Overview of Respiratory Function

The major function of lungs and pulmonary circulation as the pulmonary system is to deliver oxygen (O₂) to cells and remove carbon dioxide (CO₂) from the cells (gas exchange). The adequacy of oxygenation and ventilation is measured by partial pressure of arterial oxygen (PaO₂) and partial pressure of arterial carbon dioxide (PaCO₂). The pulmonary system also functions as a blood reservoir for the left ventricle when it is needed to boost cardiac output; as a protector for the systemic circulation by filtering debris/particles; as a fluid regulator so water can be kept away from alveoli; and as a provider of metabolic functions such as surfactant production and endocrine functions.

Terminology in Respiratory

• Alveolus—air sac where gas exchange takes place
• Apex—top portion of the upper lobes of lungs
• Base—bottom portion of lower lobes of lungs, located just above the diaphragm
• Bronchoconstriction—constriction of smooth muscle surrounding bronchioles
• Bronchus—large airways; lung divides into right and left bronchi
• Carina—location of division of the right and left main stem bronchi
• Cilia—hairlike projections on the tracheobronchial epithelium, which aid in the movement of secretions and removal of debris
• Compliance—ability of the lungs to distend and change in volume relative to an applied change in pressure (eg, emphysema—lungs very compliant; fibrosis—lungs noncompliant or stiff)
• Dead space—ventilation that does not participate in gas exchange; also known as wasted ventilation when there is adequate ventilation but no perfusion, as in pulmonary embolus or pulmonary vascular bed occlusion. Normal dead space is 150 mL.
• Diaphragm—primary muscle used for respiration; located just below the lung bases, it separates the chest and abdominal cavities
• Diffusion (of gas)—movement of gas from area of higher to lower concentration
• Dyspnea—subjective sensation of breathlessness associated with discomfort, often caused by a dissociation between motor command and mechanical response of the respiratory system as in:
  • Respiratory muscle abnormalities (hyperinflation and airflow limitation from chronic obstructive pulmonary disease [COPD]).
  • Abnormal ventilatory impedance (narrowing airways and respiratory impedance from COPD or asthma).
  • Abnormal breathing patterns (severe exercise, pulmonary congestion or edema, recurrent pulmonary emboli).
  • Arterial blood gas (ABG) abnormalities (hypoxemia, hypercarbia).
• Hemoptysis—coughing up of blood
• Hypoxemia—PaO₂ less than normal, which may or may not cause symptoms (Normal PaO₂ is 80 to 100 mm Hg on room air.)
• Hypoxia—insufficient oxygenation at the cellular level due to an imbalance in oxygen delivery and oxygen consumption (Usually causes symptoms reflecting decreased oxygen reaching the brain and heart.)
• Mediastinum—compartment between lungs containing lymph and vascular tissue that separates left from right lung
• Orthopnea—shortness of breath when in reclining position
• Paroxysmal nocturnal dyspnea—sudden shortness of breath associated with sleeping in recumbent position
• Perfusion—blood flow, carrying oxygen and CO₂ that passes by alveoli
• Pleura—serous membrane enclosing the lung; comprised of visceral pleura, covering all lung surfaces, and parietal pleura, covering chest wall and mediastinal structures, between which exists a potential space
• Pulmonary circulation—network of vessels that supply oxygenated blood to and remove CO₂-laden blood from the lungs
• Respiration—inhalation and exhalation; at the cellular level, a process involving uptake of oxygen and removal of CO₂ and other products of oxidation
• Shunt—adequate perfusion without ventilation, with deoxygenated blood conducted into the systemic circulation, as in pulmonary edema, atelectasis, pneumonia, COPD
• Surfactant—fluid secreted by alveolar cells that reduces surface tension of pulmonary fluids and aids in elasticity of pulmonary tissue
• Ventilation—movement of air (gases) in and out of the lungs
• Ventilation-perfusion (V/Q) imbalance or mismatch—imbalance of ventilation and perfusion; a cause for hypoxemia. V/Q mismatch can be due to:
  • Blood perfusing an area of the lung where ventilation is reduced or absent.
  • Ventilation of parts of lung that are not perfused.