Heart Anatomy, Cardiac Cycle, Blood Pressure & Circulation
A complete academic guide to the circulatory system — from the four-chambered heart and its valves through the cardiac cycle, coronary arteries, electrical conduction, blood pressure regulation, systemic and pulmonary circulation, cardiac output, and clinical cardiovascular disorders. Written for students at every level, from introductory biology through clinical medicine.
Every second you are alive, your heart contracts. Not once or twice — about 70 times per minute, 100,000 times per day, 2.5 billion times across an average lifetime, without pause, without rest, without conscious effort. Through those contractions, roughly five litres of blood complete an entire circuit of the body every minute — reaching the fingertips and returning to the heart in under sixty seconds. The cardiovascular system — the heart, blood vessels, and blood it propels — is not simply a pump and a network of pipes. It is the logistical infrastructure of human life: the delivery system for oxygen, nutrients, hormones, and immune cells; the waste disposal network removing carbon dioxide and metabolic byproducts; the temperature regulation highway and the pressure-sensing feedback loop that keeps every organ from the brain to the kidney functioning within its narrow physiological window.
If you are studying human biology, physiology, nursing, medicine, or any allied health science, cardiovascular anatomy and physiology is foundational to virtually everything else you will learn. Cardiac failure, hypertension, myocardial infarction, arrhythmias — these are not abstract conditions. They are what happens when the precisely engineered machinery described in this guide breaks down. Understanding how the healthy cardiovascular system works, step by step and mechanism by mechanism, is what makes pathophysiology intelligible and clinical reasoning possible.
The Cardiovascular System — Scope, Components, and Core Functions
The cardiovascular system — also called the circulatory system — consists of three integrated components: the heart, a muscular pump that generates the pressure gradient driving blood flow; the blood vessels, a closed network of tubes (arteries, arterioles, capillaries, venules, and veins) that carry blood to and from every tissue; and blood itself, the fluid medium transporting gases, nutrients, waste products, hormones, and immune cells. Together, these components perform functions that no other organ system can replicate.
Gas Transport
Haemoglobin in red blood cells binds oxygen in the pulmonary capillaries and releases it at tissues with low oxygen tension. Simultaneously, carbon dioxide produced by cellular respiration dissolves in plasma (as bicarbonate) or binds haemoglobin, travelling back to the lungs for exhalation. Without continuous circulation, the brain sustains irreversible damage within four to six minutes of oxygen deprivation.
Nutrient and Waste Delivery
Glucose, fatty acids, amino acids, and vitamins absorbed by the gastrointestinal tract enter the portal and systemic circulation for distribution to cells throughout the body. Metabolic waste — urea, creatinine, excess ions — is transported to the kidneys for filtration. Liver-bound circulation processes ingested substances and breaks down old blood cells and hormones.
Homeostatic Regulation
The cardiovascular system regulates body temperature by redistributing blood between core and peripheral tissues — vasodilation in the skin dissipates heat; vasoconstriction conserves it. It distributes hormones from endocrine glands to target organs, participates in immune surveillance by transporting white blood cells, and maintains the fluid balance between vascular and interstitial compartments.
Heart Anatomy — Location, Gross Structure, and Myocardial Layers
The heart is a hollow, muscular organ situated in the mediastinum — the central compartment of the thoracic cavity — between the two lungs, behind the sternum, and above the diaphragm. It is roughly the size of a closed fist, weighing between 250 and 350 grams in adults, and is oriented so that its apex (tip) points inferiorly and to the left, resting on the diaphragm at approximately the fifth intercostal space in the midclavicular line. The base of the heart — its posterior, superior surface — faces the vertebral column and receives the great vessels (aorta, pulmonary trunk, superior and inferior venae cavae, and pulmonary veins).
The Three Layers of the Heart Wall
The heart wall consists of three concentric layers, each with a distinct structure and function. From outermost to innermost:
Epicardium (visceral pericardium): the outermost layer, a smooth, serous membrane composed of mesothelial cells and a thin layer of connective tissue. It is the visceral layer of the pericardium — the double-walled fibroserous sac enclosing the heart. Between the epicardium and the parietal pericardium (the outer layer of the pericardial sac) lies the pericardial cavity, containing a thin film of serous fluid (approximately 15–50 mL) that reduces friction during cardiac contractions. Pericarditis (inflammation of the pericardium) and pericardial effusion (abnormal fluid accumulation) are clinically important conditions involving this structure. Cardiac tamponade — life-threatening compression of the heart by accumulated pericardial fluid — is a medical emergency.
Myocardium: the middle and thickest layer — the cardiac muscle itself. It is composed of branching, striated cardiac muscle cells (cardiomyocytes) connected by intercalated discs that contain gap junctions (allowing rapid electrical impulse propagation between cells, enabling the myocardium to function as an electrical syncytium) and desmosomes (providing mechanical cohesion between cells during contraction). The myocardium of the left ventricle is approximately 10–15 mm thick — about three times the right ventricular myocardium — reflecting the greater pressure work required of the left side. Cardiomyocytes are rich in mitochondria (comprising approximately 30% of cell volume) reflecting the heart’s almost exclusively aerobic metabolism and its dependence on a continuous oxygen supply.
Endocardium: the innermost layer — a smooth, glistening endothelial membrane lining the inner surfaces of all four chambers, the valves, and the chordae tendineae. It is continuous with the endothelium lining all blood vessels and provides a smooth, non-thrombogenic surface that prevents intracardiac clot formation under normal flow conditions. Infective endocarditis — bacterial infection of the endocardium, particularly affecting the valves — is a serious condition that can cause valve destruction and septic emboli.
The Four Chambers — Right Heart and Left Heart
The heart is divided into four chambers — two atria (singular: atrium) and two ventricles — separated by muscular walls called septa. The interatrial septum separates the right and left atria; the interventricular septum separates the two ventricles. A key anatomical feature of the interatrial septum is the fossa ovalis — the remnant of the foramen ovale, an opening present in fetal circulation that normally closes shortly after birth. Failure to close produces a patent foramen ovale (PFO), present in approximately 25–30% of the adult population and a risk factor for paradoxical embolism and cryptogenic stroke. The right-sided chambers (right atrium and right ventricle) are collectively responsible for pulmonary circulation; the left-sided chambers (left atrium and left ventricle) drive systemic circulation. The American Heart Association explains that the healthy heart is a strong, hard-working pump made of muscle tissue whose two-sided design efficiently separates oxygenated from deoxygenated blood.
