Module 10 - Respiration

• The functions of the respiratory system

  • the transport of oxygen from the air into the blood
  • the removal of carbon dioxide from the blood
  • the control of blood acidity
  • temperature regulation
  • forming a line of defense to airborne particles
• The lungs are located in the thoracic cavity
• The airway consists of the nasal cavity and mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli
• The pulmonary artery branches extensively to form a dense network of capillaries around each alveolus. These capillaries are made up of endothelial cells, large cross-sectional area, and have a low blood velocity to maximize gas exchange
• There are roughly 300 million alveoli in a healthy human lung each with a diameter of 0.3mm
• The walls of an alveoli are one cell thick and are composed of alveolar epithelial cells (Type I cells). Type II cells secrete surfactant that lines the alveoli
• The region between the alveolar space and the capillary lumen is the respiratory membrane. This membrane is where gas exchange takes place. Cells of the immune system such as macrophages and lymphocytes protect the body from airborne particles that make their way into the alveoli
• The lungs have a parietal pleura (lines the ribs) and visceral pleura (lines the lungs). This forms the intrapleural space which contains a small amount of pleural fluid
• Pleural fluid reduces friction between the two pleural membranes (visceral and parietal)
• Due to the nature of ribs and their attached muscles, they tend to spring outward, while the lungs, due to their presence of elastin tend to recoil and collapse
• The pressure inside the lungs is called alveolar pressure or intrapulmonary pressure. The pressure in the intrapleural space is called the intrapleural pressure. The atmospheric pressure outside of the body at sea level is 760 mmHg. The pressure in the intrapleural space is 756 mmHg. The chest wall and the lungs moving in opposite directions causes the lower intrapleural pressure
• Intrapulmonary pressure is equal to 760 mmHg
• The transpulmonary pressure is equal to the intrapulmonary pressure - intrapleural pressure. This pressure difference is what keeps the lungs open. In a healthy set of lungs this pressure should be positive (outward) and keeps the lungs and alveoli open
• If both the intrapulmonary and intrapleural pressures were equal (transpulmonary pressure of 0 mmHg) then the lungs would collapse and produce a pneumothoroax. This occurs when the intrapleural space gets punctured.
• Boyle's law states that when the volume of a container decreases, the pressure inside increases (and vice versa). (inversely related)
• Moving air into the lungs requires a pressure gradient. To move air into the lungs requires high pressure outside and low pressure pressure in the alveoli (and opposite to move air out). Since you cannot change the atmospheric pressure, the alveoli pressure must change
• To decrease intrapulmonary pressure, the lung volume must increase. To increase the volume, the diaphragm contracts and the external intercostal muscles of the ribs contract, lifting the rib cage up and out. - this causes a lower pressure in the lungs and higher pressure outside causing airflow into the lungs
• The mechanisms for expiration depend on whether you are relaxing or exercising
• At rest, during expiration the diaphragm and external intercostal muscles relax causing the lungs to recoil to their original size - as a result the volume decreases causing the intrapulmonary pressure to increase above the outside pressure (inside 761 mmHg and outside 760 mmHg) - this is a passive process since no muscle contractions occur
• During exercise, air must be forced out of the lungs. This requires contraction of the abdominal muscles and internal intercostal muscles of the ribs. When the muscles contract, they further decrease the volume of the lungs creating a larger pressure gradient (inside 763 mmHg and outside 760 mmHg) forcing air out
• Pulmonary compliance is the stretchability of the lungs - the volume change that occurs as a result of the change in pressure
• Pulmonary compliance is important for the ease of breathing. A lung that has decreased compliance (decreased stretchability) will be harder to inflate
• There are two major factors that influence the compliance of the lung

  • the amount of elastic tissue in the alveoli, blood vessels, and bronchi
  • the surface tension of the film of liquid that lines the alveoli
• The more elastin, the less compliant the lung - like a thick rubber brand
• two-thirds of the elastic behaviour of the lung is attributed to the surface tension of the liquid film lining of the alveoli. The surface tension of the liquid tends to collapse the alveoli, decreasing compliance and making inflation difficult
• Surface tension is the force developed at the surface of a liquid and is due to the attractive forces between water molecules
• Pulmonary surfactant is a lipoprotein substance produced by type II alveolar cells and consists mostly of phospholipids. Surfactant has a hydrophilic head and hydrophobic tail that faces away.
• The maximum amount of air our lungs can hold is 5 litres. We do not breath this much in every breath. The amount of air we inhale/exhale depends on a variety of factors including health, age, and level of activity
• The spirometer is a device used to measure lung volumes and capacities. They are also useful in helping diagnose pulmonary diseases like asthma, bronchitis, and emphysema
• There are four basic lung volumes and capacities - a lung capacity consists of two or more lung volumes

  • tidal volume
  • inspiratory reserve volume
  • Expiratory reserve volume
  • Residual volume
  • inspiratory capacity
  • functional residual capacity
  • vital capacity
  • total lung capacity
• Tidal volume - the volume of air entering or leaving the lungs during one breath at rest = (500ml)
• Inspiratory reserve volume - the maximum amount of air that can enter the lungs in addition to the tidal volume (2500 ml)
• Expiratory reserve volume - the maximum amount of air that can be exhaled beyond tidal volume (1000 ml)
• Residual volume - the remaining air in the lungs after maximal expiration (1200 ml)
• Inspiration capacity - the maximum amount of air that inhaled after exhaling the tidal volume (tidal volume + inspiratory reserve volume)
• Functional residual capacity - the amount of air still in the lungs after exhalation of the tidal volume (expiratory reserve volume + residual volume)
• Vital capacity - the maximum amount of air that can be exhaled after a maximal inhalation (inspiratory reserve volume + tidal volume + expiratory reserve volume)
• Total lung capacity - the maximum amount of air that lungs can hold (vital capacity + residual volume)
• Pulmonary ventilation is the amount of air that enters all of the conducting and respiratory zones in one minute
• The conducting zone is the area of the lungs where no gas exchange occurs

  • trachea
  • primary bronchi
  • secondary bronchi
  • bronchioles