2,701 articles
Hand
Hand: terminal organ of the human upper limb with 27 bones (8 carpals, 5 metacarpals, 14 phalanges), an opposable thumb via the saddle trapeziometacarpal joint, power and precision grips, and dense tactile innervation.
Hand–eye coordination
Hand–eye coordination (eye–hand coordination, EHC): using visual information to guide manual movements like reaching and grasping. Defined by sequential visual-to-motor processing, posterior parietal control, tests such as the Purdue Pegboard, and reliability ICCs of 0.80–0.94.
Handedness
Handedness, or hand preference, is the tendency to favor one hand for tasks; about 10.6% of people are left-handed, with the roughly 1:10 right-left ratio shaped by evolution and moderated by culture.
Hard palate
Hard palate: bony anterior two thirds of the mouth's roof, formed by maxillary palatine processes and palatine bone horizontal plates, separating oral and nasal cavities; develops from the secondary palate between weeks 6 and 12.
HARP (algorithm)
HARP (harmonic phase) MRI algorithm, by Osman and Prince, Johns Hopkins, 1999: rapid analysis of SPAMM-tagged cardiac MR images using invariant harmonic phase to track myocardial motion and strain; basis of Diagnosoft HARP software.
Hasnat Khan
Hasnat Ahmad Khan (born 1959, Jhelum, Pakistan) is a British-Pakistani cardiothoracic surgeon at Basildon University Hospital, Essex, known for his secret 1995-1997 relationship with Princess Diana, later covered in the book Diana: Her Last Love, the 2013 film Diana, and The Crown.
Head
Head: the uppermost part of the human body, containing the brain in its 22-bone skull (cranium), the face, and sensory organs; divided into neurocranium and viscerocranium, joined by sutures.
Head Movements
Head movements: voluntary or involuntary motion of the head in humans and animals (MeSH D019416). Covers gaze stabilization via the vestibulo-ocular reflex, head impulse testing, active versus passive movement effects, and nonverbal communication roles.
Head-related transfer function
Head-related transfer function (HRTF): direction-dependent acoustic filter of sound by a listener's head, pinnae, and torso, measured at the ear canal entrance; listener-specific like a fingerprint, basis of binaural VR audio, carrying ITD, ILD, and spectral localization cues.
Hearing
Hearing: the sense converting sound waves into electrical signals via the ear canal, eardrum, malleus-incus-stapes chain, and cochlear hair cells, with tonotopic coding carried by the auditory nerve to the brain; distinct from balance, the inner ear's other function.
Hearing aid
Hearing aid: wearable medical device amplifying sound for impaired hearing; air- and bone-conduction types, digital signal processing, FDA OTC category since October 2022, styles from BTE to CIC, and global use by only 17% of those who could benefit.
Hearing loss
Hearing loss (deafness): reduced hearing versus 20 dB thresholds, from mild to profound. Types sensorineural, conductive, mixed; 430 million need rehabilitation; first gene therapy (Otarmeni, OTOF) approved 2026.
Hearing range
Hearing range, the span of frequencies and intensities an ear detects; humans hear about 20 Hz to 20 kHz (adults often 15–17 kHz upper limit), best sensitivity 2000–5000 Hz, declining with age.
Heart and vessels
Heart and Vessels (Heart Vessels), English-language peer-reviewed Japanese cardiology journal founded 1985, Springer, ISSN 0910-8327, official journal of the Sakakibara Heart Foundation, MEDLINE-indexed since 1985, 2025 impact factor 1.5.
Heart murmur
Heart murmur: an extra heart sound from turbulent blood flow, heard on auscultation. Innocent murmurs are common in children; abnormal ones signal valve disease, confirmed by echocardiography.
Heart nanotechnology
Heart nanotechnology applies 1–100 nm materials to cardiac drug delivery, imaging, biosensors, gene therapy, and tissue engineering; also called cardiovascular nanomedicine, it remains mostly preclinical, with about 1% of the nanodrug market versus 20% for cancer.
Heart Protection Study
MRC/BHF Heart Protection Study (HPS): UK randomized 2×2 factorial trial of simvastatin 40 mg and antioxidant vitamins in 20,536 high-risk people; simvastatin cut major vascular events 24%, vitamins showed no benefit.
Heart rate
Heart rate, or pulse, is the number of heartbeats per minute; normal at rest is 60–100 bpm. Set by the sinoatrial node pacemaker and tuned by autonomic, hormonal, and mechanical inputs; measured manually or by wearables and chest straps.
Heart rate across the lifespan
Heart rate across the lifespan describes how heart rate changes with age: about 150 beats per minute in infancy, near 70 in adulthood, with maximum rate declining steadily.
Heart rate monitor
Heart rate monitor: device measuring beats per minute, via chest-strap ECG electrodes or wrist-worn optical PPG in smartwatches and fitness trackers. Chest straps are the validation criterion; wrist accuracy falls as exercise intensity rises.
Heart rate response to exercise and training
Heart rate response to exercise and training describes how heart rate rises during exercise, how endurance training lowers resting heart rate, and how quickly heart rate recovers afterward.
Heart rate variability
Heart rate variability (HRV), also called heart period variability, measures beat-to-beat R-R interval variation on the ECG as a non-invasive marker of autonomic nervous system function, analyzed in time and frequency domains (SDNN, RMSSD, LF, HF bands) and applied in cardiology, mental health, and sepsis prediction.
Heart Rhythm Society
Heart Rhythm Society (HRS), formerly NASPE (North American Society of Pacing and Electrophysiology), founded 1979 in Boston; professional society for cardiac pacing and electrophysiology, renamed 2004, 9,700+ members in 93 countries, based in Washington, DC.
Heart sounds
Heart sounds are transient noises from valve closure heard during auscultation. The first two sounds, S1 and S2, are normal, while S3 and S4 are usually abnormal.
Heart transplantation
Heart transplantation: surgical replacement of a failing heart with a donor heart. First done by Barnard in 1967; 1-year survival now near 90%; main risks are rejection, vasculopathy, and immunosuppression side effects.
Heart valve
Heart valve: the four cardiac valves (mitral, tricuspid, aortic, pulmonary), passive pressure-driven leaflet/cusp mechanics, valve disease epidemiology, repair versus replacement, TAVI, and 2024-2025 trial changes.
Hebbian theory
Hebbian theory, also called Hebbian learning or Hebb's rule, is the principle from Donald Hebb's 1949 book The Organization of Behavior that synapses strengthen when neurons repeatedly fire together.
Hemianopsia
Hemianopsia is the loss of one half of the vertical visual field, usually caused by stroke, brain tumor, or traumatic brain injury, occurring in homonymous or heteronymous forms depending on where the visual pathways are damaged.
Hemiazygos vein
The hemiazygos vein, also called the inferior hemiazygos vein, is an unpaired left-sided chest vein that drains the lower posterior chest and abdominal wall and empties into the azygos vein.
Hemihyperplasia-multiple lipomatosis syndrome
Hemihyperplasia-multiple lipomatosis syndrome (HHML) is a rare PIK3CA-related overgrowth disorder with asymmetric nonprogressive hemihyperplasia, multiple recurrent lipomas, vascular malformations, and possible embryonal malignancy risk.