Emphysema - Complete Explanation

1. What is COPD?

• Currently the 4th leading cause of death worldwide
• Emphysema and chronic bronchitis are the two major forms of COPD - they often coexist in the same patient because they share the same main cause: cigarette smoking
• COPD is an obstructive lung disease, meaning there is increased resistance to airflow - the FEV1/FVC ratio drops below 0.7

2. Definition of Emphysema

Irreversible (permanent) enlargement of airspaces distal to the terminal bronchiole, accompanied by destruction of their walls, without obvious fibrosis.

• Damage is beyond the terminal bronchiole (in the acini/alveoli)
• The walls are destroyed (not just stretched)
• There is no fibrosis - this distinguishes it from other lung conditions

3. Types of Emphysema

4. Pathogenesis

Main Causes

• Tobacco smoking (accounts for ~80% of COPD)
• α1-antitrypsin (α1-AT) deficiency (genetic cause)

Four Mechanisms

A. Toxic Injury and Inflammation

• Cigarette smoke damages respiratory epithelium
• Resident macrophages and epithelial cells release inflammatory mediators: leukotriene B4, IL-8, TNF
• These attract neutrophils from the circulation
• Neutrophils amplify inflammation and cause structural damage via growth factors and pro-inflammatory cytokines

B. Protease-Antiprotease Imbalance (Most Important)

• Nicotine acts as a chemoattractant for neutrophils
• Neutrophils release elastase (a protease) and reactive oxygen species (ROS)
• Elastase breaks down elastin - the elastic tissue that normally holds small airways open via radial traction during expiration
• Normally, α1-antitrypsin (produced by the liver) inhibits elastase
• In smokers or those with α1-AT deficiency: elastase exceeds inhibitor → elastic tissue destruction → airways collapse on expiration → airflow obstruction

C. Oxidative Stress

• Tobacco smoke and inflammatory cells produce oxidants (ROS) causing further tissue damage
• NRF2 is a transcription factor that normally activates antioxidant defense genes (including glutathione)
• Genetic variation in NRF2 is linked to increased susceptibility to smoking-related lung disease

D. Infection

• Does NOT initiate tissue destruction
• Superimposed bacterial/viral infections worsen existing inflammation and chronic bronchitis

5. Why Does Airflow Get Obstructed?

1. Loss of elastic tissue → loss of radial traction → small airways collapse during expiration
2. Goblet cell hyperplasia → excess mucus plugging airways
3. Inflammatory infiltrates in bronchial walls (neutrophils, macrophages, B cells, T cells)
4. Smooth muscle hypertrophy + peribronchial fibrosis → thickening of bronchiolar walls

6. α1-Antitrypsin Deficiency (Genetic Emphysema)

• Autosomal recessive disorder
• Encoded at the Pi locus on chromosome 14q
• Pi ZZ genotype (homozygous Z allele): serum α1-AT drops to <15% of normal
• ~1% of all emphysema patients have this defect

• 80% of Pi ZZ individuals develop panacinar emphysema - earlier in life and more severe if they smoke

• α1-AT is produced by hepatocytes, bone marrow cells, and lymphoid cells
• Also causes liver disease (protein accumulates in hepatocytes)

7. Morphology (What You See)

Gross (Naked Eye)

• Voluminous, enlarged lungs - often overlap and hide the heart
• Upper two-thirds more severely affected
• Blebs (<1 cm) and bullae (>1 cm) - large air-filled cysts, especially at the apex

Microscopic (Under the Microscope)

• Abnormally large alveoli separated by thin, destroyed septa
• Loss of alveolar attachments to small airway walls
• Pores of Kohn (inter-alveolar openings) become so large that septa appear to be "floating" in airspace
• Inflammatory changes around small airways

8. Clinical Features (Signs & Symptoms)

• Narrowed airways → can't fully exhale → residual air builds up → hyperinflation

9. Complications

• Pulmonary arterial hypertension - from chronic hypoxia causing prolonged vasoconstriction
• Cor pulmonale (right heart enlargement from pulmonary hypertension) → right-sided heart failure
• Pneumothorax - especially from rupture of bullae (particularly in paraseptal emphysema in young adults)

