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Oxidative phosphorylation is the mitochondrial process that couples respiratory electron transfer to the synthesis of ATP from ADP and inorganic phosphate. In human cells that contain mitochondria, it takes place across the inner mitochondrial membrane and …
An oxidoreductase is an enzyme that catalyzes an oxidation-reduction, or redox, reaction. In these reactions, one chemical species is oxidized by losing electrons or increasing its oxidation state, while another is reduced by gaining electrons or …
PNF stretching, short for proprioceptive neuromuscular facilitation stretching, is a group of stretching techniques that combines stretching with voluntary muscle contractions. Rather than simply holding a muscle in a stretched position, PNF methods use sequences of …
Pancreatic bicarbonate is bicarbonate ion, HCO₃⁻, secreted in large amounts as part of the watery fluid produced by the exocrine pancreas. It is released mainly by epithelial cells lining the pancreatic ducts, whereas pancreatic acinar cells …
Pancreatic lipase, more precisely pancreatic triglyceride lipase, is a digestive enzyme produced by the exocrine pancreas and released into the small intestine in pancreatic juice. Its main function is to hydrolyze dietary triglycerides, which make up …
Pancreatic proteases are protein-digesting enzymes produced by the exocrine pancreas and released into the small intestine. The major pancreatic proteases include trypsin, chymotrypsin, elastase, and carboxypeptidases A and B. Together, they hydrolyze peptide bonds in dietary …
Parathyroid hormone (PTH) is an 84-amino-acid peptide hormone secreted by the parathyroid glands, small endocrine glands usually located behind the thyroid gland. Its principal physiological role is the regulation of extracellular calcium and phosphate concentrations, making …
The patella, commonly called the kneecap, is a small triangular bone located at the front of the knee. It is the largest sesamoid bone in the human body, meaning that it develops within a tendon. The …
The patellofemoral joint is the articulation between the patella, or kneecap, and the trochlear surface at the front of the distal femur. Specifically, the cartilage-covered posterior surface of the patella contacts the femoral trochlea, an articular …
Pathogenic bacteria are bacteria capable of causing disease in a host. Pathogenicity refers to this disease-causing capacity. Most bacteria are not pathogenic, and whether disease develops depends on both characteristics of the bacterium and the susceptibility …
The pectoral girdle, also called the shoulder girdle, is the bony structure that connects each upper limb to the axial skeleton. In humans, each side of the pectoral girdle consists of two bones: the clavicle, or …
The pelvic girdle, also called the hip girdle, is the part of the appendicular skeleton that attaches the lower limbs to the axial skeleton. It is formed by the right and left hip bones, or coxal …
The pentose phosphate pathway (PPP) is a cytosolic pathway of glucose metabolism that branches from glucose-6-phosphate and operates alongside glycolysis. Its major products include NADPH, which provides reducing power for cellular reactions, and ribose-5-phosphate, a five-carbon …
Pepsin is a digestive enzyme that begins the breakdown of dietary proteins in the stomach. It is an aspartic endopeptidase, meaning that it hydrolyzes peptide bonds within protein chains rather than removing amino acids one at …
A peptide bond is the covalent amide linkage that joins amino acid residues in peptides and proteins. It connects the carbonyl carbon derived from the carboxyl group of one amino acid to the nitrogen derived from …
Percent Daily Value, written as %DV, is a value on U.S. Nutrition Facts and Supplement Facts labels that shows how much a nutrient in one serving contributes relative to its Daily Value. The Daily Value (DV) …
Periodization is the planned organization and variation of training over time to promote specific adaptations, manage fatigue, and prepare for particular performance goals. Rather than keeping training variables constant, a periodized program deliberately changes factors such …
Peripheral chemoreceptors are sensory structures outside the brain that monitor the chemical composition of arterial blood and help regulate breathing. They are located primarily in the carotid bodies, small organs near the bifurcation of each common …
Peristalsis is a coordinated pattern of muscular contraction and relaxation that moves material through the digestive tract. In the gastrointestinal system, it helps propel swallowed food, liquid, intestinal contents, and eventually fecal material along different regions …
The pharyngeal phase is the part of swallowing that moves a food or liquid bolus through the pharynx, or throat, toward the esophagus. It follows the oral phase, in which the bolus is prepared and moved …
Phenylketonuria (PKU) is a severe form of inherited phenylalanine hydroxylase (PAH) deficiency, a disorder of phenylalanine metabolism. PAH normally converts the amino acid phenylalanine into tyrosine in a reaction that requires the cofactor tetrahydrobiopterin. When PAH …
The phosphagen system is a rapid cellular mechanism for buffering and regenerating ATP using compounds called phosphagens. In humans, the relevant phosphagen is phosphocreatine, or creatine phosphate, and the system is therefore often called the phosphocreatine …
Phosphocreatine, also called creatine phosphate, is a phosphorylated form of creatine that serves as a rapidly available reserve of transferable phosphoryl groups in cells. It is especially important in tissues whose energy demand can change quickly, …
Physical activity is any bodily movement produced by skeletal muscles that results in energy expenditure. It includes movement performed during work, transportation, household activities, recreation, sport, and exercise. Physical activity therefore covers a broad range of …
