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The nephron is the kidney's functional unit, comprising the glomerulus, renal tubules and collecting duct, where filtration, reabsorption and secretion produce urine. Roughly one million nephrons are present in each human kidney.
Phosphofructokinase-1 converts fructose-6-phosphate to fructose-1,6-bisphosphate and is the committed, rate-limiting step, regulated by ATP, AMP, citrate and fructose-2,6-bisphosphate. Hexokinase catalyzes the preceding step but is not the committed regulatory step in most tissues.
Vitamin C (ascorbate) is a cofactor for prolyl and lysyl hydroxylase, which hydroxylate proline and lysine so collagen triple helices can cross-link. Without it, collagen is unstable, producing bleeding gums, poor wound healing and capillary fragility.
Antigen-activated B cells differentiate into plasma cells, which are terminally differentiated and devoted to high-rate synthesis and secretion of immunoglobulin. Memory B cells, by contrast, do not secrete antibody at high rate but persist for rapid recall responses.
IgG is transported across the syncytiotrophoblast by the neonatal Fc receptor, so maternal IgG reaches the fetal circulation. This confers passive humoral protection for the first months of life.
Resting potential is set largely by potassium permeability through leak channels, with the Na+/K+-ATPase maintaining the ion gradients, and lies near the potassium equilibrium potential at about −70 mV. Values near +30 mV represent the peak of an action potential.
The pulmonary artery is the only artery carrying deoxygenated blood, delivering it from the right ventricle to the capillary beds of the lungs. Oxygenated blood then returns to the left atrium through the pulmonary veins.
Citrate synthase condenses the two-carbon acetyl group with four-carbon oxaloacetate to yield six-carbon citrate. Oxaloacetate is regenerated later in the cycle, allowing it to act catalytically.
A rightward shift (Bohr effect) indicates that at any given partial pressure of oxygen, hemoglobin saturation is lower, so oxygen is released more readily to tissues. It is produced by rising PCO2 and hydrogen ion concentration, higher temperature and increased 2,3-BPG.
A competitive inhibitor occupies the active site of free enzyme and can be outcompeted by raising substrate concentration, so Vmax is unchanged while the apparent Km rises. This pattern is the classic signature on a Lineweaver-Burk plot, where the lines intersect on the y-axis.
Glucose is reabsorbed almost entirely in the proximal convoluted tubule by SGLT2 (apical) and GLUT2 (basolateral) transporters. Glucose appears in the urine only when the filtered load exceeds the transport maximum, as in uncontrolled diabetes.
Greater end-diastolic filling stretches cardiac myocytes toward optimal sarcomere length, increasing calcium sensitivity of the myofilaments and the force of contraction. Within physiologic limits, stroke volume therefore rises as preload rises.
Neisseria meningitidis is a Gram-negative, oxidase-positive diplococcus whose polysaccharide capsule is the principal virulence factor in meningococcal meningitis. Streptococcus pneumoniae is also a diplococcus but stains Gram-positive.
Beta-lactams acylate penicillin-binding proteins (transpeptidases), preventing the cross-linking of peptidoglycan strands. The weakened cell wall cannot withstand osmotic pressure, so actively dividing bacteria lyse.
MHC class I is expressed on nearly all nucleated cells and displays endogenous, cytosolic peptides to CD8+ cytotoxic T cells. Recognition of foreign peptide in this context triggers killing of the infected cell.
Insulin receptor signaling causes GLUT4-containing vesicles to fuse with the plasma membrane in skeletal muscle and adipose tissue, increasing facilitated glucose uptake. GLUT4 is therefore the insulin-responsive transporter.
In chronic respiratory acidosis the kidneys compensate over days by secreting more hydrogen ions and reabsorbing and generating more bicarbonate, partially restoring arterial pH toward normal. Plasma bicarbonate therefore rises as a compensatory response.
HMG-CoA reductase reduces HMG-CoA to mevalonate, the committed and rate-limiting step of cholesterol synthesis. It is inhibited by statins and downregulated by intracellular cholesterol.
Hydrolysis of one ATP drives the export of three sodium ions and the import of two potassium ions, producing a net outward current of one positive charge. This electrogenic action helps maintain the negative resting membrane potential and cell volume.
DNA polymerase can only add nucleotides to a free 3'-hydroxyl, so synthesis runs 5' to 3'. On the leading strand the template runs in the opposite direction, allowing the polymerase to move continuously toward the replication fork.
Because the inhibitor removes ES complex, both Vmax and apparent Km fall by the same factor, so parallel lines appear on a Lineweaver-Burk plot. The Km/Vmax ratio remains constant, distinguishing this from competitive and noncompetitive patterns.
Phase 2 reflects a near-balance between slow inward calcium current through L-type channels and outward potassium currents. The sustained calcium entry prolongs depolarization, prevents tetany by lengthening the refractory period, and triggers excitation-contraction coupling.
IL-4, with CD40-CD40L costimulation, directs B cells to switch to IgE and also promotes Th2 differentiation and eosinophilic inflammation. Blocking IL-4 signaling is therefore a therapeutic strategy in atopic disease.
Von Gierke disease (glycogen storage disease type I) results from glucose-6-phosphatase deficiency, so the liver cannot release glucose derived from glycogenolysis or gluconeogenesis. Glucose-6-phosphate accumulates and is shunted toward lactate, producing fasting hypoglycemia with lactic acidosis and hepatomegaly.
Inulin is the standard reference for GFR measurement: it is freely filtered, is not secreted or reabsorbed by the tubule, and is not metabolized, so its clearance equals the filtration rate. Creatinine is used clinically but is slightly secreted, which modestly overestimates GFR.
