E

exam 4

CHAPTER 8: The Respiratory System and Its Regulation

Overview of the Respiratory System

  • Pulmonary ventilation
    Process of moving air into and out of the lungs.

  • Pulmonary volumes
    Measurement of lung capacities and flow rates.

  • Pulmonary diffusion
    Gas exchange process in lungs between alveoli and capillaries.

  • Transport of oxygen and carbon dioxide in the blood
    Mechanisms of how O2 and CO2 are carried in the bloodstream.

  • Gas exchange at the muscles
    Exchange of gases at the level of the muscle tissues.

  • Regulation of pulmonary ventilation
    The mechanisms that maintain homeostasis of breathing.

  • Afferent feedback from exercising limbs
    Neural feedback signals that influence respiration during exercise.

  • Exercise training and respiratory function
    Impact of physical training on respiratory performance.

Respiratory System Introduction

  • Purpose:
    To carry O2 to and remove CO2 from all body tissues.

  • Four processes of respiration:

    • Pulmonary ventilation (external respiration):
      Moving air into and out of the lungs.

    • Pulmonary diffusion (external respiration):
      Exchange of gases between alveoli and blood.

    • Transport of gases via blood:
      Movement of oxygen and carbon dioxide in the bloodstream.

    • Capillary diffusion (internal respiration):
      Exchange of gases between blood and tissue cells.

Pulmonary Ventilation

  • Definition:
    The process of moving air into and out of the lungs.

  • Anatomical Zones:

    • Transport Zone:
      Includes the nose/mouth, nasal conchae, pharynx, larynx, trachea, and bronchial tree, ultimately leading to the alveoli.

    • Exchange Zone:
      Comprises the alveoli where gas exchange occurs.

Anatomy of the Lungs

  • Pleural Sacs:

    • Parietal Pleura:
      Lines the thoracic wall.

    • Visceral (Pulmonary) Pleura:
      Lines the lungs themselves.

  • Lung Expansion:
    Lungs take the size and shape of the rib cage, influenced by the anatomy of the diaphragm and rib cage.

Mechanics of Breathing

  • Inspiration:

    • An active process involving multiple muscles (e.g., diaphragm and external intercostals).

    • Diaphragm flattens, pulling lungs downward to increase thoracic cavity volume.

    • Boyle's Law states that lung volume increases, leading to a decrease in intrapulmonary pressure, which causes air to flow into the lungs.

    • Forced Breathing:
      Involves additional accessory muscles such as scalenes and sternocleidomastoid.

  • Expiration:

    • Mostly a passive process where inspiratory muscles relax and lung volume decreases leading to increased intrapulmonary pressure, forcing air out of the lungs.

    • Active Expiration:
      Involves internal intercostals and abdominal muscles to force air out more vigorously.

Respiratory Pump

  • Functions to enhance venous return to the heart due to changes in intra-abdominal and intrathoracic pressures during breathing.

    • Increasing abdominal pressure leads to venous compression, while decreased pressure facilitates filling of veins.

Work of Breathing

  • Energy demand of respiratory muscles:

    • At rest, uses less than 5% of resting VO2.

    • During intense exercise, this may increase to 30% of VO2.

  • Different activities yield different levels of breathing work:

    • Greater work in swimming due to hydrostatic pressure affecting ventilatory mechanics.

Pulmonary Volumes

  • Measurement through Spirometry:

    • Tidal Volume (TV):
      Amount of air entering and leaving the lungs with each normal breath.

    • Vital Capacity (VC):
      Greatest amount of air that can be expired after maximal inspiration.

    • Residual Volume (RV):
      Volume of air remaining in the lungs after maximal expiration.

    • Total Lung Capacity (TLC):
      Sum of all lung volumes, including residual volume.

Pulmonary Diffusion

  • Definition:
    Gas exchange between alveoli and capillaries.

  • Inspired Air Pathway:
    Air travels from the bronchial tree to alveoli where gas exchange occurs.

  • Blood Pathway:
    Right ventricle → pulmonary trunk → pulmonary arteries → pulmonary capillaries.

  • Functions:

    • Replenishes blood oxygen supply.

    • Removes carbon dioxide from blood.

Blood Flow to the Lungs at Rest

  • Resting lung blood flow: approximately 4 to 6 L/min.

  • Right ventricular cardiac output equals left ventricular cardiac output, ensuring balanced blood flow.

  • Characteristics of lung circulation:

    • Low pressure with mean arterial pressure of 15 mmHg compared to systemic circulation of approximately 95 mmHg.

    • Small pressure gradients facilitate blood flow due to thinner vessel walls.

Respiratory Membrane

  • Definition: Also referred to as alveolar-capillary membrane comprising:

    • Alveolar wall

    • Capillary wall

    • Surrounding basement membranes.

  • Features:

    • Very large surface area (approximately 300 million alveoli).

    • Minimal thickness (0.5 to 4 micrometers) aiding efficient gas exchange.

Partial Pressures of Gases in Breathing

  • Composition of Air:

    • Approximately 79.04% nitrogen, 20.93% oxygen, and 0.03% carbon dioxide.

