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Respiratory System: Ventilation, Gas Exchange, and Regulation – Study Guide

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Respiratory System: Ventilation

Muscles Involved in Ventilation

  • Primary Muscles: The diaphragm and external intercostal muscles are the main muscles responsible for normal breathing (inspiration and expiration).

  • Accessory Muscles: During forced breathing, additional muscles such as the sternocleidomastoid, scalenes, pectoralis minor, and abdominal muscles are recruited.

Roles of Muscles in Normal Inspiration and Expiration

  • Inspiration: The diaphragm contracts and moves downward, while the external intercostals contract to elevate the ribs, increasing thoracic volume.

  • Expiration: Normally passive; the diaphragm and external intercostals relax, causing the thoracic cavity to decrease in volume and air to be expelled.

Roles of Muscles in Active Expiration and Forced Inspiration

  • Active Expiration: Internal intercostal and abdominal muscles contract to force air out of the lungs.

  • Forced Inspiration: Accessory muscles (sternocleidomastoid, scalenes) further increase thoracic volume.

Changes in Thoracic Size During Breathing

  • Inspiration: Thoracic cavity increases in size (volume).

  • Expiration: Thoracic cavity decreases in size.

Pressure Changes During Breathing

  • Inspiration: Intrapulmonary (alveolar) pressure drops below atmospheric pressure; intrapleural pressure becomes more negative.

  • Expiration: Intrapulmonary pressure rises above atmospheric pressure; intrapleural pressure becomes less negative.

How Pressure Changes Drive Air Movement

  • Air moves from areas of higher pressure to lower pressure.

  • During inspiration, lower alveolar pressure draws air in; during expiration, higher alveolar pressure pushes air out.

Determinants of Lung Compliance

  • Elasticity of lung tissue

  • Surface tension within the alveoli

Importance of Surfactant

  • Surfactant reduces surface tension in alveoli, preventing collapse and making breathing easier.

  • Produced by type II alveolar cells.

Respiratory System: Gas Exchange

Oxygen and Carbon Dioxide Gradients

  • Alveolar pO2 is higher than venous blood pO2; O2 diffuses into blood.

  • Alveolar pCO2 is lower than venous blood pCO2; CO2 diffuses into alveoli.

  • Capillary pO2 is higher than tissue pO2; O2 diffuses into tissues.

  • Capillary pCO2 is lower than tissue pCO2; CO2 diffuses into blood.

Oxygen Transport in Blood

  • Dissolved in plasma: ~1.5% of O2

  • Bound to hemoglobin: ~98.5% of O2

Hemoglobin and Oxygen Binding

  • Hemoglobin (Hb): A protein in red blood cells that can bind up to 4 O2 molecules per molecule.

  • O2 Binding: Occurs in the lungs where pO2 is high, pCO2 is low, temperature is lower, and pH is higher.

  • O2 Release: Occurs in tissues where pO2 is low, pCO2 is high, temperature is higher, and pH is lower.

Factors Affecting O2 Release from Hemoglobin

  • Increased temperature

  • Increased pCO2

  • Decreased pH (Bohr effect)

  • These factors are enhanced during exercise, promoting O2 delivery to active tissues.

Carbon Dioxide Transport in Blood

  • Dissolved in plasma: ~7%

  • Bound to hemoglobin (carbaminohemoglobin): ~23%

  • As bicarbonate ion (HCO3-): ~70%

CO2 and Bicarbonate Formation

  • CO2 combines with H2O to form carbonic acid (H2CO3), catalyzed by carbonic anhydrase in red blood cells.

  • H2CO3 dissociates into HCO3- and H+.

Fate of Bicarbonate and Hydrogen Ions

  • HCO3- is transported out of red blood cells into plasma (chloride shift).

  • H+ binds to hemoglobin, buffering pH.

Factors Promoting CO2 Release from Hemoglobin

  • Low pCO2 in the lungs causes CO2 to be released from hemoglobin and exhaled.

Fetal vs. Adult Hemoglobin

  • Fetal hemoglobin has a higher affinity for O2 than adult hemoglobin, facilitating O2 transfer from mother to fetus.

Respiratory System: Regulation

Regulatory Centers in the Brain

  • Medulla oblongata and pons contain the main respiratory centers.

Sensory Systems Stimulating the Respiratory Center

  • Central chemoreceptors (in medulla, sensitive to CO2 and pH)

  • Peripheral chemoreceptors (in carotid and aortic bodies, sensitive to O2, CO2, and pH)

  • Stretch receptors in the lungs

Roles of Medullary and Pontine Centers

  • Medullary inspiratory center: Initiates inspiration.

  • Medullary expiratory center: Active during forced expiration.

  • Pneumotaxic area (pons): Modulates breathing rate and pattern.

Triggers for Breathing After Voluntary Breath-Holding

  • Rising CO2 levels (hypercapnia) stimulate the respiratory center, overriding voluntary control.

Hering-Breuer Reflex

  • A protective reflex that inhibits inspiration to prevent over-inflation of the lungs.

Effects of Hypercapnia, Hypoxia, and Acidosis on Ventilation

  • Hypercapnia (high CO2), hypoxia (low O2), and acidosis (low pH) all stimulate increased ventilation.

Chloride Shift

  • The exchange of HCO3- out of and Cl- into red blood cells to maintain electrical neutrality during CO2 transport.

Key Equations

  • CO2 hydration and dissociation:

  • Oxygen binding to hemoglobin:

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