Respiratory System

Carbon dioxide in the body
CO₂ combines with water to form H₂CO₃ in the RBC, catalyzed by enzyme carbonic anhydrase. H⁺, from H₂CO₃, combines with hemoglobin (protein buffer) and HCO₃^- leaves the cell into plasma. When the RBC reaches the lungs, its hemoglobin releases H⁺ and H⁺ combines with a bicarbonate ion to reform CO₂.

Oxygen in the body

Unlike carbon dioxide, oxygen is not very soluble in plasma, but is carried on the hemoglobin of RBCs. In the oxygen-hemoglobin dissociation curve, hemoglobin easily gives up oxygen to tissues where there is low pO₂ (40 mmHg), yet binds well to oxygen in the alveoli where there is high pO₂ (60-100 mmHg). Hemoglobin also releases more O₂ when pCO₂ is elevated, when pH is low, or when temperature is high (where oxygen needs may be increased). This is represented by the dissociation curve shifting right, for reduced affinity.

How do cells control the use of oxygen?
The level of ADP limits O₂ consumption. This means when ADP is low (and there is abundant ATP), the cell doesn't need as much O₂. Less O₂ reacts in oxidative phosphorylation to produce ATP.

How does the brain control respiration? 
Carbon dioxide and H⁺
Increased blood CO₂ or H⁺ level stimulate the brain stem to increase respiration in order to remove excess CO₂ and decrease blood acidity. Carotid and aortic bodies are also stimulated to increase firing, which relays neural messages to the brain stem to increase respiration.

Oxygen
Decreased blood pO₂ increases the firing of the carotid and aortic bodies. This information is relayed to the brain stem.

Hering-Breuer reflex
Stretch in the lungs at the bronchiolar and bronchial tree tells the brain stem to inhibit inspiration.

Note: Remember that the CO₂ control mechanism also balances blood pH. The O₂ control mechanism is simple and does no more than balancing O₂ level.

Respiratory problems 
Inspiratory muscles / Control of respiration
Examples: brain stem injury, spinal cord injury, stroke, polio.

Pneumothorax
Air wrongly enters into pleural space, causing lung collapse due to inadequate negative pressure. Causes include obstruction of respiratory passage, thick membrane preventing efficient gas diffusion (pulmonary edema), fluid in alveolar space (pulmonary edema, pneumonia).

Diminished surface area of alveolar space from damage
Examples: collapsed alveoli (atelectasis), scarring of the lung (pulmonary fibrosis), alveolar destruction from smoking (emphysema).

Abnormally low blood oxygen (arterial hypoxemia)
Oxygen fails to diffuse into the lungs, due to low air ventilation or poor blood perfusion at the alveolar membrane.

Inefficient cardiovascular perfusion of tissues
Oxygen supply cannot meet the tissue energy needs during intense exercise.

Respiratory effort terminology

• Tidal Volume (TV), the average breath of air inhaled or exhaled.
• Inspiratory Reserve Volume (IRV), the extra amount of air that can be inhaled after TV.
• Expiratory Reserve Volume (ERV), the extra amount of air that can be exhaled after TV.
• Residual Volume (RV), the air left in lungs after strongest expiration.
• Functional Residual Capacity (FRC) = RV + ERV. (After TV exhalation, the total remaining volume of air in the lungs). 
• Inspiratory Capacity (IC) = TV + IRV. (After TV exhalation, the maximal volume of air that can be inhaled). 
• Vital Capacity (VC), the maximum amount of air you can ever inhale or exhale, an exchange between maximal intake and most forceful expiration.
• Total Lung Capacity (TLC), maximal volume of air in the lungs (after maximal inhalation).