Cardiovascular and Respiratory Physiology
Cardiovascular and respiratory physiology work as one integrated oxygen-delivery system. Master the relationships between pressure, flow, resistance, and gas exchange. You will then reason about nearly every cardiopulmonary clinical scenario.
Core Cardiac Concepts
Cardiac output equals heart rate times stroke volume. A normal adult at rest has roughly 5 L/min, which can increase fivefold during exercise. Stroke volume (blood ejected per beat) depends on preload, afterload, and contractility, with a normal value around 70 mL.
Ejection fraction divides stroke volume by end-diastolic volume. Normal values range from 55-70%. A reading below 40% defines systolic heart failure. The Frank-Starling law explains how stretching myocardial fibers increases contraction force, up to a physiologic limit.
Pressure and Regulation
Mean arterial pressure (MAP) is the average pressure during a cardiac cycle (DBP + 1/3 × (SBP − DBP)), normally around 93 mmHg. Baroreceptors in the carotid sinus and aortic arch sense blood pressure and signal through cranial nerves IX and X to adjust heart rate and vascular tone.
Preload is ventricular end-diastolic volume or pressure. Volume overload increases it; hemorrhage or venodilators decrease it. Afterload is the resistance the ventricle overcomes to eject blood, approximated by systolic blood pressure and aortic impedance.
Respiratory Mechanics and Gas Exchange
Pulmonary ventilation is the mechanical process of breathing. The diaphragm and intercostal muscles create pressure gradients for inhalation and exhalation. Tidal volume (air moved per breath at rest) is normally around 500 mL.
The FEV1/FVC ratio measures what percentage of vital capacity exhales in the first second. A ratio below 70% indicates obstructive lung disease. The V/Q ratio compares ventilation to perfusion. A normal average is 0.8. High V/Q indicates dead space; low V/Q indicates a shunt.
The oxygen-hemoglobin dissociation curve plots oxygen saturation against arterial oxygen pressure in a sigmoidal shape. A right shift (the Bohr effect) occurs with increased CO2, H+, temperature, or 2,3-BPG, which releases oxygen to tissues.
Pulmonary surfactant, a phospholipid mixture from type II pneumocytes, reduces alveolar surface tension and prevents collapse. Dead space includes ventilated but unperfused lung regions, both anatomic (airways) and alveolar.
Chemoreceptor control relies on two sensor types. Central chemoreceptors in the medulla respond to CSF pH. Peripheral chemoreceptors in the carotid and aortic bodies respond to low arterial oxygen pressure.
| Term | Meaning |
|---|---|
| Cardiac output | Heart rate × stroke volume. Normal adult resting value is approximately 5 L/min. Increases with exercise up to 5-fold. |
| Stroke volume | Blood ejected per beat. Determined by preload, afterload, and contractility. Normal: ~70 mL. |
| Ejection fraction | Stroke volume divided by end-diastolic volume. Normal: 55-70%. <40% defines systolic heart failure. |
| Frank-Starling law | Greater end-diastolic volume stretches myocardial fibers and increases force of contraction up to a physiologic limit. |
| Mean arterial pressure (MAP) | Average arterial pressure during a cardiac cycle. MAP = DBP + 1/3 × (SBP − DBP). Normal ~93 mmHg. |
| Baroreceptors | Stretch receptors in the carotid sinus and aortic arch that sense BP and signal through CN IX and X to regulate heart rate and vascular tone. |
| Preload | Ventricular end-diastolic volume/pressure. Increased by volume overload, decreased by hemorrhage or venodilators. |
| Afterload | Resistance the ventricle must overcome to eject blood. Approximated by systolic BP and aortic impedance. |
| Pulmonary ventilation | Mechanical process of inhalation and exhalation driven by diaphragm and intercostal muscles creating pressure gradients. |
| Tidal volume | Volume of air moved per breath during quiet respiration. Normal: ~500 mL. |
| FEV1/FVC ratio | Percentage of vital capacity exhaled in the first second. <70% indicates obstructive lung disease. |
| V/Q ratio | Ratio of ventilation to perfusion. Normal average is 0.8. High V/Q = dead space; low V/Q = shunt. |
| Oxygen-hemoglobin dissociation curve | Sigmoidal curve of SaO2 vs PaO2. Right shift (Bohr effect): increased CO2, H+, temperature, 2,3-BPG releases O2 to tissues. |
| Pulmonary surfactant | Phospholipid mixture produced by type II pneumocytes that reduces alveolar surface tension and prevents collapse. |
| Dead space | Ventilated but not perfused lung regions. Includes anatomic (conducting airways) and alveolar dead space. |
| Chemoreceptor control | Central chemoreceptors in the medulla respond to CSF pH; peripheral (carotid/aortic bodies) respond to low PaO2. |
Renal and Gastrointestinal Physiology
The kidney and gut are the body's interface with the outside world. They regulate water, electrolytes, acid-base balance, and nutrient absorption. Memorize nephron function by segment and you master more than half of renal physiology in one study session.
