Spirometry: Lung Volumes and Capacities

Spirometry

• Tidal volume (VT): Volume of air inhaled or exhaled during a normal breath, typically 500 ml.
• Total lung capacity (TLC): Maximum volume of air the lungs can hold, roughly 6 liters.
• Functional residual capacity (FRC): Air remaining in the lungs after a normal exhalation.
• Expiratory reserve volume (ERV): Extra air that can be exhaled forcefully after a normal breath.
• Residual volume (RV): Air remaining in the lungs after a maximum exhalation, cannot be expelled.
• Inspiratory reserve volume (IRV): Extra air that can be inhaled forcefully before a normal breath.
• Vital capacity (VC): Maximum volume of air that can be exhaled from a maximum inhalation, calculated as IRV + VT + ERV.
• Forced vital capacity (FVC): Measures the volume of air that can be exhaled forcefully and maximally.
• Forced expiratory volume in 1 second (FEV1): Proportion of the FVC exhaled in the first second, typically around 80% of the FVC.
• FEF25-75: Measure of airflow rate during the middle portion of the FVC.
• Reduced lung capacity is associated with conditions like bronchitis, emphysema, and asthma, leading to various symptoms. Symptoms can include hypoxemia (low blood oxygen), hypercapnia (high blood carbon dioxide), and increased pulmonary vascular resistance

Spirometry Lab Session Reminder

• Watch the provided PowerPoint presentation on spirometry.

Special Circulations

• Cardiac circulation: Blood flow to the heart can increase 4-5 fold during exercise. Metabolic products like adenosine act as local vasodilators. Nitric oxide (NO) from endothelial cells also supports vasodilation. Coronary blood flow is reduced during systole.
• Cerebral circulation: Good autoregulation of blood flow occurs at blood pressures ranging from 55-155 mm Hg. Carbon dioxide (CO2) and potassium (K+) trigger dilation of blood vessels.
• Small intestine circulation: Blood flow increases during digestion, proportional to metabolic need. Vasodilation occurs due to the release of adenosine and nitric oxide (NO). Sympathitic stimulation of vasoconstriction reduces blood flow during exercise.
• Hepatic circulation: Blood from both the arterial supply and portal vein mixes in the capillaries. Capillary endothelial cells are leaky, so liver vasoconstriction can cause significant blood volume shifting.

Lung Ventilation

• Respiration: The exchange of oxygen and carbon dioxide between the atmosphere and tissues in the lungs.

• Pulmonary capillaries enable gas exchange between the blood and the atmosphere.

• Systemic capillaries enable gas exchange between the blood and tissues.

• Cellular respiration uses oxygen to produce carbon dioxide during metabolic chemical reactions.

• Respiratory system structure: Large, highly branching airway trees and vascular trees. Conducting zone has branching bronchi and bronchioles with cartilage for support.

• Respiratory zone has respiratory bronchioles and alveolar ducts, closely associated with pulmonary capillaries for efficient gas exchange.

• Gas pressures: Atmospheric pressure is about 760 mmHg.

• Alveolar ventilation (VA): Volume of fresh air reaching the functional alveoli per minute.

• Dead space volume (VD): Volume of air in the conducting zone that does not participate in gas exchange.

Alveolar Ventilation

• Breathing involves pressure changes in the thoracic cavity.
• The diaphragm and rib muscles expand the thoracic cavity.
• Forced inspiration or exhalation increases the pressure change in the thoracic cavity.

Pressure changes during the breathing cycle

• Inspiration: Respiratory muscles relax, alveolar pressure is zero, transpulmonary pressure = 5, diaphragm contracts, and pleural pressure decreases to about -8 cm H2O. Alveolar pressure becomes negative, air flows into the lungs. At end of inspiration, alveolar pressure equals atmospheric pressure.
• Expiration: Diaphragm relaxes, pleural pressure increases (less negative). Alveolar volume decreases and pressure increases above atmospheric pressure. Air flows out of lungs until alveolar pressure equals atmospheric pressure.

Gas Exchange in Systemic Capillaries

• Interstitial fluid partial pressure of oxygen (PO2): Averages about 40 mm Hg.

• Partial pressure of oxygen in blood arriving at systemic capillaries (PaO2) averages about 95 mmHg.

• Tissue cells need a PO2 of 1–3 mm Hg (probably >20 mmHg).

• Partial pressure of oxygen in blood leaving systemic capillaries (PcvO2) averages about 40 mmHg.

• Tissue partial pressure of carbon dioxide (PCO2) is about 45 mm Hg.

• Arterial blood arriving at systemic capillaries (Pco2): about 40 mm Hg

• Venous blood leaving systemic capillaries (Pco2): about 45 mm Hg.

• Diffusing capacities are higher during exercise, due to increased capillary dilation and better ventilation-perfusion ratio.

Explore lung volumes and capacities with spirometry. This includes tidal volume, total lung capacity, and functional residual capacity. Also learn about expiratory reserve volume, residual volume, inspiratory reserve volume, and vital capacity.