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217 lessons in 16 series. Each series builds one idea at a time, so start at the first lesson. Filter by course or system, or search the titles.
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Renal & acid–base
USMLE Step 1
5:35ABG interpretation, made simple
ABG interpretation from three numbers: pH, PaCO₂ and HCO₃⁻. Learn the normal ranges, the 20 : 1 ratio behind pH, and a three-step method for naming the primary disorder and its compensation.
5:25Anion gap explained: high vs normal gap metabolic acidosis
How to calculate the anion gap, why an added acid widens it while bicarbonate loss does not, and how a low albumin can hide a high-gap acidosis.
5:10Winters' formula: the expected PaCO₂ in metabolic acidosis
Winters' formula tells you how far the PaCO₂ should fall in a metabolic acidosis. Compare the measured value with that window and a hidden respiratory disorder shows itself.
5:09Delta ratio: find the hidden second acid–base disorder
The delta ratio compares the rise in the anion gap with the fall in bicarbonate. In a high anion gap acidosis, it shows whether a second metabolic disorder is hiding.
5:23New bicarbonate: how the kidney excretes acid with ammonium
The kidney reclaims the bicarbonate it filters, but that never makes new bicarbonate. Only excreting acid does, on two urinary buffers: titratable acid and ammonium.
5:54High anion gap metabolic acidosis causes, sorted by the anion
A high anion gap shows that an unmeasured anion has arrived, but not which one. Sort the causes of high anion gap metabolic acidosis by the anion that is actually there.
5:31Osmolar gap: how toxic alcohols hide from the anion gap
The osmolar gap is measured minus calculated osmolality. It picks up uncharged particles such as methanol and ethylene glycol, which raise it early, before the anion gap opens.
5:33Urine anion gap: telling diarrhea from RTA
Two patients can share the same normal anion gap acidosis, one from the gut and one from the kidney. The blood can't tell them apart, but the urine anion gap can.
5:24Renal tubular acidosis: tell RTA types 1, 2 and 4 apart
Renal tubular acidosis gives the same normal anion gap blood picture in three different ways. The mechanism, the urine pH and the serum potassium separate type 1, type 2 and type 4.
5:30Metabolic alkalosis: generation, maintenance and urine chloride
A metabolic alkalosis needs both an event that raises bicarbonate and a kidney that keeps it. Urine chloride shows which kind of maintenance is at work and whether saline will help.
5:36Vomiting metabolic alkalosis: why the urine turns acid
Vomiting removes gastric acid and chloride, leaving a hypochloraemic, hypokalaemic metabolic alkalosis. Yet the urine is acid, a finding called paradoxical aciduria.
5:21Bartter vs Gitelman: NKCC2, NCC and urine calcium
Bartter syndrome behaves like an inherited loop diuretic and Gitelman syndrome like an inherited thiazide. Both cause metabolic alkalosis, but they split on urine calcium.
5:22Respiratory acidosis compensation: acute vs chronic
In respiratory acidosis the lungs are the problem, so compensation comes from buffers first and from the kidney later. Learn the expected HCO₃⁻ for the acute and the chronic phase.
5:33Respiratory alkalosis compensation: acute vs chronic
Two patients can breathe the same way and still have different bicarbonate. Respiratory alkalosis compensation comes in two waves, and the kidneys need days to deliver the second.
5:12Metabolic alkalosis compensation: the expected PaCO₂
In metabolic alkalosis the lungs compensate by breathing less, but only weakly. Use the expected PaCO₂ to tell appropriate compensation from a second respiratory disorder.
5:31Salicylate poisoning acid-base: a classic mixed disorder
Salicylate hits the brain and the mitochondria, so two primary disorders appear together. Use the anion gap and Winters' expected PaCO₂ to recognise the mix.
5:33Type A vs type B lactic acidosis: split it by mechanism
The same lactate can come from a patient in shock or from one with normal perfusion. Ask one question, is perfusion adequate, to separate type A from type B lactic acidosis.
5:42DKA anion gap explained: from high gap to hyperchloraemic acidosis
In diabetic ketoacidosis the anion gap starts high. During treatment it can close while the patient is still acidotic, because ketoanions leave in the urine and chloride takes their place.
5:43Acidosis and hyperkalemia: how pH shifts potassium
Two patients can share the same blood pH and have different potassium. In acidosis and hyperkalemia, the type of acid matters, not just the pH, and the anion gap tells you which acid you have.
5:32ABG interpretation in 6 steps: the acid-base algorithm
One fixed sequence makes ABG interpretation systematic. A normal pH does not clear a gas, and a missed compensation window or a high delta ratio exposes the second disorder.
5:44Acid-base practice cases: three ABG vignettes, one algorithm
The same six steps solve one, two or three acid-base disorders. Work through two ABG vignettes in full, then try the practice case yourself.
Renal physiology
USMLE Step 1
5:25GFR and Starling forces at the glomerulus
Three pressures across the capillary wall set the net filtration pressure, and GFR is Kf times that number. Use them to predict why a kidney stone lowers filtration.
5:16Afferent vs efferent arteriole: GFR, RPF and filtration fraction
Two resistors set the pressure in the glomerulus. Learn the 2×2 table of afferent and efferent constriction and dilation, and use it to explain NSAID and ACE inhibitor kidney injury.
5:20Renal clearance: inulin, creatinine and PAH
Renal clearance is a bookkeeping volume, not a real flow. Use the same equation to estimate GFR with inulin and creatinine, effective renal plasma flow with PAH, and to read what the tubule did.
4:55Glucose transport maximum, splay and SGLT2
Glucosuria is not a leak in a healthy pipe. It appears when the filtered load exceeds a finite carrier capacity, the glucose transport maximum, and splay makes it start earlier than the simple threshold predicts.
