Respiratory Physiology and Physics Flashcards

Anatomy and Overview of the Respiratory System

• Scope of the Lecture: While the content touches on respiratory anatomy and general function, the primary focus is on pulmonary physiology, specifically the physics of airflow, pressure, volume, and the principles regulating respiratory rhythms.

• Anatomical Components Review:

  • Larynx: The voice box located above the trachea.

  • Trachea: The windpipe leading from the larynx into the lungs.

  • Primary Bronchi: The first division of the trachea entering each lung.

  • Bronchioles: Minute branches into which a bronchus divides, eventually leading to the alveoli.

  • Alveoli: Tiny air sacs at the end of the bronchioles where gas exchange occurs. These structures are microscopic; however, their size is significant relative to the bronchioles that supply them.

• General Mechanics of Inhalation (Inspiration):

  • Occurs when the diaphragm and intercostal muscles contract.

  • This contraction forces the diaphragm downward into the abdominal cavity.

  • The result is an increase in the total volume of the thoracic cage.

Mechanics of Breathing: Pressure and Volume Relationships

• Inspiration Dynamics:

  • As the volume of the thoracic cavity increases, the internal air pressure within the cavity decreases.

  • This pressure drop creates a gradient that allows air to flow into the lungs.

• Expiration Dynamics:

  • Occurs when the diaphragm and the intercostal muscles relax.

  • The diaphragm moves upward and returns to its natural dome shape.

  • The rib cage moves downward and inward.

  • The volume of both the thoracic cavity and the lungs decreases.

  • This reduction in volume causes the air pressure within the lungs to increase, forcing air to flow out of the lungs.

• Pressure Locations in the Lung:

  • Intrapulmonary Pressure: The pressure located inside the lungs.

  • Intrapleural Pressure: The pressure existing between the pleura (the membranes surrounding the lungs).

  • Transpulmonary Pressure: The pressure measured across the wall of the lungs.

Physical Laws Governing Lung Function: Boyle's Law

• Definition of Boyle's Law: This physical law states that for a fixed amount of gas at a constant temperature, the pressure and volume are inversely proportional.

• Mathematical Formula: $P_1 \times V_1 = P_2 \times V_2$

• Application to the Lungs:

  • When lung volume increases ($V \uparrow$), the internal pressure decreases ($P \downarrow$).

  • When lung volume decreases ($V \downarrow$), the internal pressure increases ($P \uparrow$).

• Balloon Analogy: The pressure exerted on the walls of a balloon depends on its volume; a large, well-inflated balloon exhibits different pressure characteristics compared to a smaller one. Alveoli function similarly to these small, balloon-like structures.

Key Factors Influencing Pulmonary Physiology

There are three major factors that dictate how the lungs function in respect to the inhalation and exhalation of air:

1. Compliance

• Definition: The ease with which the lungs can be inflated. It describes the pulmonary volume change per unit of pressure change ($\text{transpulmonary pressure}$).

• Analogy: Being compliant at work means doing what is asked with minimal effort; compliant lung tissue inflates easily when air is taken in.

2. Elasticity

• Definition: The ability of the lung's elastic tissues to recoil during expiration.

• Resting State: Lungs must return to their resting state easily to ensure sufficient gas exchange.

• Rubber Band Analogy: Just like a stretched rubber band returns to its original shape, the lungs must return to their original shape after being stretched.

• Degradation: Elasticity can be lost due to aging or the inhalation of various toxins.

3. Surface Tension

• Definition: A force created when gases in the air interact with liquids.

• Mechanism: In the lungs, surface tension is created by the interaction between inspired air and the fluids surrounding the lung tissue and alveoli.

• Belly Flop Analogy: A belly flop into a swimming pool hurts because the air trapped between the body and the water prevents the surface tension from breaking, causing a high-impact smack. Conversely, diving head-first or breaking the water with arms first breaks the surface tension and eliminates that impact.

Intrathoracic Pressure and Elastic Recoil

• Intrathoracic Pressure: The combination of positive and negative pressure occurring within the thoracic cavity.

  • Inspiration: Characterized by a negative internal pressure.

  • Expiration: Characterized by a positive internal pressure.

• Effort vs. Passive Process:

  • Breathing in (inhalation) requires effort because the tiny alveoli must be inflated.

  • Breathing out (exhalation) is normally a passive process aided by the elastic recoil of the alveoli.

  • Analogy: It takes effort to blow up a balloon, but the balloon deflates on its own once released.

• Clinical Correlation (Emphysema):

  • In emphysema, the elastic recoil of the alveoli is compromised.

  • Patients find it difficult to exhale and must do so forcibly.

  • Symptom: Patients may exhibit coughing or an "expiratory wheeze" during exhalation.

