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Human skin color
Human skin color is melanin-based pigmentation shaped by UV radiation via the vitamin D–folate hypothesis: dark equatorial skin conserves folate, depigmented high-latitude skin aids vitamin D synthesis, evolving polygenically across SLC24A5, SLC45A2, MC1R, and KITLG.
Human vaginal size
Human vaginal size: measurable length and width of the vaginal canal, also called vaginal dimensions. MRI baselines: 62.7 mm undistended length, widths 26–33 mm, large inter-individual variation; size does not affect sexual function.
Humerus
Humerus: the single long bone of the upper arm, largest bone of the upper limb, articulating with the scapula at the shoulder and radius and ulna at the elbow; fracture-prone at the surgical neck, radial groove, and supracondylar region.
Humoral immunity
Humoral immunity: the antibody-mediated arm of adaptive immunity, driven by B cells and plasma cells, protecting extracellular spaces; includes germinal center affinity maturation, five immunoglobulin classes, active versus passive immunity, and its 19th-century origins with Behring and Ehrlich.
Humorism
Humorism (humoralism, four humors): ancient Greek-Roman medical system of blood, phlegm, yellow bile, black bile; health as balance, disease as imbalance; Hippocrates, Galen, Avicenna; dominant until 19th-century germ theory; survives in Unani medicine.
Huntington's disease
Huntington's disease (HD), also called Huntington's chorea, is an autosomal dominant neurodegenerative disease caused by CAG repeat expansion in the HTT gene, causing chorea, cognitive decline, and behavioral symptoms, typically starting in middle age with no cure.
Hyaline cartilage
Hyaline cartilage, also called articular or glassy cartilage, is the most common cartilage type: a type II collagen and aggrecan matrix lining joints, airways, ribs, and the fetal bone model.
Hyalocyte
Hyalocytes (of Hannover, Balazs) are yolk sac-derived resident macrophages of the eye's vitreous body, first identified in 1840, that maintain vitreous transparency, remodel the hyaloid vasculature, and drive proliferative vitreoretinal disease.
Hybrid coronary revascularization
Hybrid coronary revascularization (HCR) combines minimally invasive LIMA-LAD bypass grafting with PCI for non-LAD lesions in multivessel coronary disease; introduced in the mid-1990s, it lowers perioperative morbidity versus CABG but raises repeat revascularization.
Hydralazine/isosorbide dinitrate
Hydralazine/isosorbide dinitrate (BiDil) is a fixed-dose vasodilator combination for heart failure with reduced ejection fraction, best evidenced in self-identified Black patients by A-HeFT, which showed 43% lower mortality.
Hymen
Hymen: thin tissue at the vaginal opening, also called the maidenhead, with no known function. Its appearance cannot indicate sexual history; WHO and UN agencies condemn hymen-based virginity testing as unscientific and harmful.
Hyoid bone
Hyoid bone: free-floating U-shaped neck bone of body, greater horns, and lesser horns, anchoring tongue and swallowing muscles; fractures of the greater horns indicate neck violence, and shape varies widely between individuals.
Hyperacusis
Hyperacusis is reduced tolerance to everyday sounds, perceived as too loud or painful despite normal hearing. Prevalence 9-15%; subtypes include loudness and pain hyperacusis; treated with sound desensitization and counseling.
Hyperaemia
Hyperaemia (hyperemia) is increased blood flow to tissue causing engorgement; forms include active (functional), reactive (post-ischaemic), and passive (congestion). Covers mediators (NO, adenosine, K+, CO2), measurement (RHI, FMD, plethysmography), coronary FFR ≤0.80 and iFR ≤0.89 thresholds, and stress agents.
Hypertensive heart disease
Hypertensive heart disease, also called hypertensive cardiomyopathy, is heart damage from long-standing high blood pressure, ranging from left ventricular hypertrophy to heart failure and sudden death.
Hypertensive retinopathy
Hypertensive retinopathy, also called fundus hypertonicus, is damage to the retinal blood vessels caused by high blood pressure, producing narrowed arterioles, hemorrhages, and exudates; treatment is mainly blood pressure control.