  • Standard Atmospheric Pressure:

    • Total air pressure is 760 mmHg.

  • Dalton's Law:

    • Total air pressure is the sum of individual partial pressures:

      • P{N2} = 760 imes 79.04 ext{%} = 600.7 ext{ mmHg}

      • P{O2} = 760 imes 20.93 ext{%} = 159.1 ext{ mmHg}

      • P{CO2} = 760 imes 0.03 ext{%} = 0.2 ext{ mmHg}

  • Henry’s Law:

    • States that gas solubility in liquids relates to its partial pressure regardless of fluid characteristics.

  • Importance of Partial Pressure Gradient in Gas Exchange:

    • The partial pressure gradient is the driving force for gas diffusion.

Gas Exchange in the Alveoli

  • Oxygen Exchange:

    • Alveolar PO2 averages 105 mmHg, while pulmonary artery PO2 stands at 40 mmHg, creating a gradient of 65 mmHg that facilitates diffusion.

    • Fick’s Law:

      • Rate of diffusion is proportional to surface area and the partial pressure gradient: Rate \, ext{of diffusion} \ ext{∝} A (P1 - P2)

    • Diffusion constant varies for each gas; CO2 has a greater diffusion constant than O2 even with a lower gradient, allowing it to diffuse more readily.

  • Carbon Dioxide Exchange:

    • Alveolar PCO2 is 40 mmHg while pulmonary artery PCO2 is around 46 mmHg. The gradient of 6 mmHg is sufficient for diffusion due to CO2’s high diffusion constant.

Transport of Oxygen in the Blood

  • Carrying Capacity:

    • Approximately 20 mL O2 can be carried per 100 mL of blood, with >98% bound to hemoglobin (Hb) in red blood cells as oxyhemoglobin.

  • Oxygen-Hemoglobin Curve:

    • High PO2 in lungs results in loading of oxygen, with Hb saturation nearing full at approximately 0.75 seconds transit time.

    • Low PO2 in tissues allows for unloading of O2, where substantial shifts in Hb saturation occur with small changes in PO2.

Factors Affecting Hemoglobin Saturation

  • Blood pH:

    • More acidic conditions shift the O2-Hb curve to the right (Bohr effect), enhancing O2 unloading in active muscles.

  • Blood Temperature:

    • Increased temperatures during exercise shift the curve to the right, promoting O2 release in tissues.

Carbon Dioxide Transport

  • CO2 is released as a waste product of cellular metabolism, transported in blood through three primary mechanisms:

    • As bicarbonate ions (60-70% of CO2 transport).

    • Dissolved in plasma (7-10% of CO2).

    • Bound to hemoglobin as carbaminohemoglobin (20-33% of CO2).

  • Bicarbonate Formation:

    • CO2 combines with water to form carbonic acid, which dissociates into bicarbonate ions and hydrogen ions:

      • CO2 + H2O
        ightarrow H2CO3
        ightarrow HCO_3^- + H^+

    • Bicarbonate diffuses into plasma, and hydrogen ions bind to hemoglobin, contributing to the Bohr effect.

Gas Exchange at the Muscles

  • (a-v)O2 difference:

    • The measure of oxygen extraction by tissues. Higher extraction rates mean lower venous oxygen content.

    • Arterial oxygen content is around 20 mL O2 per 100 mL blood, while mixed venous O2 content ranges from:

    • Rest: 4 to 5 mL O2 per 100 mL blood

    • Heavy exercise: 15 to 16 mL O2 per 100 mL blood.

  • Oxygen in Muscle:

    • Transported via myoglobin, which has a higher affinity for O2 than hemoglobin.

    • Myoglobin's dissociation curve is steeper at lower PO2 levels, facilitating oxygen release in muscle tissues.

Regulation of Pulmonary Ventilation

  • Homeostatic Balance:

    • The body maintains equilibrium between blood PO2, PCO2 and pH through the coordination of multiple systems, particularly respiratory and cardiovascular.

  • Central Mechanisms:

    • Respiratory Centers: Located in the brain stem; they control the rate and depth of breathing via signals to respiratory muscles.

    • Central Chemoreceptors: Respond to increased H+ in cerebrospinal fluid, promoting deeper and faster breathing to remove excess CO2.

  • Peripheral Mechanisms:

    • Peripheral Chemoreceptors: Located in carotid and aortic bodies; sensitive to blood levels of PO2, PCO2, and H+.

    • Mechanoreceptors: In the lungs that react to excessive stretch, inhibiting breathing depth to prevent over-inflation.

Afferent Feedback from Exercising Limbs

  • Neural feedback from active limbs significantly contributes to the initiation of breathing during exercise, proportional to the frequency of limb movement.

Exercise Training and Respiratory Function

  • Regular aerobic training shows minimal impact on the respiratory system in healthy individuals, while improvements can be noted through size of tidal volume and maximal ventilation.

  • Aging and Respiratory Function:

    • Aging may affect the recruitment and distension of pulmonary capillaries, but this is less pronounced in highly fit older adults.

  • Asthma and Pneumonia:

    • Routine aerobic exercise is beneficial in managing symptoms and risk of these conditions due to improved respiratory efficacy.