1. Coronary artery disease
2. Respiratory failure
3. Right-sided heart failure
4. Massive lung collapse (pneumothorax)

10. Diagnosis

11. Treatment

1. Smoking cessation - most important intervention
2. Oxygen therapy
3. Long-acting bronchodilators + inhaled corticosteroids
4. Chest physiotherapy
5. Surgical options:
   • Bullectomy (removal of bullae)
   • Lung volume reduction surgery
   • Lung transplantation
6. α1-AT replacement therapy - under evaluation for genetic cases

12. Other Special Forms

Quick Summary

Emphysema = permanent destruction of alveolar walls distal to terminal bronchioles → air trapping → hyperinflation → obstructive airflow pattern. Main causes are smoking (centriacinar type) and α1-AT deficiency (panacinar type). The core mechanism is elastase-antiprotease imbalance destroying lung elastic tissue. Complications include cor pulmonale, respiratory failure, and pneumothorax.

• Cigarette smoke recruits neutrophils, which release elastase
• α1-Antitrypsin (produced by the liver) keeps elastase in check
• Elastin is preserved, small airways stay open via radial traction, and expiration is normal

• Heavy smoking OR genetic α1-AT deficiency (Pi ZZ genotype, chromosome 14q) tips the balance
• Elastase overwhelms the available α1-AT
• Elastin in alveolar walls is digested and destroyed
• Without elastic support, small airways collapse during expiration
• Air traps in the lungs → hyperinflation → permanent alveolar wall destruction = Emphysema

1. Protease-Antiprotease Imbalance (Primary Mechanism)

• Cigarette smoke and noxious particles recruit neutrophils and macrophages into the lung
• These cells release proteases - mainly elastase and matrix metalloproteinases (MMPs)
• Elastase directly digests elastin, the structural protein that makes up the alveolar wall framework
• Normally, α1-antitrypsin neutralizes elastase - but in smokers or Pi ZZ individuals, elastase activity overwhelms the inhibitor
• Result: the connective tissue scaffold of alveolar walls is broken down and cannot be repaired

2. Oxidative Stress

• Tobacco smoke and activated inflammatory cells (neutrophils, macrophages) flood the lung with reactive oxygen species (ROS)
• ROS directly damage alveolar epithelial cells and endothelial cells
• ROS also inactivate α1-antitrypsin, making the protease-antiprotease imbalance even worse
• The antioxidant defense (controlled by NRF2) is overwhelmed in heavy smokers

3. Alveolar Cell Apoptosis (Programmed Cell Death)

• Oxidative stress and inflammatory mediators trigger apoptosis (cell death) of:
  • Type I pneumocytes (cover alveolar surface)
  • Type II pneumocytes (repair and surfactant-producing cells)
  • Capillary endothelial cells
• When these cells die faster than they can be replaced, the alveolar wall physically disintegrates
• This is a key reason why emphysematous damage is irreversible - the repair machinery is itself destroyed

4. Chronic Inflammation

• Persistent inflammatory cells in the alveolar walls release:
  • TNF-α, IL-8, leukotriene B4 - sustain neutrophil/macrophage recruitment
  • Growth factors that paradoxically cause structural remodeling rather than proper repair
• This chronic low-grade inflammation keeps the destructive cycle going even after smoking stops

The End Result

Gross Morphology (Naked Eye)

Overall Appearance

• Lungs are voluminous and over-inflated - they are larger and heavier than normal
• They overlap and hide the heart when the chest is opened (a classic finding)
• Lungs feel spongy and may not collapse normally when the chest is opened

Distribution

• Upper two-thirds of the lungs are more severely affected (especially in centriacinar/smoking-related emphysema)
• Panacinar (α1-AT deficiency) preferentially affects the lower lobes and bases

Surface Features

• Blebs - air-filled spaces less than 1 cm, just under the pleura (subpleural)
• Bullae - larger air-filled cysts greater than 1 cm, often at the apex
• Bullae are especially characteristic of:
  • Paraseptal (distal acinar) emphysema
  • Irregular emphysema secondary to scarring

Cut Surface

• Large, abnormally wide alveoli visible to the naked eye
• Thin, wispy alveolar walls between the enlarged spaces
• Loss of the normal fine sponge-like texture

Microscopic Morphology (Histology)