Hypertonia
Hypertonia, also called spasticity or rigidity in its main forms, is abnormally increased resistance to passive limb movement caused by loss of supraspinal control of stretch reflexes, assessed clinically with Ashworth and Tardieu scales.
Hypertrophic cardiomyopathy
Hypertrophic cardiomyopathy (HCM), also called idiopathic hypertrophic subaortic stenosis, is a genetic heart disease with unexplained left ventricular thickening, prevalence about 1 in 500, first modern description by Teare in 1958, and treated with beta blockers, mavacamten, myectomy, and ICDs.
Hypertrophic chondrocyte
Hypertrophic chondrocyte: terminally differentiating growth plate cartilage cell, marked by type X collagen and Runx2, that enlarges about 20-fold, mineralizes matrix, and drives cartilage-to-bone conversion via Ihh-PTHrP, VEGF, and MMP signaling.
Hyperventilation
Hyperventilation (overbreathing) is ventilation exceeding metabolic needs, lowering PaCO2 and causing respiratory alkalosis; chronic symptomatic forms are called hyperventilation syndrome, linked to anxiety and asthma.
Hyphema
Hyphema is blood in the eye's anterior chamber, usually from blunt trauma; graded 0–IV (microhyphema to eight-ball), complicated by rebleeding, raised intraocular pressure, and corneal blood staining.
Hypnic jerk
Hypnic jerk, also called sleep start or hypnagogic jerk: a brief involuntary myoclonic muscle contraction at sleep onset, stage N1, with falling sensation; affects about 70% of people, triggered by stress, caffeine, and sleep deprivation.
Hypogastric plexus
The hypogastric plexus is a network of nerve fibers in the pelvis, divided into superior and inferior parts, that supplies autonomic innervation to the bladder, rectum, and reproductive organs.
Hypoglossal nerve
Hypoglossal nerve (cranial nerve XII): pure motor nerve from the medulla's hypoglossal nucleus, exiting via the hypoglossal canal, supplying all tongue muscles except palatoglossus; lesions deviate the tongue toward the lesion.
Hypothalamic gonadotropin-releasing hormone neuron
Hypothalamic GnRH neuron, also called GnRH-1 neuron, a neuroendocrine cell born in the nasal placode that migrates into the forebrain and secretes pulsatile GnRH controlling LH/FSH release; its failure causes Kallmann syndrome and congenital hypogonadotropic hypogonadism.
Hypothalamic–pituitary–adrenal axis
Hypothalamic–pituitary–adrenal (HPA) axis: neuroendocrine system linking hypothalamus, anterior pituitary, and adrenal cortex. CRH/AVP drive ACTH and cortisol; glucocorticoid negative feedback, circadian and hourly ultradian rhythms, stress response, and suppression by chronic glucocorticoids.
Hypothalamic–pituitary–gonadal axis
Hypothalamic–pituitary–gonadal (HPG) axis, also called the reproductive endocrine axis: GnRH pulses drive LH/FSH, which drive gonadal steroids with feedback; active in fetal life and mini-puberty, quiescent in childhood, reactivated at puberty.
Hypothalamus
Hypothalamus: small ventral brain region below the thalamus, about 2% of brain volume, controlling the pituitary via releasing hormones (TRH, GnRH, CRH, dopamine) and homeostasis, appetite, temperature, and circadian rhythm; mapped in 2025 as HYPOMAP with 452 cell types.
Hypotrichosis-deafness syndrome
Hypotrichosis-deafness syndrome, or hypotrichosis-hearing loss syndrome, is a rare autosomal dominant GJB2 (connexin 26) disorder with erythrokeratoderma, sparse hair, nail dystrophy, and sensorineural deafness.
Hypovolaemic shock
Hypovolemic shock is a life-threatening condition in which loss of blood or plasma, most often from trauma or bleeding, leaves too little circulating volume for adequate tissue perfusion.