9/26/25 Circulation of the Heart

Blood components and oxygen transport

• Blood is composed of plasma and cells: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

• Erythrocytes contain hemoglobin, the main oxygen carrier.

• Normal hematocrit is about 45% (the percentage of red blood cells in blood).

• Red blood cell lifespan ≈ 120 days.

• Hematocrit (Hct) definition: the percentage of red blood cells in whole blood:
  $\text{Hct} = \frac{V<em>{\text{RBC}}}{V</em>{\text{Total}}} \times 100\%.$

• Red blood cell production occurs in bone marrow (e.g., femur is a major site).

• Blood donation:

  • Whole blood donation intervals roughly every 6–8 weeks.

  • Plasma donation is also common and in high demand.

• Normal hemoglobin (Hb) ~ $\text{Hb} \approx 14\text{--}15\ \text{g/dL}.$

• When trauma causes blood loss, hematocrit is used to gauge blood loss and oxygen-carrying capacity.

• Leukocytes (white blood cells) protect against infection; normal WBC count ~ $4{,}000\text{--}11{,}000\,/\text{µL}.$

• CBC with differential (CBC diff) breaks down leukocytes into subsets:

  • Granulocytes: neutrophils, eosinophils, basophils.

  • Agranulocytes: T cells and B cells (monocytes are a separate category).

• Neutrophils are the most common WBCs, comprising roughly $40\%\text{--}70\%$ of WBCs.

• Eosinophils are elevated in allergic responses, especially asthma; basophils participate in allergic responses too.

• In allergic/asthmatic situations, histamine release causes swelling and airway irritation; antihistamines can mitigate this.

• Eosinophils (and especially eosinophilic asthma) are a key marker in asthma-related leukocyte response.

• Agranular leukocytes (lymphocytes: T cells and B cells) are central to adaptive immunity; monocytes also participate in infection control.

• HIV/AIDS and other immunocompromising conditions specifically deplete T cells and B cells; chemotherapy can suppress many leukocytes.

• Thrombocytes (platelets) are the smallest plasma components and are essential for clotting; disorders like hemophilia affect clotting.

• Plasma is the liquid fraction (~45% of blood volume) and mostly water with proteins; it can be used as a volume extender.

Heart anatomy and basic physiology

• The heart is a muscular pump located in the center-left thorax within the mediastinum.

• Four chambers: two atria (upper) and two ventricles (lower).

  • Right atrium (receives deoxygenated blood from body via the superior and inferior vena cavae).

  • Right ventricle (pumps to the lungs via the pulmonary artery).

  • Left atrium (receives oxygenated blood from the lungs via the pulmonary veins).

  • Left ventricle (pumps to the systemic circulation via the aorta).

• Inter-chamber septum separates atria from ventricles.

• The heart is enclosed by the pericardium; pericardial effusion or tamponade can compress the heart and impede pumping.

• Heart wall layers from outside to inside:

  • Epicardium (outer layer)

  • Myocardium (thick muscular middle layer; the actual pump)

  • Endocardium (inner lining of chambers and valves)

• Myocardial infarction (heart attack): loss of blood flow to myocardium, causing tissue death; “MI” is preferred over lay term “heart attack.”

• Coronary arteries supply the heart muscle; major arteries include the left coronary arteries and the left anterior descending (LAD).

• Blockage of a coronary artery reduces blood flow to the supplied myocardium; left ventricle is the workhorse and most critical for systemic blood pressure.

• Atherosclerotic plaque buildup can rupture or compress a coronary artery, causing myocardial infarction.

• Left anterior descending (LAD) artery is known as the “widow maker” when acutely occluded due to its supply to the left ventricle.

• Coronary bypass grafting (CABG): surgically bypass blocked coronary segments using vessels from the leg or chest.

  • CABG may involve multiple bypasses (e.g., CABG times four or five).

  • Goal is to reroute blood flow around blocked segments to restore myocardium perfusion.

• Angioplasty (PCI): catheter-based therapy in the cath lab where a balloon is inflated to compress plaque and reopen the artery; sometimes followed by stent placement.

• Cardiac catheterization: uses contrast dye to visualize coronary arteries; goal is to get to the catheterization lab quickly (within ~1 hour) for chest pain with suspected MI.

• Cardiac surgery ecologies:

  • Some hospitals have cath labs without surgical capability; transfer to surgical center may be required.

• Valve anatomy and replacement options:

  • Tricuspid valve: right atrioventricular valve.

  • Mitral valve: left atrioventricular valve.

  • Aortic valve: controls blood flow from left ventricle to aorta.