Alveolar Changes

• Abnormally enlarged alveoli - the defining feature
• Thinned and destroyed alveolar septa - walls are sparse and fragile
• Loss of alveolar attachments to the outer wall of small airways - the airways lose their structural support

Pores of Kohn

• The inter-alveolar pores (Pores of Kohn) become so enlarged that adjacent alveoli merge
• Septa appear to be "floating" freely in airspace, or protrude blindly into the lumen with a club-shaped end - a hallmark microscopic sign

Vascular Changes

• Reduced capillary bed in alveolar walls
• Thickening of pulmonary arterial walls in advanced disease (due to pulmonary hypertension)

Airway / Inflammatory Changes

• Inflammatory infiltrates in bronchial and bronchiolar walls:
  • Neutrophils, macrophages, B cells (sometimes forming follicles), T cells
• Goblet cell hyperplasia - excess mucus-secreting cells
• Smooth muscle hypertrophy in bronchiolar walls
• Peribronchial fibrosis - thickening of tissue around small airways

Side-by-Side Comparison

Key Exam Points

• Gross: Voluminous lungs, hidden heart, subpleural bullae at apex
• Micro: Enlarged alveoli + thin destroyed septa + floating/club-ended septal remnants + inflammatory infiltrates in airways
• The absence of fibrosis microscopically is what separates emphysema from other destructive lung conditions
• Bullae >1 cm rupturing into the pleural space = spontaneous pneumothorax (classic in young adults with paraseptal emphysema)

Step-by-Step Formation

Step 1 - Alveolar Wall Destruction

• Elastase and ROS destroy the walls between adjacent alveoli
• Multiple small alveoli lose their separating septa and merge into one larger space
• This is called confluence - the merged space holds more air than any single alveolus

Step 2 - Ball-Valve / Check-Valve Effect

• The small airways feeding these enlarged spaces become partially obstructed (inflamed walls, mucus, loss of elastic support)
• Air can flow in during inspiration (airways are pulled open by negative pressure)
• But during expiration, the already-collapsing airway closes before all the air escapes
• Result: more air enters than leaves with each breath cycle - progressive air trapping

Step 3 - Progressive Pressure Build-Up

• With each breath, a little more air accumulates in the trapped space
• The pressure inside the merged airspace gradually exceeds the surrounding lung pressure
• The thin, weakened walls of the space stretch outward

Step 4 - Bleb or Bulla Forms

• The expanding air pocket pushes outward toward the pleural surface (path of least resistance)
• It dissects through the loose connective tissue just under the visceral pleura
• A bleb forms when this air pocket is small (<1 cm) - essentially a subpleural air bubble
• With continued air trapping and expansion, the bleb grows into a bulla (>1 cm)

Why the Subpleural Location?

• Central lung parenchyma is surrounded and reinforced by adjacent alveoli on all sides
• Peripheral alveoli near the pleura have no support on one side
• Air preferentially dissects into the subpleural plane
• This is why bullae are almost always found at the apex and along pleural surfaces

Why the Apex Specifically?

• Pleural pressure is most negative (most sub-atmospheric) at the apex due to gravity
• This means apical alveoli are already subjected to greater distending pressure at baseline
• Any air trapping amplifies this further at the apex
• Blood flow is also lowest at the apex, so inflammatory repair is poorest there

Blebs vs Bullae - The Distinction

Which Type of Emphysema Forms Bullae?

The Danger - Pneumothorax

• Air escapes into the pleural space (which is normally a potential space under negative pressure)
• The negative pressure of the pleural space is lost
• The lung collapses on that side
• This is spontaneous pneumothorax - the classic complication of bullous/paraseptal emphysema, typically in tall, thin young men with apical bullae

Formula

Why Does a "Gap" Exist?