5:02Countercurrent multiplier: how the kidney concentrates urine
No pump in the kidney moves water, yet urine can be several times as concentrated as plasma. The countercurrent multiplier, urea recycling and vasa recta build the medullary gradient that ADH lets the duct use.
5:20Aldosterone and ADH in the distal nephron
Two hormones act on the same stretch of tubule but do different jobs. Aldosterone moves sodium in and potassium and hydrogen out; ADH only opens water channels.
5:53Diuretics by nephron segment: loop, thiazide and more
Each diuretic class blocks one transporter in one nephron segment, and that transporter predicts the whole lab pattern. Map the four stops, then split loops from thiazides on calcium.
5:23RAAS explained: renin, angiotensin II and aldosterone
The renin–angiotensin–aldosterone system explained step by step: three triggers for renin release, four jobs for angiotensin II, and what aldosterone does in the collecting duct.
Cardiovascular physiology
USMLE Step 1
5:00Pressure-volume loop explained: four phases, four valve events
One heartbeat drawn as a single shape. Learn which corner of the pressure-volume loop is which valve, which way the loop runs, and how to read stroke volume and ejection fraction straight off it.
5:04Preload on the pressure-volume loop: EDV, stroke volume and the EDPVR
Give fluid or lose blood, and the same heart draws a different loop. See how preload slides one corner of the pressure-volume loop, and why the other side barely moves.
4:49Afterload on the pressure-volume loop
Afterload on the pressure-volume loop is a line, not a number. Pivot it and the end-systolic point slides along a fixed ESPVR: higher afterload means a taller, narrower loop and less blood out.
5:17Contractility on the pressure-volume loop: the ESPVR explained
Contractility on the pressure-volume loop shows up as the slope of the ESPVR. More contractility steepens it, so ejection runs to a smaller volume; less contractility flattens it.
5:30Wiggers diagram explained: the cardiac cycle step by step
A Wiggers diagram is a few traces on one time axis. One rule decodes it: blood moves down a pressure gradient, and each valve is a one-way door that obeys the gradient.
4:44The Frank-Starling curve: preload, inotropy and heart failure
The Frank-Starling curve plots stroke volume against preload. Filling moves the heart along one curve; inotropy and heart failure shift the curve itself.
5:41Aortic stenosis vs mitral regurgitation on the pressure-volume loop
Aortic stenosis is pressure overload: a tall, narrow loop. Mitral regurgitation is volume overload: a wide loop shifted right, with no true isovolumetric sides. Read height first, then width.
5:07Fick principle: how to calculate cardiac output
The Fick principle uses oxygen as a tracer to find cardiac output from oxygen consumption and the arteriovenous oxygen content difference. Learn the formula, the sampling sites and the units trap.
5:33HFrEF vs HFpEF on the pressure-volume loop
HFrEF vs HFpEF comes down to which curve moves: a flatter ESPVR breaks emptying, a steeper EDPVR breaks filling. Both raise end-diastolic pressure and can congest the lungs.
5:15Poiseuille's law and vascular resistance: why arterioles matter
A normal pump can still produce a high pressure. Mean arterial pressure is cardiac output times resistance, and Poiseuille's law shows why arteriolar radius controls that resistance.
5:11Venous return and cardiac function curves: where they cross
Right atrial pressure is not a free input: it is set by where the cardiac function curve and the venous return curve cross. Learn which curve each change moves.
5:17The baroreceptor reflex: from stretch to vagal and sympathetic output
Stretch receptors in the carotid sinus and aortic arch report arterial pressure to the medulla, which opposes any change. Learn the arc and predict standing, bleeding and carotid massage.
4:50Myocardial oxygen supply and demand: why tachycardia causes ischaemia
Heart rate, contractility and wall tension set demand; coronary flow, mostly in diastole, sets supply. Speeding up the heart raises one and shrinks the other.
4:50Unstable angina vs NSTEMI vs STEMI: from plaque to ACS
Acute coronary syndrome starts with a clot on a disrupted plaque. How much of the artery that clot blocks decides whether you see unstable angina, NSTEMI or STEMI.
4:56Antianginal drugs: nitrates, beta blockers, calcium channel blockers
Each anti-ischaemic drug turns different dials of myocardial oxygen demand. Learn which dial each one turns, and why some of them speed the heart up.
4:59Complications of myocardial infarction, by timeline
Most post-MI complications follow what the dead wall is doing at that time. Learn the time windows and one-line signatures for arrhythmia, ruptures, aneurysm and Dressler syndrome.
Heart sounds, murmurs and ECG
USMLE Step 1
5:26S2 splitting and heart sounds S1–S4, explained
Heart sounds are valves closing. Read S2 splitting with two questions: which valve is late, and does the gap move with breathing?
5:15Heart murmurs and dynamic manoeuvres, from preload and afterload
A murmur follows the flow: manoeuvres that change preload or afterload change how much blood crosses the lesion. Hypertrophic obstructive cardiomyopathy is the exception that flips the rule.
5:31HOCM vs aortic stenosis murmur: telling them apart at the bedside
Both murmurs are harsh and systolic. The difference is the obstruction: a fixed valve in aortic stenosis, a dynamic muscle and mitral leaflet in HOCM. The manoeuvres follow from that.
5:15Ventricular action potential: phases 0 to 4 and their ion currents
A ventricular muscle cell takes about a quarter of a second to fire. Match each phase to its ion current, then to the antiarrhythmic class that acts on it.
5:17SA node pacemaker potential: phase 4 sets the heart rate
Sinoatrial nodal cells have no stable resting potential. Heart rate is the slope of phase 4, and the vagus and sympathetic nerves simply move that slope.
