BIOL 2420 Lecture Notes - Lecture 4: Functional Residual Capacity, External Intercostal Muscles, Lung Volumes
Unit 5 – Lecture 4
Lung Capacities
- Capacity: the sum of two or more lung volumes
o Vital capacity (VC): sum of the inspiratory reserve volume, expiratory reserve volume,
and tidal volume
▪ Represents the maximum amount of air that can be voluntarily moved into or
out of the respiratory system with one breath
▪ Decreases with age as muscles weaken and the lungs become less elastic
▪ To measure VC
• Asked to take in as much air as possible, then blow out as fast as
possible
• Forced VC test allows the clinician to measure how fast air leaves the
airways in the first second of expiration
o FEV1 (forced expiratory volume in 1 second)
▪ decreases in certain lung diseases
• asthma
▪ decreases with age
▪ VC plus the residual volume = total lung capacity (TLC)
o Inspiratory capacity = tidal volume + inspiratory reserve volume
o Functional residual capacity = expiratory reserve volume + residual volume
During Ventilation, Air Flows Because of Pressure Gradients
- Breathing requires muscle contraction and is an active process
o Air flows into the lungs because of pressure gradients created by a pump
▪ muscles of the thoracic cage and diaphragm function as the pump because most
lung tissue is thin exchange epithelium
▪ when muscles contract, the lungs expand, held to the inside of the chest wall by
the pleural fluid
- primary muscles involved in quiet breathing (breathing at rest) are
o diaphragm
o external intercostals
o scalenes
- during forced breathing, other muscles of the chest and abdomen may be recruited to assist
o examples: exercise, playing a wind instrument, blowing up a balloon
- air flow in the respiratory tract obeys the same rule as blood flow
Flow ∆P/R
1. air flows in response to a pressure gradient ∆P
2. flow decreases as the resistance ® of the system to flow increases
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