Receiving Chamber for Deoxygenated Blood
The right atrium (RA) receives systemic venous return — deoxygenated blood — from three sources: the superior vena cava (SVC), draining the head, neck, arms, and upper thorax; the inferior vena cava (IVC), draining the abdomen, pelvis, and lower limbs; and the coronary sinus, returning blood from the coronary veins that drain the myocardium itself. The RA wall is thin (approximately 2 mm) reflecting its low-pressure role as a receiving reservoir. Its inner surface shows the crista terminalis, a muscular ridge separating the smooth-walled sinus venarum from the trabeculated muscular part containing the pectinate muscles. The sinoatrial (SA) node — the heart’s primary pacemaker — lies at the superior end of the crista terminalis, adjacent to where the SVC joins the RA.
Pumping Station for Pulmonary Circulation
The right ventricle (RV) receives blood from the RA through the tricuspid valve and pumps it through the pulmonary valve into the pulmonary trunk, which divides into left and right pulmonary arteries carrying deoxygenated blood to the respective lungs. The RV has a crescent-shaped cross-section and a wall approximately 3–5 mm thick — sufficient to generate the relatively low pressures needed to perfuse the pulmonary circuit (normal pulmonary artery systolic pressure: ~25 mmHg), but not the high pressures of the systemic circulation. The inner surface of the RV is extensively trabeculated (muscular ridges called trabeculae carneae) and contains the papillary muscles that anchor the tricuspid valve leaflets via chordae tendineae. Pulmonary hypertension — chronically elevated pulmonary arterial pressure — causes progressive RV hypertrophy and ultimately right heart failure.
Receiving Chamber for Oxygenated Blood
The left atrium (LA) receives oxygenated blood from the lungs via four pulmonary veins — typically two from the right lung and two from the left lung — and passes it through the mitral valve into the left ventricle. The LA wall is slightly thicker than the RA (approximately 3 mm) reflecting somewhat higher operating pressures on the left side. The LA is the most posterior cardiac chamber and is directly anterior to the oesophagus — a clinically important relationship because LA enlargement (as occurs in mitral stenosis or atrial fibrillation) can compress the oesophagus causing dysphagia, and the proximity allows transoesophageal echocardiography (TOE/TEE) to provide excellent LA imaging. Atrial fibrillation — the most common sustained cardiac arrhythmia — primarily involves the LA and frequently generates clots within the left atrial appendage, a muscular pouch that is now the target of surgical or catheter-based occlusion devices to reduce stroke risk.
The Systemic Circulation Power Pump
The left ventricle (LV) is the primary working chamber of the heart — the most muscular of the four chambers, with a wall 10–15 mm thick — responsible for generating the high-pressure output required to perfuse the systemic circulation. It receives blood from the LA through the mitral valve and ejects it through the aortic valve into the aorta at a peak systolic pressure of approximately 120 mmHg. In cross-section, the LV is circular or elliptical, in contrast to the crescent-shaped RV. The LV contains two papillary muscles (anterolateral and posteromedial) anchoring the mitral valve leaflets. Normal LV ejection fraction (EF) — the proportion of end-diastolic volume ejected with each beat — is 55–70%. An EF below 40% defines heart failure with reduced ejection fraction (HFrEF), the most common form of systolic heart failure. Cardiac MRI and echocardiography are the primary imaging modalities for assessing LV structure, function, and ejection fraction.
The Four Cardiac Valves — Structure, Function, and Heart Sounds
The four cardiac valves are the one-way gates that enforce unidirectional blood flow through the heart. They open and close passively in response to pressure gradients — opening when pressure is higher on the upstream side and closing when pressure is higher on the downstream side. No muscular or neural effort is required to operate the valves; the pressure changes generated by myocardial contraction and relaxation drive all valve motion automatically. The closing of these valves generates the characteristic heart sounds heard through a stethoscope — sounds that remain the first line of cardiovascular assessment in clinical medicine.
The Cardiac Cycle — Systole, Diastole, and Pressure-Volume Relationships
The cardiac cycle is the complete sequence of mechanical events constituting a single heartbeat — from the beginning of one ventricular contraction to the beginning of the next. It is the fundamental unit of cardiac function, and every parameter of cardiac performance — heart rate, stroke volume, cardiac output, ejection fraction — is a property of this cycle. At a resting heart rate of 70 beats per minute, the entire cycle lasts approximately 0.857 seconds. According to StatPearls’ detailed account of the cardiac cycle, the cycle originates as electrochemical changes within the myocardium that result in concentric muscle contraction, with valves directing blood movement in an organised pattern through each chamber.
Isovolumic Relaxation (Early Diastole)
Diastole begins at the moment the aortic valve closes (S2). All four valves are closed. Ventricular pressure falls rapidly as the myocardium relaxes — an active, energy-requiring process that depends on calcium reuptake by the sarcoplasmic reticulum (SERCA pump). Ventricular volume remains constant (isovolumic) because no blood is entering or leaving. This phase ends when ventricular pressure falls below atrial pressure, triggering the AV valves to open. Impaired relaxation — diastolic dysfunction — is the mechanism of heart failure with preserved ejection fraction (HFpEF), the increasingly prevalent form of heart failure in hypertensive and elderly patients.
Rapid Ventricular Filling (Early-Mid Diastole)
The mitral and tricuspid valves open when atrial pressure exceeds ventricular pressure. Blood flows rapidly from the atria into the ventricles driven by the atrioventricular pressure gradient. Approximately 70–80% of ventricular filling occurs passively in this phase — blood flows in because the pressure gradient favours it, not because the atria contract. At the beginning of this phase, the ventricular myocardium is still actively relaxing, creating a suction effect (ventricular untwisting) that accelerates early filling. Slowed early filling velocity (E wave) on Doppler echocardiography is a sensitive marker of diastolic dysfunction.
Reduced Filling / Diastasis (Mid-Late Diastole)
As the pressure gradient between atria and ventricles equalises, blood flow into the ventricles slows substantially — this slow phase is called diastasis. It is shorter or absent at higher heart rates (when the cardiac cycle is compressed). End-diastolic volume (EDV) — the volume of blood in each ventricle at the end of diastole — is approximately 120–130 mL at rest. EDV is the physiological definition of preload, and it is the starting point for applying the Frank-Starling mechanism: greater EDV stretches myocardial fibres, producing stronger contraction and therefore a larger stroke volume.
