NPB 101: Lecture 28 - Respiratory System 2: Compliance & Exchange

Learning Objectives: Distinguish airway resistance from lung compliance and explain how asthma and pulmonary fibrosis affect breathing. Explain how surface tension and alveolar radius influence alveolar stability, including the role of surfactant. Identify the major lun volumes and capacities and calculate the minute and alveolar ventilation from tidal volume, respiratory rate, and dead space. Predict how changes in breathing depth, breathing frequency, and dead space affect effective alveolar ventilation. Relate gas composition and atmospheric pressure to partial pressure, and use partial-pressure gradients to explain oxygen and carbon dioxide exchange. Predict how changes in alveolar ventilation, the diffusion barrier, and pulmonary capillary transit time affect gas exchange.

Asthma

A common long-term (chronic) inflammatory disease of the airways that causes them to swell, narrow, and produce extra mucus.

• Caused by intermittent episodes of airway smooth muscle excessive contraction

• Generally associated with inflammation in airways

  • Leads to hyper-responsive airway smooth muscle contraction

What therapies would we use to treat Asthma?

• Steroids

• Epinephrine

  • relaxes smooth muscle and increases blood flow

• Bronchodilators

  • reduces inflammation

  • e.g. inhalers

Lung Compliance

A measure of the lung’s ability to stretch, expand, and distend in response to pressure changes.

• Compliance = Δ Lung volume / Δ(P_alv - P_ip) = ΔV/ ΔP_tp

• Inverse of stiffness

  • More stiff → less compliant

• Depends on:

  • Connective tissues

  • Surface tension within alveoli

• Excessive compliance reduces elastic recoil

• Low compliance increases required work for breathing

What happens in pulmonary fibrosis?

• Decreased compliance

  • presence of scar tissue = more work required to breathe

Surfactant

A mixture of lipids and proteins secreted by Type II alveolar cells.

• Reduces cohesive forces between water molecules on the alveolar surface

• Lowers surface tension

  • Increases lung compliance

Why do premature infants often need to be intubated?

• Surfactant production occurs in late gestation so premature infant lungs have low compliance

• Intubation is needed to add pressure

Law of Laplace

Describes the physical relationship between the internal pressure of a hollow vessel, the radius of the vessel, and the tension or stress within its wall.

• P = 2T/r

  • P = pressure

  • T = surface tension

  • r = Radius

• Smaller alveoli have increased pressure compared to large alveoli

  • To prevent smaller alveoli from collapsing into larger alveoli (positive feedback loop)…

  • The smaller the alveoli, the greater the surfactant concentration

    • ↓ Surface Tension, ↑ Lung Compliance

Tidal volume

The amount of air that moves in or out of the lungs with each normal respiratory cycle.

Inspiratory reserve volume

The maximum extra volume of air that can be forcibly inhaled into the lungs after a normal, quiet tidal inhalation.

Expiratory reserve volume

The extra amount of air you can forcefully exhale past a normal, quiet breath out.

Residual volume

The amount of air that stays in the lungs even after you breathe out as hard as you can.

• Lungs never fully collapse

Vital capacity

The maximum volume of air a person can exhale from the lungs after taking a maximal inhalation.

Minute ventilation (Ve)

The total volume of gas inhaled or exhaled from a person’s lungs in one minute.

• Ve = TV * f

  • Minute ventilation = Tidal volume x respiratory rate

• Typically mL/min

Dead space (Vd)

The volume of inspired air that does not participate in gas exchange with the blood. Only about ~350 mL of air participate in gas exchange.

• Conducting zone = anatomical dead space (Vd)

  • ~150 mL

• V_A = (V_t - V_D) * f

If you had a long enough snorkel, could you go 10 meters under water?

• No; the snorkel serves as an extension of the trachea. You are only increasing dead space; fresh air won’t get to where exchange needs to occur.

Partial Pressure

The independent pressure exerted by a particular gas contained within a mixture of gases

• Directly proportional to percentage of that gas in the total air mixture

Is there less percentage of oxygen molecules in the air on top of Mt. Everest?

• No; the percentage stays the same. Only pressure drops; partial pressure drops as well.

O₂ exchange

• O₂ diffuses from alveoli to blood

  • Change in pressure = 65 mmHg

• O₂ diffuses from blood to tissues

CO₂ exchange

• CO₂ diffuses from tissue to blood

• CO₂ diffuses from blood to alveoli

  • Pressure = 6 mmHg

O₂ and CO₂ exchange

In a healthy person at rest, O₂ is transferred to the blood early in the capillaries.

• What happens when fluid fills the lungs?

  • Pulmonary Edema

• Alveolar partial pressures are a function of alveolar ventilation rate

• Hyper/hypo ventilation is relative to metabolism

Which way does the curve shift during exercise when cardiac output increases?

• Right

How can you prepare to hold your breath underwater?

• Hyperventilate before