Blood Oxygen Capacity Calculation Quiz

Ventilation and Perfusion in the Lungs

• Ventilation and perfusion differ depending on the region of the lung
• The balance between alveolar ventilation and perfusion determines the partial pressures of oxygen and carbon dioxide in the alveolus and, subsequently, in arterial blood

Regional Variations in Blood Flow in the Lungs

• Blood flow in the lungs is determined by the relationship between pulmonary capillary pressures (at the arterial and venous ends) and alveolar air pressure
• In the normal upright lung, blood flow decreases from the bottom to the top due to differences in hydrostatic pressure (due to gravity) on pulmonary vessels
• The lung can be divided into three zones based on alveolar, arterial, and venous pressures:
  • Zone 1: No blood flow (higher alveolar air pressures)
  • Zone 2: Intermittent blood flow (only with peak systolic pressures ~25mm Hg)
  • Zone 3: Blood flow occurs continuously

Oxygen Transport in the Body

• Oxygen is transported in the blood in two ways:
  • Physically dissolved oxygen (0.003 mL O2/100 mL blood at a PO2 of 40 mmHg)
  • Chemically bound to hemoglobin (Hb)
• The oxyhemoglobin dissociation curve relates the percent of O2 saturation to the partial pressure of oxygen (PO2)
• Factors that affect the oxyhemoglobin dissociation curve include:
  • Temperature
  • pH
  • 2,3-DPG
  • Anemia
  • Nitric oxide levels
  • Carbon monoxide levels
  • Presence of other forms of Hb (Methemoglobin and Fetal hemoglobin)

Carbon Dioxide Transport in the Body

• Carbon dioxide is transported in the blood in three ways:
  • Dissolved carbon dioxide (7%)
  • Carbamino compounds (25%)
  • Bicarbonate ions (70%)

Hypoxia and Hypoxemia

• Hypoxia: insufficient oxygen in the tissues
• Hypoxemia: insufficient oxygen in the blood
• Causes of hypoxia can be classified into four groups:
  • Hypoxic hypoxia
  • Anemic hypoxia
  • Hypoperfusion hypoxia
  • Histotoxic hypoxia

Control of Respiration

• Breathing is involuntary and controlled by specific regions within the brainstem
• The control of breathing is complex due to the anatomical location and the complex organization of neural networks

Pulmonary Gas Exchange and Transport of Oxygen and Carbon Dioxide

• Exchange I: Atmosphere to Lung (Ventilation and Alveolar Ventilation)
• Exchange II: Lung to Blood (Pulmonary Gas Exchange and Oxygen and Carbon Dioxide transport in the blood)
• Exchange III: Blood to Cells (Cellular Respiration via Mitochondria)
• The ventilation-perfusion relationship is the ratio of alveolar ventilation and blood perfusion rates of the lung
• Ideally, each alveolus in the lungs would receive the same amount of ventilation and pulmonary capillary blood flow (perfusion)

Ventilation-Perfusion Relationship of the Lungs

• Regional differences in lung ventilation are due to variations in pleural pressures (and transpulmonary pressures) that exist between the apex and base of the lungs

• Alveoli in upper lung have a higher transpulmonary pressure, are near-maximally inflated and relatively noncompliant

• Alveoli in lower lung are smaller and have a lower transpulmonary pressure, are more compliant and undergo greater expansion during inspiration### Pulmonary Gas Exchange

• Blood is diverted from poorly ventilated alveoli to better ventilated ones through hypoxic pulmonary vasoconstriction to maintain a matched V/Q in the lungs.

• Pulmonary gas exchange occurs at the alveolar-capillary membrane, which forms the blood-gas interface.

• Henry's Law states that the quantity of a slightly soluble gas that dissolves in a given mass of liquid at a particular temperature is proportional to the partial pressure of that gas.

• Fick's First Law of Diffusion states that the diffusion rate increases when the surface area, the ionic concentration gradient, and permeability increase.

• Pulmonary diffusing capacity measures the ability of the alveolar-capillary membranes to conduct gases.

Oxygen Transport in the Body

• O2 in the blood is transported in two ways: physically dissolved in the blood and chemically bound to hemoglobin (Hb).
• The physically dissolved O2 is very low, with a solubility coefficient of 0.003mL/100mL of blood at a normal body temperature.
• Hemoglobin (Hb) is a complex molecule with a tetrameric structure consisting of 4 polypeptide chains (globin), each of which contains a protoporphyrin (heme) group that can bind one molecule of oxygen.
• Each Hb molecule can bind 4 molecules of O2.
• Normal adult Hb (HbA) consists of two α-chains and two β-chains.
• Abnormal variants of Hb, such as HbS, can cause sickle cell disease.

Oxygen Loading (Content) in the Lungs

• The oxyhemoglobin dissociation curve relates the percent of O2 saturation to the partial pressure of oxygen (PO2).
• The curve has a sigmoidal shape, plotting all 4 reactions of oxygen binding to the Hb molecule.
• At a normal arterial P02 of 100mmHg, the Hb saturation is 98%.
• Percent saturation is calculated by dividing the content of oxygen in the blood by the oxygen carrying capacity of Hb.

Alterations in the Oxyhemoglobin Dissociation Curve

• Shifts in the oxyhemoglobin dissociation curve can be best described by the P50, which is the PO2 at which 50% of the Hb present in the blood is deoxyhemoglobin and the other 50% is oxyhemoglobin.
• Alterations in temperature, pH, PCO2, and 2,3-DPG can cause shifts in the oxyhemoglobin dissociation curves.
• The Bohr effect explains why cells oxygen release or red blood cells unload oxygen in tissues, with higher CO2 values leading to more oxygen release.

Carbon Dioxide Transport in the Body

• Carbon dioxide is transported in the blood in three ways: dissolved carbon dioxide (7%), carbamino compounds (25%), and bicarbonate ions (70%).

Respiratory Stress: Hypoxia and Hypoxemia

• Hypoxia and hypoxemia are types of respiratory stress, with hypoxemia referring to insufficient oxygen in the blood and hypoxia referring to insufficient oxygen in the tissues.
• The causes of hypoxia can be classified into four groups: hypoxic hypoxia, anemic hypoxia, hypoperfusion hypoxia, and histotoxic hypoxia.

Control of Respiration

• Breathing is involuntary and controlled by specific regions within the brainstem.
• The control of breathing is complex due to the anatomical location and complex organization of neural networks.