Respiratory Physiology: Key Concepts

• Respiratory physiology is crucial for maintaining homeostasis through the respiratory system

ABG Normal Ranges

• Arterial Blood Gases (ABGs) should be within a normal range to maintain homeostasis
• Normal PO2 is 100 mmHg
• Normal PCO2 is 40 mmHg
• Normal pH is 7.4
• Normal ABG levels indicate normal respiratory function, but do not guarantee it

Respiratory System Organization

• Has two zones: the conducting zone and the respiratory zone
• The respiratory zone starts at the terminal bronchioles and alveolar sacs
• Alveolar sacs are located at the 23rd branch

ATP Production

• Anaerobic glycolysis in the cytosol converts glucose into 2 pyruvates, producing 2 ATP
• The citric acid cycle (Krebs cycle) and electron transport chain in the mitochondria produce 36 ATP

Hypoxia

• Hypoxia occurs when there is a deficiency of oxygen reaching the tissues
• Can be caused by low oxygen availability or poor oxygen utilization (due to toxins or other factors)

Atmospheric pressure

• Atmospheric pressure (measured at sea level or 0m) is the force exerted by the weight of the air
• P=F/A.
• Atmospheric pressure decreases with increasing altitude
• At sea level, atmospheric pressure is 760 mmHg
• At 5.5 km above sea level, atmospheric pressure is halved to 380 mmHg
• At 11 km above sea level, atmospheric pressure is halved again to 190 mmHg

Partial Pressures of Inspired Air

• The following partial pressures are found in inspired air at sea level:
• Total pressure is 760 mmHg
• O2 is 21% or 160 mmHg
• N2 is 79% or 600 mmHg
• CO2 & H2O is 0%
• At 5.5km above sea level, the PO2 is 80mmHg

The Conducting Zone

• One of two zones that the respiratory system is composed of
• Also known as the anatomical dead zone
• It is Called a dead zone because there is no gas exchange
• The conducting zone is a living zone that is involved in the conduction of air, humidification, warming, and filtering
• Humidification involves adding H2O vapor to prevent damage to the alveoli if dry air reaches them
• The air is warmed to 37°C (body temperature)
• Dust cells filter out dust particles and pathogens
• The volume of anatomical dead space is 2ml/kg of body weight

Humidification

• Water vapor is introduced at the trachea by goblet cells
• It should be accounted for in calculations (47 mmHg) and is a constant at 37°C as it is not affected by atmospheric pressure
• The higher the altitude, the greater the portion of pressure taken by PH2O
• At the summit of Mount Everest, PO2 is around 45 mmHg, causing breathing difficulties

The Respiratory Zone

• Gas exchange occurs at the respiratory bronchioles and alveoli, which are covered with capillary beds
• Oxygen diffuses to blood, and CO2 diffuses 20x more than oxygen in the blood to alveoli direction
• It is Compliant, meaning changes in pressure can cause drastic changes in the volume of inhaled or exhaled air

Airway Resistance

• Open airways (conducting zone) are necessary for normal breathing
• Blockages increase resistance (R) and make airflow more difficult
• Resistance is directly proportional to the length (L) of the airway and the viscosity (η) of the blood
• Resistance is inversely proportional to the radius to the fourth power (r4)
• Small decreases in airway radius can cause huge increases in resistance
• Obstructions in the conducting zone lead to obstructive lung diseases like Chronic Obstructive Pulmonary Disease (COPD)
• COPD increases airway resistance, making it difficult to breathe, especially when exhaling
• Bronchodilators help reduce resistance by opening the airways

Important terminology

• Inflatable is the same as compliant
• The respiratory zone must be compliant
• Obstruction affects the conducting zone
• Restriction affects the respiratory zone

Lung Diseases

• Falls into three main categories:
• Conducting zone (obstructive diseases like COPD)
• Respiratory zone diseases (restrictive; pulmonary fibrosis)
• Vascular zone diseases (pulmonary hypertension)
• Conducting zone diseases make up 70% of cases
• Respiratory zone diseases make up 20% of cases
• Vascular zone diseases makes up 10 % of cases
• Oxygen, nitrogen, and water vapor contribute to the total pressure in the conducting zone
• Total atmospheric pressure is 760 mmHg
• Water vapor accounts for 47 mmHg
• PO2 = 21% of 713 = 150 mmHg
• PN2 = 79% of 713 = ~563 mmHg
• The PO2 in the anatomical dead space is 150 mmHg (close to atmospheric PO2 of 160 mmHg) since the air has only been humidified

Gas Values Inside the Alveoli

• After air reaches the alveoli, gas exchange changes the composition:
• PO2 = 100 mmHg
• PCO2 = 40 mmHg
• PH2O = 47 mmHg
• PN2 = 760 – (100 + 40 + 47) = 573 mmHg

Key Facts

• Nitrogen is biologically inactive
• No need to memorize the value for nitrogen, you can easily calculate it from other gases
• Focus on PO2 and PCO2 because they constantly change due to gas exchange
• Physiological temperature is 37 Celsius
• PH2O in the respiratory system is always = 47mmHg

Abbreviations

• PvO2 - venous o2 pressure
• Pv-O2 (v bar) - mixed venous pressure
• PAO2 - alveolar pressure
• PaO2 - arterial pressure
• PeO2 - expiratory pressure
• PiO2 - interstitial pressure

The Vascular System

• The partial pressure of oxygen in mixed venous blood (PvO2) is 40 mmHg
• Mixed venous blood is a combination of venous blood from the superior and inferior vena cava travels to the right atrium, then to the right ventricle, and then to the pulmonary artery
• Oxygen diffuses from areas of high partial pressure to low partial, binding to hemoglobin in red blood cells

RBC time in pulmonary capillaries linked to heart rate:

• 0.8 seconds at rest (HR = 75 bpm)
• 0.4 seconds during exercise (HR = 150 bpm)
• 0.2 seconds in extreme conditions (HR = 300 bpm)
• When RBCs spend 0.3 seconds in the capillaries, PO2 reaches 100 mmHg
• One-third of the respiratory membrane is used, while two-thirds act as reserve, especially during exercise

The respiratory membrane

• Also called the alveolar-capillary membrane or air-blood barrier, consists of six layers
• These are:
  • Surfactant (reduces surface tension)
  • Alveolar epithelium
  • Basement membrane of alveoli
  • Interstitial space
  • Basement membrane of capillaries
  • Capillary endothelium
• The membrane is thin (0.25 – 0.6 µm) to keep gas exchange rapid
• Hypoxia due to diffusion issues are rare
• CO2 is 20 times more diffusible than oxygen

Lung Diseases

• Oxygen is affected first because it diffuses more slowly than CO2
• In Type 1 respiratory failure, only oxygenis affected
• In Type 2 respiratory failure, both oxygen and CO2 are impaired

Systemic Circulation and Oxygen Diffusion

• In the lungs, oxygen moves from high PO2 (alveoli = 100 mmHg) to low PO2 (pulmonary capillaries = 40 mmHg)
• In circulation, oxygen continues diffusing: PaO2 (100 mmHg) → PiO2 (40 mmHg) → cells (PcellO2 < 40 mmHg)
• Venous PO2 equals interstitial PO2 due to volume differences

Blood Distribution

(5L total):

• 60% (3L) → Systemic veins
• 15% (750mL) → Systemic arteries
• 7% (350mL) → Capillaries
• 9% (450mL) → Lungs
• 7% (350mL) → Heart

Water Distribution

• 60% of body weight, for ex: 70kg x 60% =42L
  • 2/3 intracellular (28L)
  • 1/3 extracellular (14L)
    • 11L interstitial + 3L plasma