Kidney Function by Nephron Segment
Glomerular filtration rate (GFR) is the volume filtered from glomerular capillaries into Bowman's capsule per minute. Normal GFR is about 125 mL/min and is estimated from creatinine levels.
The proximal convoluted tubule reabsorbs approximately 65% of filtered sodium, water, glucose, amino acids, and bicarbonate. Loop of Henle has two key functions. The thick ascending limb reabsorbs sodium, potassium, and chloride via the NKCC2 transporter (target of loop diuretics) and generates the medullary concentration gradient.
The distal convoluted tubule reabsorbs sodium and chloride via NCC (the thiazide target) and performs PTH-sensitive calcium reabsorption. The collecting duct is where ADH regulates water reabsorption through aquaporin-2 channels and aldosterone regulates sodium and potassium exchange.
Hormonal Regulation
Renin is released by juxtaglomerular cells when blood pressure drops, sodium drops, or sympathetic stimulation occurs. It initiates the renin-angiotensin-aldosterone system (RAAS). Aldosterone, an adrenal cortex hormone, increases sodium reabsorption and potassium and hydrogen ion secretion in the collecting duct.
Antidiuretic hormone (ADH) is released from the posterior pituitary when plasma osmolarity increases. It inserts aquaporin-2 channels into collecting duct cells. Acid-base regulation occurs when the kidneys excrete hydrogen ions and reabsorb or regenerate bicarbonate to compensate for respiratory and metabolic disturbances.
Gastrointestinal Secretion and Hormones
Gastric acid secretion comes from parietal cells that use the H+/K+ ATPase pump. Stimulation occurs via gastrin, acetylcholine (M3 receptors), and histamine (H2 receptors).
Cholecystokinin (CCK), released from duodenal I cells when fats and proteins arrive, triggers gallbladder contraction and pancreatic enzyme release. Secretin, released from duodenal S cells when pH drops, stimulates bicarbonate-rich pancreatic secretion. Gastrin, from G cells in the antrum, stimulates gastric acid secretion and mucosal growth (elevated in Zollinger-Ellison syndrome).
Digestion and Absorption
Pancreatic enzymes include amylase (starch), lipase (triglycerides), and trypsin/chymotrypsin (proteins). They activate in the duodenum. Bile composition contains bile acids, cholesterol, phospholipids, and bilirubin. Bile emulsifies dietary fats for absorption.
Enterohepatic circulation is the recycling pathway. Bile acids are secreted into the duodenum, reabsorbed in the terminal ileum, and returned to the liver for reuse.
| Term | Meaning |
|---|---|
| Glomerular filtration rate (GFR) | Volume of fluid filtered from glomerular capillaries into Bowman's capsule per minute. Normal: ~125 mL/min. Estimated from creatinine. |
| Proximal convoluted tubule | Reabsorbs ~65% of filtered Na+, water, glucose, amino acids, and bicarbonate. Site of action of acetazolamide. |
| Loop of Henle | Thick ascending limb reabsorbs Na+, K+, 2Cl- via NKCC2. Target of loop diuretics. Generates medullary concentration gradient. |
| Distal convoluted tubule | Reabsorbs Na+ and Cl- via NCC (thiazide target). PTH-sensitive calcium reabsorption. |
| Collecting duct | Site of ADH-regulated water reabsorption (aquaporin-2) and aldosterone-regulated Na+/K+ exchange. |
| Renin | Released by juxtaglomerular cells in response to low BP, low Na+, or sympathetic stimulation. Initiates RAAS. |
| Aldosterone | Adrenal cortex hormone increasing Na+ reabsorption and K+/H+ secretion in the collecting duct. |
| Antidiuretic hormone (ADH) | Released from posterior pituitary in response to increased plasma osmolarity. Inserts aquaporin-2 in collecting ducts. |
| Acid-base regulation | Kidneys excrete H+ and reabsorb/regenerate bicarbonate to compensate for respiratory and metabolic disturbances. |
| Gastric acid secretion | Parietal cells secrete HCl via H+/K+ ATPase. Stimulated by gastrin, ACh (M3), and histamine (H2). |
| Cholecystokinin (CCK) | Released from duodenal I cells in response to fats and proteins. Triggers gallbladder contraction and pancreatic enzyme release. |
| Secretin | Released from duodenal S cells in response to low pH. Stimulates bicarbonate-rich pancreatic secretion. |
| Gastrin | Released from G cells in the antrum. Stimulates gastric acid secretion and mucosal growth. Elevated in Zollinger-Ellison. |
| Pancreatic enzymes | Amylase (starch), lipase (triglycerides), trypsin/chymotrypsin (proteins). Activated in the duodenum. |
| Bile composition | Bile acids, cholesterol, phospholipids, bilirubin. Emulsifies dietary fats for absorption. |
| Enterohepatic circulation | Bile acids secreted into duodenum are reabsorbed in the terminal ileum and returned to the liver for reuse. |
Endocrine and Neurophysiology
Endocrine and neurophysiology round out the classic physiology curriculum. These domains emphasize feedback loops, receptor signaling, and electrochemical gradients. These concepts reappear throughout medicine and pharmacology.