Atrial Systole (Late Diastole)
The SA node fires an action potential that spreads across both atria, causing atrial contraction. This active atrial kick contributes the final 15–25% of ventricular filling — less critical at rest but increasingly important during exercise (when diastolic filling time shortens) and in conditions of impaired relaxation (where the passive phase is reduced and atrial contribution compensates). The loss of atrial systole in atrial fibrillation reduces cardiac output by approximately 15–25% and can precipitate acute decompensation in patients with diastolic dysfunction or mitral stenosis. Atrial contraction produces the a-wave visible in jugular venous pressure (JVP) tracings.
Isovolumic Contraction (Early Systole)
Systole begins at the moment the mitral and tricuspid valves close (S1). All four valves are again closed as ventricular pressure rises rapidly with no change in volume — the myocardium is generating force without ejecting blood. This phase ends when ventricular pressure exceeds arterial pressure (approximately 80 mmHg in the aorta at diastole; approximately 10 mmHg in the pulmonary artery), forcing open the semilunar valves. Isovolumic contraction time (IVCT) — measurable on tissue Doppler echocardiography — reflects ventricular systolic function and is prolonged in heart failure.
Rapid and Reduced Ventricular Ejection (Systole)
The aortic and pulmonary valves open as ventricular pressure exceeds arterial pressure. Blood is ejected rapidly in the first third of systole (rapid ejection phase, when approximately two-thirds of the stroke volume is ejected), then more slowly as the ventricles approach the end of contraction (reduced ejection phase). Peak aortic pressure (~120 mmHg) is reached near the end of rapid ejection. At the end of systole, the end-systolic volume (ESV) remains in the ventricle — typically approximately 50–60 mL at rest. Stroke volume = EDV − ESV ≈ 70 mL. Ejection fraction (EF) = SV/EDV × 100% ≈ 55–65% normally. When ventricular pressure falls below arterial pressure, the semilunar valves close (S2), ending systole and beginning the next cycle.
Resting values (70 kg adult, HR = 70 bpm): Cycle duration = 0.857 s (60 s ÷ 70 bpm) Diastole = ~0.50 s (atria: systole during LV diastole) Systole = ~0.35 s Ventricular volumes: End-Diastolic Volume (EDV) = 120–130 mL ← preload / filling End-Systolic Volume (ESV) = 50–60 mL ← residual blood Stroke Volume (SV) = EDV − ESV ≈ 70 mL Ejection Fraction (EF) = SV/EDV × 100 ≈ 55–65% Cardiac Output and derivatives: CO = HR × SV = 70 × 70 = 4,900 mL/min ≈ 5 L/min Cardiac Index (CI) = CO ÷ BSA ≈ 2.5–4.0 L/min/m² Double Product = HR × SBP (index of myocardial O₂ demand) Pressures (mmHg): LV systolic peak ≈ 120 mmHg (= aortic systolic BP) LV end-diastolic ≈ 8–12 mmHg (= LVEDP, index of preload) Aortic diastolic ≈ 80 mmHg (maintained by aortic recoil + TPR) PA systolic ≈ 25 mmHg RA pressure ≈ 0–8 mmHg (= CVP, central venous pressure)
The Cardiac Conduction System — Electrical Physiology and the ECG
The cardiac conduction system is a network of specialised cells capable of spontaneously generating and rapidly transmitting electrical impulses — action potentials — that trigger the coordinated, sequential contraction of the myocardium. Unlike skeletal muscle, which requires continuous motor nerve input to contract, cardiac muscle is myogenic — it generates its own rhythmic electrical activity through spontaneous depolarisation of pacemaker cells. This intrinsic automaticity is what allows the heart to continue beating even when denervated (as in a transplanted heart), and is what makes ventricular fibrillation so dangerous — once the organised conduction system fails, uncoordinated pacemaker activity can sustain lethal arrhythmias without restoration.
Sinoatrial (SA) Node — the Primary Pacemaker
Located in the wall of the right atrium at the junction with the superior vena cava, the SA node is a cluster of approximately 10,000 specialised pacemaker cells with an inherent firing rate of 60–100 impulses per minute under normal conditions. Pacemaker cells have an unstable resting membrane potential — they spontaneously depolarise through slow inward sodium and calcium currents (the ‘funny’ current, If), generating action potentials rhythmically without neural input. The SA node is the dominant pacemaker because it depolarises fastest. It is innervated by both divisions of the autonomic nervous system: sympathetic stimulation (via noradrenaline acting on β₁ receptors) accelerates the rate of spontaneous depolarisation, increasing heart rate (positive chronotropy); parasympathetic stimulation (via acetylcholine acting on M₂ receptors) slows the rate, decreasing heart rate (negative chronotropy). SA node dysfunction — sick sinus syndrome — causes bradycardia, tachycardia-bradycardia alternation, and may require pacemaker implantation.
Atrioventricular (AV) Node — the Gatekeeper
Located at the junction of the atria and ventricles in the interatrial septum (Koch’s triangle, above the coronary sinus ostium), the AV node is the only normal electrical connection between atria and ventricles — the fibrous skeleton of the heart electrically insulates all other atrio-ventricular connections. The AV node deliberately slows conduction (by approximately 0.1 seconds — 100 milliseconds), creating the PR interval on the ECG. This delay allows atrial contraction to complete before ventricular systole begins — ensuring the atria deliver their stroke to the ventricles before the ventricles eject. The AV node has an inherent pacemaker rate of 40–60 bpm (its escape rhythm) and serves as a backup pacemaker if the SA node fails. AV block (first, second, or third degree) reflects impaired conduction through the AV node or His-Purkinje system and is readily identifiable from the surface ECG.
Bundle of His and Bundle Branches
From the AV node, the impulse enters the Bundle of His (atrioventricular bundle) — a compact bundle of specialised conductive cells that penetrates the fibrous skeleton to enter the interventricular septum. The Bundle of His divides at the crest of the septum into the right bundle branch (conducting to the right ventricle) and the left bundle branch (which further divides into the left anterior fascicle and the left posterior fascicle, supplying the left ventricle). Bundle branch blocks — caused by ischaemia, fibrosis, or other pathology — delay conduction to one ventricle, causing asynchronous ventricular activation and a wide QRS complex on the ECG. Left bundle branch block (LBBB) is particularly important clinically as it alters ECG morphology in ways that can mask or mimic myocardial infarction patterns, complicating diagnosis.
