TLDR;
This video is a comprehensive review of key concepts in physiology, intended as a last-minute revision tool for medical students preparing for exams. The instructor emphasizes focusing on high-yield topics and recent past year questions (PYQs) rather than attempting to learn new material. The session covers general cell physiology, digestive system, respiratory system, cardiovascular system, renal system, and central nervous system, providing mnemonics, key facts, and exam-oriented insights.
- Focus on revising notes and PYQs from 2020-2024.
- Cell membrane structure and function, including the Fluid Mosaic Model.
- Body fluid compartments and homeostasis.
- Digestive system layers, enteric nervous system, and gastrointestinal movements.
- Respiratory mechanics, lung volumes, and gas exchange.
- Cardiovascular physiology, cardiac cycle, and blood pressure regulation.
- Renal physiology, including filtration, reabsorption, and secretion.
- Central nervous system, including neurons, glial cells, and sensory systems.
General Cell Physiology [1:18]
The cell membrane, described by the Singer-Nicholson Fluid Mosaic Model, primarily consists of lipids and proteins, with proteins making up 55-60% of the structure. Phospholipids arrange in a bilayer with polar heads facing outwards and hydrophobic tails inwards, while proteins are interspersed for enzymatic and transport functions. Lipids maintain membrane integrity, flexibility, and solubility, allowing lipid-soluble substances like gases and steroid hormones to cross easily. Proteins facilitate the transport of water-soluble substances, such as ions via channels and pumps, and larger molecules via carrier proteins. Water-soluble hormones and neurotransmitters use cell surface receptors, with prolactin and insulin utilizing tyrosine kinase receptors, and others using GPCRs. Membrane proteins also function as enzymes, antigens (HLA, MSC, blood group antigens), and structural components.
Body Fluids and Homeostasis [5:25]
Body water constitutes 60% of body weight, divided into intracellular fluid (ICF) and extracellular fluid (ECF), with ICF being two-thirds and ECF one-third in adults. In newborns, ICF is less than ECF. ECF is further divided into plasma (inside vessels) and interstitial fluid (75%), which acts as the tissue fluid between cells and blood vessels. Lymph is formed from tissue fluid, similar to plasma but lacking proteins. Interstitial fluid facilitates the exchange between cells and vessels, maintaining a constant internal environment through homeostasis. Negative feedback mechanisms restore values to normal, while positive feedback amplifies changes in the same direction, exemplified by oxytocin's role in uterine contractions during childbirth. Body fluids are measured using dye dilution methods, with heavy water (D2O, T2O) for total body water, inulin and sucrose for ECF, and albumin Evans blue for plasma volume.
Transport Across Cell Membranes [10:48]
Transport across cell membranes is categorized into active and passive processes. Active transport requires ATP and moves substances against their concentration gradient, including primary active transport via pumps like the sodium-potassium pump and secondary active transport where movement is coupled to a pump, such as sodium-amino acid symport. Passive transport does not require energy and follows Fick's Law, moving substances down their concentration gradient. Simple diffusion involves no carrier, as seen with gases, while facilitated diffusion uses a carrier, like glucose transporters (GLUT). Cell junctions include gap junctions made of connexin proteins for ion current spread in electrical synapses, tight junctions (zonula occludens) made of occludin and claudin for barriers like the blood-brain barrier, and desmosomes (macula adherens) made of cadherin proteins for cell adhesion. Cell adhesion molecules (CAMs) like integrins and selectins maintain cell cohesion.
Digestive System Anatomy and Control [15:34]
The digestive system consists of four layers: mucosa (innermost with villi for absorption), submucosa (containing vessels, nerves, and glands), muscularis (smooth muscle for contraction), and serosa (attachment to mesentery, absent in the esophagus). The enteric nervous system controls GIT function via the submucosal plexus (secretion) and myenteric plexus (Auerbach's, for contraction). The autonomic nervous system influences GIT activity, with sympathetic stimulation decreasing function and parasympathetic increasing contractions and movement. Interstitial cells of Cajal (ICC) in the Auerbach plexus act as pacemakers, generating slow waves that, when combined with stimuli like food, cause spikes and increase calcium, leading to muscle contraction. Stimuli for contraction include food, distension, calcium, ACh, and GIT hormones.