Endocrine Regulation and Hormones
The hypothalamic-pituitary axis is the master control center. Hypothalamic releasing hormones regulate the anterior pituitary, which in turn regulates peripheral endocrine glands via negative feedback.
Insulin, released by beta cells, promotes glucose uptake, glycogen storage, lipogenesis, and protein synthesis in response to elevated glucose. Glucagon, from alpha cells, counteracts hypoglycemia by promoting glycogenolysis, gluconeogenesis, and lipolysis.
Thyroid hormone (T3 and T4) increases basal metabolic rate, cardiac output, and sympathetic activity. T3 is the more metabolically active form. Cortisol, an adrenal glucocorticoid, increases gluconeogenesis, suppresses inflammation and immunity, and follows a diurnal rhythm.
Parathyroid hormone (PTH) increases serum calcium through bone resorption, renal calcium reabsorption, and activation of vitamin D. Calcitonin, from thyroid C cells, lowers serum calcium by inhibiting osteoclasts but plays a minor role in humans.
Growth hormone, from the anterior pituitary, increases IGF-1 secretion from the liver. It promotes linear growth, lipolysis, and causes insulin resistance.
Neurophysiology and Synaptic Function
An action potential is a rapid depolarization and repolarization sequence in excitable cells. It propagates along axons via voltage-gated sodium and potassium channels. The resting membrane potential is typically around negative 70 millivolts in neurons, maintained by the Na+/K+ ATPase and high potassium permeability.
Synaptic transmission begins when an action potential triggers calcium influx at the presynaptic terminal. This releases neurotransmitter, which binds postsynaptic receptors. At the neuromuscular junction, acetylcholine released from motor neurons binds nicotinic receptors on muscle, triggering depolarization and contraction.
Autonomic Nervous System
The sympathetic nervous system produces fight-or-flight responses. It increases heart rate, blood pressure, pupil dilation, and bronchodilation by releasing norepinephrine at most targets. The parasympathetic nervous system produces rest-and-digest responses. It slows heart rate, constricts pupils, and increases GI motility using acetylcholine at muscarinic receptors.
Central Nervous System
Cerebrospinal fluid (CSF) is produced by the choroid plexus. It circulates through ventricles and the subarachnoid space, then gets reabsorbed at arachnoid granulations. The blood-brain barrier consists of tight junctions between endothelial cells that restrict molecular entry into the central nervous system. Lipophilic molecules cross freely.
