Respiratory

RESPIRATORY PHYSIOLOGY STUDY NOTES
OUTLINE

  • Mechanics of Breathing

  • Assessing Ventilation

  • Gas Exchange

  • Transport of Respiratory Gases by Blood

  • Control of Respiration

MECHANICS OF BREATHING

1. Pressure Relationships in Thoracic Cavity

  • The mechanics of breathing are influenced greatly by pressure relationships within the thoracic cavity, which is an enclosed space that contains the lungs. The thoracic cavity's pressure dynamics are crucial for efficient ventilation by creating pressure gradients that facilitate airflow during inhalation and exhalation.

2. Pulmonary Ventilation (Breathing)

  • Pulmonary Ventilation refers to the movement of air into and out of the lungs through the process of inhalation and exhalation, which is essential for maintaining adequate gas exchange. It typically involves two major phases: inspiration (inhalation) where air is drawn into the lungs, and expiration (exhalation) where air is expelled. The integration of lung volumes and capacities effectively supports this process, enabling the body to meet continual oxygen demands.

3. Physical Factors Influencing Pulmonary Ventilation

  • The mechanics of breathing are influenced by several physical factors:

    • Airway Resistance: Resistance in the airways, which can be affected by diameter and the presence of obstructions (e.g., mucus, foreign bodies), dictates airflow efficiency.

    • Alveolar Surface Tension: The innate surface tension caused by liquid molecules in the alveoli can lead to their collapse if not counteracted by surfactants, which reduce this tension and enhance lung compliance.

    • Lung Compliance: This term refers to the ability of the lung tissue to stretch and expand in response to pressure changes. Factors such as age, disease (e.g., pulmonary fibrosis), and surface tension directly influence compliance levels.

PRESSURE RELATIONSHIPS

Atmospheric Pressure (Patm)

  • Defined as the pressure exerted by air and gases surrounding the body; its standard value at sea level is 760 mm Hg or 1 atm. This serves as a baseline reference for evaluating other respiratory pressures. Atmospheric pressure plays a vital role in determining the direction of airflow into or out of the lungs based on relative pressure differences.

Intrapulmonary Pressure (Ppul)

  • Refers to the pressure within the alveoli, fluctuating during the phases of breathing:

    • During Expiration: As the volume in the thoracic cavity decreases, Ppul rises above atmospheric pressure, creating a positive pressure that drives air out of the lungs (Ppul > Patm).

    • During Inspiration: The increase in thoracic volume leads to a decrease in Ppul below atmospheric pressure, creating a vacuum effect that allows air to flow into the lungs (Ppul < Patm).

Intrapleural Pressure (Pip)

  • The pressure within the pleural cavity, which fluctuates with respiratory movements but is typically maintained 4 mmHg less than Ppul. The negative pressure in the pleural cavity is essential for preventing lung collapse, as it creates a suction that keeps the lungs expanded. A balance must be maintained; if Pip equals Patm, it may result in a pneumothorax (lung collapse).

Causes of Negative Intrapleural Pressure

  • The negative intrapleural pressure is a result of:

    • Inward Pull of Lungs: This is due to the elastic recoil of lung tissue and the high surface tension of the alveolar fluid that promotes collapse.

    • Outward Pull of Chest Wall: The chest wall inherently wants to expand outward due to its structure.

    • Pleural Fluid: This fluid creates a cohesive bond between the parietal and visceral pleura, essential for maintaining the pressure differential required for lung inflation.

Transpulmonary Pressure

  • Defined as the difference between intrapulmonary and intrapleural pressures:

    • Pextpul−Pextip=4extmmHgP ext{pul} - P ext{ip} = 4 ext{ mmHg}

    • Transpulmonary pressure is a critical factor that keeps airways open and helps prevent alveolar collapse. A greater transpulmonary pressure indicates better lung expansion and compliance.

PULMONARY VENTILATION

Boyle’s Law

  • This law describes the fundamental relationship between pressure and volume of a gas at constant temperature:

    • PimesV=kP imes V = k (where k is a constant).