Purkinje Fibre Network — Rapid Ventricular Activation
The bundle branches divide into the subendocardial Purkinje fibre network — a dense meshwork of large-diameter, rapidly conducting specialised cells that distributes the electrical impulse simultaneously throughout the inner surface of both ventricles. Purkinje fibres have extremely rapid conduction velocity (approximately 1–4 m/s, compared to 0.01 m/s through the AV node), enabling nearly simultaneous activation of all ventricular myocardial cells. Depolarisation spreads from the subendocardium to the subepicardium and from the apex toward the base — producing the tight, narrow QRS complex (normally <120 ms) that represents synchronised ventricular activation on the ECG. Purkinje cells also have an intrinsic pacemaker rate of 20–40 bpm — the ventricular escape rhythm, which sustains cardiac output in third-degree heart block but at too slow a rate to maintain normal perfusion.
The electrocardiogram (ECG/EKG) records the electrical activity of the heart from the body surface. Each deflection on the ECG corresponds to a specific electrical event in the cardiac cycle: the P wave represents atrial depolarisation (SA node firing, impulse spreading across both atria) — immediately followed by atrial contraction. The PR interval (P wave onset to QRS onset, normally 120–200 ms) represents conduction through the AV node. The QRS complex (normally ≤120 ms) represents ventricular depolarisation — the rapid spread of the impulse through the Purkinje network and myocardium — immediately followed by ventricular contraction (systole). The ST segment represents ventricular repolarisation — the period of maintained depolarisation (plateau phase). ST elevation is the hallmark of acute myocardial infarction (STEMI), reflecting cellular injury to the myocardium. The T wave represents ventricular repolarisation (return to resting membrane potential), followed by ventricular relaxation (diastole begins).
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Cardiac Output, Stroke Volume, and the Frank-Starling Mechanism
Cardiac output (CO) is the single most important integrative measure of cardiac performance — it represents the volume of blood delivered by the heart to the circulation per unit time. CO equals heart rate (HR) multiplied by stroke volume (SV): CO = HR × SV. Normal resting CO of approximately 5 L/min must increase to 20–25 L/min during maximal exercise in trained athletes — a fourfold-to-fivefold increase achieved through simultaneous increases in both HR and SV. Understanding the determinants of each component is central to understanding how the heart adapts to physiological demands and how it fails in disease.
Heart Rate
Normal: 60–100 bpm. Primarily controlled by SA node automaticity modified by autonomic tone. Sympathetic ↑HR (positive chronotropy via β₁ receptors); Parasympathetic ↓HR (negative chronotropy via M₂ receptors). Also affected by temperature, thyroid hormone, and circulating catecholamines.
Stroke Volume
Normal: ~60–80 mL at rest. SV = EDV − ESV. Determined by three factors: preload, afterload, and contractility. Increases during exercise (sympathetic stimulation + Frank-Starling), decreases in heart failure, hypovolaemia, and conditions increasing afterload (e.g. hypertension, aortic stenosis).
Ejection Fraction
EF = SV÷EDV × 100%. Normal: 55–70%. A primary marker of systolic function. HFrEF: EF <40%. HFmrEF (mildly reduced): 40–49%. HFpEF (preserved): EF ≥50% but diastolic dysfunction present. Measured by echocardiography, cardiac MRI, or nuclear imaging.
Ventricular Filling
Preload = EDV = the volume of blood stretching the ventricle at the end of diastole. Increased by: increased venous return, fluid loading, regurgitant valve lesions, bradycardia (longer filling time). Decreased by: haemorrhage, dehydration, positive pressure ventilation, vasodilators reducing venous return.
Resistance to Ejection
Afterload = the pressure the ventricle must overcome to eject blood = primarily aortic diastolic pressure (= TPR × CO). Increased afterload (hypertension, aortic stenosis) reduces SV and increases wall stress. The LV compensates by hypertrophy — adaptive initially but pathological long-term, leading to fibrosis and diastolic dysfunction.
Length-Tension Relationship
Within physiological limits, ↑ EDV → ↑ stretch of myocardial fibres → ↑ calcium sensitivity of contractile proteins → ↑ force of contraction → ↑ SV. This intrinsic mechanism allows the heart to automatically match output to venous return — the Starling mechanism is why the two ventricles, in series, pump equal volumes over time despite having different pressures.
Heartbeats in a 70-year lifetime — and the Frank-Starling mechanism keeps them perfectly matched to demand for every one
The Frank-Starling law of the heart — formulated independently by Otto Frank (1895) and Ernest Starling (1918) — established that cardiac muscle, like skeletal muscle, contracts more forcefully when stretched within physiological limits. Unlike skeletal muscle (where the length-tension relationship reflects optimal myosin-actin overlap), the cardiac mechanism primarily operates through calcium sensitivity: greater sarcomere length increases the sensitivity of the contractile protein troponin C to calcium, amplifying the contractile response for the same intracellular calcium transient. This intrinsic mechanism balances right and left ventricular output automatically: if one side transiently pumps more, the other fills more, stretches more, and responds with increased force to match.
Blood Pressure — Measurement, Determinants, and Regulation
Blood pressure (BP) is the force exerted by circulating blood against the inner walls of the blood vessels, expressed as two values: systolic BP (the peak arterial pressure during ventricular ejection) over diastolic BP (the minimum arterial pressure during ventricular relaxation). Conventionally measured in the brachial artery using a sphygmomanometer and stethoscope (auscultatory method) or automated oscillometric devices, BP is expressed in millimetres of mercury (mmHg). According to StatPearls’ physiology review, mean arterial pressure (MAP) — the average pressure across the cardiac cycle — is calculated as MAP = diastolic BP + 1/3 × pulse pressure, and equals cardiac output × total peripheral resistance (MAP = CO × TPR), which are the two key determinants of blood pressure.
Blood Pressure Classification (AHA/ACC 2017 Guidelines)
Normal: systolic <120 mmHg and diastolic <80 mmHg. Elevated: systolic 120–129 mmHg and diastolic <80 mmHg. Stage 1 Hypertension: systolic 130–139 or diastolic 80–89 mmHg. Stage 2 Hypertension: systolic ≥140 or diastolic ≥90 mmHg. Hypertensive crisis: systolic >180 and/or diastolic >120 mmHg — requires immediate evaluation. Isolated systolic hypertension (ISH) — systolic ≥140 with diastolic <90 — is the most common pattern in patients over 65 years, reflecting loss of aortic compliance (arterial stiffening) with age rather than increased peripheral resistance. Approximately 1.28 billion adults worldwide have hypertension, making it the most prevalent modifiable cardiovascular risk factor globally.
