NPB 101: Respiratory System 2: Lec 28- Compliance and Exchange

Asthma

Episodes of airway smooth muscle excessive contraction

• Resistance increases, producing “wheezing” sound

Generally associated w/ inflammation in airways

What therapies would we use to treat Asthma?

• Steroids → Reduce inflammatory response

• Bronchodilators → Medication that relaxes smooth muscles around airway (bronchi)

• Epinephrine → Relaxes bronchial smooth muscle (bronchodilation via β₂ receptors)

Lung Compliance

Opposite of stiffness

• Less stiff → more compliant → easy to stretch out

• More stiff → less compliant

Lung Compliance Equation

Compliance = Δ Lung Volume / Δ (P_alv - P_ip) = Δ V / ΔP_tp

• Compliance = Change in lung volume / change in transpulmonary pressure

Lung Compliance depends on:

• Connective tissues → Holding capillaries to alveoli, has to stretch out

• Surface tension within alveoli

Excessive Compliance

Reduces elastic recoil → Inflates lungs too much, can cause rupture

Low Compliance

Increases required work for breathing → Big changes in transpulmonary pressure in order to move same amount of air

What happens in pulmonary fibrosis?

Pulmonary fibrosis → Build up of connective tissues , more glue = harder to inflate

• Decreased compliance → Lungs become stiffer = Harder to inflate them (work harder to breathe)

Surfactant

Type II alveolar cells secrete detergent like substance surfactant

• Reduces cohesive forces bw H₂O molecules on the alveolar surface

Surfactant effect on Surface Tension

Surfactant thus lowers surface tension → Increases lung compliance

• Interrupt interaction bw cells / reduce surface tension → increase compliance → easier to expand lung

Pathway of Pulmonary Surfactant

Lowers surface tension of H₂O layer at alveolar surface → increases lung compliance → easier for lungs to expand

Law of Laplace

P = 2T / r

• P = Pressure

• T = Surface tension

• r = Radius

Smaller alveoli will collapse into larger alveoli

Surfactant / Law of Laplace

Surfactant dynamically lowers surface tension in small alveoli

• Prevents small alveoli from collapsing , no uncontrolled expansion → helps stabilize smaller alveoli

Important facts ab Pulmonary Surfactant

• Mixture of phospholipids / protein

• Secreted by type II alveolar cells

• Lowers surface tension of H₂O → increases lung compliance → easier for lungs to expand

• Effect is greater in smaller alveoli → stabilizes the alveoli

Production of Surfactant in Fetal Lung

Production occurs in late gestation , stimulated by the increase in cortisol secretion that occurs then

Why do premature infants often need to be intubated?

Born before late gestation → surfactant not produced

• Surfactant absence → low compliance → more stiffness = need more pressure in order to inflate

• Diaphragm muscles not strong enough to combat low compliance → need intubation until they can start making low surfactant

Lung Volumes and Capacity

• Tidal Volume → Amount of volume that goes in/out each breathe

• Inspiratory reserve volume → Maximum possible inspiration (air in)

• Expiratory reserve volume → Maximum voluntary expiration (air out)

• Vital Capacity → How much air you can move in/out in total (can adapt based on contraction of skeletal muscles)

Minute Ventilation (Ve)

• Ve = TV * f → Tidal volume x respiratory rate

• f = (Respiratory rate)

• Typically mL/min

Is it more effective to breathe twice as fast or twice as deep?

Twice as deep → End goal : Deliver more O₂

Dead Space (Vd)

• Volume of inspired air that isn’t involved in gas exchange

• Alveolar dead space

• Some air gets stuck in conducting zone (mouth/nose, trachea, bronchi)

  • Air that doesn’t participate in exchange, just stays in conducting zone and leaks during exhalation

Dead Space Formula

Alveolar ventilation = ( Tidal volume - Dead Space) x Respiratory rate

• V_A (mL/ min) = (V_t (mL/breathe) - V_D (mL/breathe) ) x f

  • Substracts the anatomical dead space (airways like trachea/bronchi where no gas exchange occurs.

How is alveolar ventilation different than minute ventilation?

Minute Ventilation

• Total volume of air inhaled or exhaled in one minute. Measures all air moving in/out of respiratory system, including anatomical dead space (Vd)

Alveolar Ventilation

• Volume of air that actually reaches alveoli per minute to participate in gas exchange. Subtracts anatomical dead space (where no gas exchange occurs)

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

No!

• The snorkel becomes dead space, conducting zone extends.

  • Too long = more dead space → no air reaching alveoli to participate in gas exchange

• If you dilate exchange part, conducting zone increases and decreases ability to exchange

Partial Pressure

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

• Total atm. pressure = 760 mmHg

  • Partial pressure of N₂ = 600 mmHg (79% N₂)

  • Partial pressure of O₂ = 160 mmHg (21% O₂ )

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

Pressure drops, air pressure is lower → % of O₂ molecules ab the same → partial pressure decreases

O₂ and CO₂ Exchange

• O₂ diffuses from alveoli to blood

  • Change in pressure = 65 mmHg

• O₂ diffuses from blood to tissues

• CO₂ diffuses from tissue to blood

• CO₂ diffuses from blood to alveoli

  • Pressure = 6 mmHg

Pulmonary Edema

Fluid fills the lungs

• Limit ability of O2 to diffuse across → not able to get as much air into the blood

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

Right → Oxygenated later on, flowing through a lot faster than it can actually use

Hypoventilation

Underbreathing, Breathe too slowly or too shallowly → Taking in less O₂ than you are using

• Retains CO₂ → drop in blood pH → often low blood oxygen

Hyperventilation

Overbreathing, Taking in more O₂ than body’s metabolic demand

• Alveolar partial pressure increases, gets rid of more CO₂ → low blood CO₂ and rise in blood pH