    • According to Boyle's Law, an increase in the lung volume during inspiration causes the intrapulmonary pressure to decrease, promoting airflow into the lungs, while a decrease in lung volume during expiration leads to an increase in pressure, driving air out of the lungs.

Actions of Muscles During Inspiration and Expiration

  • Diaphragm: The principal muscle of respiration; contraction causes it to move downward, effectively increasing thoracic cavity height and volume, thereby reducing intrapulmonary pressure.

  • External Intercostal Muscles: These muscles assist in elevating the ribs, further increasing the volume of the thoracic cavity during deeper inspirations.

  • Internal Intercostal Muscles: Often engaged during forced expiration, these muscles pull the ribs downward and inward; they work against the diaphragm's relaxation.

  • Normal Expiration: A predominantly passive process that relies on the elastic recoil of lung and thoracic structures, reducing the thoracic volume and consequently increasing intrapulmonary pressure leading to airflow out of the lungs.

Pressure Changes During Inspiration

  • Following diaphragm contraction, lung volume increases, leading to an intrapulmonary pressure drop by about 1 mm Hg below atmospheric pressure. Air flows into the lungs until equilibrium with atmospheric pressure is reached.

  • The intrapleural pressure drops to approximately -6 mm Hg, facilitating lung expansion.

Pressure Changes During Expiration

  • The passive recoil from lung tissue causes a reduction in thoracic volume, which in turn increases intrapulmonary pressure above atmospheric levels, leading to air expulsion from the lungs until pressures equalize.

FACTORS INFLUENCING PULMONARY VENTILATION

1. Airway Resistance

  • Total airflow through the respiratory tract is influenced by the magnitude of pressure differences divided by resistance in the airways:

    • F=rac(P2−P1)RF = rac{(P_2 - P_1)}{R}.

    • Larger airway diameters reduce resistance and enhance airflow, while conditions like asthma result in bronchoconstriction, significantly affecting airflow and resistance levels.

2. Alveolar Surface Tension

  • Surface Tension: At the gas-liquid interface, liquid molecules are more strongly drawn to each other than to gas molecules, which creates a tendency for alveoli to collapse. Surfactant produced by type II alveolar cells counteracts this effect, facilitating stability during the respiratory cycle and preventing collapse, especially at low lung volumes.

3. Lung Compliance

  • Lung Compliance: This refers to the ease with which the lungs expand under pressure. Conditions such as pulmonary fibrosis significantly reduce compliance, making it difficult for lungs to expand, consequently limiting lung volume and efficiency. Higher compliance values are ideal as they indicate that less work is required to ventilate the lungs effectively.

ASSESSING VENTILATION

Spirometry Measures

  • Spirometry is a critical tool used to quantify and evaluate lung function by measuring:

    • Respiratory Volumes: Measuring various lung volumes and their combinations provides insight into both the capacity and functionality of the lungs.

RESPIRATORY VOLUMES

  • Tidal Volume (TV): The volume of air exchanged during normal, quiet breathing, typically about 500 mL.

  • Inspiratory Reserve Volume (IRV): The extra air that can be inhaled forcefully after a normal tidal volume, averaging 2100-3200 mL.

  • Expiratory Reserve Volume (ERV): The additional air that can be forcibly exhaled after normal expiration, averaging 1000-1200 mL.

  • Residual Volume (RV): The volume of air remaining in the lungs after maximal expiration, roughly 1200 mL, essential for preventing alveolar collapse.

RESPIRATORY CAPACITIES

  • Inspiratory Capacity (IC): The total volume of air that can be inspired following normal expiration (TV + IRV, average of 3600 mL).

  • Functional Residual Capacity (FRC): The remaining volume in the lungs after normal expiration (RV + ERV, average of 2400 mL).

  • Vital Capacity (VC): The total volume of air that can be exchanged in one respiratory cycle (TV + IRV + ERV, average of 4800 mL).

  • Total Lung Capacity (TLC): The complete volume of the lungs (TV + IRV + ERV + RV, about 6000 mL).