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Systemic Circulation — Arteries, Arterioles, Capillaries, Venules, and Veins
Systemic circulation is the circuit that carries oxygenated blood from the left ventricle to every organ and tissue in the body (except the lungs) and returns deoxygenated blood to the right atrium. It operates at high pressure — the left ventricle generates systolic pressures of approximately 120 mmHg — and must distribute blood through a branching network of vessels whose total cross-sectional area increases enormously from the aorta to the capillaries, dramatically slowing blood velocity to allow adequate time for nutrient and gas exchange.
Blood pressure and velocity across the systemic vascular network
Conducting Arteries — the Elastic Pressure Buffer
The aorta and its primary branches (subclavian, common carotid, common iliac) are large-diameter elastic arteries with walls rich in elastin fibres. During ventricular systole, they distend to absorb the ejected stroke volume, storing pressure energy. During diastole, they recoil — the Windkessel effect — converting pulsatile systolic ejection into a more continuous forward flow. This elastic buffering maintains diastolic BP and smooth coronary filling during diastole (when coronary flow is greatest). Loss of aortic compliance with age (arteriosclerosis) is the primary cause of isolated systolic hypertension in the elderly, increasing pulse pressure and cardiac workload.
Arterioles — the Resistance Vessels
Arterioles are small muscular vessels (50–300 µm diameter) whose walls contain a thick layer of smooth muscle innervated by the sympathetic nervous system. They are the primary site of vascular resistance — their contraction or relaxation determines total peripheral resistance (TPR), which is the main determinant of diastolic BP and the principal mechanism for redistributing blood flow between organs. Local metabolic regulation (CO₂, H⁺, adenosine, hypoxia) causes arteriolar dilation in active tissues, overriding sympathetic vasoconstriction to increase blood flow to where it is most needed — the mechanism of exercise hyperaemia.
Capillaries — the Exchange Network
Capillaries are the functional units of the circulatory system — single-cell-thick tubes (endothelium only, ~8 µm diameter) where all exchange of gases, nutrients, metabolites, hormones, and fluid between blood and tissues occurs. Their enormous total number (~10 billion) and cross-sectional area dramatically slows blood velocity (to ~0.03 cm/s), maximising transit time for diffusion. Exchange mechanisms include simple diffusion (O₂, CO₂, lipid-soluble molecules), facilitated transport (glucose), vesicular transcytosis (large proteins), and bulk flow of fluid driven by Starling forces (hydrostatic and oncotic pressure differences across the capillary wall).
Venous Return — Getting Blood Back to the Heart
Veins are thin-walled, compliant vessels that contain approximately 65–70% of the total circulating blood volume at any time — functioning as a capacitance reservoir. Despite very low pressure in the venous system (~5–10 mmHg), venous return is maintained by the respiratory pump (inspiration reduces intrathoracic pressure, drawing blood into the great thoracic veins and right atrium); the skeletal muscle pump (contracting leg muscles compress deep veins, propelling blood toward the heart — one-way venous valves prevent backflow); and the cardiac suction effect (ventricular relaxation during diastole creates slight negative pressure in the atria that assists venous filling). Venous insufficiency (failure of valves allowing backflow) causes chronic venous hypertension, varicose veins, oedema, and venous ulcers.
Starling Forces — Fluid Exchange at the Capillary
The Starling forces governing fluid movement across the capillary wall are: capillary hydrostatic pressure (Pc — favours filtration out of capillary), interstitial hydrostatic pressure (Pi — usually negative or near zero, slightly favours filtration), plasma oncotic pressure (πp — due to plasma proteins, especially albumin, favours reabsorption), and interstitial oncotic pressure (πi — small, slightly favours filtration). Net filtration = Kf [(Pc − Pi) − σ(πp − πi)], where Kf is the filtration coefficient and σ is the reflection coefficient for proteins. At the arterial end of the capillary, net filtration predominates; at the venous end, net reabsorption predominates. Hypoalbuminaemia (low plasma oncotic pressure) causes oedema by reducing the reabsorptive force; elevated venous pressure (heart failure) causes oedema by increasing capillary hydrostatic pressure.
Pulmonary Circulation — the Gas Exchange Circuit
Pulmonary circulation is the low-pressure circuit carrying deoxygenated blood from the right ventricle to the lungs for gas exchange, then returning oxygenated blood to the left atrium. Despite receiving the same cardiac output as the systemic circulation (the two circuits are connected in series), the pulmonary circulation operates at dramatically lower pressures — normal pulmonary artery pressure is approximately 25/10 mmHg (systolic/diastolic), compared to 120/80 mmHg in the systemic circuit. This low-pressure design is essential because the delicate pulmonary capillaries surrounding the alveoli would be damaged by high hydrostatic pressures, and pulmonary oedema (fluid flooding the alveolar air spaces) would result — exactly what happens in left heart failure when elevated left ventricular filling pressures are transmitted backward through the pulmonary veins to the pulmonary capillaries.
Pulmonary hypertension (PH) is defined as a mean pulmonary arterial pressure (mPAP) ≥20 mmHg at rest. It is classified into five WHO groups based on pathophysiology: Group 1 (pulmonary arterial hypertension — idiopathic, heritable, drug/toxin-associated); Group 2 (PH due to left heart disease — the most common cause); Group 3 (PH due to lung disease and/or hypoxia — COPD, interstitial lung disease); Group 4 (chronic thromboembolic pulmonary hypertension — CTEPH); Group 5 (PH with unclear or multifactorial mechanisms). Untreated, pulmonary hypertension leads to progressive right ventricular pressure overload, RV hypertrophy, dilation, tricuspid regurgitation, and eventual right heart failure (cor pulmonale). Targeted therapies for Group 1 PAH — prostacyclin analogues, endothelin receptor antagonists, and phosphodiesterase-5 inhibitors — have dramatically improved outcomes over the past two decades.
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A critical feature of the pulmonary circulation is hypoxic pulmonary vasoconstriction (HPV) — the local constriction of pulmonary arterioles in response to alveolar hypoxia. This is the opposite of the systemic vasculature response (which dilates in response to hypoxia) and serves an important ventilation-perfusion matching function: when an area of lung is poorly ventilated (low O₂), blood flow is redirected away from that area to better-ventilated regions, optimising gas exchange. In conditions of global alveolar hypoxia (altitude, severe COPD, sleep apnoea), HPV becomes generalised, causing pulmonary hypertension — the mechanism of altitude-related right heart strain and chronic cor pulmonale in COPD.