5:29ECG waves and intervals: what P, PR, QRS, ST and QT mean
Each ECG wave and interval maps to an electrical event in the heart. Read the gaps and you can tell where a conduction delay sits.
Respiratory physiology
USMLE Step 1
5:26Lung volumes and capacities, explained
Every lung volume and capacity on a spirogram, and why residual volume, FRC and total lung capacity need more than spirometry.
5:23Obstructive vs restrictive lung disease: FEV1/FVC and loops
How the FEV1/FVC ratio, total lung capacity and the shape of the flow-volume loop separate obstructive from restrictive lung disease in a patient who is short of breath.
5:23Lung compliance, Laplace and surfactant: why alveoli stay open
A film of water lines every alveolus and pulls inward on it. Lung compliance, the law of Laplace and surfactant explain why the lungs do not collapse at the end of each breath.
5:18V/Q mismatch: shunt, dead space and the oxygen test
Shunt, dead space and low V/Q all mismatch ventilation and perfusion, yet only shunt barely responds to oxygen. The oxygen test sorts the three.
5:39A-a gradient: the alveolar gas equation and the causes of hypoxaemia
A low arterial PO₂ does not say whether the lung is failing or the patient is under-breathing. The A-a gradient, calculated with the alveolar gas equation, does.
5:19Oxygen-hemoglobin dissociation curve: P50, shifts and content
The oxygen-hemoglobin dissociation curve plots saturation against oxygen pressure. Learn to read it, predict its shifts, and see why a normal saturation can hide a low oxygen content.
5:29CO₂ transport: bicarbonate, the chloride shift, Bohr vs Haldane
Blood carries carbon dioxide in three forms, mostly as bicarbonate made inside the red cell. One rule then separates the Bohr effect from the Haldane effect.
5:27Central vs peripheral chemoreceptors and oxygen-induced hypercapnia
What the central and peripheral chemoreceptors sense, why the classic hypoxic-drive story is incomplete, and how to think about oxygen in COPD.
5:41Hypoxic pulmonary vasoconstriction and cor pulmonale
Low oxygen constricts the small pulmonary arteries. In one poorly ventilated patch this protects gas exchange; across the whole lung, it strains the right ventricle.
5:34CO, cyanide and methaemoglobin: why SpO₂ and PaO₂ fool you
CO, cyanide and methaemoglobin can all starve tissue of oxygen while the pulse oximeter or the arterial PaO₂ looks reassuring. Each poison leaves a different fingerprint.
Neuroanatomy and lesion localisation
USMLE Step 1
5:08Upper vs lower motor neuron lesions: signs and localisation
Weak is weak, but the muscle tells you which neuron broke. Learn the signs of upper and lower motor neuron lesions and use them to place the damage.
5:01Dorsal column vs spinothalamic tract: where each pathway crosses
Both sensory pathways start in the same ganglion and end in the same cortex, but one crosses in the medulla and the other in the cord. That one fact predicts the pattern of sensory loss.
5:06Spinal cord syndromes compared: rebuild each from three tracts
Every spinal cord syndrome is the same cross-section with a different shape of damage. Three tracts and the cord's blood supply are enough to predict each pattern.
5:10Cranial nerve nuclei: a brainstem map by level and position
Ten pairs of cranial nerves have nuclei in the brainstem, and they look like a list to memorise. Two rules sort them, level and position from the midline, with one exception: the trigeminal nuclei run the whole length.
4:50Brainstem crossed findings: the localisation rule
A cranial nerve sign on one side with weakness on the other points to the brainstem. Learn which side each tract carries and how the cranial nerve names the level.
5:30Medial vs lateral medullary syndrome: artery, structures and signs
Split the medulla at the olive level into a medial strip and a lateral part. Each has its own artery, its own structures and its own signs, and the two syndromes mirror each other.
4:59Facial nerve palsy: why the forehead tells you the lesion
Two droops can look alike. The forehead separates an upper motor neuron lesion from a lower motor neuron one, and the nerve's branches show where along the nerve the lesion sits.
4:41Stroke syndromes by artery: ACA, MCA and PCA
Each cerebral artery supplies a patch of cortex, and the deficits of a stroke follow from that patch and from the body map inside it. Place the territory, then read the syndrome.
4:54Lacunar stroke syndromes: pure motor, pure sensory and more
A small deep infarct can cause a dense deficit with no cortical signs. Learn the five classic lacunar stroke syndromes and match each one to its small vessel and site.
4:42Visual pathway lesions and field defects
Predict the visual field defect from the site of the lesion. Before the chiasm, one eye; at the chiasm, both outer fields; after the chiasm, the same side in both eyes, opposite the lesion.
5:19Basal ganglia direct and indirect pathways: D1, D2 and movement
The basal ganglia act as a brake held on the thalamus. The direct and indirect pathways and dopamine set that brake, and Parkinson and Huntington disease are what happens when it is too strong or too weak.
4:42Cerebellar lesion localisation: vermis vs hemisphere
Name the site of a cerebellar lesion from the pattern of ataxia. The midline steadies the trunk, the hemispheres steady the limbs, and hemisphere signs stay on the same side.
4:56Brain herniation syndromes: what is squeezed, and where
Brain herniation syndromes are told apart by the structure compressed and the signs it produces: subfalcine, uncal, central, tonsillar and upward, plus the Kernohan notch and Cushing triad.
5:12CSF flow and hydrocephalus: where the flow fails
Trace cerebrospinal fluid from the choroid plexus to the dural venous sinuses, then use the point of failure to separate communicating, non-communicating and normal pressure hydrocephalus.
4:59Corticospinal tract: from cortex to anterior horn
The corticospinal tract carries voluntary motor commands from the cortex to the spinal cord. Knowing its course lets you predict the deficit from a lesion at each level.
