Respiratory System & Gas Pressure

• Physiology professors illustrate the respiratory system as a bulb-shaped balloon
• The straight part of the balloon represents the ADS (anatomical dead space) or airway
• The bulb shape represents the alveoli
• The respiratory system is an aggregation of alveoli

Common Abbreviations

• ADS stands for anatomical dead space
• A refers to alveolar
• "a" refers to arterial
• Cap. refers to capillaries

Respiratory Gases Pressure

• Oxygen diffuses from the alveoli to the bloodstream
• Alveolar pressure must be higher than arterial pressure for oxygen to diffuse down the pressure gradient
• PO2 A=100 and PO2 a=40
• Oxygen rapidly diffuses, increasing arterial pressure until it reaches 100 (equilibrium) in 0.8 seconds normally
• The rate of diffusion depends on the heart's bpm (beats per minute)
• In normal conditions (0.8 secs), there's sufficient time for RBCs to oxygenate hemoglobin
• Elevated heart rate still ensures hemoglobin oxygenation, requiring only 0.25 seconds

PAO2 and PaO2 Values

• PAO2=100 and PaO2=40 allows O2 to diffuse from the alveoli to artery until PaO2=100 (equilibrium)
• Only a portion of the capillary length is used, needing only 0.25 seconds for hemoglobin oxygenation
• PaO2 remains normal during exercise (PAO2=100)
• ABGs are unaffected during exercise unless an underlying condition exists
• Hemoglobin remains oxygenated even with increased heart rates (150-190 bpm)

Impact on Respiratory Gases

• Oxygen is the first respiratory gas affected
• PaCO2=45mmHg and PACO2=40mmHg allows CO2 to diffuse from artery to alveoli in 0.25 seconds

Oxygen Movement

• Oxygen moves across biological membranes based on partial pressure gradient, not concentration gradient
• Partial pressure does not indicate the "amount" of oxygen
• Anemia patients may show normal PAO2 (100mmHg) but have insufficient oxygenated hemoglobin due to low RBC count, leading to symptoms like shortness of breath and fatigue

Spirometer Use

• Spirometer: a device to measure the volume of air coming "in and out" (inspired or expired)
• Spirometers measures lung functions, especially the volume of air a person can inhale and exhale, and the speed of exhalation
• Spirometers used to diagnose and monitor respiratory diseases like asthma, chronic obstructive pulmonary disease, and other lung conditions

Systemic Circulation and Pressure

• Pressure in the interstitium (space between cells and capillaries) must be greater than 45 mmHg for CO2 to diffuse into the capillary
• Blood filled with CO2 goes into the vein with PvCO2= 45mmHg

CPR and Expiration

• Cardio pulmonary resuscitation is the process of exhaling through the patient's mouth
• Expiratory air contains oxygen
• Tidal volume (VT) is the air volume inhaled or exhaled per breath
• The ADS volume equals 2ml/kg
• In a person weighing 75 kg, the ADS volume is 150ml
• In CPR, one inhales and then exhales into the patient's mouth

Air Composition and Inspiration

• 2800 ml of air in the respiratory zone
• 500 ml of expired air
• PCO2 and PO2 in the ADS match the respiratory zone (PA02=100 mmHg, PACO2=40 mmHg)
• Alveolar ventilation: the amount of volume of fresh air that enters the lungs per minute

Inspiration Process

• Inhaling 150ml of new fresh air pushes 150ml of old air into the respiratory zone
• Additional 150ml of new air further pushes old air
• Inhaling 500ml of new air pushes the old air back into the respiratory zone

Air Composition After Inspiration

• The composition of air in the ADS after inspiration is similar to room air
• PCO2=0 mmHg and PO2=150 mmHg (lower than 160 due to humidification at ADS)

Expiration Process

• Expired air consists of of new air from ADS + old air that was in the respiratory zone (volume becomes 2300)
• Given we want to deliver CPR we will expire air into the patient's mouth

Mixed Air Calculation

• 150ml has PO2=150
• 350ml has PO2=100
• Calculate the PO2 of mixed air (old + new) by considering the percentage of old air relative to new air
• The PO2 of mixed expiratory air is 116 mmHg
• PO2 is highest in mixed expired air

Mechanics of Breathing

• Breathing involves the flow of air during inhalation and expiration
• Flow requires a "driving force" due to a pressure gradient
• Alveoli and atmospheric pressure have PO2
• A difference of at least 1mmHg is required to allow O2 flow

Solutions to Create Pressure difference

• Increasing outer atmospheric pressure to 101mmHg compared to alveolar 100mmHg ensures flow
• This is possible in ventilators (artificial breathing)
• Ventilators control pressure for breathing

"Negative Pressure"

• Creation inside the alveoli
• Increased lung volume leads to decreased pressure
• Diaphragm muscle contraction constricts the abdomen, allowing the lung to puff up and inflate
• Takes ATP energy
• Lungs inflate and pulls air inside

Inspiration Sequence

• Contraction of the diaphragm>>Increases thoracic cavity>>Intrapleural pressure drops>>The lungs inflate>> Intrapulmonary pressure drops (0 to -1)
• Sequence explains the process of inhalation

Lung Elasticity

• Elasticity, allowing them to stretch and return to resting length
• Negative pressure in the pleural cavity prevents collapse if lungs aren't elastic
• Plural pressure makes the lungs more negative because it is already negative by the diaphragm
• The pressure difference between the left ventricle and the aorta is 5mmHg

Anatomical Note

• Alveoli-arterial <=5 is ok