Coronary Circulation — the Heart’s Own Blood Supply
For all its sophisticated work of pumping blood to every organ, the myocardium cannot extract its required oxygen directly from the blood within the cardiac chambers — the wall of the heart is too thick for diffusion to be adequate. Instead, the heart has its own dedicated blood supply through the coronary circulation. The coronary arteries arise from the sinuses of Valsalva at the base of the aorta, just above the aortic valve cusps, and encircle the heart in the atrioventricular and interventricular grooves, sending penetrating branches into the myocardium. At rest, the myocardium extracts approximately 60–70% of available oxygen from coronary blood — compared to approximately 25% in most systemic tissues — leaving almost no reserve for increased extraction during stress. Increased myocardial oxygen demand must therefore be met almost entirely by increased coronary blood flow.
Right Heart and Inferior Wall Supplier
Arises from the right aortic sinus, courses in the right atrioventricular groove. Supplies: right atrium, right ventricle, the SA node (in ~60% of individuals), the AV node (in ~90% of individuals — explaining why right coronary occlusion frequently causes AV block), the posterior descending artery (PDA) in right-dominant circulation (~70% of people), and the posterior/inferior wall of the left ventricle via posterior branches. RCA occlusion in right-dominant patients causes inferior MI (ST elevation in leads II, III, aVF) and may cause right ventricular infarction and AV nodal block.
Anterior Wall and Septum — “The Widow Maker”
The LAD is a branch of the left main coronary artery (LMCA), coursing in the anterior interventricular groove. It supplies the anterior wall of the left ventricle, the anterior two-thirds of the interventricular septum, and the apex. It gives off diagonal branches (lateral wall) and septal perforating branches (interventricular septum and bundle branches — explaining why LAD occlusion causes bundle branch blocks). LAD occlusion is the most haemodynamically devastating coronary occlusion: it is responsible for the largest myocardial infarctions, involving the most LV muscle, and carries the highest mortality of all acute coronary syndromes.
Lateral and Posterior Wall Supplier
The LCx branches from the LMCA and courses in the left atrioventricular groove, giving off obtuse marginal branches to the lateral and posterior LV wall. In left-dominant circulation (~10% of people), the LCx gives rise to the PDA and supplies the AV node. LCx occlusion causes lateral or posterior MI — often producing minimal or subtle ECG changes (ST depression in V1-V3 for true posterior MI, or lateral ST changes), making it the most commonly missed MI on standard 12-lead ECG. Posterior leads (V7-V9) are needed to directly demonstrate posterior ST elevation.
Matching Flow to Metabolic Demand
Coronary blood flow is tightly autoregulated to match myocardial oxygen demand despite variations in perfusion pressure. Local metabolic factors — adenosine (released by ischaemic myocardium), CO₂, H⁺, and K⁺ — cause coronary arteriolar vasodilation proportional to metabolic activity, increasing flow three-to-five-fold during exercise. Endothelial nitric oxide (NO) provides baseline vasodilatory tone. Coronary flow occurs predominantly during diastole (especially in the left ventricle), because the compressive forces of ventricular systole compress intramyocardial vessels — the reason tachycardia (shortening diastole) and elevated LVEDP (increasing compressive force) both reduce coronary perfusion and can precipitate angina or ischaemia.
Blood Vessel Anatomy — Wall Layers, Types, and Structural Specialisations
Blood vessels are not simply passive tubes — they are dynamic, hormonally responsive structures capable of active contraction, relaxation, remodelling, and new vessel growth (angiogenesis). All blood vessels except capillaries share a common three-layer wall architecture (the tunica layers), though the thickness and composition of each layer varies dramatically with function.
Autonomic Regulation of the Cardiovascular System
The autonomic nervous system (ANS) is the primary neural mechanism for short-term cardiovascular regulation, allowing the heart and blood vessels to respond within seconds to changes in posture, physical activity, emotional state, blood loss, or environmental temperature. Both divisions of the ANS — sympathetic and parasympathetic — innervate the heart, while vascular smooth muscle receives predominantly sympathetic innervation.
Sympathetic Nervous System — “Fight or Flight”
Sympathetic fibres from the thoracic spinal cord (T1–T5) reach the heart via the cardiac plexus, innervating the SA node, AV node, and myocardium. Noradrenaline (norepinephrine) released from sympathetic nerve terminals, and adrenaline (epinephrine) released from the adrenal medulla, act on cardiac β₁ adrenergic receptors to produce: positive chronotropy (↑ heart rate), positive dromotropy (↑ AV node conduction velocity), positive inotropy (↑ myocardial contractility through increased intracellular calcium), and positive lusitropy (↑ rate of myocardial relaxation during diastole, facilitating faster filling at higher heart rates). Sympathetic stimulation of vascular smooth muscle (predominantly α₁ receptors) causes vasoconstriction — increasing total peripheral resistance and redirecting blood from skin and gut to working muscles during exercise. β₂ receptors in skeletal muscle arterioles respond to circulating adrenaline with vasodilation — the mechanism of exercise-related skeletal muscle hyperaemia.
Parasympathetic Nervous System — “Rest and Digest”
Parasympathetic fibres reach the heart via the vagus nerve (CN X), innervating primarily the SA node and AV node (with minimal ventricular innervation compared to the sympathetic system). Acetylcholine (ACh) acts on cardiac M₂ muscarinic receptors to produce: negative chronotropy (↓ heart rate — slows SA node depolarisation by increasing potassium conductance), negative dromotropy (↓ AV node conduction — prolonged PR interval, basis of therapeutic vagal manoeuvres for supraventricular tachycardia). Resting heart rate is primarily the result of dominant parasympathetic (vagal) tone — a healthy heart at rest has substantial vagal suppression of the intrinsic SA node rate (~100 bpm). Trained endurance athletes have very high resting vagal tone, producing resting bradycardia (40–50 bpm). Atropine — a muscarinic receptor antagonist — blocks vagal effects and increases heart rate, used clinically to treat symptomatic bradycardia.
