Respiratory Physiology II

• Alveolar Surface Tension:

  • A thin layer of fluid in alveoli causes inwardly directed force known as surface tension
  • This force causes alveoli to remain as small as possible
  • Surfactant, a detergent-like substance produced by Type II alveolar cells, lowers alveolar surface tension and prevents alveoli collapse
  • Insufficient surfactant in premature babies leads to alveoli collapse at the end of each exhalation
• Pulmonary Surfactant:

  • Source: secreted by Type II (septal cells) alveolar & Clara cells
  • Chemistry: Oily secretion with a lipoprotein complex formed by lipids (phospholipids) and proteins (SP-A, B, C, and D) and ions
  • Formation: Precursor is tubular myelin
  • Factors: Glucocorticoids play a significant role
  • Deficiency leads to respiratory distress syndrome where alveoli collapse after each exhalation
• Ventilation: The Role of Surfactant:

  • Functions include reducing surface tension, stabilizing alveoli, aiding in lung inflation after birth, and contributing to lung defense
• Dead Space:

  • Anatomical dead space: volume of the conducting respiratory passages (150 ml)
  • Alveolar dead space: alveoli that cease to act in gas exchange due to collapse or obstruction
  • Total dead space: sum of alveolar and anatomical dead spaces
• Gas Exchange:

  • External Respiration involves the exchange of gases through diffusion between alveolar air and pulmonary capillary blood
  • Driven by partial pressure (P) gradients for O2 and CO2, solubility of gas, and temperature
  • Oxygen is transported in blood dissolved in plasma or bound to hemoglobin inside RBCs
• A Summary of the Primary Gas Transport Mechanisms:

  • Oxygen pickup: oxygen is transported in blood dissolved in plasma or bound to hemoglobin inside RBCs
  • Oxygen delivery: oxygen enters the blood at the alveolar-capillary interface and is transported to cells in peripheral tissues
  • Carbon dioxide pickup: carbon dioxide is transported dissolved, bound to hemoglobin, or as HCO3- in the blood
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen
• Gas exchange in the body involves the transport of oxygen and carbon dioxide between the lungs and cells
• Oxygen enters the blood at the alveolar-capillary interface and is transported dissolved in plasma or bound to hemoglobin inside red blood cells
• Carbon dioxide is transported dissolved, bound to hemoglobin, or as bicarbonate ions
• Hypoxia occurs when there is a decrease in the amount of oxygen reaching the tissues
• Factors causing low arterial PO2 include not enough oxygen reaching alveoli, problems with exchange between alveoli and pulmonary capillaries, and not enough oxygen being transported in the blood
• Hypoxia problems can arise from not enough oxygen in alveoli, interference with alveolar capillary exchange, and conditions like anemia
• Pulmonary pathologies like emphysema, fibrotic lung disease, pulmonary edema, and asthma can affect alveolar ventilation and gas exchange
• Oxygen in the blood can remain dissolved in plasma or bind to hemoglobin to form oxyhemoglobin
• Hemoglobin allows for the total oxygen content in the blood to exceed the oxygen requirement at rest
• Oxygen quickly associates with hemoglobin, allowing blood to carry an extra amount of oxygen
• The binding of oxygen to hemoglobin is influenced by factors like temperature, PCO2, pH, and 2,3-DPG
• Carbon dioxide is transported in blood by being dissolved, converted to bicarbonate ions, or attached to hemoglobin
• Regulation of ventilation involves local factors like rising PCO2 levels and the ventilation-to-perfusion ratio coordinating lung perfusion with alveolar ventilation
• Factors influencing ventilation include lung perfusion (blood flow to alveoli), alveolar ventilation (airflow), PCO2 levels, and the control of bronchoconstriction and bronchodilation
• CO2, O2, and pH levels all influence ventilation rates
• CO2 has the biggest influence on ventilation rates, while O2 and H+ (pH) are influencing factors to a smaller extent
• Ventilation is monitored by peripheral and central chemoreceptors:

  • Peripheral chemoreceptors are located in the carotid and aortic bodies
  • Central chemoreceptors are located in the medulla oblongata
• Respiratory centers in the brain stem control ventilation rates:

  • When oxygen demand rises, respiratory rates increase under neural control
  • There are voluntary and involuntary components that affect respiratory centers in the pons and medulla oblongata
  • Three pairs of nuclei in the reticular formation of the medulla oblongata and pons regulate the frequency and depth of pulmonary ventilation in response to sensory information
• Respiratory rhythmicity centers in the medulla oblongata establish respiratory rate and rhythm:

  • The dorsal respiratory group (DRG) is the inspiratory center that functions in quiet and forced breathing
  • The ventral respiratory group (VRG) has inspiratory and expiratory centers that function only in forced breathing
• Apneustic and pneumotaxic centers of the pons adjust the output of respiratory rhythmicity centers to regulate the depth and rate of respiration:

  • The apneustic center provides continuous stimulation to its DRG center
  • Pneumotaxic centers inhibit apneustic centers and promote passive or active exhalation
• Respiratory centers respond to sensory information from chemoreceptors sensitive to PCO2, PO2, or pH of blood or cerebrospinal fluid, baroreceptors sensitive to changes in blood pressure, and stretch receptors that respond to changes in lung volume
• Homeostasis in respiration is maintained through various mechanisms:

  • Hypercapnia: an increase in arterial PCO2 stimulates chemoreceptors to accelerate breathing cycles, increasing respiratory rate and encouraging CO2 loss at the lungs
  • Hypocapnia: a decrease in arterial PCO2 inhibits chemoreceptors, decreasing the rate of respiration and increasing arterial PCO2
• In Physiology practical sessions, students need to understand how spirometry is used to assess lung function, different lung volumes and capacities, and the interpretation of flow-volume curves in different pathological conditions