Physical fitness is a set of measurable attributes related to a person's ability to perform physical activity. It is not a single ability or measurement. Instead, it is a multidimensional concept that describes attributes of the …
Physiological cardiac hypertrophy is an adaptive increase in heart muscle mass that occurs in response to increased physiological demand rather than disease-related stress. It is seen most clearly with sustained exercise training and during pregnancy. In …
Phytate refers to the negatively charged forms and salts of phytic acid, also called myo-inositol hexakisphosphate or InsP6. The molecule consists of an inositol ring carrying six phosphate groups, which give it a strong negative charge …
Plasma tonicity is the effective osmotic concentration of plasma, meaning the concentration of dissolved substances that can produce sustained water movement across cell membranes and thereby change cell volume. It depends on solutes that remain sufficiently …
Plyometric training is a form of exercise built around rapid stretch-shortening cycles, in which an active muscle-tendon unit is lengthened and then quickly transitions into shortening. Common plyometric movements include jumping, hopping, bounding, and other explosive …
Polyphenols are a large and chemically diverse group of compounds characterized by multiple phenolic structural features, including aromatic rings bearing hydroxyl groups. They occur widely as metabolites in plants, where they participate in functions such as …
Portal circulation is a vascular arrangement in which blood passes through one capillary bed and then travels through a portal vein to a second capillary bed before returning to the heart. In digestive physiology, the term …
Scapular posterior tilt is a three-dimensional rotational movement of the scapula around an approximately medial-to-lateral axis. During posterior tilt, the superior portion of the scapula moves posteriorly while the inferior angle moves anteriorly toward the thorax. …
Postprandial glucose is the concentration of glucose in the blood during the period after eating a meal. It reflects the balance between glucose entering the circulation, endogenous glucose production, and glucose uptake by the liver and …
Preabsorptive signals are sensory and physiological signals generated during eating or drinking before absorbed nutrients or water have produced their later systemic effects. They provide the nervous system with rapid information about what is being consumed …
Progesterone is a steroid hormone involved in the menstrual cycle, implantation, and pregnancy. It is synthesized from cholesterol and is produced mainly by the corpus luteum of the ovary after ovulation. During pregnancy, the placenta becomes …
Progressive overload is the principle of gradually increasing or adjusting the training stimulus as the body adapts to exercise. In resistance training, a workload that initially challenges a muscle can become less demanding as strength and …
Propionate is the negatively charged conjugate base of propionic acid, a three-carbon short-chain fatty acid. In the human large intestine, it is one of the major short-chain fatty acids produced by the gut microbiota, together with …
Proprioception is the sense that provides information about the position, movement, and mechanical state of the body and its parts. It allows the nervous system to estimate where the limbs and trunk are and how they …
A protein is a biological macromolecule built from one or more chains of amino acids joined together by peptide bonds. These amino acid chains are called polypeptides. Proteins are among the major functional molecules of cells …
Protein aggregation is the association of multiple protein molecules into larger assemblies. It often occurs when proteins become misfolded or partially unfolded, exposing regions that are normally buried within their native structures. These exposed regions can …
Protein denaturation is the disruption of a protein's native structure, causing it to lose some or all of the conformation associated with its normal function. Denaturation primarily alters secondary, tertiary, or quaternary structure by disturbing interactions …
A protein network is a representation of relationships among proteins, commonly their physical interactions with one another. In a protein-protein interaction (PPI) network, each protein is represented as a node and each represented interaction as a …
The protein-sparing effect refers to reduced use of amino acids for energy and reduced loss of body protein when energy is supplied from other fuels, particularly carbohydrate and fat. By reducing amino acid oxidation and protein …
The psoas major is a long, thick skeletal muscle that runs along each side of the lumbar spine and continues through the pelvis toward the upper femur. It has attachments to the bodies, intervertebral discs, and …
The pubis, or pubic bone, is the anterior and inferior part of each hip bone. It is one of the three bones that form the hip bone, together with the ilium and ischium. These bones begin …
Pulmonary ventilation is the movement of air into and out of the lungs. During inspiration, air flows from the atmosphere through the conducting airways into the lungs. During expiration, air moves in the opposite direction. Ventilation …
The quadratus lumborum is a paired skeletal muscle of the posterior abdominal wall, positioned on either side of the lumbar spine. It extends between the iliac crest, the transverse processes of the lumbar vertebrae, and the …
The quadriceps tendon is the strong connective tissue structure that links the quadriceps femoris muscles at the front of the thigh to the superior part of the patella, or kneecap. It is formed as the tendinous …