Cardiovascular Disorders — Pathophysiology of Common Conditions
The precision of cardiovascular physiology — the exact timing of valve opening and closing, the precise matching of cardiac output to venous return, the tight regulation of blood pressure, the autoregulation of coronary flow — means that disruption of any component can cause disease. Cardiovascular disease (CVD) encompasses a spectrum of conditions affecting the heart and blood vessels, collectively responsible for approximately 17.9 million deaths per year globally, representing approximately 32% of all deaths. Understanding the pathophysiology of each major condition in terms of the normal physiology it disrupts makes clinical reasoning far more accessible.
| Condition | Pathophysiology | Key Physiological Disruption | Clinical Consequence |
|---|---|---|---|
| Myocardial Infarction (MI) | Atherosclerotic plaque rupture → coronary artery thrombosis → ischaemia → myocardial cell death (necrosis). ST elevation (STEMI) vs. non-ST elevation (NSTEMI). | Loss of myocardial contractility in infarcted zone → ↓ stroke volume → ↓ CO. Compensatory neurohormonal activation (RAAS, sympathetic) → tachycardia, vasoconstriction, fluid retention. | Cardiogenic shock (massive MI); arrhythmias (re-entrant VT/VF from ischaemic border zone); ventricular rupture; mitral regurgitation (papillary muscle infarction); heart failure. |
| Heart Failure (HFrEF) | Loss of viable myocardium (post-MI, cardiomyopathy, valvular disease) → ↓ systolic function → ↓ EF → chronic neurohormonal activation → adverse cardiac remodelling (LV dilation, hypertrophy, fibrosis). | ↓ CO → ↓ tissue perfusion (fatigue, reduced exercise tolerance); ↑ LVEDP → pulmonary venous hypertension → pulmonary oedema (breathlessness, orthopnoea, PND); ↑ RVEDP → systemic venous congestion → peripheral oedema, hepatomegaly, ascites. | Progressive deterioration; high mortality (5-year survival ~50%); hospitalisation cycles. Treated with ACE inhibitors/ARBs, beta-blockers, aldosterone antagonists, SGLT2 inhibitors, and device therapy (ICD, CRT). |
| Hypertension | Increased total peripheral resistance (most commonly) ± increased cardiac output. Secondary causes in ~5%: renal artery stenosis (RAAS activation), primary aldosteronism, phaeochromocytoma, obstructive sleep apnoea. | Chronically elevated afterload → LV concentric hypertrophy → diastolic dysfunction (HFpEF) → systolic dysfunction (end-stage). Vascular injury: endothelial dysfunction, accelerated atherosclerosis, arteriolar remodelling. | LV hypertrophy and failure; coronary artery disease; stroke (haemorrhagic and ischaemic); hypertensive nephropathy (CKD); hypertensive retinopathy; aortic dissection (hypertension is the primary risk factor). |
| Atrial Fibrillation (AF) | Chaotic, high-frequency re-entrant electrical circuits in atrial myocardium (often initiating from pulmonary vein-LA junctions) → loss of organised atrial contraction → irregular, often rapid ventricular response via AV node. | Loss of atrial kick → ↓ CO by 15–25%; stasis of blood in LA (especially LAA) → thrombosis → embolic stroke risk; rapid ventricular rate → tachycardia-mediated cardiomyopathy (reversible with rate control). | Stroke (5× increased risk — the primary driver of anticoagulation with warfarin, direct oral anticoagulants); heart failure; palpitations; reduced exercise tolerance. Most common sustained arrhythmia — prevalence ~2–4% in adults, rising with age. |
| Aortic Stenosis | Progressive calcification and stiffening of aortic valve cusps (degenerative — most common; or bicuspid aortic valve — congenital). Narrowed orifice increases resistance to LV outflow. | Increased afterload → concentric LV hypertrophy → increased wall stress, impaired subendocardial perfusion, diastolic dysfunction. Fixed ↓ CO during exercise → classic triad: angina (demand-supply mismatch), syncope (failure to increase CO with exercise → cerebral hypoperfusion), heart failure (LV compensation exhausted). | Severe AS (valve area <1.0 cm², gradient >40 mmHg) has poor prognosis without intervention. Treated with surgical aortic valve replacement (SAVR) or transcatheter aortic valve implantation/replacement (TAVI/TAVR) — now the most common structural heart intervention in high-income countries. |
| Pulmonary Embolism (PE) | Thrombus (most commonly from DVT) lodges in pulmonary arterial tree → mechanical obstruction of pulmonary blood flow + vasoconstrictive response → acute ↑ pulmonary vascular resistance → acute RV pressure overload. | Acute RV dilation and failure (cor pulmonale) → RV–LV interdependence → septal shift → ↓ LV filling → ↓ CO → systemic hypotension (massive PE). Ventilation-perfusion (V/Q) mismatch → hypoxaemia. Pleuritic chest pain, haemoptysis (pulmonary infarction). | Massive PE: haemodynamic collapse, cardiac arrest, high mortality. Sub-massive PE: RV dysfunction without haemodynamic compromise. Low-risk PE: normal RV function. Diagnosed by CT pulmonary angiography (CTPA). Treated with anticoagulation; thrombolysis or catheter-directed therapy for massive/sub-massive PE. |
Cardiovascular Science Academic Support at Every Level
Whether you are completing a second-year physiology assignment on the cardiac cycle, writing a nursing care plan for a patient in heart failure, or composing a postgraduate dissertation on coronary physiology — our specialist team in anatomy, physiology, and clinical nursing science covers every topic in this guide and beyond.
Cardiac Muscle Cell Physiology — Action Potential, Calcium, and Contraction
Every heartbeat originates from the action potential of a single pacemaker cell in the SA node, but the mechanical contraction that pumps blood depends on the action potentials and calcium-handling of billions of cardiomyocytes in the working myocardium. Understanding cardiac cellular physiology — the cardiac action potential, excitation-contraction coupling, and the role of intracellular calcium — explains why drugs targeting specific ion channels or pumps produce precisely the therapeutic effects they do in arrhythmia management, heart failure, and angina treatment.
Five Phases of Cardiac Depolarisation
The ventricular cardiomyocyte action potential has five phases: Phase 0 — rapid depolarisation (fast Na⁺ channels open, membrane potential rises from −90 to +30 mV in milliseconds); Phase 1 — brief repolarisation (Na⁺ channels inactivate, Ito K⁺ channels open); Phase 2 — plateau (characteristic of cardiac muscle, absent in other tissues — L-type Ca²⁺ channels open, balancing K⁺ efflux and prolonging the action potential, creating the refractory period that prevents tetanic contraction); Phase 3 — rapid repolarisation (K⁺ channels dominate, Ca²⁺ channels inactivate); Phase 4 — resting membrane potential (~−90 mV, maintained by the inward rectifier K⁺ channel). The long plateau phase is the basis of the QT interval on the ECG — QT prolongation (from drugs, electrolyte abnormalities, or genetic channelopathies) predisposes to torsades de pointes, a potentially fatal polymorphic ventricular tachycardia.