5:24Hypothalamic nuclei and their functions
One small structure runs water, food, heat and the body clock. Match each hypothalamic nucleus to its job and a lesion becomes a single missing function you can predict.
Immunology
USMLE Step 1
5:03MHC I vs MHC II: how antigen presentation works
MHC I vs MHC II comes down to one rule: class I shows what the cell made inside, class II shows what the cell swallowed from outside. Each class has its own machinery and its own T cell.
4:58T-cell activation: the two-signal model
A T cell can recognise its antigen perfectly and still stay asleep. T-cell activation needs a second signal, and the antigen-presenting cell decides whether it is given.
5:13Th1, Th2, Th17 and Treg: helper T-cell subsets
Th1, Th2, Th17 and Treg are chosen by the cytokines around a naive CD4 T cell. Each has an inducer, a master transcription factor, a product and a target.
5:13Antibody isotypes and class switching
Every antibody class has one main job. A B cell can change its class without changing what it recognises, and the helper T cell and the enzyme AID decide how.
5:26Central vs peripheral tolerance: how lymphocytes learn self-tolerance
Receptors are built at random, so some will recognise the body's own molecules. Central tolerance removes them in the thymus and bone marrow; peripheral tolerance catches those that slip through.
5:24Complement system explained: three pathways, one ending
The complement system has three ways in and one way out. Compare how the classical, lectin and alternative pathways start, then follow the shared steps that all three produce.
5:15Complement deficiencies: C1q, C3, C5–C9 and C1 inhibitor
Each missing complement component predicts its own disease. Work out the infection or syndrome from the job that is lost, and the CH50, C3 and C4 pattern follows.
5:20Hypersensitivity types I to IV: the four effectors
Hypersensitivity types I to IV sort allergic and immune damage by what does the harm: IgE, antibody on cells, immune complexes or T cells. Timing is the clue.
5:24B-cell vs T-cell immunodeficiency: predict the infections
Two babies are both immunodeficient, yet their infections differ. Work out which arm of immunity is lost, antibody or cellular, and the pattern of infection follows.
5:19Hyper IgM syndrome: CD40 ligand and class switching
High IgM, yet a deficiency. In its commonest form, the B cell is present but cannot switch class, because the T-cell signal through CD40 ligand fails.
5:20Chronic granulomatous disease: NADPH oxidase and the respiratory burst
The neutrophils are present in normal number, yet they cannot kill. Follow one oxygen molecule through the respiratory burst to see why infections persist in chronic granulomatous disease.
5:01Leukocyte adhesion deficiency: CD18 and neutrophil extravasation
A newborn has a very high white cell count and an infection with no pus. Follow a neutrophil out of the blood to see which step fails in leukocyte adhesion deficiency type 1, and how type 2 differs.
5:51Transplant rejection explained: hyperacute, acute, chronic and GVHD
Is the host attacking the graft, or the graft attacking the host? How to tell the three types of transplant rejection and graft-versus-host disease apart by timing and immune mechanism.
5:41Vaccine types explained: live, inactivated, conjugate and mRNA
Each vaccine platform shows the immune system an antigen in a different way. Match the type to the response it triggers, and see why live vaccines are avoided in immunodeficiency.
Pharmacology: principles and autonomic drugs
USMLE Step 1
5:17Potency vs efficacy: EC50, Emax and agonist types
Potency vs efficacy: potency is where a dose-response curve sits, efficacy is how high it reaches. Learn to read EC50 and Emax, and to tell full, partial and inverse agonists apart.
5:01Competitive vs noncompetitive antagonists: effect on EC50 and Emax
Two antagonists can block the same receptor, yet only one can be beaten by more agonist. The shape of the dose-response curve tells them apart.
5:01Therapeutic index explained: TD50 / ED50
Two drugs can be equally potent and very different in safety. The therapeutic index compares the toxic dose with the effective dose, and explains why some drugs need monitoring.
5:18First-pass metabolism and bioavailability: calculating oral F from AUC
A swallowed drug passes through the gut wall and liver before it reaches the circulation. Learn how first-pass metabolism lowers bioavailability and how to calculate oral F from AUC and dose.
5:17Volume of distribution: why Vd can be bigger than the body
Volume of distribution is the amount of drug in the body divided by the plasma concentration. It is an apparent volume, so it can be far larger than the body itself.
5:01Clearance and half-life: t½ = 0.693 × Vd / CL
How clearance and volume of distribution together set a drug's half-life, and why the half-life is the clock for reaching steady state and for washout.
5:12Zero-order vs first-order elimination explained
First-order elimination removes a constant fraction per unit time; zero-order removes a constant amount. Saturation of metabolizing enzymes turns one into the other.
5:02Loading dose and maintenance dose, explained
A drug with a long half-life takes days to reach target on its own. The loading dose fills the volume of distribution; the maintenance dose replaces what clearance removes.
5:16CYP450 inducers and inhibitors: how they change drug levels
A drug's dose can stay the same while its level changes. Cytochrome P450 inducers and inhibitors turn the clearance dial, and the level, and the effect, follow.
5:33Autonomic receptors: alpha, beta and muscarinic
The same transmitter can do opposite things in different organs, because the receptor and its G protein decide the response. Learn the code and the tissue list once, and the effects follow.
5:24Beta blocker selectivity: beta-1, ISA and alpha blockade
Every beta blocker slows the heart, yet side effects differ. Three questions, selectivity, intrinsic sympathomimetic activity and alpha-1 blockade, predict what each drug does.
5:46Vasopressors compared: receptors, pressure and heart rate
Four adrenergic drugs, four different effects on mean arterial pressure and heart rate. Work them out from the receptors and the baroreflex instead of memorising them.
5:36Organophosphate poisoning: muscarinic vs nicotinic signs
One poison, two halves of the picture, two antidotes. Learn the muscarinic and nicotinic features of organophosphate poisoning and how atropine and pralidoxime work.
