Chapter 22: The Respiratory System

Respiratory System

• The respiratory system is responsible for ventilation of the lungs and gas exchange within the lungs.

• Principal organs include the nose, pharynx, larynx, trachea, bronchi, and lungs. Within the lungs, air flows down a dead-end path from the bronchi through bronchioles to alveoli. The alveoli are the site of gas exchange between the air and the blood.

Pulmonary Ventilation

• Ventilation consists of a repetitive cycle of inspiration and expiration.

• One complete breath is called a respiratory cycle.

• Air flows in and out due to variations in pressure in the thoracic cavity caused by changes in the volume of the thoracic cavity.

The Respiratory Muscles

• The principal muscles of respiration are the diaphragm and the intercostal muscles.

• Contraction enlarges the thoracic cavity and decreases the pressure inside that cavity.

• This leads to an inflow of air through the airways

Resistance to Airflow

• Resistance to airflow is regulated by the diameter of the bronchioles and by pulmonary compliance.

• Epinephrine and sympathetic nerves (norepinephrine) stimulate bronchodilation to increase airflow.

• Parasympathetic nerves, cold air, and chemical irritants stimulate bronchoconstriction.

  Pulmonary Compliance

• Pulmonary compliance refers to the ease with which the lungs expand.

• Higher compliance means the lungs are more likely to expand.

• Compliance can be reduced by lung diseases such as tuberculosis.

• One major limitation on pulmonary compliance is the thin film of water found on the respiratory epithelium. The film is necessary for gas exchange, but it creates a surface tension (due to hydrogen bonds between the water molecules) that must be overcome to allow the alveoli to expand.

• Pulmonary surfactant disrupts the hydrogen bonds and reduces surface tension.

• Premature infants often have a lack of pulmonary surfactant and therefore have difficulty breathing (infant respiratory distress syndrome).

Alveolar Gas Exchange

• For oxygen to move into the blood from the alveolus, it must dissolve in the thin film of water covering the epithelium of the alveolus and then pass across the wall of the alveolus into the blood.

• Carbon dioxide must move in the opposite direction and then diffuse out of the film of water into the air inside the alveolus.

Gas Exchange

Movements of oxygen and carbon dioxide into and out of the blood are driven by differences in the partial pressures of those individual gases.

Oxygen Transport

• About 98.5% of oxygen is bound to hemoglobin as it is transported; the rest is dissolved in the blood plasma.

• Hemoglobin can be called oxyhemoglobin if it is carrying oxygen or deoxyhemoglobin if it is not.

• The utilization coefficient refers to the percentage of oxygen that is released from hemoglobin at the systemic capillaries

Carbon Dioxide Transport

1. About 90% of carbon dioxide reacts with water to form carbonic acid, which dissociates into biacarbonate and hygrogen ions:

   CO2 + H20 —→ H2CO3 ——→ HCO3- + H+

2. About 5% is bound to hemoglobin

3. About 5% is dissolved in the blood.

Carbon Dioxide Loading

• The conversion of carbon dioxide to bicarbonate and hydrogen ions occurs much more quickly inside RBC’s due to the presence of carbonic anhydrase.

• The bicarbonic gets pumped out of the RBC in exchange for a chloride ion. This is called the chloride shift.

• The hydrogen ion binds to hemoglobin and promotes the release of oxygen.

Acidosis and Alkalosis

• A bkiid pH lower than 7.35 is called acidosis , while a blood pH greater than 7.45 is called alkalosis.

• One can correct acidosis. by hyperventilation, thus driving off carbon dioxide. this shifts the carbonic acid reaction to the left.

• CO2 + H2O C2CO3 HCO3- + H+

• One can correct alkalosis by hypoventilating; this allows for the accumulation of carbon dioxide. This drives the same reaction to the right.

• CO2 + H2O → C2CO3 →HCO3- + H+