3.1 Cardiopulmonary Anatomy & Physiology

The cardiovascular and pulmonary systems work together to deliver oxygen to tissues and remove carbon dioxide. Physical therapists must understand these systems to safely prescribe exercise and manage patients with cardiopulmonary conditions.

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Cardiac Anatomy and Physiology

Heart Chambers, Valves, and Blood Flow

Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation):

• Isovolumetric contraction: All valves closed, pressure builds in ventricles
• Ventricular ejection: Semilunar valves open, blood ejected into aorta/pulmonary artery
• Isovolumetric relaxation: All valves closed, pressure drops in ventricles
• Ventricular filling: AV valves open, blood fills ventricles passively and with atrial contraction (atrial kick)

Cardiac Output and Hemodynamics

Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR)

• Normal resting CO: ~5 L/min
• During maximal exercise: CO can increase to 20-25 L/min in healthy adults

Blood Pressure = Cardiac Output x Total Peripheral Resistance

• Systolic BP: Pressure during ventricular contraction (normal <120 mmHg)
• Diastolic BP: Pressure during ventricular relaxation (normal <80 mmHg)
• Mean Arterial Pressure (MAP) = DBP + 1/3(SBP - DBP); normal ~93 mmHg; MAP <60 mmHg indicates inadequate organ perfusion

Coronary Arteries

Critical Point: Coronary perfusion occurs primarily during diastole. An increased heart rate shortens diastole, reducing coronary blood flow — this is why tachycardia can trigger angina in patients with coronary artery disease.

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Pulmonary Anatomy and Physiology

Mechanics of Breathing

Lung Volumes and Capacities

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Arterial Blood Gas (ABG) Interpretation

ABG Interpretation Steps:

1. Check pH: acidosis (<7.35) or alkalosis (>7.45)?
2. Check PaCO2: respiratory component
3. Check HCO3: metabolic component
4. Determine primary cause and whether compensation is present

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Oxygen Transport and the Oxyhemoglobin Dissociation Curve

Oxygen is carried in two forms: roughly 98.5% bound to hemoglobin and 1.5% dissolved in plasma (the dissolved fraction is what PaO2 measures). Each gram of fully saturated hemoglobin carries about 1.34 mL of oxygen. The oxyhemoglobin dissociation curve plots SaO2 against PaO2 and is S-shaped.

A right shift (lower hemoglobin affinity, oxygen unloaded to tissues) occurs with increased temperature, increased PaCO2, increased 2,3-DPG, and decreased pH (acidosis) — the Bohr effect. A left shift (higher affinity, oxygen held by hemoglobin) occurs with the opposite: hypothermia, hypocapnia, decreased 2,3-DPG, and alkalosis. Exercising muscle is warm, acidic, and high in CO2, so the curve shifts right precisely where unloading is needed.

Frank-Starling Mechanism and Ejection Fraction

The Frank-Starling law states that increased ventricular filling (preload) stretches myocardial fibers and increases the force of contraction, raising stroke volume up to a physiologic limit. Preload is end-diastolic stretch; afterload is the resistance the ventricle pumps against (aortic pressure/systemic vascular resistance); contractility is the intrinsic strength of contraction independent of preload/afterload.

Ejection fraction (EF) = stroke volume / end-diastolic volume; normal is 55-70%. An EF below 40% indicates systolic heart failure (HFrEF), a high-yield concept for safe exercise progression. Stroke volume itself is determined by preload, afterload, and contractility — increasing preload or contractility raises SV, while increasing afterload lowers it.

Ventilation-Perfusion Relationships and Compensation

Efficient gas exchange requires matched ventilation (V) and perfusion (Q). In an upright lung, the bases receive more of both ventilation and blood flow than the apices, but perfusion increases more steeply, so the V/Q ratio is highest at the apex and lowest at the base. A pulmonary embolism creates dead space (ventilation without perfusion); a mucus plug or pneumonia creates shunt (perfusion without ventilation).

When interpreting ABGs, after identifying the primary disorder, check for compensation: the lungs compensate for metabolic disorders within minutes to hours (changing PaCO2), while the kidneys compensate for respiratory disorders over hours to days (changing HCO3). If the pH has returned to normal, the disorder is fully compensated; if pH is still abnormal but the opposing system has moved, it is partially compensated. A patient with chronic COPD typically shows a chronically elevated HCO3 (renal compensation) keeping pH near normal despite a high PaCO2 — a key trap when a "normal" pH masks severe underlying disease.