  • Valve replacement options include mechanical and bioprosthetic (pig/cow) valves; mechanical valves require long-term anticoagulation.

  • Transcatheter Aortic Valve Replacement (TAVR): less invasive valve replacement via catheter, often robotic-assisted or percutaneous; suitable for selected patients.

• Valve function basics:

  • Valves open/close to ensure unidirectional flow; regurgitation occurs when valves fail to seal.

  • Postoperative auscultation can detect mechanical valve clicks due to prosthetic valve opening/closing.

• Perfusionist and heart-lung machine:

  • Perfusionists operate the cardiopulmonary bypass machine during open-heart surgery, temporarily taking over circulation and oxygenation.

  • They manage blood flow, oxygenation, and CO2 removal during procedures.

• Nitric oxide in cardiac surgery:

  • Potent pulmonary vasodilator used to reduce pulmonary hypertension and improve right ventricular perfusion during surgery.

Pathway of blood flow through the heart and major vessels

• Major pathway (systemic-to-pulmonary-to-systemic):

  • Inferior vena cava (from lower body) and Superior vena cava (from upper body) deliver deoxygenated blood to the Right Atrium (RA).

  • RA → Tricuspid valve → Right Ventricle (RV).

  • RV → Pulmonary valve → Pulmonary artery → Lungs (gas exchange occurs in pulmonary capillaries).

  • Oxygenated blood returns from lungs via Pulmonary Veins → Left Atrium (LA).

  • LA → Mitral valve → Left Ventricle (LV).

  • LV → Aortic valve → Aorta → Systemic circulation (body).

  • Aorta branches to head and limbs via carotids, subclavians, etc.

• Pressures and hemodynamics:

  • Right heart operates at lower pressures (pumping into the lungs).

  • Left ventricle generates much higher pressures (e.g., systolic ~ $120\,\text{mmHg}$) to maintain systemic circulation.

• Arteries vs veins polarity:

  • Arteries typically carry oxygenated blood (except pulmonary artery).

  • Veins return deoxygenated blood to the heart (except pulmonary veins).

• Gas exchange in the lungs:

  • Oxygen diffuses from alveolar air into the blood at the alveolar-capillary membrane and binds to hemoglobin in RBCs.

  • Carbon dioxide diffuses from blood into alveolar air to be exhaled.

• Heart valves in flow: three valves relevant to this pathway include:

  • Tricuspid valve (RA to RV)

  • Mitral valve (LA to LV)

  • Aortic valve (LV to aorta)

• Additional valve considerations:

  • Valve replacement options and the concept of prosthetic valve clicks indicating mechanical devices.

  • Transcatheter approaches (TAVR) as less invasive alternatives in selected patients.

Hemodynamics, cardiac output, and related metrics

• Key hemodynamic terms:

  • Systole: ventricles contract and eject blood; highest arterial pressure.

  • Diastole: ventricles relax and fill with blood.

  • S1 (lub) corresponds to AV valve closure; S2 (dub) corresponds to semilunar valve closure.

• Stroke volume (SV): amount of blood ejected by the left ventricle with each beat:

  • $SV = ext{volume ejected per beat}$

• Cardiac output (CO): volume of blood pumped per minute; CO is the product of stroke volume and heart rate (HR):

  • $CO = SV \times HR$

• Normal ranges and clinical relevance:

  • Normal CO roughly $CO \approx 5\text{--}8\ \text{L/min}$.

  • Ejection fraction (EF): fraction of end-diastolic volume (EDV) ejected with each beat; $EF = \frac{SV}{EDV} \times 100\%.$

  • Ejection fraction values: some patients with severe systolic dysfunction may have EF markedly reduced (e.g., $EF \approx 20\%$).

• Hemodynamic implications:

  • Hypotension triggers compensatory tachycardia and increased SV to maintain CO.

  • Ventricle diastolic filling (preload) and contractility influence SV and CO.

• Clinical measurement tools:

  • Echocardiography to estimate EF (noninvasive): provides LV outflow assessment and ejection fraction.

  • Cardiac catheterization and angiography provide direct measurements of coronary anatomy and ventricle pressures.

• Oxygen transport relevance:

  • Adequate CO must meet tissue oxygen demand; when blood flow is inefficient (e.g., LV failure or CAD), tissues become hypoxic.

Clinical conditions, emergencies, and treatment concepts

• Atherosclerosis and plaques:

  • Plaques can progressively narrow coronary arteries, reducing myocardial blood flow; acute rupture can cause thrombosis and MI.