The mitral valve guards the left atrioventricular orifice and prevents backflow from the left ventricle into the left atrium during systole. The aortic and pulmonary valves are semilunar valves located at the ventricular outflow tracts.
The posterior pituitary stores and releases antidiuretic hormone and oxytocin, which are synthesized in the hypothalamus. The other listed hormones are produced and released by the anterior pituitary.
Vitamin K is the cofactor for gamma-glutamyl carboxylase, which converts glutamate residues to gamma-carboxyglutamate in factors II, VII, IX, and X and in proteins C and S. This modification is necessary for calcium binding and normal coagulation.
Streptococcus pneumoniae is an encapsulated Gram-positive diplococcus and remains the leading cause of community-acquired pneumonia in adults. Haemophilus influenzae and Mycoplasma pneumoniae are less common causes, while Staphylococcus aureus is more typically associated with hospital-acquired or post-viral pneumonia.
IgG accounts for roughly 75% to 80% of serum immunoglobulin and is the main antibody in secondary immune responses. IgA is predominant in mucosal secretions, while IgE and IgD are present in much lower serum concentrations.
Naloxone competitively antagonizes opioid receptors and rapidly reverses opioid-induced respiratory depression and sedation. It has high affinity for mu, kappa, and delta opioid receptors.
The ascending limb of the loop of Henle is water-impermeable and reabsorbs sodium chloride, thereby diluting the tubular fluid. The descending limb is water-permeable, and the proximal tubule reabsorbs water isosmotically with solutes.
The inner mitochondrial membrane contains the electron transport chain and ATP synthase, which together carry out oxidative phosphorylation. The other listed organelles are mainly involved in protein processing, degradation, or lipid metabolism.
Cortisol is a lipophilic steroid hormone that diffuses across the plasma membrane and binds to intracellular receptors. The hormone-receptor complex then translocates to the nucleus and alters gene transcription.
Phosphofructokinase-1 catalyzes the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate and is the major regulated, rate-limiting step of glycolysis. Hexokinase catalyzes the first step, but it is not the committed rate-limiting step of the pathway.
Vagal acetylcholine acts on muscarinic M2 receptors in the SA node, which are Gi-coupled and reduce cAMP, slowing the phase 4 depolarization rate. Beta-1 adrenergic stimulation has the opposite effect and increases heart rate.
Clostridioides difficile overgrows after broad-spectrum antibiotic therapy disrupts normal colonic flora and produces toxins that damage the colonic mucosa, causing pseudomembranous colitis. Escherichia coli and Vibrio cholerae cause other forms of diarrhea, and Staphylococcus epidermidis is a skin commensal.
Dendritic cells are professional antigen-presenting cells that constitutively express high levels of MHC class II and costimulatory molecules, enabling them to activate CD4+ T cells. Erythrocytes, neurons, and platelets do not constitutively express MHC class II.
The proximal convoluted tubule reabsorbs approximately 65% to 70% of filtered sodium, mainly through Na+/K+-ATPase-driven transporters and cotransport mechanisms. Distal segments reabsorb smaller fractions and fine-tune sodium balance under hormonal control.
The phrenic nerve arises from cervical roots C3 to C5 and provides the motor innervation to the diaphragm. Its integrity is essential for normal spontaneous ventilation.
Acute hypoxia depletes ATP, causing Na+/K+-ATPase failure and influx of sodium and water, which produces cellular swelling. This change is reversible if oxygen delivery is restored before irreversible membrane and mitochondrial injury occur.
S-adenosylmethionine is the major methyl-group donor for methylation of DNA, proteins, and other molecules. It is generated in the methionine cycle and transfers its methyl group to diverse acceptors.
Aminoglycosides accumulate in proximal tubular cells and in cochlear and vestibular hair cells, causing nephrotoxicity and ototoxicity. These adverse effects are important reasons for therapeutic drug monitoring.
Parathyroid hormone increases serum calcium by promoting osteoclastic bone resorption, renal calcium reabsorption, and activation of vitamin D. Calcitonin has the opposite effect and lowers serum calcium.
Epstein-Barr virus infects B lymphocytes via CD21 and is the primary cause of heterophile-positive infectious mononucleosis. Cytomegalovirus can produce a heterophile-negative mononucleosis-like illness but is less commonly the primary cause.
Lesch-Nyhan syndrome results from deficiency of hypoxanthine-guanine phosphoribosyltransferase, which impairs purine salvage and causes hyperuricemia, neurologic dysfunction, and self-injurious behavior. Adenine phosphoribosyltransferase deficiency causes kidney stones, and xanthine oxidase deficiency causes xanthinuria.
Pneumonia produces consolidated, poorly ventilated alveoli that remain perfused, creating ventilation-perfusion mismatch and hypoxemia. Alveolar hypoventilation would typically raise PCO2, and an intracardiac shunt is not caused by pneumonia.
Glucuronidation transfers glucuronic acid from UDP-glucuronic acid to drugs and endogenous compounds via UDP-glucuronosyltransferase. Acetylation uses acetyl-CoA, methylation uses S-adenosylmethionine, and sulfation uses PAPS.
The lectin pathway is initiated when mannose-binding lectin or ficolins bind carbohydrate patterns on microbial surfaces and activate MBL-associated serine proteases. The classical pathway is triggered mainly by antibody-antigen complexes, and the alternative pathway is activated spontaneously on microbial surfaces.
The suprachiasmatic nucleus of the anterior hypothalamus receives light input through the retinohypothalamic tract and coordinates circadian rhythms, including melatonin secretion. The amygdala, hippocampus, and ventromedial hypothalamus serve other functions.