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Homeostasis is the physiological regulation that keeps important features of an organism's internal environment within ranges compatible with normal function despite internal or external disturbances. Regulated variables can include body temperature, blood pressure, blood glucose, blood …
Hormonal contraception refers to reversible methods that use hormones to prevent pregnancy. Most hormonal contraceptives contain a progestin, a compound with progesterone-like activity, either alone or combined with an estrogen. They are available in several forms, …
Hybrid fibers are skeletal muscle fibers that express two or more myosin heavy chain, or MyHC, isoforms within the same muscle fiber. By contrast, fibers classified as pure Type I, Type IIa, or Type IIx predominantly …
Hydrostatic weighing, also called underwater weighing or hydrodensitometry, is a method for estimating body composition from body density. It is based on Archimedes' principle: a submerged body experiences an upward buoyant force equal to the weight …
Hyperpnea is an increase in the depth and often the rate of breathing that raises ventilation in response to increased metabolic demand. It is commonly seen during exercise, when working muscles consume more oxygen and produce …
Hypertension describes persistently elevated arterial blood pressure, distinguishing a sustained condition from a single rise during stress or activity.
Hypertrophic cardiomyopathy (HCM) is a heart muscle disease characterized primarily by abnormal thickening of the left ventricular wall that cannot be adequately explained by another cardiac, systemic, or metabolic condition. In adults, a clinical diagnosis is …
Hyperventilation is breathing that increases alveolar ventilation, the amount of fresh air reaching the gas-exchanging regions of the lungs, beyond that required for the body's current rate of carbon dioxide production. As a result, carbon dioxide …
Hypocapnia is an abnormally low partial pressure of carbon dioxide in arterial blood, called PaCO₂. In adults, it is commonly defined as a PaCO₂ below about 35 mm Hg, although reference ranges can vary slightly. Hypocapnia …
Hypohydration is a physiological state in which the body contains less water than its euhydrated state. It can develop when water losses exceed water intake, such as through sweating, increased urine production, gastrointestinal losses, or restricted …
Hyponatremia is a lower-than-normal concentration of sodium in the blood, conventionally defined as a plasma or serum sodium concentration below 135 mmol/L. Plasma sodium concentration is closely related to the balance between body water and exchangeable …
The hypothalamus is a small region of the diencephalon located below the thalamus and closely associated with the third ventricle and pituitary gland. It acts as an important integration center between the nervous and endocrine systems, …
Hypothermia is a condition in which the body's core temperature falls below 35°C (95°F). Core temperature refers to the temperature of the body's internal tissues and organs rather than the temperature of the skin. Hypothermia develops …
Hypotonicity is the state in which a body fluid has a lower effective osmotic concentration, or tonicity, than the fluid on the other side of a cell membrane. In human physiology, the term commonly refers to …
Hypoxemia is an abnormally low level of oxygen in arterial blood. It is most directly assessed by the arterial partial pressure of oxygen, or PaO₂, measured in an arterial blood gas sample. Oxygen saturation is closely …
The I band is the lighter region of a sarcomere that contains thin, actin-containing filaments but no thick, myosin-containing filaments. It lies on either side of the darker A band in striated skeletal and cardiac muscle. …
Ia afferents, or group Ia sensory fibers, are large, rapidly conducting sensory nerve fibers that arise from the primary endings of muscle spindles. Muscle spindles are sensory receptors located within skeletal muscles that detect muscle length …
IgE, or immunoglobulin E, is one of the five major classes of antibodies in humans. Like other antibodies, an IgE molecule contains two heavy chains and two light chains. Its heavy chains are the epsilon type, …
IgE-mediated food allergy is a type of food allergy in which immunoglobulin E, or IgE, antibodies mediate an allergic reaction to specific food allergens, usually proteins. It is characterized by reproducible symptoms after exposure to the …
The iliac crest is the curved superior border of the ilium, the largest and uppermost of the three bones that form each hip bone. It forms the prominent upper rim of the pelvis that can often …
The ilium is the largest and uppermost of the three bones that form each hip bone, or os coxae. The other two are the ischium and pubis. In adults, these three bones are fused, but their …
Incremental area under the curve summarizes a response relative to baseline, with meaning that depends on the sampling window and calculation convention.
Indispensable amino acids are amino acids that humans cannot synthesize in amounts sufficient to meet physiological requirements and therefore must obtain from the diet. They are also commonly called essential amino acids. In humans, the nine …
Inflammatory cytokines carry signals across interacting immune networks, so their effects depend on tissue and timing rather than fixed harmful labels.