Measured cations:    Na⁺ (main), K⁺ (often omitted)
Measured anions:     Cl⁻ + HCO₃⁻
Unmeasured anions:   Albumin, phosphate, sulfate, organic acids
Unmeasured cations:  Ca²⁺, Mg²⁺, K⁺

Using AG to Classify Metabolic Acidosis

High AG Metabolic Acidosis (AG > 12)

Normal AG (Hyperchloremic) Metabolic Acidosis (AG 8-12)

Corrected AG for Albumin

Delta-Delta Ratio (Δ/Δ)

Quick Clinical Example

• Na⁺ = 138, Cl⁻ = 100, HCO₃⁻ = 10
• AG = 138 - (100 + 10) = 28 → High AG
• Albumin = 4 g/dL (normal, no correction needed)
• Diagnosis: High AG metabolic acidosis → DKA most likely

Summary Table

Recent Research on Anion Gap in Clinical Practice

1. AG as a Dynamic Prognostic Biomarker in Sepsis

2. Albumin-Corrected AG (ACAG) is Superior to Plain AG

• ACAG predicted 30-day mortality with AUC 0.643 vs. plain AG at 0.604
• ACAG had a linear relationship with mortality - higher ACAG = higher death risk
• ACAG was mediated through worsening renal function (BUN, creatinine) and disease severity scores

3. Mixed Acid-Base Disorders - Updated 2025 Curriculum

• Mixed acid-base disorders are frequently missed because opposing disturbances can bring pH back toward normal
• A systematic approach is mandatory: history + exam + AG + electrolytes + urine parameters together
• The delta-delta ratio and compensatory response evaluation remain the key tools
• Eight clinical cases are provided to illustrate real-world mixed disorder diagnosis

4. Negative Anion Gap - An Emerging Recognition

5. Fluid Choice in DKA Affects the AG

Summary of Key Clinical Takeaways from Recent Research

Location

Overview

The Five Complexes

Complex I - NADH Dehydrogenase

• Accepts electrons from NADH (from matrix)
• Transfers electrons to ubiquinone (CoQ)
• Pumps 4 H⁺ from matrix → intermembrane space
• Contains FMN (flavin mononucleotide) and iron-sulfur clusters
• Inhibited by rotenone (insecticide), metformin, amytal

Complex II - Succinate Dehydrogenase

• Accepts electrons from FADH₂ (from Krebs cycle - succinate → fumarate)
• Transfers electrons to ubiquinone (CoQ)
• Does NOT pump H⁺ - this is why FADH₂ generates less ATP than NADH
• Also part of Krebs cycle (dual function)
• Inhibited by malonate (competitive inhibitor of succinate)

Ubiquinone (Coenzyme Q / CoQ₁₀)

• Mobile electron carrier - freely diffuses within the inner membrane
• Collects electrons from Complexes I and II
• Carries them to Complex III
• Exists in three forms: ubiquinone (oxidized), semiquinone (radical), ubiquinol (reduced)

Complex III - Cytochrome bc₁ Complex

• Accepts electrons from ubiquinol (CoQH₂)
• Transfers electrons to cytochrome c via the Q cycle
• Pumps 4 H⁺ into intermembrane space per 2 electrons
• Contains cytochrome b, cytochrome c₁, and Rieske iron-sulfur protein
• Inhibited by antimycin A

Cytochrome c

• Mobile electron carrier - small protein in intermembrane space
• Carries electrons one at a time from Complex III → Complex IV
• Also a key trigger of apoptosis when released into cytoplasm

Complex IV - Cytochrome c Oxidase

• Accepts electrons from cytochrome c
• Transfers them to O₂ (the final electron acceptor) → forms H₂O
• Pumps 2 H⁺ into intermembrane space
• Contains cytochrome a, cytochrome a₃, and copper centers (CuA, CuB)
• Inhibited by cyanide, carbon monoxide, azide (bind to cytochrome a₃ and block O₂ binding)

Complex V - ATP Synthase (F₀F₁-ATPase)

• Not part of electron transport - but couples to it
• H⁺ ions flow back into matrix through the F₀ subunit (proton channel)
• This rotation drives the F₁ subunit to synthesize ATP from ADP + Pᵢ
• Called chemiosmosis (Mitchell's chemiosmotic hypothesis)
• Inhibited by oligomycin (blocks F₀ proton channel)

The Full Sequence

NADH → Complex I → CoQ ←── Complex II ← FADH₂
                    ↓
               Complex III
                    ↓
              Cytochrome c
                    ↓
               Complex IV → O₂ → H₂O
                    
H⁺ gradient → Complex V → ATP

H⁺ Pumping Summary

ATP Yield

• ~2.5 H⁺ needed to make 1 ATP by Complex V
• NADH → 10 H⁺ → ~2.5 ATP
• FADH₂ → 6 H⁺ → ~1.5 ATP

