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Respiratory System. 11 (Pressure Relationships in the Thoracic Cavity…

- - - - Pharynx: Passageway for air and food; facilitates exposure of immune system to inhaled antigens
      - Paranasal sinsues: Lighten skull; also may warm, moisten and filter incoming air
      - Nose (external nose and nasal cavity): produces mucus; filters, warms and moistens incoming air, resonance chamber for speech; receptors for sense of smell
    - - Bronchial tree: Air passageways connecting trachea with alveoli; cleans, warms and moistens incoming air
      - Alveoli: Main sites of gas exchange; reduces surface tension; helps prevent lung collapse; have 6 times more surface area than skin
      - Trachea: Air passageway; cleans, warms, moistens incoming air
      - Lungs: House respiratory passages smaller than the main bronchi
      - Larynx (divides upper and lower): Air passageway; prevents food from entering lower respiratory tract; voice production
      - Pleurae: Produce lubricating fluid and compartmentalize
  - - - Alveolar sacs contain clusters of alveoli: ~300 million alveoli make up most of lung volume; sites of actual gas exchange
  - - - Alveolar pores connect adjacent alveoli: Equalize air pressure throughout lung and provide alternate routes in case of blockages
      - Alveolar macrophages keep alveolar surfaces sterile: 2 million dead macrophages/hour carried by cilia to throat and swallowed
      - Surrounded by fine eleastic fibers and pulmonary capillaries
- - - - Lungs' natural tendency to recoil: Because of elasticity, lungs always try to assume smallest size
      - Surface tension of alveolar fluid: surface tension pulls on alveoli to try to reduce alveolar size
  - - - Negative P_ip must be maintained to keep lungs inflated
- - - - Action of diaphragm: when dome-shaped diaphragm contracts, it moves inferiorly and flattens out (results in increase in thoracic volume)
      - Action of intercostal muscles: when external intercostals contract, rib cage is lifted up and out (results in increase in thoracic volume)
      - As thoracic cavity volume increases, lungs are stretched as they are pulled out with thoracic cage
        
        Because of difference between atmospheric and intrapulmonary pressure, air flows into lungs, down its pressure gradient, until P-pul = P_ atm
        
        Causes intrapulmonary pressure to drop by 1 mmHg (P_pul < P_atm)
        
        During same period, P_ip lowers to about 6 mmHg less than P_atm
      - Forced (deep) inspirations can occur during vigorous exercise or in people with COPD
      - Accessory muscles are also activated
        
        Scalenes, sternocleidomastoid and pectoralis minor: erector spinae muscles of back also help to straighten thoracic curvature
        
        Act to further increase thoracic cage size, creating a larger pressure gradient so more air is drawn in
    - - Quiet expiration normally is passive process
        
        Volume decrease causes intrapulmonary pressure (P_pul) to increase by +1mmHg
        
        P_pul > P_atm so air flows out of lungs down its pressure gradient until P_pul = P_atm
        
        Inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil
      - Forced expiration is an active process that uses oblique and transverse abdominal muscles, as well as internal intercostal muscles
- - - - Oxyhemoglobin (HbO2)
        As O2 binds, Hb changes shape, increasing its affinity for O2 increases
      - Reduced hemoglobin (deoxyhemoglobin) (HHb):
        As O2 is released, Hb shape changes causes a derease in affinity for O2
    - - Partial Pressure of O2:
        ♢ Plotting %Hb saturation against P_O2 concentrations results in an S-shaped curve known as oxygen-hemoglobin dissociation curve
        ♢In arterial blood; P_O2 is 100 mm Hg andcontains 20 volume % oxygen. Hb is 98% saturated
        ♢In venous blood; P_O2 is 40 mm Hg and contains 15 volume %oxygen. Hb is still 75% saturated (venous reserve)
      - Other Factors:
        ♢Increases in temperature, H+, P_CO2, and BPG can modify structures of hemoglobin resulting in decrease for Hb's affinity for O2 and enhances O2 unloading. Overall shift in curve to right
        ♢Decreases in these factors shifts curve to left