• Left anterior descending (LAD) artery:

  • Occlusion of LAD can cause significant myocardial ischemia in the left ventricle; termed a high-risk event (the “widow maker”).

• Acute interventions:

  • Cardiac catheterization (cath) lab within ~1 hour for chest pain with suspected MI to assess blockage and consider immediate PCI or surgical intervention.

  • Angioplasty (PCI) to open clogged arteries; stents may be placed to keep vessel open.

• Coronary bypass grafting (CABG):

  • Open-heart procedure; chest opened, pericardium opened; bypass around blocked coronary segments using grafts from the leg or chest.

  • Sometimes multiple bypasses (e.g., CABG times four or five).

  • Indicated for multivessel disease or high-grade blockages not amenable to PCI.

• Valve disease and replacement:

  • Mitral and/or aortic valve disease may require replacement with mechanical or bioprosthetic valves.

  • Transcatheter options (TAVR) allow valve replacement without open-heart surgery in selected patients.

• Open-heart surgery logistics:

  • A perfusionist runs the heart-lung machine during bypass.

  • Oxygenation, CO2 removal, and perfusion are temporarily managed outside the heart to allow surgical repair.

  • Nitric oxide can be used during certain surgeries to dilate pulmonary vasculature and aid right heart function.

• Prosthetic valve considerations:

  • Mechanical valves require lifelong anticoagulation; bioprosthetic valves (animal tissue) have different durability and anticoagulation needs.

• Postoperative and recovery notes:

  • Robotic-assisted valve replacements and minimally invasive approaches reduce recovery times for some patients.

  • Some patients may require LVADs (left ventricular assist devices) as a bridge to transplant or destination therapy.

• Heart transplant:

  • First successful human heart transplant performed in 1967 by Christiaan Barnard; widespread success improved over subsequent decades.

  • Donor organs come from brain-dead or organ-donor families; ethics and organ allocation are critical topics in medicine.

  • Pediatric heart transplants are possible; waiting times and outcomes vary with donor availability and recipient condition.

  • Long-term survival varies but many transplant recipients live meaningful lives; lifelong immunosuppression is required.

  • Alternatives while waiting include implanted devices (e.g., LVADs) to support circulation.

• Ethical and logistical considerations:

  • Organ donation consent and brain death determination are essential ethical issues in transplant medicine.

  • CON (certificate of need) and regulatory environments influence availability and development of heart programs (evolving landscape mentioned).

Practical notes, anecdotes, and professional insight

• Clinical roles and interprofessional teams:

  • Perfusionists are central to open-heart surgery and work with surgeons to manage cardiopulmonary bypass.

  • Respiratory therapists collaborate in cardiac surgery settings, including nitric oxide administration and intraoperative ventilation strategies.

  • Robotic and minimally invasive techniques require specialized training and teams.

• Real-world considerations:

  • Many communities have specialized heart centers with varying capabilities for cath lab and surgical services.

  • Outcomes and availability can influence where patients receive procedures like CABG or valve replacements.

  • Lifestyle factors (diet, smoking, exercise) contribute significantly to plaque formation and cardiovascular risk.

• Ethical reflection prompts:

  • Balancing organ availability with patient need; the ethics of organ allocation.

  • The impact of advanced therapies (e.g., LVADs, heart transplant) on quality of life and cost.

Quick reference: key definitions and formulas

• Hematocrit: $\text{Hct} = \frac{V<em>{\text{RBC}}}{V</em>{\text{Total}}} \times 100\%.$

• Hemoglobin: $\text{Hb} \approx 14\text{--}15\ \text{g/dL}.$

• RBC lifespan: ≈ 120 days.

• White blood cell count: $4{,}000\text{--}11{,}000\,/\text{µL}.$

• Neutrophil proportion: $\text{neutrophils} \approx 40\%\text{--}70\%$ of WBCs.

• Cardiac output: $CO = SV \times HR,$ with typical resting values around $CO \approx 5\text{--}8\ \text{L/min}.$

• Stroke volume (SV): amount of blood ejected from the left ventricle with each beat.

• Ejection fraction: $EF = \frac{SV}{EDV} \times 100\%.$

• Blood flow pathway summary: inferior vena cava and superior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → systemic circulation.

• Pathophysiology notes:

  • LAD occlusion can cause a life-threatening MI (the “widow maker”).

  • Aortic rupture or aneurysm with trauma carries very poor survival without rapid intervention; tamponade may occur when blood accumulates in the pericardial sac.

• Diagnostic and treatment timelines:

  • Chest pain: cath lab within ~1 hour when indicated.

  • PCI vs. CABG decisions depend on anatomy, risk, and available facilities.