• Glycolysis: 2 ATP
• Pyruvate decarboxylation + Krebs: 2 ATP
• ETC: ~28 ATP
• Grand total: ~30-32 ATP

Chemiosmosis - The Driving Force

1. Chemical gradient - high H⁺ concentration in intermembrane space vs. matrix
2. Electrical gradient - intermembrane space is more positive than matrix

Uncouplers

Clinically Important Inhibitors

Reactive Oxygen Species (ROS)

• Electrons occasionally "leak" from Complexes I and III and react with O₂ to form superoxide (O₂•⁻)
• This is converted to H₂O₂ by superoxide dismutase (SOD), then to H₂O by catalase/glutathione peroxidase
• Excess ROS causes oxidative stress → DNA damage, lipid peroxidation, protein damage
• Relevant to aging, neurodegeneration, and ischemia-reperfusion injury

Oxygen is the Final Electron Acceptor

4 e⁻  +  4 H⁺  +  O₂  →  2 H₂O

• 4 electrons arrive via cytochrome c (one at a time)
• 4 H⁺ are pulled from the matrix
• 1 O₂ molecule accepts all 4 electrons simultaneously
• Product: 2 molecules of water

Why Oxygen Must Accept the Electrons

What Happens if Oxygen is Absent

• Complex IV has nowhere to send electrons
• Cytochrome c backs up - cannot donate electrons to Complex IV
• Complex III backs up - cannot pass electrons to cytochrome c
• CoQ backs up - reduced CoQH₂ accumulates
• Complexes I and II stop - cannot pass electrons to CoQ
• NADH and FADH₂ accumulate - cannot be reoxidized
• Krebs cycle halts - needs NAD⁺ and FAD to keep running
• H⁺ pumping stops entirely - proton gradient collapses
• Complex V stops - no gradient = no ATP synthesis
• Cell switches to anaerobic glycolysis → lactic acid builds up

• Hypoxia (low O₂ delivery)
• Cyanide/CO poisoning (O₂ present but Complex IV is blocked - same result)
• Ischemia (no blood flow → no O₂)

The Partial Reduction Problem - ROS

Summary of Oxygen's Exact Role

Start Here - What NADH Actually Carries

Hydrogen atom  =  1 proton (H⁺)  +  1 electron (e⁻)

NAD⁺  +  2H  →  NADH  +  H⁺

What Happens When NADH Donates to Complex I

NADH  →  NAD⁺  +  H⁺  +  2e⁻

What Happens at Ubiquinone (CoQ) - Key Step

CoQ  +  2e⁻  (from Complex I/II)  +  2H⁺  (from matrix)  →  CoQH₂

CoQH₂  →  CoQ  +  2H⁺ (into intermembrane space)  +  2e⁻ (to cytochrome c)

The Q Cycle (Complex III) - Amplifies Proton Pumping

• One CoQH₂ donates electrons and releases 2 H⁺ into intermembrane space
• One CoQ picks up 2 H⁺ from the matrix to become CoQH₂ again
• Net result: 4 H⁺ pumped per 2 electrons passing through

Complex I and IV - Direct Pumping

• Complex I: electron flow through iron-sulfur clusters causes shape changes that pump 4 H⁺
• Complex IV: as electrons reduce O₂ to H₂O, the reaction also pumps 2 H⁺ and consumes 4 H⁺ from the matrix (vectorial chemistry)

Full Picture - Where Each H⁺ Comes From

Simple Analogy

• The oxygen part = the electron, goes into the electrical circuit (the chain)
• The hydrogen part = the proton, gets pumped across the membrane

One-Line Summary

Sites of Erythropoiesis (Changes with Age)

The Developmental Sequence

Pluripotent Stem Cell (HSC)
        ↓
  Common Myeloid Progenitor (CMP)
        ↓
  BFU-E  (Burst Forming Unit - Erythroid)
        ↓         ← EPO starts acting here
  CFU-E  (Colony Forming Unit - Erythroid)
        ↓         ← EPO most sensitive here
  Pronormoblast (Proerythroblast)
        ↓
  Basophilic Normoblast (Early)
        ↓
  Polychromatophilic Normoblast (Intermediate)
        ↓
  Orthochromatic Normoblast (Late) ← nucleus ejected here
        ↓
  Reticulocyte  ← released into blood
        ↓ (1-2 days in blood)
  Mature Erythrocyte (RBC)