DEAD SPACE VOLUME (DSV)

  • Anatomical Dead Space: The volume of air in the conducting zone that does not participate in gas exchange, approximately 150 mL.

  • Alveolar Dead Space: This is the volume of air in non-functional alveoli that do not participate in gas exchange.

  • Total Dead Space: The sum of anatomical and alveolar dead space provides insight into overall pulmonary efficiency and gas exchange effectiveness.

PULMONARY FUNCTION TESTS

  • Forced Vital Capacity (FVC): Represents the total volume of air expelled after maximum inhalation followed by forceful exhalation; a normal FVC is about 5 L in healthy adults.

  • Forced Expiratory Volume (FEV1): Represents the volume of air exhaled in the first second of the forced expiration, with healthy individuals typically exhaling around 4 L within this timeframe.

  • FEV1/FVC Ratio: This ratio assists in differentiating types of restrictive and obstructive respiratory diseases, with a normal ratio being about 0.8.

GAS EXCHANGE

Basic Properties of Gases

  • Dalton’s Law: States that the total pressure exerted by a mixture of gases is the sum of the pressures each gas would exert independently, allowing a better understanding of how gases exchange in relation to their partial pressures.

  • Henry’s Law: Indicates that the quantity of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid at a given temperature, underscoring the importance of solubility for gas exchange processes in the lungs.

Gas Exchange in the Body

  1. External Respiration: Involves the exchange of gases between the alveoli and blood in the pulmonary capillaries, which is essential for the uptake of oxygen and the release of carbon dioxide.

  2. Internal Respiration: This process occurs at the tissue level, where oxygen diffuses from blood to tissues, and carbon dioxide diffuses from tissues back to the blood for transport to the lungs for expiration.

Factors Influencing External Gas Exchange

  • Partial Pressure Gradients: The key factor driving diffusion, gases move from areas of high partial pressure to areas of low partial pressure, which plays a fundamental role in both external and internal respiration.

  • Thickness and Surface Area of Membranes: The efficiency of gas exchange is influenced by how thin the alveolar-capillary membrane is and the extensive surface area of alveoli available for exchange. Pathological conditions such as pulmonary edema or thickening of the membrane can severely impair gas exchange.

  • Ventilation-Perfusion Coupling: A crucial mechanism ensuring that airflow (ventilation) is appropriately matched with blood flow (perfusion) to optimize gas exchange efficiency. Proper coupling enables maximized oxygen intake and carbon dioxide removal from the blood.

Internal Respiration

  • Internal respiration refers to the gas exchange occurring in systemic capillaries within tissues. Oxygen diffuses from the blood into the tissues due to a concentration gradient, while carbon dioxide produced by cellular metabolism diffuses from the tissues to the blood for transport back to the lungs.

Transport of Respiratory Gases by Blood

  1. Oxygen Transport:

    • Approximately 98.5% of oxygen is transported bound to hemoglobin molecules within red blood cells.

    • The remaining 1.5% is dissolved directly in plasma, which is important for immediate oxygen availability during high metabolic demands.

  2. Carbon Dioxide Transport:

    • About 7-10% of carbon dioxide is dissolved in plasma, accounting for the rapid transport of CO2.

    • Roughly 20% of CO2 is carried bound to hemoglobin, while the majority (70%) is transported as bicarbonate ions (HCO3-) following a reaction primarily occurring in red blood cells, which helps buffer the blood pH.

Influence of CO2 on Blood pH

  • The Carbonic Acid-Bicarbonate Buffer System plays a crucial role in maintaining pH balance within the blood. Elevated levels of carbon dioxide lead to acidosis (lower pH), while a reduced respiratory rate can also affect carbon dioxide removal, resulting in a potential alkalosis (higher pH) condition, which can compromise physiological functions.

These detailed notes encapsulate the principles of respiratory physiology, emphasizing the mechanisms of breathing, gas exchange processes, and their physiological implications, along with critical assessment methods and factors influencing respiratory efficiency.