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Aerobic exercise is planned, structured physical activity in which large muscle groups typically move rhythmically for a sustained period, with a substantial proportion of the required energy supplied through aerobic metabolism. Common forms include walking, running, …
Aerobic metabolism is the set of energy-yielding metabolic processes in which molecular oxygen ultimately serves as the final electron acceptor. In human cells, it allows carbohydrates, fatty acids, and some amino acids to be extensively oxidized …
An afferent pathway is a neural pathway that carries sensory information toward the central nervous system, meaning the spinal cord and brain. Afferent signals originate when sensory receptors detect changes inside or outside the body, such …
Agility, in sport and exercise science, is the ability to make a rapid whole-body movement involving a change in speed or direction in response to a stimulus. The stimulus may come from an opponent, ball, target, …
Agonist–antagonist pairing describes the functional relationship between muscles or muscle groups that produce opposing actions around a joint. For a particular movement, the agonist is the muscle or group contributing force in the intended direction, while …
Alpha-linolenic acid (ALA) is an essential polyunsaturated fatty acid containing 18 carbon atoms and three cis double bonds. It is written in fatty-acid shorthand as 18:3 n-3, or 18:3 omega-3. The omega-3 designation means that the …
Alpha-tocopherol, or α-tocopherol, is a fat-soluble compound and the form of vitamin E used to define human vitamin E requirements. Vitamin E is a broader family that includes four tocopherols and four tocotrienols, so alpha-tocopherol and …
Alveolar pressure is the pressure of gas inside the lung’s alveoli, the microscopic air spaces where gas exchange occurs. It is commonly expressed relative to atmospheric pressure, with atmospheric pressure treated as zero. When alveolar pressure …
Alveolar ventilation is the volume of fresh air that reaches the gas-exchanging regions of the lungs each minute and is available for the exchange of oxygen and carbon dioxide. It represents the effective portion of total …
An amino acid is an organic molecule containing an amino group and a carboxyl group. The amino acids used to build proteins are primarily alpha-amino acids, in which these groups are attached to the same central …
Anabolism is the part of metabolism that builds larger or more complex molecules from smaller precursors. Anabolic pathways synthesize cellular components such as proteins, nucleic acids, glycogen, and lipids. Because constructing these molecules is often energetically …
Anaerobic metabolism refers to metabolic processes that can regenerate ATP without depending directly on oxygen-dependent mitochondrial respiration. In human cells, major sources of rapidly available ATP that do not directly require oxygen include the phosphocreatine system …
Anandamide is an endocannabinoid messenger that acts locally across tissues, with effects shaped by receptors and context rather than one pleasure function.
Anaphylaxis is a serious systemic hypersensitivity reaction that usually begins rapidly and can become life-threatening. Severe anaphylaxis can compromise the airway, breathing, or circulation and may cause death. Although hives, flushing, swelling, or other skin and …
Angiotensin II is an eight-amino-acid peptide hormone and a major biologically active effector of the classical renin-angiotensin system. This hormonal system helps regulate blood pressure, sodium balance, and extracellular fluid volume. Angiotensin II acts in the …
Anorexia of aging is a reduction in appetite that occurs more commonly in older adulthood and can lead to reduced food intake. In this context, “anorexia” means loss of appetite and does not refer to anorexia …
Anthocyanins give many plants red, purple or blue colors, while differences in chemical structure and processing shape their stability and biological exposure.
ApoB, or apolipoprotein B, is a large structural protein found on several classes of lipoprotein particles that transport lipids through the blood. Two main forms occur in humans: apoB-100, produced primarily by the liver, and apoB-48, …
Arterial CO₂ usually refers to the partial pressure of carbon dioxide in arterial blood, abbreviated PaCO₂. It reflects the partial pressure of dissolved carbon dioxide and is commonly measured as part of an arterial blood gas …
Aspartame is a high-intensity sweetener used to provide sweetness in foods and beverages. Chemically, it is the methyl ester of a dipeptide formed from the amino acids aspartic acid and phenylalanine. Aspartame is about 200 times …
Atherosclerosis is a chronic disease of medium and large arteries characterized by the development of plaques within the innermost layer of the arterial wall, called the intima. These plaques contain lipids, inflammatory cells, smooth muscle cells, …
Athlete’s heart is the collection of physiological structural, functional, and electrical adaptations that can develop in response to sustained exercise training. It reflects exercise-induced cardiac remodeling in response to repeated increases in circulatory demand rather than …
An autoimmune disease is a disorder in which immune responses directed against the body’s own molecules, cells, or tissues contribute to pathological damage or dysfunction. Normally, the immune system maintains self-tolerance through mechanisms that limit harmful …
Autoimmune gastritis damages acid-producing stomach tissue, potentially impairing iron and vitamin B12 absorption before anemia develops.