Each Stage Explained

1. Pronormoblast (Proerythroblast)

• Largest cell in the series (~20 µm)
• Large nucleus, prominent nucleoli
• Deeply basophilic cytoplasm (lots of ribosomes, making hemoglobin machinery)
• Hemoglobin synthesis begins here

2. Basophilic Normoblast

• Smaller than pronormoblast
• Intensely blue cytoplasm (heavy ribosome content)
• Nucleus condensing, nucleoli disappearing
• Active hemoglobin synthesis

3. Polychromatophilic Normoblast

• Cytoplasm is mixed blue and pink (basophilic ribosomes + eosinophilic hemoglobin)
• "Polychromatic" = multiple colors
• Nucleus is smaller and more condensed
• This is the last stage that can divide

4. Orthochromatic Normoblast (Late Normoblast)

• Cytoplasm is now predominantly pink - mostly hemoglobin
• Nucleus becomes pyknotic (dense, dark, small)
• Nucleus is then extruded (ejected) - this is unique to mammalian RBCs
• The cell is now anucleate

5. Reticulocyte

• Anucleate but still contains residual RNA and organelles (mitochondria, ribosomes)
• Appears slightly larger than mature RBC
• Stains with supravital stains (new methylene blue) showing a reticular network
• Spends 1-2 days in bone marrow, then released into blood
• Circulates for 1-2 days in peripheral blood, then loses remaining organelles
• Normal reticulocyte count: 0.5-2.5% of RBCs
• Elevated reticulocytes = sign of active erythropoiesis (good response to anemia)

6. Mature Erythrocyte

• Biconcave disc, 7-8 µm diameter
• No nucleus, no organelles
• Packed with hemoglobin (~33 g/dL intracellular)
• Lifespan: 120 days

Regulation of Erythropoiesis

Erythropoietin (EPO) - The Master Regulator

• Glycoprotein hormone, 90% produced by peritubular fibroblasts of the kidney (cortex), 10% by liver
• Triggered by: hypoxia → HIF-1α (Hypoxia Inducible Factor) → EPO gene transcription
• Acts on: CFU-E (most sensitive) and BFU-E
• Effects:
  • Promotes survival of erythroid progenitors (inhibits apoptosis)
  • Stimulates proliferation and differentiation
  • Speeds up maturation
  • Promotes early reticulocyte release from marrow

Feedback Loop

Hypoxia / Anemia
      ↓
Kidney senses low PaO₂
      ↓
HIF-1α activated → EPO released
      ↓
Bone marrow → increased RBC production
      ↓
O₂ delivery restored → EPO falls (negative feedback)

Other Regulators

Hemoglobin Synthesis (Happens During Erythropoiesis)

Heme Synthesis

1. Succinyl-CoA + Glycine → ALA (δ-aminolevulinic acid) - by ALA synthase (rate-limiting step, in mitochondria)
2. ALA → Porphobilinogen → Uroporphyrinogen → Coproporphyrinogen → Protoporphyrin IX (in cytoplasm)
3. Protoporphyrin IX + Fe²⁺ → Heme (in mitochondria, by ferrochelatase)

Globin Chains

• Alpha chains: encoded on chromosome 16
• Beta chains: encoded on chromosome 11
• Heme + Globin → Hemoglobin

Hemoglobin Types

Iron Metabolism in Erythropoiesis

• Dietary iron absorbed in duodenum as Fe²⁺ (reduced by vitamin C)
• Transported in blood bound to transferrin
• Taken up by erythroid precursors via transferrin receptor (TfR1)
• Stored as ferritin (intracellular) or hemosiderin (excess)
• Regulated by hepcidin (liver) - master iron regulator:
  • High iron → high hepcidin → blocks ferroportin → less iron absorption
  • Anemia/hypoxia → low hepcidin → more iron available

Key Clinical Correlations

One-Line Summary

Overview of Phases

DIASTOLE (0.5 sec)          SYSTOLE (0.3 sec)
├─ Isovolumetric relaxation  ├─ Isovolumetric contraction
├─ Rapid filling             ├─ Rapid ejection
├─ Slow filling (diastasis)  └─ Slow ejection
└─ Atrial systole