Gastrointestinal Motility and Secretion [19:16]
Peristalsis, initiated by food distension, involves neurotransmitter release from the myenteric plexus, with VIP causing relaxation in front and ACh/serotonin/substance P causing contraction behind to propel food forward. Anti-peristalsis is reverse peristalsis, causing vomiting. Migrating motor complexes (MMC), or hunger pangs, occur every 90-100 minutes, cleaning the GIT between meals and are stimulated by motilin. Mass movements in the colon lead to defecation, which is absent in Hirschsprung's disease. Dietary fiber increases stool bulk, aiding bowel movements. Digestion occurs in three phases: cephalic (30% HCl secretion via parasympathetic stimulation), gastric (gastrin release triggering maximum HCl secretion and gastric emptying), and intestinal (GIT hormones inhibiting HCl secretion and gastric emptying, while increasing bicarbonate-rich juices and insulin release).
Saliva and Gastric Digestion [25:10]
Choleretics, like bile salts and acids, stimulate bile secretion. Saliva, primarily from the submandibular gland, is acidic at rest but becomes alkaline upon eating due to bicarbonate secretion. Saliva functions as a lubricant, aids in taste, mastication, swallowing, and provides antibacterial action via IgA, zinc, defensins, and lysozyme. Salivary amylase (ptyalin) breaks down starch into sugars, activated by chloride, with an optimal pH of 6.7. Gastric glands contain parietal cells (fundus area) that secrete HCl via proton pumps and intrinsic factor for vitamin B12 binding in the duodenum, though absorption occurs in the terminal ileum. Chief cells release pepsinogen, activated to pepsin. Vegas, ECL cells (histamine), and G cells (gastrin) stimulate HCl secretion. Brunner's glands in the duodenum protect against acid with alkaline secretions.
Digestion and Absorption of Nutrients [29:52]
Proteins are converted to amino acids, absorbed via sodium-amino acid symport. Fats are converted to long-chain fatty acids by lipases, emulsified by bile salts into micelles for absorption. Carbohydrates, mainly starch, are broken down by amylase and glucosidases into monosaccharides, absorbed via SGLT1 (glucose, galactose) and GLUT5 (fructose). Maximum absorption occurs in the jejunum, except for short-chain fatty acids (colon), B12 and bile salts (ileum), and iron and folate (duodenum). Iron absorption involves conversion of Fe3+ to Fe2+ by reductase, entry via DMT1, storage as ferritin, and exit via ferroportin, regulated by hepcidin. Vitamin C and HCl increase iron absorption, while tannins and phytates decrease it. The liver receives substances via enterohepatic circulation, with Kupffer cells killing bacteria, Ito cells storing fat and vitamin A, and hepatocytes performing metabolism and making plasma proteins.
Respiratory System: Airway and Mechanics [35:13]
The respiratory system begins with the trachea (generation 0), branching into bronchi and bronchioles, with terminal bronchioles at generation 16. Respiratory bronchioles start at generation 17, transitioning to alveolar ducts (generation 20-22) and alveoli (generation 23). The conduction zone (trachea to terminal bronchioles) has no gas exchange, while the respiratory zone (generation 17 onward) does. Terminal bronchioles have the highest smooth muscle content, regulating airway resistance. Boyle's Law dictates that pressure is inversely proportional to volume. Inspiration is active, using the diaphragm (75%) and external intercostals, while expiration is passive, relying on elastic recoil. Forceful expiration uses abdominal and internal intercostal muscles.