From Electrical Signal to Mechanical Contraction
Calcium is the transducer linking electrical excitation to mechanical contraction in cardiomyocytes. The process — excitation-contraction (EC) coupling — occurs in four steps: (1) Action potential depolarisation opens L-type (dihydropyridine receptor, DHPR) calcium channels in the T-tubule membrane — a small amount of calcium enters the cell. (2) This trigger calcium binds ryanodine receptors (RyR2) in the sarcoplasmic reticulum (SR) membrane, causing massive calcium release from SR stores — calcium-induced calcium release (CICR), amplifying the initial signal approximately tenfold. (3) Released calcium binds troponin C (the calcium-sensitive subunit of troponin), causing conformational changes that remove tropomyosin’s inhibition of myosin-actin cross-bridge cycling — contraction proceeds. (4) Relaxation requires removal of cytosolic calcium: the SR Ca²⁺-ATPase (SERCA2a) pumps 70% of calcium back into the SR; the sarcolemmal Na⁺/Ca²⁺ exchanger (NCX) extrudes 28% across the plasma membrane. Phospholamban (PLN) normally inhibits SERCA2a — sympathetic stimulation causes PKA-mediated PLN phosphorylation, relieving inhibition and accelerating relaxation (positive lusitropic effect).
Critical Structural and Functional Differences
Cardiac muscle differs from skeletal muscle in six key ways that are frequently tested: (1) Myogenic — cardiac muscle contracts spontaneously without neural input (skeletal muscle requires continuous motor nerve stimulation); (2) Involuntary — not under conscious control; (3) Branching interconnected cells with intercalated discs (gap junctions + desmosomes) forming a functional syncytium; (4) Absolute refractory period extends through most of the action potential plateau — cardiac muscle cannot tetanise (essential for efficient pumping); (5) Frank-Starling mechanism — cardiac muscle length-tension relationship is used physiologically to match output to venous return (skeletal muscle operates near optimal length); (6) Entirely aerobic metabolism — cardiac muscle has abundant mitochondria (~30% cell volume) and negligible glycogen stores, making it exquisitely sensitive to ischaemia.
How Drugs Exploit Cardiac Physiology
The molecular machinery of cardiac cells provides the targets for the most commonly used cardiac drugs: Beta-blockers (metoprolol, bisoprolol) — block β₁ adrenergic receptors → ↓ HR, ↓ contractility, ↓ conduction velocity → anti-ischaemic, anti-arrhythmic, and anti-heart-failure effects. Calcium channel blockers (amlodipine, diltiazem, verapamil) — block L-type Ca²⁺ channels → vasodilation (DHPs); ↓ HR and AV conduction (non-DHPs). Digoxin — inhibits Na⁺/K⁺-ATPase → ↑ intracellular Na⁺ → ↑ NCX Ca²⁺ efflux reduced → ↑ intracellular Ca²⁺ → positive inotropy; also enhances vagal tone → ↓ HR. Class I anti-arrhythmics (lidocaine, flecainide) — block fast Na⁺ channels → slow Phase 0 depolarisation. Class III anti-arrhythmics (amiodarone, sotalol) — block K⁺ channels → prolong Phase 3 repolarisation → lengthen refractory period → prevent re-entrant arrhythmias.
Specialised Circulations — Cerebral, Renal, and Portal
While all organs receive their blood from the systemic circulation, several have specialised circulatory arrangements reflecting their unique functional requirements. Understanding these specialised circulations is critical for clinical reasoning in neurology, nephrology, hepatology, and surgery.
Cerebral Circulation and the Blood-Brain Barrier
The brain receives approximately 15% of cardiac output (750 mL/min) and accounts for approximately 20% of total oxygen consumption despite comprising only 2% of body weight. Cerebral blood flow (CBF) is tightly autoregulated (maintained at ~50 mL/100g/min) over a wide range of perfusion pressures (MAP 60–160 mmHg) through myogenic and metabolic mechanisms. The brain cannot tolerate ischaemia: irreversible neuronal damage begins within four minutes of complete CBF cessation. The cerebral vasculature is the site of stroke — ischaemic (thrombotic, embolic, lacunar) or haemorrhagic (hypertensive rupture of small penetrating arteries or aneurysm rupture). Cerebral perfusion pressure (CPP) = MAP − intracranial pressure (ICP), making raised ICP (brain tumour, haemorrhage, oedema) as dangerous as low MAP for cerebral perfusion. The blood-brain barrier (tight junctions of brain capillary endothelium + astrocyte end-feet) restricts passage of large molecules and many drugs to the central nervous system, making CNS drug delivery a major pharmacological challenge.
Renal Circulation — the Blood Pressure Barometer
The kidneys receive approximately 20–25% of cardiac output (1.0–1.2 L/min) — disproportionately high relative to their mass (~0.4% of body weight) — reflecting their role in continuous blood filtration and plasma volume regulation. The glomerular capillaries are fenestrated and arranged between two arterioles (afferent and efferent), creating a unique portal-like arrangement that allows precise control of glomerular filtration rate (GFR) by changing the relative resistance of each arteriole. Angiotensin II preferentially constricts the efferent arteriole, maintaining GFR despite reduced renal perfusion pressure — the mechanism by which ACE inhibitors can precipitate acute kidney injury in renal artery stenosis by removing this protective efferent constriction. The juxtaglomerular apparatus — a specialised structure in the afferent arteriole adjacent to the macula densa — is the primary sensor for RAAS activation, releasing renin in response to reduced renal perfusion pressure.
Portal Circulation — First-Pass Metabolism
The hepatic portal circulation is unique in receiving blood from two sources: the hepatic portal vein (carrying nutrient-rich, partially deoxygenated blood from the gastrointestinal tract and spleen — approximately 75% of hepatic blood flow) and the hepatic artery (oxygenated blood from the systemic circulation — approximately 25% of hepatic blood flow). This dual supply means the liver receives ingested nutrients and drugs directly from the gut before they reach the systemic circulation — the basis of first-pass metabolism, by which the liver metabolises (and often inactivates) orally administered drugs during their initial transit through the hepatic portal system. Portal hypertension — raised portal venous pressure (>10 mmHg) from cirrhosis, pre-hepatic causes (portal vein thrombosis), or post-hepatic causes (Budd-Chiari syndrome) — causes oesophageal varices (risk of catastrophic haemorrhage), ascites, splenomegaly, and portosystemic encephalopathy.
Frequently Asked Questions About the Cardiovascular System
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