer system.
bicuspid: Having two cusps or leafs; refers to a premolar tooth, which has two roots, or to the left AV valve, which has two cusps.
bicuspid valve: The left atrioventricular (AV) valve, also called mitral valve.
bifurcate: To branch into two parts.
bile: The exocrine secretion of the liver; stored in the gallbladder and ejected into the duodenum.
bile salts: Steroid derivatives in bile; responsible for the emulsification of ingested lipids.
bilirubin: A pigment that is the byproduct of hemoglobin catabolism.
bioenergetics: The analysis of energy production and utilization by cells.
biofeedback: The use of artificial signals to provide feedback about unconscious, visceral motor activities.
biopsy: The removal of a small sample of tissue for pathological analysis.
bipennate: A muscle whose fibers are arranged on either side of a common tendon.
bladder: A muscular sac that distends as fluid is stored and whose contraction ejects the fluid at an appropriate time; used alone, the term usually refers to the urinary bladder.
blastocoele: A fluid-filled cavity within a blastocyst.
blastocyst: An early stage in the developing embryo, consisting of an outer trophoblast and an inner cell mass.
blastomere: One of the cells in the morula; a collection of cells produced by the division of the zygote.
blockers/blocking agents: Drugs that block membrane pores or prevent binding to membrane receptors.
blood–brain barrier: The isolation of the central nervous system from the general circulation; primarily the result of astrocyte regulation of capillary permeabilities.
blood clot: A network of fibrin fibers and trapped blood cells.
blood–CSF barrier: The isolation of the cerebrospinal fluid from the capillaries of the choroid plexus; primarily the result of specialized ependymal cells.
blood pressure: A force exerted against vessel walls by the blood in the vessels, due to the push exerted by cardiac contraction and the elasticity of the vessel walls; usually measured along one of the muscular arteries, with systolic pressure measured during ventricular systole and diastolic pressure during ventricular diastole.
blood–testis barrier: The isolation of the interior of the seminiferous tubules from the general circulation, due to the activities of the sustentacular (Sertoli) cells.
Bohr effect: The increased oxygen release by hemoglobin in the presence of elevated carbon dioxide levels.
boil: An abscess of the skin, normally involving a sebaceous gland.
bolus: A compact mass; usually refers to compacted ingested material on its way to the stomach.
bone: See osseous tissue.
botulinus toxin: A toxin, produced by the anaerobic bacterium Clostridium botulinum, that can cause severe food poisoning.
bowel: The intestinal tract.
Bowman's capsule: The cup-shaped initial portion of the renal tubule; surrounds the glomerulus and receives the glomerular filtrate.
Boyle's law: The principle that, in a gas, pressure and volume are inversely related.
brachial: Pertaining to the arm.
brachial plexus: A network formed by branches of spinal nerves C5–T1 en route to innervating the upper limb.
brachium: The arm.
bradycardia: A slow heart rate, below 50 bpm.
brain stem: The brain minus the cerebrum, diencephalon, and cerebellum.
brevis: Short.
Broca's area: The speech center of the brain, normally located on the neural cortex of the left cerebral hemisphere.
bronchial tree: The trachea, bronchi, and bronchioles.
bronchitis: An inflammation of the bronchial passageways.
bronchodilation: The dilation of the bronchial passages; can be caused by sympathetic stimulation.
bronchodilators: Drugs that produce bronchodilation; some are used clinically in treating asthma.
bronchoscope: A fiber-optic instrument used to examine the bronchial passageways.
bronchus/bronchi: A branch of the bronchial tree between the trachea and bronchioles.
buccal: Pertaining to the cheeks.
buffer: A compound that stabilizes the pH of a solution by removing or releasing hydrogen ions.
buffer system: Interacting compounds that prevent increases or decreases in the pH of body fluids; includes the carbonic acid–bicarbonate buffer system, the phosphate buffer system, and the protein buffer system.
bulbar: Pertaining to the brain stem.
bulbourethral glands: Mucous glands at the base of the penis that secrete into the penile urethra; the equivalent of the greater vestibular glands of females; also called Cowper’s glands.
bundle branches: Specialized conducting cells in the ventricles that carry the contractile stimulus from the bundle of His to the Purkinje fibers.
bundle of His: Specialized conducting cells in the interventricular septum that carry the contracting stimulus from the AV node to bundle branches and then to Purkinje fibers.
bursa: A small sac filled with synovial fluid that cushions adjacent structures and reduces friction.
bursitis: A painful inflammation of one or more bursae.