Respiratory Pressures and Lung Volumes [40:07]
Intra-alveolar pressure (IAP) is equal to atmospheric pressure (zero) and becomes negative during inspiration and positive during expiration. Intrapleural pressure (ITP) is always negative, preventing lung collapse due to elastic recoil. Pneumothorax occurs when ITP becomes zero or positive, causing lung collapse. Compliance is the ability to stretch the lungs, reduced in restrictive lung diseases (RLD) and increased in emphysema. Surface tension, caused by fluid inside the alveoli, is reduced by surfactant produced by type II pneumocytes, preventing RDS. Surfactant production starts at 18 weeks, action at 28 weeks, and maximum maturity at 34-35 weeks. Lung volumes include tidal volume (500 ml), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV). Capacities include inspiratory capacity (IC), expiratory capacity (EC), functional residual capacity (FRC), and vital capacity (VC).
Pulmonary Function Tests and Gas Exchange [53:43]
In restrictive lung disease (RLD), all lung volumes are decreased, while in obstructive lung disease (OLD), FEV1/FVC ratio is decreased. Alveolar ventilation is calculated as (tidal volume - dead space) x respiratory rate. The V/Q ratio compares ventilation to perfusion, with a normal ratio of 1 indicating perfect exchange. High V/Q mismatch occurs in pulmonary embolism and emphysema, increasing physiological dead space. Hypoxic hypoxia results from lung diseases, while anemic hypoxia is due to low hemoglobin. Histotoxic hypoxia occurs when tissues fail to use oxygen, as in cyanide poisoning. CO2 transport primarily occurs as bicarbonate (75%), while oxygen transport is mainly via hemoglobin. The oxygen-hemoglobin dissociation curve shifts right with decreased PO2 and increased CO2, lactic acid, 2,3-BPG, H+, and temperature, and shifts left in the lungs.
Respiratory Control and Regulation [1:07:55]
The pre-Bötzinger complex in the medulla is the pacemaker for respiration, with the DRG causing normal respiration and the BRG causing forceful respiration. The apneustic center increases the depth of respiration, while the pneumotaxic center inhibits the apneustic center and increases the rate of respiration. Regulation of respiration is done by chemoreceptors that sense blood changes like hypoxia and hypercapnia. Central chemoreceptors in the ventral medulla are stimulated by H+, while peripheral chemoreceptors in the carotid and aortic arteries sense hypoxia, hypercapnia, and acidosis.
Cardiovascular System: Cardiac Cycle and Regulation [1:12:19]
The SA node is the heart's pacemaker due to its high impulse frequency, driven by pacemaker potential. Cardiac action potentials involve phases 0 (depolarization), 1 (early repolarization), 2 (plateau), 3 (repolarization), and 4 (resting membrane potential). The conducting pathway is SA node → AV node → bundle of His → bundle branches → Purkinje fibers. The AV node is the slowest, causing AV delay. The cardiac cycle includes systole (S1-S2) and diastole (S2-S1). S1 involves MV/TV closure (isovolumetric contraction), while S2 involves AV/PV closure (isovolumetric relaxation). Filling occurs in diastole, with S3 (early filling) and S4 (late filling) sounds. A2P2 split occurs due to higher aortic pressure. JVP waves include A (atrial contraction), C (ventricular contraction), X (atrial relaxation), and V (vena cava filling).
Cardiovascular System: Volumes, Pressures, and Regulation [1:20:47]
End-diastolic volume (EDV) is the volume of blood in the ventricle at the end of diastole, representing preload. Stroke volume (SV) is the amount of blood ejected, while end-systolic volume (ESV) is the remaining blood. Ejection fraction (EF) is the percentage of blood pumped. Cardiac output (CO) is SV x heart rate, normally 5 L/min. Cardiac reserve is the increase in CO during exercise. The Frank-Starling law states that increased EDV causes increased SV. Positive inotropic agents (epinephrine, dobutamine, digoxin) increase contractility and SV. Beta-1 stimulation increases heart rate (chronotropic), stroke volume (inotropic), conduction (dromotropic), and excitability (bathmotropic). Blood vessels include elastic arteries (aorta), capacitance vessels (veins), exchange vessels (capillaries), and resistance vessels (arterioles).