Phase-by-Phase Breakdown

PHASE 1 - Isovolumetric Relaxation

• Duration: ~0.08 sec
• Trigger: Aortic valve closes (marks end of systole)
• Ventricle begins to relax, pressure drops rapidly
• All valves are CLOSED - mitral, tricuspid, aortic, pulmonary
• Ventricular volume stays constant (no blood entering or leaving)
• Pressure falls from ~80 mmHg → ~8 mmHg (left ventricle)
• Ends when ventricular pressure drops below atrial pressure → mitral valve opens

PHASE 2 - Rapid Ventricular Filling

• Duration: ~0.11 sec
• Mitral and tricuspid valves open
• Blood rushes from atria → ventricles passively (pressure gradient)
• ~70-80% of ventricular filling occurs in this phase
• Ventricular pressure is low (suction effect from elastic recoil)
• Produces the 3rd heart sound (S3) - heard in rapid filling, normal in children/young adults; pathological in heart failure in adults

PHASE 3 - Slow Filling (Diastasis)

• Duration: ~0.19 sec
• Blood flow from atria to ventricles slows down
• Pressures in atria and ventricles nearly equalize
• Shortest and most variable phase - disappears at high heart rates

PHASE 4 - Atrial Systole (Atrial Kick)

• Duration: ~0.11 sec
• SA node fires → atria contract
• Pushes final 20-30% of blood into the ventricles
• Raises ventricular end-diastolic pressure slightly
• Produces the 4th heart sound (S4) when ventricle is stiff (hypertrophied) - always pathological
• Ends when atrial contraction is complete
• End-diastolic volume (EDV) = ~120-130 mL

PHASE 5 - Isovolumetric Contraction

• Duration: ~0.05 sec
• Ventricles receive electrical signal → begin to contract
• Ventricular pressure rises sharply
• All valves are CLOSED - AV valves close (mitral, tricuspid) but aortic/pulmonary haven't opened yet
• Volume stays constant (isovolumetric)
• Closure of AV valves produces 1st heart sound (S1) - "LUB"
• Pressure rises from ~8 mmHg → 80 mmHg (left ventricle)
• Ends when ventricular pressure exceeds aortic pressure (~80 mmHg) → aortic valve opens

PHASE 6 - Rapid Ejection

• Duration: ~0.09 sec
• Aortic and pulmonary valves open
• Blood is forcefully ejected into aorta and pulmonary artery
• ~70% of stroke volume ejected in this phase
• Aortic pressure rises to peak systolic pressure (~120 mmHg)
• Ventricular pressure = Aortic pressure (valves open, no gradient)

PHASE 7 - Slow (Reduced) Ejection

• Duration: ~0.13 sec
• Ventricular contraction weakens
• Ejection slows - remaining ~30% of stroke volume ejected
• Ventricular pressure starts to fall slightly below aortic pressure
• Blood flow momentarily reverses → aortic valve closes
• Closure of semilunar valves produces 2nd heart sound (S2) - "DUB"
• End-systolic volume (ESV) = ~50-60 mL

Key Volume and Pressure Values

Valve Events Summary

The Four Heart Sounds

• S3 gallop = "Ken-tuck-Y" cadence = volume overload
• S4 gallop = "Ten-nes-see" cadence = pressure overload

Wiggers Diagram - What to Know

1. Aortic pressure - rises during ejection, shows dicrotic notch at S2
2. Left ventricular pressure - rises steeply in isovolumetric contraction, falls in isovolumetric relaxation
3. Left atrial pressure - shows a, c, v waves
4. Ventricular volume - flat during isovolumetric phases, falls during ejection, rises during filling
5. ECG - P wave (atrial depol) → QRS (ventricular depol) → T wave (ventricular repol)
6. Heart sounds - S1 after QRS, S2 after T wave

Atrial Pressure Waves (a, c, v)

ECG Correlation

Effect of Heart Rate on the Cycle

• Diastole shortens more than systole - specifically the slow filling (diastasis) phase disappears first
• At very high heart rates, filling time is so short that EDV falls → stroke volume falls
• Compensated by increased contractility (sympathetic stimulation)
• This is why coronary artery disease worsens with exertion - most coronary filling happens in diastole

Clinical Correlations