Alveolar Histology and Surface Tension Mechanics

• Surface Tension Sources: Normal mucus produced by goblet cells and the tissue surrounding the alveoli creates surface tension for each individual alveolus.

• Cohesiveness of Water: Surface tension is driven by the cohesive properties between liquid molecules. Examples include:

  • Water beading on a windshield.

  • Water moving from the roots to the tips of plant leaves.

  • Insects like spiders walking across water.

• Measurement Unit: Surface tension is measured in dynes per centimeter ($dyn/cm$).

  • Definition of a Dyne: A unit of force required to accelerate a mass of $1\,g$ at a rate of $1\,cm/s^2$.

• Types of Pneumocytes:

  • Type 1 Pneumocytes: Consist of simple squamous epithelium. These cells are thin, making them ideal for filtration and diffusion. They form the air-blood barrier where gases diffuse easily.

  • Type 2 Pneumocytes: Consist of simple cuboidal cells. These cells secrete surfactant, a sticky substance that reduces surface tension by breaking the cohesive properties of water.

• Alveolar Radius ($R$):

  • Not all alveoli are uniform in size.

  • During inhalation, the radius typically increases from an average of $0.05\,mm$ to an average of $0.1\,mm$ in diameter.

The Law of Laplace

• Function: The Law of Laplace dictates the pressure required to keep an alveolus open based on its radius and surface tension.

• Formula: $P = \frac{2 \times T}{R}$

  • $P$: Pressure required to keep the alveolus open.

  • $T$: Surface tension.

  • $R$: Alveolar radius (total size).

• Proportional Relationships:

  • Directly Proportional: Alveolar pressure is directly proportional to surface tension ($P \propto T$). If surface tension increases, pressure increases; if surface tension decreases, pressure decreases.

  • Inversely Proportional: Alveolar pressure is inversely proportional to the radius ($P \propto \frac{1}{R}$). If the radius increases, pressure decreases. If the radius decreases, pressure increases.

• Inverse Relationship Summary: As the alveolar radius ($R$) increases, alveolar pressure ($P$) decreases, and subsequently, surface tension ($T$) decreases.

Surfactant: Composition and Role

• Chemical Composition: Surfactant is a lipoprotein complex. Its primary ingredient is dipomatoyl phosphatidylcholine ($DPPC$).

• Mechanism: Surfactant reduces the attractiveness of hydrogen bonds in water, thereby lowering surface tension.

• Impact on Pressure: Surfactant reduces surface tension in the lungs by a factor of approximately 15. This allows the lungs to inflate with as little as $1\,mmHg$ of pressure difference.

• Compliance: By lowering surface tension, surfactant increases the lung's compliance, making expansion easier.

• Prevention of Collapse: Surfactant lowers surface tension more effectively as the alveolar radius decreases. This prevents smaller alveoli from collapsing and forcing their air into larger nearby alveoli, which would happen if pressure were too high in small sacs.

• Wooden Spoon Analogy: Placing a wooden spoon over a boiling pot prevents it from boiling over because of surfactants naturally found in wood. If the spoon is washed in a dishwasher, the detergents remove these surfactant oils, and it loses this property.

Neonatal Respiration and Clinical Applications

• The First Breath: An infant's first breath is the most difficult because all alveoli are small and have never been fully inflated (collapsed state).

• Overcoming Resistance:

  • Smaller balloons have a greater force of elastic recoil than larger, prestretched ones.

  • Per Laplace's Law, if a balloon has half the radius of another, it requires twice as much pressure to initiate stretching.

  • In a fetus, alveoli are coated with fluid similar to water, having much higher surface tension than normal surfactant.

• Stimulating the First Breath: Doctors may apply a fingernail to the plantar (bottom) region of the infant's foot. This irritation triggers the phrenic nerve, causing the diaphragm to flatten and the thoracic volume to increase, which drops pressure ($\text{Boyle's Law}$) and allows air to enter.

• Infant Respiratory Distress Syndrome (IRDS):

  • Treated by spraying natural or synthetic surfactant into the respiratory passages.

  • Usually, only one treatment is given to encourage the infant's lungs to begin producing their own surfactant.

• Discussion: Amelia's Case Study (Evolution of Respiratory Support):

  • Amelia's Journey: Amelia required surfactant and was intubated for her first 44 days. She progressed through several devices: oscillator, mechanical ventilator, and finally non-invasive NAVA.

  • Non-Invasive NAVA: Uses an electrode placed along the interior of an NG (nasogastric) feeding tube.

  • Function: The electrode reads signals from the phrenic nerve sent by the brain to the diaphragm.

  • Benefit: If the brain sends a signal for the diaphragm to flatten, the machine allows the child to breathe independently. If the brain fails to send a signal (due to medullary prematurity/failure), the machine breathes for the child.

  • Outcome: Children using this device can achieve independent breathing maturation up to twice as fast as those without it.