5:01Receptor desensitization, downregulation and upregulation
Receptors are regulated, not fixed. Learn why a drug can work less with repeated use, why chronic blockade builds extra receptors, and why beta-blockers are tapered.
5:04Beta-lactams and vancomycin: enzyme versus substrate
Beta-lactams and vancomycin both stop the bacterial cell wall being built, but one grips the enzyme and the other grips the substrate. That single difference explains their reach and their resistance patterns.
5:07Protein synthesis inhibitors: 30S vs 50S antibiotics
Sort protein synthesis inhibitors by ribosomal subunit and the step they block, then link each class to its signature adverse effect.
Endocrinology
USMLE Step 1
5:05Hypothalamic-pituitary axes and feedback: primary, secondary, tertiary
Every hypothalamic-pituitary axis is a three-step chain held in check by negative feedback. Use that logic to predict which hormones rise or fall when the fault is in the gland, the pituitary or the hypothalamus.
5:13Primary vs secondary endocrine disorders: paired hormone levels
Two numbers, read together, tell you whether the gland or the pituitary is failing. Learn the two-number rule for primary vs secondary endocrine disorders across the adrenal, thyroid and gonadal axes.
5:06Insulin secretion in the beta cell: from glucose to granule
A beta cell has no eyes and no taste buds, yet it answers a rise in glucose within minutes. Follow the six steps, and see where sulfonylureas and diazoxide act on the KATP channel.
5:19Insulin vs glucagon signalling: receptors and effects
Insulin vs glucagon: two islet hormones, two receptor families, and three target tissues. Why a glucagon injection rescues hypoglycaemia through the liver, not muscle.
5:58DKA vs HHS pathophysiology: why only one makes acid
Both crises are hyperglycaemic and dehydrated, yet only diabetic ketoacidosis produces acid. The difference is how much insulin is left.
5:58Diabetes drugs by mechanism: where each class acts
Every antidiabetic drug lowers glucose, but each one acts at a different site and carries its own signature side effect. Sort them by mechanism and the side effects follow.
5:14Thyroid hormone synthesis, step by step
Six steps turn iodide into T₄ and T₃, and each antithyroid drug is defined by the step it blocks. Learn the sequence once and the drug map follows.
5:28Calcium homeostasis: PTH, vitamin D and calcitonin
One hormone, PTH, raises calcium yet lowers phosphate. Calcium homeostasis makes sense once you follow PTH, vitamin D and calcitonin across bone, kidney and gut.
5:29Adrenal steroidogenesis: zones, enzymes and the 17α-hydroxylase fork
One gland and one cholesterol pool make three hormone families. Each cortical zone has its own driver and its own enzymes, and 17α-hydroxylase decides which path a steroid takes.
5:44Congenital adrenal hyperplasia: three enzymes, three patterns
Congenital adrenal hyperplasia is one disease family with opposite blood pressures. Ask what piles up, what falls and where it spills to predict the pattern for each enzyme.
5:29Cushing syndrome: the dexamethasone suppression test and ACTH
A normal morning cortisol does not rule out Cushing syndrome. Use screening tests, ACTH and dexamethasone suppression to decide whether the source is the pituitary, an ectopic tumour or the adrenal.
5:27Primary hyperaldosteronism: high aldosterone, low renin
Primary hyperaldosteronism is autonomous aldosterone. Follow it into the collecting duct to explain the hypertension, hypokalaemia, metabolic alkalosis and suppressed renin.
5:44SIADH vs diabetes insipidus: central vs nephrogenic DI
SIADH and diabetes insipidus are both ADH problems that run in opposite directions. Serum sodium, urine osmolality and the response to desmopressin tell them apart.
5:13MEN1, MEN2A and MEN2B: match the gene to its tumours
Two genes explain three syndromes. MEN1 loses a brake (menin); MEN2 jams an accelerator (RET). Once you know the gene, the core tumours follow; the extra features separate MEN2A from MEN2B.
Haematology and oncology
USMLE Step 1
5:37Microcytic anaemia: four causes, four iron fingerprints
In microcytic anaemia, small red cells look alike under the microscope, but each cause breaks a different step of haemoglobin making. The iron studies, the red cell count and HbA₂ show which step failed.
5:34Hepcidin: why iron falls in inflammation
Hepcidin is the liver hormone that locks iron inside cells. It explains why serum iron drops in inflammation without any bleeding, and how ferritin and iron-binding capacity tell it apart from iron deficiency.
5:23B12 vs folate deficiency: MMA, homocysteine and the nerves
How to tell B12 deficiency from folate deficiency using methylmalonic acid, homocysteine and the nervous system, and why folic acid alone can fix the blood while the nerves worsen.
5:27Intravascular vs extravascular haemolysis: site and labs
Both kinds of haemolysis destroy red cells too early. The site of destruction, and what haptoglobin, LDH, bilirubin and urine show, tells them apart.
5:36G6PD deficiency vs hereditary spherocytosis, by mechanism
Both cause haemolysis with jaundice, but one is an antioxidant enzyme failure and the other a membrane scaffold failure. The smear, the timing and the antiglobulin test sort them.
5:26Sickle cell disease pathophysiology: HbS to vaso-occlusion
How one amino acid change makes haemoglobin S polymerise when deoxygenated, what promotes sickling, and how the polymer causes haemolysis, vaso-occlusion and functional asplenia.
5:53Primary haemostasis: platelet adhesion, activation and aggregation
Primary haemostasis is the platelet plug, built in three steps. Learn where each antiplatelet drug acts and why aspirin's effect outlasts the tablet.