Inorganic phosphate, commonly abbreviated Pi, refers to orthophosphate that is not covalently attached to an organic molecule. It is derived from phosphoric acid and exists in aqueous biological fluids mainly as a mixture of dihydrogen phosphate …
Inorganic arsenic is a group of arsenic compounds in which arsenic is combined with elements such as oxygen, chlorine, or sulfur, rather than forming the carbon-containing compounds classified as organic arsenic. Arsenic itself is a naturally …
Insulin is a peptide hormone produced and secreted by beta cells in the pancreatic islets. Mature human insulin consists of two amino-acid chains, called the A and B chains, linked by disulfide bonds. It is produced …
Intercalated discs are specialized regions of cell-to-cell contact that connect neighboring cardiac muscle cells, or cardiomyocytes. They are located mainly where the ends of adjacent cardiomyocytes meet and contain several types of junctions organized together. These …
Interleukin-6 supports regulated host-defense responses, while persistent dysregulated production can contribute to disease in different biological settings.
The internal oblique is a paired skeletal muscle forming the middle layer of the three flat muscles of the anterolateral abdominal wall. It lies deep to the external oblique and superficial to the transversus abdominis. In …
Intra-abdominal pressure describes pressure inside the abdominal cavity, while sustained pathological elevation defines distinct clinical syndromes.
Intrapleural pressure, also called pleural pressure, is the pressure within the pleural space between the visceral pleura covering the lungs and the parietal pleura lining the inside of the chest wall. During normal quiet spontaneous breathing, …
Iodine is a chemical element and an essential trace nutrient required for the production of thyroid hormones. In the body and in many foods, iodine is commonly present as iodide, the negatively charged ion I⁻, rather …
Iron deficiency develops when available iron cannot meet physiological needs, and depleted stores can precede the low hemoglobin that defines anemia.
The ischial tuberosity is a large bony prominence on the posterior and inferior part of the ischium, one of the three bones that form each hip bone. It projects from the lower rear portion of the …
The ischium is the posteroinferior, or lower rear, part of each hip bone. It is one of the three bones that form the hip bone, together with the ilium and pubis. These three bones are separate …
An isometric contraction is a muscle contraction in which force is produced without an appreciable change in the overall length of the muscle-tendon unit or movement of the associated joint. Because there is no visible shortening …
Joint torque is the rotational effect of forces acting about a joint. In biomechanics, the closely related term joint moment is often used interchangeably with joint torque. The magnitude of a torque depends on both the …
Ketogenesis is the metabolic pathway that produces ketone bodies from acetyl-CoA. In humans, it occurs primarily in the mitochondria of liver cells. The principal ketone bodies released into the circulation are acetoacetate and beta-hydroxybutyrate, while acetone …
LDL, or low-density lipoprotein, is a class of cholesterol-rich lipoprotein particles that circulates in the blood. Lipoproteins allow poorly water-soluble lipids such as cholesterol to be transported through plasma. An LDL particle contains a core rich …
LDL-C, or low-density lipoprotein cholesterol, is the concentration of cholesterol carried within low-density lipoprotein particles in the blood. It is usually reported in milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L). LDL-C is a standard …
Lactate threshold (LT) is an exercise intensity identified from a systematic rise in blood lactate concentration during progressively harder exercise. At lower intensities, lactate is continuously produced and removed, and its blood concentration remains relatively stable. …
Lacteals are specialized lymphatic capillaries located within the villi of the small intestine. Intestinal villi contain blood capillaries and typically a central lacteal. Like other initial lymphatic vessels, lacteals collect fluid and material from surrounding tissue, …
A lapse is a temporary return to an earlier behavior, but research definitions vary and a brief setback does not inevitably become relapse.
Left ventricular hypertrophy (LVH) is an increase in the mass of the muscular wall of the heart's left ventricle, the chamber that pumps blood into the systemic circulation. It generally develops through enlargement of individual cardiac …
Leptin is a peptide hormone produced primarily by adipocytes, especially in white adipose tissue. Its circulating concentration generally reflects the amount of stored body fat, although it also changes with short-term energy availability. Leptin acts as …
The limbic system is a traditional neuroanatomical concept referring to a group of interconnected cortical and subcortical brain structures associated with functions such as memory, emotion, motivation, and the coordination of behavior with internal physiological states. …
A limiting amino acid is the indispensable amino acid supplied in the lowest proportion relative to the amount specified in a reference amino acid pattern. When the amino acid composition of a dietary protein is compared …
