A Band
The A band is the region of a sarcomere that spans the full length of the thick, myosin-containing filaments in striated muscle. It lies near the center of each sarcomere and appears darker than the neighboring …
Browse the journal
Knowledge
Follow journal articles connected to Alpekin knowledge topics.
376
published entries
Explore articles
Knowledge
A1 Receptor
The A1 receptor, or A1 adenosine receptor, is a cell-surface receptor that responds to the signaling molecule adenosine. It is one of four known adenosine receptor subtypes, alongside A2A, A2B, and A3. All four belong to …
A2A Receptor
The A2A receptor, or A2A adenosine receptor, is a cell-surface receptor activated by the signaling molecule adenosine. It is one of four adenosine receptor subtypes, alongside A1, A2B, and A3, and belongs to the G-protein-coupled receptor …
ADP
ADP, or adenosine diphosphate, is a nucleotide that plays a central role in cellular energy metabolism. It consists of adenosine, which contains the nitrogenous base adenine and the sugar ribose, attached to two phosphate groups. ADP …
ATP
ATP, or adenosine triphosphate, is a nucleotide that serves as a central carrier of usable chemical energy in cells. It consists of adenosine, which contains the nitrogenous base adenine and the sugar ribose, attached to a …
ATP Hydrolysis
ATP hydrolysis is the chemical reaction in which adenosine triphosphate (ATP) reacts with water and is converted into products with lower free energy. Most commonly, ATP is hydrolyzed to adenosine diphosphate (ADP) and inorganic phosphate. ATP …
Acceptable Macronutrient Distribution Range
The Acceptable Macronutrient Distribution Range (AMDR) is a recommended range for the proportion of total dietary energy that comes from a macronutrient. It was developed to identify intake ranges associated with adequate intake of essential nutrients …
Acetabulum
The acetabulum is the cup-shaped socket on the lateral aspect of the hip bone that receives the head of the femur, or thigh bone. Together, the acetabulum and femoral head form the hip joint, a ball-and-socket …
Acetate
Acetate is the negatively charged conjugate base of acetic acid, a two-carbon short-chain fatty acid. In human physiology, it is one of the principal short-chain fatty acids associated with the gut, together with propionate and butyrate. …
Acetylcholine
Acetylcholine, abbreviated ACh, is a small-molecule neurotransmitter used for chemical signaling in both the central and peripheral nervous systems. It is especially important at the neuromuscular junction, where motor neurons release acetylcholine to activate skeletal muscle …
Actin
Actin is a highly conserved protein that forms one of the major components of the cytoskeleton in eukaryotic cells. Individual actin molecules, called globular actin or G-actin, can assemble into long helical polymers known as filamentous …
Action Potential
An action potential is a rapid, temporary change in the electrical potential across the membrane of an excitable cell. Action potentials allow electrical signals to travel along neurons and also occur in other excitable cells, including …
Adaptive Thermogenesis
Adaptive thermogenesis is a regulated change in energy expenditure that occurs in response to changes in energy balance. In body-weight research, the term commonly refers to a change in energy expenditure that differs from what would …
Adenosine
Adenosine is a purine nucleoside composed of the nitrogen-containing base adenine joined to the five-carbon sugar ribose. A nucleoside consists of a nitrogenous base and a sugar but no phosphate group. Addition of phosphate groups to …
Adenosine Receptor Antagonist
An adenosine receptor antagonist is a substance that binds to an adenosine receptor and reduces or prevents its activation by adenosine. Adenosine is an endogenous signaling molecule that acts through four G-protein-coupled receptor subtypes: A1, A2A, …
Adiposity
Adiposity refers to the amount or proportion of fat in the body. It is a body-composition concept and can be expressed as total fat mass or as body fat percentage, which describes fat mass relative to …
Aerobic Exercise
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
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 …
Afferent Pathway
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
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
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
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
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
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
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 …
Amino Acid
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
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
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 …
Anaphylaxis
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
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
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 …
ApoB
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
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
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
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
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 …
Autoimmune Disease
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 …
Axial Skeleton
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
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 …
Base of Support
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
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
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
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 …
Biomarker
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
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 …
Body Density
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
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
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 …