Cardiovascular System: Blood Flow and Regulation [1:27:55]
Alpha-1 stimulation causes vasoconstriction, while beta-2 stimulation causes vasodilation. Arterioles regulate peripheral resistance, influencing afterload. Blood flow is regulated by F = P/R, with autoregulation allowing organs to control their blood flow. Resistance is determined by vessel radius (most important), viscosity, and length. Blood pressure (BP) is CO x peripheral resistance. Systolic BP is due to ventricular contraction, while diastolic BP is due to elastic recoil. Pulse pressure is the difference between systolic and diastolic BP. Regulatory centers in the medulla include the vasomotor center (VMC) and cardioinhibitory center (CIC). Baroreceptors sense BP changes, activating negative feedback to maintain BP. Bainbridge reflex increases heart rate with fluid infusion, while Bezold-Jarisch reflex causes bradycardia, apnea, and hypotension. Cushing's reflex increases BP but decreases heart rate.
Renal Physiology: Filtration, Reabsorption, and Secretion [1:35:22]
The kidney contains afferent arterioles leading to glomerular capillaries (GC) and efferent arterioles leading to peritubular capillaries (PC). Glomerular capillaries filter plasma based on size, with good substances like glucose reabsorbed and waste products secreted. Filtration occurs through endothelial fenestrations, a negatively charged basement membrane, and podocytes. Substances less than 4 nm are freely filtered, while larger substances are not. The PCT reabsorbs most substances, except magnesium, and secretes drugs, toxins, and acid. The loop of Henle concentrates urine via countercurrent mechanisms, with the descending limb permeable to water and the ascending limb permeable to solutes.
Renal Physiology: Tubular Function and Regulation [1:43:38]
The DCT contains NCC, absorbing sodium and chloride, blocked by thiazides. The collecting duct (CD) contains I cells (acid secretion) and P cells (sodium reabsorption, potassium secretion). P cells are stimulated by aldosterone, leading to metabolic alkalosis. ADH increases water and urea reabsorption in the CD via aquaporins. The kidney receives 1200 ml of blood, with a GFR of 125 ml/min. Clearance is UV/P, with inulin clearance equaling GFR. Creatinine clearance is slightly higher than GFR due to secretion. Glucose is 100% reabsorbed up to a transport maximum (Tm) of 375 mg/min. The JG apparatus contains lacis cells, JG cells (releasing renin), and macula densa cells (sensing GFR). Renin converts angiotensinogen to angiotensin I, then to angiotensin II, increasing BP.
Central Nervous System: Cells and Neurophysiology [1:53:28]
The CNS contains neurons and glial cells. Glial cells include microglia (macrophages), oligodendrocytes (myelin in CNS), Schwann cells (myelin in PNS), ependymal cells (ventricle lining), and astrocytes (support, BBB). CSF is produced by choroid plexuses and absorbed by arachnoid villi. Neurons consist of dendrites, a cell body, an axon hillock, and nerve endings. Nissl granules (ribosomes) perform protein synthesis. The resting membrane potential (RMP) is maintained by potassium and chloride permeability. Depolarization is EPSP, while hyperpolarization is IPSP. Glutamate is the main excitatory neurotransmitter, while GABA and glycine are inhibitory.
Central Nervous System: Action Potentials and Sensory Systems [2:02:15]
Action potentials occur when a threshold stimulus is reached, following the all-or-none law. Depolarization is caused by sodium influx, while repolarization is caused by potassium efflux. Memory is stored in synapses of the neocortex through synaptic plasticity. Sensory systems consist of receptors, pathways, and cortical centers. Receptors have a threshold, specificity, and undergo adaptation. Weber-Fechner Law relates stimulus intensity to sensation. Vision involves rods and cones, with color vision in area V4. Audition involves hair cells in the cochlea, with the auditory pathway involving the cochlear nerve, nucleus, olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body, and auditory cortex. Taste involves papillae, with different receptors for salty, sour, sweet, bitter, and umami. Somatosensory receptors include Merkel discs, Meissner corpuscles, Pacinian corpuscles, and Ruffini endings.