| Term | Meaning |
|---|---|
| Hypothalamic-pituitary axis | Hypothalamic releasing hormones regulate anterior pituitary, which in turn regulates peripheral endocrine glands via negative feedback. |
| Insulin | Beta cell hormone promoting glucose uptake, glycogen storage, lipogenesis, protein synthesis. Secreted in response to glucose. |
| Glucagon | Alpha cell hormone counteracting hypoglycemia by promoting glycogenolysis, gluconeogenesis, and lipolysis. |
| Thyroid hormone (T3/T4) | Increases basal metabolic rate, cardiac output, and sympathetic activity. T3 is the more active form. |
| Cortisol | Adrenal cortex glucocorticoid. Increases gluconeogenesis and suppresses inflammation and immunity. Follows diurnal rhythm. |
| Parathyroid hormone (PTH) | Increases serum calcium via bone resorption, renal reabsorption, and activation of vitamin D. |
| Calcitonin | Thyroid C-cell hormone that lowers serum calcium by inhibiting osteoclasts. Minor role in humans. |
| Growth hormone | Anterior pituitary hormone increasing IGF-1 secretion from liver. Promotes linear growth, lipolysis, and insulin resistance. |
| Action potential | Rapid depolarization/repolarization sequence in excitable cells. Propagates along axons via voltage-gated sodium and potassium channels. |
| Resting membrane potential | Typically −70 mV in neurons. Maintained by Na+/K+ ATPase and K+ permeability. |
| Synaptic transmission | Action potential triggers calcium influx at presynaptic terminal, releasing neurotransmitter that binds postsynaptic receptors. |
| Neuromuscular junction | Acetylcholine released from motor neurons binds nicotinic receptors on muscle, triggering depolarization and contraction. |
| Sympathetic nervous system | Fight or flight: increases HR, BP, pupil dilation, bronchodilation. Releases norepinephrine at most targets. |
| Parasympathetic nervous system | Rest and digest: slows HR, constricts pupils, increases GI motility. Uses acetylcholine at muscarinic receptors. |
| Cerebrospinal fluid (CSF) | Produced by choroid plexus, circulates through ventricles and subarachnoid space, reabsorbed at arachnoid granulations. |
| Blood-brain barrier | Tight junctions between endothelial cells restrict molecular entry into the CNS. Lipophilic molecules cross freely. |
How to Study physiology Effectively
Mastering physiology requires the right study approach, not just more hours. Cognitive science research consistently shows three techniques produce the best learning outcomes.
Three Evidence-Based Study Techniques
The first is active recall: testing yourself rather than re-reading. The second is spaced repetition: reviewing at scientifically-optimized intervals. The third is interleaving: mixing related topics rather than studying one in isolation. FluentFlash is built around all three.
When you study with our FSRS algorithm, every term is scheduled for review at the exact moment you're about to forget it. This maximizes retention while minimizing study time.
Why Passive Review Fails
The most common mistake students make is relying on passive review methods. Re-reading notes, highlighting textbook passages, or watching lectures feels productive but delivers poor results. Studies show these methods produce only 10-20% of the retention that active recall achieves.
Flashcards force your brain to retrieve information, which strengthens memory pathways far more than recognition alone. Pair this with spaced repetition scheduling and you learn in 20 minutes what takes hours of passive review.
Practical Study Plan
Start by creating 15-25 flashcards covering the highest-priority concepts. Review them daily for the first week using our FSRS scheduling. As cards become easier, intervals automatically expand from minutes to days to weeks.
You're always working on material at the edge of your knowledge. After 2-3 weeks of consistent practice, physiology concepts become automatic rather than effortful to recall.
Daily Study Steps
- Generate flashcards using FluentFlash AI or create them manually from your notes
- Study 15-20 new cards per day, plus scheduled reviews
- Use multiple study modes (flip, multiple choice, written) to strengthen recall
- Track your progress and identify weak topics for focused review
- Review consistently, daily practice beats marathon sessions
- 1
Generate flashcards using FluentFlash AI or create them manually from your notes
- 2
Study 15-20 new cards per day, plus scheduled reviews
- 3
Use multiple study modes (flip, multiple choice, written) to strengthen recall
- 4
Track your progress and identify weak topics for focused review
- 5
Review consistently, daily practice beats marathon sessions
Why Flashcards Work Better Than Other Study Methods for physiology
Flashcards are one of the most research-backed study tools for any subject, including physiology. The reason comes down to how memory works.
Memory and the Testing Effect
When you read a textbook passage, your brain stores that information in short-term memory. Without retrieval practice, it fades within hours. Flashcards force retrieval, which is the mechanism that transfers information from short-term to long-term memory.
The testing effect, documented in hundreds of peer-reviewed studies, shows that students using flashcards consistently outperform those who re-read by 30-60% on delayed tests. This isn't because flashcards contain more information. It's because retrieval strengthens neural pathways in a way that passive exposure cannot.
Every time you successfully recall a physiology concept from a flashcard, you make that concept easier to recall next time. Your brain is rewiring the memory pathway.
FSRS Spaced Repetition
FluentFlash amplifies the testing effect with the FSRS algorithm, a modern spaced repetition system. It schedules reviews at mathematically-optimal intervals based on your actual performance.
Cards you find easy get pushed further into the future. Cards you struggle with come back sooner. Over time, this builds remarkable retention with minimal time investment. Students using FSRS-based systems typically retain 85-95% of material after 30 days, compared to roughly 20% retention from passive review alone.