Automaticity describes how a response starts with less conscious deliberation, often as repeated actions become linked to recurring cues.
The axial skeleton is the central framework of the human skeleton, forming the longitudinal axis of the head and trunk. In an adult, it consists of 80 bones and includes the skull, vertebral column, and thoracic …
Ballistic stretching is a stretching technique that uses relatively rapid, momentum-driven movements, often involving bouncing or repeated movements toward the end of a joint’s available range of motion. Unlike a sustained stretch, the relevant soft tissues …
The base of support (BoS) is the region over which a body is mechanically supported through contact with the environment. In human standing, it is determined mainly by the position and contact areas of the feet. …
Bile salts are amphipathic molecules derived from bile acids that play a central role in the digestion and absorption of dietary lipids. Bile acids are synthesized from cholesterol in the liver and are commonly conjugated with …
Bioavailability describes the extent to which a substance becomes available for biological use after it is ingested or otherwise administered. In nutrition, it generally refers to the proportion of an ingested nutrient that is absorbed and …
Bioelectrical impedance analysis (BIA) is a noninvasive method used to estimate body composition by measuring how the body opposes the flow of a small alternating electrical current. This opposition is called electrical impedance. Because electrical current …
A biomarker is a defined characteristic that is measured as an indicator of a normal biological process, a disease-related process, or a biological response to an exposure or intervention, including treatment. Biomarkers may include molecular measurements, …
Biomechanics is the scientific study of biological systems using principles of mechanics, the branch of physics concerned with forces, motion, and deformation. It examines how mechanical forces act on or are generated within living organisms and …
Blanching is a brief heat treatment in which vegetables are exposed to boiling water or steam. When preparing vegetables for freezing, this heating is followed by rapid cooling to stop further cooking. The treatment also softens …
Body density is the mass of the body divided by its total volume. In body-composition assessment, it is a derived quantity that can be used to estimate the relative amounts of fat mass and fat-free mass. …
Body mass index, or BMI, is a calculated measure of body weight relative to height. It is obtained by dividing a person’s weight in kilograms by the square of their height in meters, giving units of …
Body composition refers to the amounts and proportions of the different components that make up the human body. Rather than treating body weight as a single quantity, body composition separates it into components that can be …
In digestive physiology, a bolus is a cohesive mass of chewed food mixed with saliva and formed in the mouth for swallowing. As food is chewed, it is mechanically broken into smaller pieces and mixed with …
Bone mineral density estimates mineral content in bone, but fracture risk also depends on structural features that a standard DXA scan does not capture.
Bone resorption is the physiological process by which existing bone tissue is broken down and removed. It is carried out primarily by osteoclasts, large multinucleated cells derived from the monocyte-macrophage lineage. Bone resorption is a normal …
The Borg Rating of Perceived Exertion scale, or Borg RPE scale, is a 15-point scale used to rate how hard physical exercise feels. Developed by Gunnar Borg, it ranges from 6, representing no exertion, to 20, …
Brain age models estimate age from imaging features, so the difference from calendar age does not directly measure years of biological brain aging.
BDNF supports neuronal maintenance and plasticity, but blood concentrations do not directly measure its activity within a particular brain region.
The brainstem is the part of the brain that connects the diencephalon and other higher brain regions with the spinal cord. It lies in front of the cerebellum and consists of three major regions: the midbrain, …
Butyrate, also called butanoate, is the negatively charged conjugate base of butyric acid, a four-carbon short-chain fatty acid. In the human colon, butyrate is produced mainly by members of the gut microbiota during anaerobic fermentation of …
C-reactive protein rises in response to inflammatory signals, but a blood measurement alone cannot identify the cause or location of inflammation.
Caffeine is a stimulant of the central nervous system belonging to a chemical family called methylxanthines. It occurs naturally in plants including coffee and tea and can also be added to products. Its familiar effect is …
Caffeine half-life describes how slowly caffeine levels fall during elimination, and individual differences mean earlier doses can remain for hours.