5:21Coagulation cascade: PT vs aPTT, pathway by pathway
PT and aPTT each test part of the coagulation cascade. Learn which factors sit behind each test, so a long result points to a pathway and not just to a bleeding risk.
5:32Von Willebrand disease vs haemophilia: the labs that separate them
Both conditions can give a long PTT, so the PTT alone cannot separate them. The bleeding pattern, the bleeding time and the factor levels can.
5:34Anticoagulants: target, monitoring test and reversal agent
Every anticoagulant in this lesson ends up blocking thrombin or factor Xa. Learn each drug's mechanism and the monitoring test and reversal agent follow from it.
5:29Heparin-induced thrombocytopenia: why a low count clots
In heparin-induced thrombocytopenia (HIT), an antibody against PF4–heparin complexes lowers the platelet count and, at the same time, drives thrombin generation, so the danger is thrombosis, not bleeding.
5:33DIC vs TTP vs HUS: mechanism and the PT/PTT
DIC vs TTP vs HUS: schistocytes and a low platelet count can appear in all three. What separates disseminated intravascular coagulation, TTP and HUS is what forms the clot, and that decides the labs.
5:44AML vs ALL vs CML vs CLL: leukemia classification made simple
Sort the four main leukemias by lineage, maturity and age, and learn why a high white cell count can be the milder disease and a low one the emergency.
5:17Oncogenes vs tumour suppressor genes: one hit or two
Oncogenes switch on with a single gain-of-function hit. Tumour suppressor genes classically must lose both copies. Knudson's two-hit hypothesis explains why the same gene can cause early or late tumours.
Microbiology
USMLE Step 1
5:39Gram-positive vs gram-negative cell walls
The Gram stain looks like a dye test, but it is a wall test. Compare the two cell envelopes and use them to predict the stain and antibiotic susceptibility.
5:01Acid-fast stain and the mycobacterial cell wall
Mycobacteria wrap their wall in wax. That one feature explains why the acid-fast stain works, why Gram staining does not, and why many antibiotics struggle to get in.
5:46Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids
Endotoxin is a piece of the gram-negative outer membrane. An exotoxin is a protein that living bacteria secrete. Both can cause shock, but they differ in source, chemistry, mechanism and effect.
5:54Bacterial toxins that raise cAMP: cholera, pertussis, anthrax
Four toxins from four bacteria all push cyclic AMP up, but each takes a different route. Knowing the route, and the cell it acts on, predicts the clinical picture.
5:22Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile
Tetanus and botulinum toxins cut the same SNARE proteins but give opposite pictures. Compare the clostridial toxins by their target and their clinical picture.
5:23Staph vs strep: catalase, coagulase and haemolysis
Two tests do most of the work in staph vs strep identification. Catalase separates the genera, then each genus has its own fork: coagulase and novobiocin, or haemolysis and a susceptibility disc.
5:08Encapsulated organisms and the spleen: asplenia and conjugate vaccines
A missing spleen turns a minor illness into an emergency because encapsulated bacteria depend on it for clearance. Conjugate vaccines work by bringing in T cell help.
5:13Bacterial genetics: how bacteria swap genes
Bacteria divide by binary fission, yet genes still move sideways between cells. Each route of horizontal gene transfer has one identifying clue.
5:13Beta-lactamase and antibiotic resistance: three ways bacteria win
Bacteria resist beta-lactams by destroying the drug, changing its target or keeping it out. Only the first can be beaten by a beta-lactamase inhibitor such as clavulanate.
5:48DNA vs RNA virus replication: which enzyme each genome needs
Where DNA and RNA viruses replicate, what positive-sense and negative-sense mean, and the one rule that tells you which enzyme a virion must carry.
5:39HIV life cycle and drug targets, step by step
Six steps take HIV from attachment to a mature virion, and antiretroviral drug classes block them. Learn the order, the drugs and why the integrated provirus cannot be cleared.
5:17Hepatitis B serology: reading the panel
A hepatitis B panel is built from three viral antigens and their antibodies. Learn what each marker means and how the combinations separate acute, chronic, recovered and vaccinated patients.
5:26Antifungal drugs explained: mechanisms of action
Fungi are eukaryotes like us, so antifungal drugs must hit the few things human cells lack. Classify the five classes by the fungal target each one inhibits.
5:19Bacterial meningitis organisms by age and risk group
The cerebrospinal fluid tells you the meningitis is bacterial, but not which bacterium. Age and risk group predict the organism, and the Gram stain confirms it.
GI physiology and pathology
USMLE Step 1
5:14GI hormones: gastrin, CCK, secretin, GIP and motilin
Each gut hormone answers one question: what has just arrived, and what must the gut do about it? Match every hormone to its source cell, its stimulus and its actions.
4:52Gastric acid secretion: the parietal cell, step by step
Three signals converge on one proton pump, and acid also brakes its own release. Follow the parietal cell, then see where PPIs, H₂ blockers and somatostatin act.
5:14H. pylori and peptic ulcer disease: how mucosal defence fails
More than a third of people worldwide carry H. pylori, yet only a minority develop an ulcer. This lesson builds the balance between acid and mucosal defence, then shows how H. pylori and NSAIDs break it.
5:44Bilirubin metabolism and jaundice: pre-hepatic, hepatic, post-hepatic
Jaundice is one sign with three places to fail. Follow bilirubin from haem to the gut, then use the fraction, the enzymes, the urine and the stool to find where the pathway breaks.
Emergency medicine
USMLE Step 2 CK
5:21Chest pain approach: the first 10 minutes
A simple chest pain approach for any adult with acute chest pain: a three-rung ladder of stability, a 12-lead ECG within 10 minutes with the six life threats, then troponin and risk.
5:23High-sensitivity troponin: myocardial injury vs infarction
How to use high-sensitivity troponin in suspected ACS without ST elevation: serial ECGs, the repeat sample and the rise or fall, and how myocardial injury differs from myocardial infarction.
