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Rectus femoris muscle
Rectus femoris (Musculus rectus femoris): superficial two-headed quadriceps muscle of the anterior thigh, originating at the anterior inferior iliac spine and supraacetabular groove, extending the knee and flexing the hip, innervated by the femoral nerve (L2-L4).
Recurrent idiopathic neuroretinitis
Recurrent idiopathic neuroretinitis (RINR), a rare inflammatory optic neuropathy with recurrent painless visual loss, optic disc edema, and macular star; poor prognosis, treated with long-term azathioprine and prednisone.
Recurrent laryngeal nerve
Recurrent laryngeal nerve (RLN, nervus laryngeus recurrens): vagus nerve branch supplying the larynx's intrinsic muscles; right loops under the subclavian artery, left under the aortic arch; injury is a key thyroidectomy complication.
Red eye
Red eye: redness from inflammation of the eye's anterior segment; also called conjunctival hyperaemia. Most cases are benign conjunctivitis, blepharitis, or subconjunctival haemorrhage, but pain, photophobia, or reduced vision signal sight-threatening disease needing urgent ophthalmology referral.
Red pulp
Red pulp (pulpa rubra) is the blood-filtering compartment of the spleen, about 75% of its tissue, made of cords of Billroth and venous sinuses where macrophages cull and pit aged red cells, recycle iron, and store platelets.
Reelin
Reelin (RELN gene) is a large extracellular matrix glycoprotein controlling neuronal migration and cortical layering via ApoER2/VLDLR-Dab1 signaling; absent in reeler mice, lost in human lissencephaly, and protective COLBOS variants delay Alzheimer's disease.
Referred pain
Referred pain, also called pain referral, is pain felt at a site distant from its source, such as cardiac pain in the left arm or gallbladder pain in the right scapula, explained mainly by convergence projection.
Reflex
A reflex is an automatic, involuntary response to a stimulus mediated by a fixed neural pathway, the reflex arc, ranging from the knee jerk to protective withdrawal.
Reflex arc
A reflex arc, or reflex circuit, is the neural pathway that produces an automatic, involuntary response to a stimulus, integrating it in the spinal cord or brainstem without conscious involvement.
Refractory angina
Refractory angina is chronic angina lasting at least 3 months that cannot be controlled by medication, angioplasty, or coronary bypass surgery, affecting about 5–10% of patients with stable coronary artery disease.
Refractory period (physiology)
Refractory period (physiology): post-action-potential interval in neurons and muscle, with absolute phase (Na+ channel inactivation, no second spike) and relative phase (elevated K+ conductance, stronger stimulus needed); limits firing rate and ensures unidirectional conduction.
Regulatory T cell
Regulatory T cell (Treg, FOXP3+CD4+CD25+): suppressive CD4+ T cell maintaining immune tolerance; discovered by Sakaguchi 1995, Foxp3 identified 2001; 2025 Nobel Prize; first approved Treg therapy Tregzi (2026).
Renal artery
Renal artery: paired artery from the abdominal aorta (L1-L2 level) supplying each kidney; divides into anterior/posterior divisions; accessory renal arteries occur in about 21% of people; main disease is renal artery stenosis causing renovascular hypertension.
Renal artery stenosis
Renal artery stenosis (RAS), also called atherosclerotic renovascular disease, is narrowing of one or both renal arteries, about 90% from atherosclerosis and most of the rest from fibromuscular dysplasia, causing renovascular hypertension and ischemic nephropathy; trials (ASTRAL, CORAL) show stenting adds no benefit over medical therapy for most patients.
Renal Circulation
Renal circulation: blood flow through the kidney's vessels, about 20% of cardiac output (~1 L/min), with glomerular and peritubular capillaries in series and myogenic plus tubuloglomerular feedback autoregulation.
Renal physiology
Renal physiology: how the kidneys filter blood, reabsorb and secrete solutes, and regulate homeostasis. Key facts: normal GFR 100-125 mL/min/1.73 m², myogenic and tubuloglomerular autoregulation, countercurrent multiplication, renal functional reserve.
Renal vein
The renal vein, known in Latin as vena renalis, drains each kidney into the inferior vena cava; the left vein is much longer than the right.
Renal vein thrombosis
Renal vein thrombosis (RVT) is clot formation in the renal veins, causing kidney injury. Main causes are nephrotic syndrome and malignancy; also a neonatal disease. Treated with anticoagulation, thrombolysis, or thrombectomy.
René Favaloro
René Favaloro, born René Gerónimo Favaloro, was an Argentine cardiac surgeon who pioneered coronary artery bypass surgery at the Cleveland Clinic in 1967 and returned to Argentina to found the Fundación Favaloro.
Renshaw cell
Renshaw cell: spinal inhibitory interneuron (named for Birdsey Renshaw, 1941) in the ventral horn's lamina VII/IX; excited cholinergically by α-motoneuron collaterals, it inhibits motoneurons glycinergically, mediating recurrent inhibition.
Reperfusion
Reperfusion is restoration of blood supply to ischemic tissue, mainly via fibrinolysis, primary PCI, or thrombectomy in heart attack and stroke; time-critical, but can itself cause reperfusion injury through oxidative stress, calcium overload, and no-reflow.
Reperfusion therapy
Myocardial reperfusion restores blood flow to ischemic heart muscle, mainly via primary PCI or fibrinolysis for STEMI. Key limits: 90/120-minute targets, reperfusion injury, and no-reflow.
Reproductive system
The reproductive system, also called the genital system, is the organ system producing offspring: in women the ovaries, fallopian tubes, and uterus, and in men the testes and sperm-carrying ducts.
Residual Volume
Residual volume (RV) is the gas remaining in the lungs after maximal exhalation; measured indirectly as FRC minus ERV or TLC minus vital capacity, it prevents alveolar collapse and rises in obstructive air trapping.
Respiration (physiology)
Respiration (physiology): gas exchange supplying oxygen and removing carbon dioxide, covering pulmonary versus cellular respiration, diffusion, ventilation–perfusion matching, Bohr and Haldane effects, breathing control, and responses to exercise and altitude.
Respiratory center
Respiratory center: the brainstem neuronal network (medullary pre-Bötzinger complex, Bötzinger complex, RTN/pFRG, pontine respiratory group) that generates and modulates the automatic breathing rhythm, shaped by CO2/pH chemoreceptor and lung mechanoreceptor feedback.
Respiratory rate
Respiratory rate (RR), breathing rate, breaths per minute: a core vital sign, normal 12–20 in adults, scored in NEWS2; manual counting is error-prone, capnography is the gold standard.
Respiratory system
Respiratory system: the organ system exchanging oxygen and carbon dioxide across the alveolar blood-gas barrier via ventilation and perfusion. Key facts: V/Q matching governs efficiency; COPD caused 3.4 million deaths in 2023.
Respiratory tract
Respiratory tract: the airways carrying air from nose and mouth to the alveoli. Divided into upper (nose, sinuses, pharynx, larynx) and lower (trachea, bronchi, lungs) tracts, with conducting and gas-exchange zones.
Resting potential
Resting potential (resting membrane potential, RMP): the inside-negative voltage across a cell membrane at rest, about -70 mV in neurons, set mainly by K+ leak permeability and maintained by the Na+/K+-ATPase, described by Nernst and GHK equations.