5:18STEMI vs NSTEMI: the reperfusion decision
Same crushing chest pain, two different ECGs. One sends the patient for reperfusion within minutes; the other starts a risk assessment. Here is how to tell them apart and what to do first.
4:44Chest pain next best step: STEMI or aortic dissection?
Inferior ST elevation meets STEMI criteria, but the rest of the stem points somewhere else. Walk a chest pain case line by line and see why the next best step is to image the aorta.
3.1 Biological molecules
A-level Biology
3:10Condensation and Hydrolysis Reactions
How monomers join into polymers by condensation (a bond forms and water is released) and how hydrolysis breaks them apart again using water.
3:41Maltose, Sucrose, Lactose: Glycosidic Bonds
How two monosaccharides join by condensation to form a glycosidic bond, and which pairs make the three disaccharides: maltose, sucrose and lactose.
4:55Alpha vs Beta Glucose: Starch vs Cellulose
Why alpha-glucose gives starch and glycogen (storage) while beta-glucose gives cellulose (structure), and how each structure fits its function.
3:55Benedict's and Iodine Tests Explained
How to test for reducing sugars, non-reducing sugars and starch with Benedict's reagent and iodine, including the acid hydrolysis route for non-reducing sugars.
4:03Triglycerides: Saturated vs Unsaturated Fats
How a triglyceride forms from glycerol and three fatty acids by condensation (ester bonds), and how saturated and unsaturated fatty acids differ.
4:14Phospholipids vs Triglycerides: Emulsion Test
How the hydrophilic head and hydrophobic tails of a phospholipid make it form layers on water while triglycerides form droplets, plus the emulsion test for lipids.
3:31Amino Acids and Peptide Bonds Explained
The general structure of an amino acid, how the 20 amino acids differ in their R group, and how a condensation reaction forms a peptide bond in a dipeptide.
4:53Protein Structure: Primary to Quaternary
The four levels of protein structure (primary, secondary, tertiary, quaternary), the bonds that hold each level, and how one changed amino acid can alter the final shape.
3:45Globular vs Fibrous Proteins and Biuret Test
How protein shape decides its job, comparing globular and fibrous proteins (collagen), and how the biuret test shows that protein is present.
4:35How Enzymes Work: Active Site and Induced Fit
How enzymes speed up reactions by lowering activation energy at the active site, why each enzyme is specific, and how the induced fit model replaced lock and key.
4:36Enzymes: Temperature and pH Effects
How temperature and pH change the rate of an enzyme-controlled reaction, and why high temperature or extreme pH denatures the enzyme by breaking bonds in its tertiary structure.
3:49Enzyme Rate Graphs: Why the Plateau
Why the rate of an enzyme-controlled reaction rises with substrate concentration then levels off, and which factor is limiting at the plateau.
4:25Competitive vs Non-Competitive Inhibitors
How competitive and non-competitive inhibitors slow an enzyme-controlled reaction, where each binds, and what raising substrate concentration does in each case.
4:18Required Practical 1: Enzyme Rates
How to investigate the effect of temperature on the rate of an enzyme-controlled reaction using catalase: variables, water baths, controls, initial rate and repeats.
4:37DNA and RNA Nucleotides: Structure Compared
What a nucleotide is made of (pentose, phosphate, organic base) and how DNA and RNA nucleotides differ, plus the role of each molecule.
4:15DNA Double Helix and Phosphodiester Bonds
How nucleotides join by phosphodiester bonds into polynucleotides, how two strands pair by hydrogen bonds into a double helix, and why scientists once doubted DNA.
5:06DNA Replication: Helicase and Polymerase
The steps of semi-conservative DNA replication: helicase unwinding and breaking hydrogen bonds, base pairing of free nucleotides, and DNA polymerase joining them.
4:27Meselson and Stahl Experiment Explained
How Meselson and Stahl used nitrogen isotopes and density gradient centrifugation to test the conservative, semi-conservative and dispersive models of replication.
4:06ATP: Structure, Hydrolysis and Resynthesis
The structure of ATP, how its hydrolysis releases energy for energy-requiring reactions, how phosphorylation works, and how ATP is resynthesised.
3:31Water: Five Properties Linked to Function
Five properties of water (metabolite, solvent, high heat capacity, latent heat of vaporisation, cohesion) and how each helps organisms, all explained by hydrogen bonding.
3:12Inorganic Ions and Their Roles
The roles of four inorganic ions: hydrogen ions and pH, iron ions in haemoglobin, sodium ions in co-transport, and phosphate ions in DNA and ATP.
3.2 Cells
A-level Biology
3:53Nucleus, Mitochondria and Chloroplasts
The structure and function of the nucleus, mitochondria, chloroplasts and ribosomes in eukaryotic cells, with cell theory as the unifying idea.
3:54Rough ER, Golgi and Lysosomes Explained
How the rough endoplasmic reticulum, Golgi apparatus, Golgi vesicles and lysosomes work together to make, modify, package and secrete proteins.
3:43Cell Wall, Vacuole and Specialised Cells
The cell-surface membrane, plant cell wall and vacuole, and how specialised cells, tissues, organs and systems are organised, with an adaptation example.
4:37Prokaryotic vs Eukaryotic Cells
The structural differences between prokaryotic and eukaryotic cells: size, DNA, organelles, ribosomes, murein cell wall, plasmids, capsule and flagella.
2:20Viruses: Structure and Non-Living Status
The structure of a virus particle (genetic material, capsid, attachment proteins) and why viruses are described as acellular and non-living.
3:43Optical, TEM and SEM Microscopes Compared
How optical, transmission electron and scanning electron microscopes work, why electron microscopes resolve more detail, and the limitations of each.
4:49Magnification vs Resolution Calculations
How to use magnification = image size / real size, convert units, and calibrate an eyepiece graticule against a stage micrometer to measure cells.
3:51Cell Fractionation and Ultracentrifugation
How organelles are separated from tissue: homogenisation, filtration, then ultracentrifugation in isotonic, cold, buffered solution, with pellets forming in order of density.
4:47Mitosis and the Cell Cycle: Stages
The cell cycle and what chromosomes do in interphase, prophase, metaphase, anaphase and telophase, plus cytokinesis and how to identify stages under the microscope.
2:27Cancer: Uncontrolled Cell Division
How loss of control of the cell cycle forms tumours and cancers, and how treatments such as chemotherapy and radiotherapy target the rate of cell division.
3:23Binary Fission and Virus Replication
How bacteria reproduce by binary fission (circular DNA and plasmids) and why viruses cannot divide, instead using a host cell to make new virus particles.
4:13Required Practical 2: Root Tip Squash
How to prepare a stained root tip squash, identify stages of mitosis under a microscope, calculate the mitotic index and work out actual cell size.
3:59Fluid-Mosaic Model: Cell Membrane Structure
The fluid-mosaic model: phospholipid bilayer, intrinsic and extrinsic proteins, glycoproteins, glycolipids, and the role of cholesterol in restricting movement.
3:38Simple vs Facilitated Diffusion
How substances cross membranes by simple diffusion and facilitated diffusion using channel and carrier proteins, and the factors that affect the rate of diffusion.
4:42Osmosis and Water Potential Explained
How osmosis is described in terms of water potential: water moves across a partially permeable membrane from a less negative to a more negative water potential, and what that does to plant cells.
4:35Active Transport and Co-transport (Ileum)
How carrier proteins and ATP hydrolysis drive active transport, and how the sodium-potassium pump sets up sodium-glucose co-transport in the ileum.
4:24Why Cells Transport Faster: Microvilli
How microvilli, extra channel and carrier proteins and steeper gradients increase the rate of transport across cell membranes.
4:37Required Practical 3: Water Potential
How to make a dilution series, use potato cylinders, calculate percentage change in mass and read the water potential from a calibration curve.
4:29Required Practical 4: Membrane Permeability
How to investigate the effect of temperature on membrane permeability using beetroot discs and a colorimeter, and how to explain the results.
3:38Antigens and Cell Recognition
What antigens are, how the immune system tells self from non-self, and why antigen variability matters for disease and vaccines.
3:38Phagocytosis: How Phagocytes Work
The steps of phagocytosis, from engulfing a pathogen into a phagosome to lysosome fusion and digestion by lysozymes.
4:40T Lymphocytes: The Cellular Immune Response
How antigen-presenting cells activate helper T cells, and how helper T cells stimulate cytotoxic T cells, B cells and phagocytes.
4:59B Cells, Clonal Selection and Antibodies
How a B lymphocyte is selected, divides into a clone of plasma cells, and how antibodies cause agglutination and phagocytosis.
3:50Primary and Secondary Immune Response
How plasma and memory cells explain the slow first antibody peak and the faster, larger second response, with graph interpretation.
4:11Vaccines, Herd Immunity, Active vs Passive
How a vaccine produces memory cells, how herd immunity protects a population, and the differences between active and passive immunity.
4:18HIV Replication and How It Causes AIDS
The structure of HIV, how reverse transcriptase lets it replicate in helper T cells, why that leads to AIDS, and why antibiotics do not work on viruses.
3:30Monoclonal Antibodies in Medicine
How monoclonal antibodies deliver a drug to cancer cells and show a substance is present in a sample, using specific binding to one antigen.
4:02The ELISA Test Explained
The stages of the ELISA test, from antigen binding to enzyme-linked antibody, washing and the substrate colour change, and what the result shows.
3:49Vaccine and Antibody Ethics and Evidence
How to give a balanced evaluation of vaccine and monoclonal antibody issues, using ethics, animal testing and trial data such as sample size and controls.
3.3 Exchange with the environment
A-level Biology
4:18Surface Area to Volume Ratio Explained
How to calculate surface area to volume ratio using cubes, why the ratio falls as size increases, and what that means for diffusion in cells.
3:42SA:V Ratio, Body Size and Metabolic Rate
How the surface area to volume ratio explains heat loss, body shape and metabolic rate in animals, and why large animals need exchange systems.
4:06Gas Exchange: Single Cells and Insects
How diffusion supplies single-celled organisms, and how the insect tracheal system uses spiracles, tracheae and tracheoles for gas exchange.
4:03Fish Gills and the Counter-current Principle
How gill filaments and lamellae exchange gases, and why counter-current flow keeps a diffusion gradient along the whole lamella.
3:47Gas Exchange in Leaves: Mesophyll and Stomata
How the stomata, guard cells, air spaces and mesophyll of a dicotyledonous leaf allow gas exchange by diffusion, day and night.
4:18Gas Exchange vs Water Loss: Xerophytes
How insects and xerophytes such as marram grass balance gas exchange against water loss, with each adaptation linked to its effect.
3:55Human Gas Exchange System and Alveoli
The pathway of air from trachea to alveoli, and the features of the alveolar epithelium that make it an efficient gas exchange surface.
4:39Ventilation: How Breathing Works
How the diaphragm and antagonistic intercostal muscles change thoracic volume and pressure for inspiration and expiration.
4:13Lung Disease and Gas Exchange
How fibrosis, asthma, emphysema and tuberculosis change diffusion distance, surface area, elasticity and ventilation.
4:40Smoking, Pollution and Lung Disease Data
How to interpret data on risk factors for lung disease, tell correlation from causation and evaluate the evidence behind smoking restrictions.